View source for Physics:T2K experiment

{{Short description|Long-baseline neutrino experiment in Japan}}
'''T2K''' ("Tokai to Kamioka") is a [[Physics:Particle physics|particle physics]] experiment studying the oscillations of the accelerator neutrinos. The experiment is conducted in {{wipe|Japan}} by the international cooperation of about 500 physicists and engineers with over 60 research institutions from several countries from Europe, Asia and North America<ref>{{cite web |title=T2K experiment official page – T2K collaboration |url=https://t2k-experiment.org/t2k/collaboration/ |access-date=2020-03-31 |archive-date=2020-04-17 |archive-url=https://web.archive.org/web/20200417050829/https://t2k-experiment.org/t2k/collaboration/ |url-status=live }}</ref> and it is a recognized [[Organization:CERN|CERN]] experiment (RE13).<ref>{{cite web |url=https://greybook.cern.ch/greybook/experiment/recognized |title=Recognized Experiments at CERN |website=The CERN Experimental Programme |publisher=CERN |access-date=9 March 2021 |archive-date=22 April 2019 |archive-url=https://web.archive.org/web/20190422232203/https://greybook.cern.ch/greybook/experiment/recognized |url-status=live }}</ref><ref>{{cite web |url=https://greybook.cern.ch/greybook/experiment/detail?id=RE13 |title=RE13/T2K : The long-baseline neutrino experiment |website=The CERN Experimental Programme |publisher=CERN |access-date=20 January 2020 |archive-date=30 April 2020 |archive-url=https://web.archive.org/web/20200430034507/https://greybook.cern.ch/greybook/experiment/detail?id=RE13 |url-status=live }}</ref> T2K collected data within its first phase of operation from 2010 till 2021. The second phase of data taking (T2K-II) is expected to start in 2023 and last until commencement of the successor of T2K – the Hyper-Kamiokande experiment in 2027.<ref name="panic2021">{{cite web |last1=Vilela |first1=Cristovao |title=The status of T2K and Hyper-Kamiokande experiments |url=https://indico.lip.pt/event/592/contributions/3550/ |website=PANIC 2021 Conference |date=September 5–10, 2021 |access-date=2021-09-29 |archive-date=2021-09-29 |archive-url=https://web.archive.org/web/20210929161023/https://indico.lip.pt/event/592/contributions/3550/ |url-status=live }}</ref>{{rp|12,20}}

T2K was the first experiment which observed the appearance of [[Physics:Electron neutrino|electron neutrino]]s in a [[Physics:Muon neutrino|muon neutrino]] [[Physics:Particle beam|beam]].<ref name="1106.2822">{{Cite journal|arxiv=1106.2822 |author1=T2K Collaboration |title=Indication of Electron Neutrino Appearance from an Accelerator-produced Off-axis Muon Neutrino Beam |journal=Physical Review Letters |volume=107 |issue=4 |pages=041801 |year=2011 |doi=10.1103/PhysRevLett.107.041801 |pmid=21866992 |bibcode = 2011PhRvL.107d1801A |s2cid=16654679 }}</ref> It also provided the world best measurement of oscillation parameter ''θ''<sub>23</sub><ref name="1403.1532">{{Cite journal|arxiv=1403.1532 |author1=T2K Collaboration | title=Precise Measurement of the Neutrino Mixing Parameter θ23 from Muon Neutrino Disappearance in an Off-Axis Beam | journal=Phys. Rev. Lett. |volume=112 | pages=181801 |year=2014 |issue=18 | doi=10.1103/PhysRevLett.112.181801 |pmid=24856687 |bibcode=2014PhRvL.112r1801A |s2cid=11484010 }}</ref> and a hint of a significant [[Physics:CP violation|matter-antimatter asymmetry]] in neutrino oscillations.<ref>{{Cite journal|arxiv=1502.01550 |author1=T2K Collaboration | title=Measurements of neutrino oscillation in appearance and disappearance channels by the T2K experiment with 6.6E20 protons on target | journal=Phys. Rev. |volume=D91 | pages=072010 |year=2015 | doi=10.1103/PhysRevD.91.072010 |s2cid=34184232 }}</ref><ref name="cpnature">{{cite journal |title=Constraint on the matter–antimatter symmetry-violating phase in neutrino oscillations |journal=Nature |date=15 April 2020 |volume=580 |pages=339–344 |doi=10.1038/s41586-020-2177-0 |url=https://www.nature.com/articles/s41586-020-2177-0 |arxiv=1910.03887 |access-date=16 April 2020 |archive-date=16 April 2020 |archive-url=https://web.archive.org/web/20200416073026/https://www.nature.com/articles/s41586-020-2177-0 |url-status=live |last1=Abe |first1=K. |last2=Akutsu |first2=R. |last3=Ali |first3=A. |last4=Alt |first4=C. |last5=Andreopoulos |first5=C. |last6=Anthony |first6=L. |last7=Antonova |first7=M. |last8=Aoki |first8=S. |last9=Ariga |first9=A. |last10=Arihara |first10=T. |last11=Asada |first11=Y. |last12=Ashida |first12=Y. |last13=Atkin |first13=E. T. |last14=Awataguchi |first14=Y. |last15=Ban |first15=S. |last16=Barbi |first16=M. |last17=Barker |first17=G. J. |last18=Barr |first18=G. |last19=Barrow |first19=D. |last20=Barry |first20=C. |last21=Batkiewicz-Kwasniak |first21=M. |last22=Beloshapkin |first22=A. |last23=Bench |first23=F. |last24=Berardi |first24=V. |last25=Berkman |first25=S. |last26=Berns |first26=L. |last27=Bhadra |first27=S. |last28=Bienstock |first28=S. |last29=Blondel |first29=A. |last30=Bolognesi |first30=S. |issue=7803 |pmid=32296192 |bibcode=2020Natur.580..339T |s2cid=203951445 |display-authors=1 }}</ref> The measurement of the neutrino-antineutrino oscillation asymmetry may bring us closer to the explanation of the existence of our [[Astronomy:Baryon asymmetry|matter-dominated]] Universe.<ref>{{cite journal |last1=Fukugita |first1=M. |last2=Yanagida |first2=T. |title=Barygenesis without grand unification |journal=Physics Letters B |date=June 1986 |volume=174 |issue=1 |pages=45–47 |doi=10.1016/0370-2693(86)91126-3|bibcode=1986PhLB..174...45F }}</ref><ref>{{cite journal|last1=Mohapatra |first1=R N|last2=Antusch|first2=S|display-authors=1 |title=Theory of neutrinos: a white paper |journal=Reports on Progress in Physics |date=1 November 2007 |volume=70 |issue=11 |pages=1757–1867 |doi=10.1088/0034-4885/70/11/R02|arxiv=hep-ph/0510213|bibcode=2007RPPh...70.1757M|s2cid=119092531}}</ref>

The intense beam of muon neutrinos is produced in the [[Organization:J-PARC|J-PARC]] facility (Japan Proton Accelerator Research Complex) in Tokai on the east coast of Japan. The beam is directed towards the [[Astronomy:Super-Kamiokande|Super-Kamiokande]] far detector located {{convert|295|km}} away in the city of Hida, Gifu prefecture. The properties and composition of the neutrino flux are first measured by a system of near detectors located {{convert|280|m}} from the beam production place at the J-PARC site, and then again in the Super-Kamiokande detector. Comparison of the content of different neutrino flavours in these two locations allows measurement of the oscillations probability on the way between near and far detectors. Super-Kamiokande is able to detect interactions of both, muon and electron neutrinos, and thus measure the disappearance of muon neutrino flux, as well as electron neutrino appearance in the beam.<ref name="t2knim">{{Cite journal|author=T2K Collaboration |year=2011 |title=The T2K Experiment |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |volume=659 |issue=1 |pages=106–135 |arxiv=1106.1238 |doi=10.1016/j.nima.2011.06.067 |bibcode = 2011NIMPA.659..106A |s2cid=55962579 }}</ref>

==Physics program==

T2K experiment was proposed in 2003 with the following measurement goals:<ref name="t2knim"/>
* The discovery of the {{SubatomicParticle|Muon neutrino}} → {{SubatomicParticle|Electron neutrino}} [[Physics:Neutrino oscillation|oscillations]], and thus the confirmation that the last unknown mixing angle ''θ''<sub>13</sub> is not zero.
* Precise measurement of the oscillation parameters Δ''m''{{su|b=23|p=2}} and ''θ''<sub>23</sub> via muon neutrino disappearance studies.
* Search for [[Physics:Sterile neutrino|sterile neutrino]] oscillations, which could be observed as a deficit of [[Physics:Neutral current|neutral current]] neutrino interactions.
* Measurements of various interaction [[Physics:Cross section|cross-sections]] for different types of neutrinos and targets in an energy range of few GeV.

Since the start of the data taking in 2010, the T2K experiment succeeded to provide a list of world-class results:
* The confirmation of electron neutrino appearance in the muon neutrino beam ({{SubatomicParticle|Muon neutrino}}→{{SubatomicParticle|Electron neutrino}}), which was the first time when neutrinos produced in one flavour was explicitly observed in another flavour.<ref name="1106.2822" /><ref name="1304.0841">{{cite journal |author=T2K Collaboration |title=Evidence of electron neutrino appearance in a muon neutrino beam |journal=Physical Review D |date=5 August 2013 |volume=88 |issue=3 |pages=032002 |doi=10.1103/PhysRevD.88.032002|arxiv=1304.0841|bibcode=2013PhRvD..88c2002A |s2cid=53322828 }}</ref>
* The most precise measurement of the ''θ''<sub>23</sub> parameter.<ref name="1403.1532" />
* Limits on a [[Physics:Sterile neutrino|sterile neutrino]] oscillation parameters based on studies in the near ND280<ref>{{cite journal |author=T2K Collaboration |title=Search for short baseline nue disappearance with the T2K near detector |journal=Physical Review D |date=16 March 2015 |volume=91 |issue=5 |pages=051102 |doi=10.1103/PhysRevD.91.051102 |arxiv=1410.8811|bibcode=2015PhRvD..91e1102A |s2cid=54629105 }}</ref> and far Super-Kamiokande<ref>{{cite journal |author=T2K Collaboration |title=Search for light sterile neutrinos with the T2K far detector Super-Kamiokande at a baseline of 295 km |journal=Physical Review D |date=30 April 2019 |volume=99 |issue=7 |pages=071103 |doi=10.1103/PhysRevD.99.071103 |arxiv=1902.06529|bibcode=2019PhRvD..99g1103A|s2cid=119331309 }}</ref> detectors.
* Various [[Physics:Cross section|cross-section]] measurements of electron<ref>{{cite journal | author=T2K Collaboration |title=Measurement of the charged-current electron (Anti-)neutrino inclusive cross-sections at the T2K off-axis near detector ND280 |journal=Journal of High Energy Physics |date=27 February 2020 |volume=2020 |issue=10 |page=114 |doi=10.1007/JHEP10(2020)114 | arxiv=2002.11986|bibcode=2020JHEP...10..114T |s2cid=211532582 }}</ref><ref>{{cite journal |author=T2K Collaboration |title=Measurement of the electron neutrino charged-current interaction rate on water with the T2K ND280 pi0 detector |journal=Physical Review D |date=19 June 2015 |volume=91 |issue=11 |pages=112010 |doi=10.1103/PhysRevD.91.112010|bibcode=2015PhRvD..91k2010A |doi-access=free |arxiv=1503.08815 }}</ref> and muon neutrinos and antineutrinos, including inclusive [[Physics:Charged current|charged current]] (CC) interactions,<ref>{{cite journal | author=T2K Collaboration |title=Measurement of the inclusive numu charged current cross section on carbon in the near detector of the T2K experiment |journal=Physical Review D |date=7 May 2013 |volume=87 |issue=9 |page=092003 |doi=10.1103/PhysRevD.87.092003|arxiv=1302.4908|bibcode=2013PhRvD..87i2003A |s2cid=54889936 }}</ref> CC interactions without pions<ref>{{cite journal | author=T2K Collaboration |title=Measurement of double-differential muon neutrino charged-current interactions on C8H8 without pions in the final state using the T2K off-axis beam |journal=Physical Review D |date=21 June 2016 |volume=93 |issue=11 |pages=112012 |doi=10.1103/PhysRevD.93.112012|arxiv=1602.03652|bibcode=2016PhRvD..93k2012A |s2cid=35063421 }}</ref><ref>{{cite journal |author=T2K Collaboration|title=Measurement of the numu charged-current quasielastic cross section on carbon with the ND280 detector at T2K |journal=Physical Review D |date=11 December 2015 |volume=92 |issue=11 |page=112003 |doi=10.1103/PhysRevD.92.112003|arxiv=1411.6264|bibcode=2015PhRvD..92k2003A |s2cid=118125266 }}</ref><ref>{{cite journal |author=T2K Collaboration |title=First combined measurement of the muon neutrino and antineutrino charged-current cross section without pions in the final state at T2K |journal=Physical Review D |date=21 February 2020 |volume=101 |issue=11 |page=112001 |doi=10.1103/PhysRevD.101.112001 |arxiv=2002.09323|bibcode=2020PhRvD.101k2001A |s2cid=211252681 }}</ref> and with single pion in the final state,<ref>{{cite journal |author=T2K Collaboration |title=First measurement of the muon neutrino charged current single pion production cross section on water with the T2K near detector |journal=Physical Review D |date=26 January 2017 |volume=95 |issue=1 |pages=012010 |doi=10.1103/PhysRevD.95.012010|arxiv=1605.07964|bibcode=2017PhRvD..95a2010A |s2cid=16748058 }}</ref> coherent pion production,<ref>{{cite journal |author=T2K Collaboration |title=Measurement of Coherent π+ Production in Low Energy Neutrino-Carbon Scattering |journal=Physical Review Letters |date=4 November 2016 |volume=117 |issue=19 |pages=192501 |doi=10.1103/PhysRevLett.117.192501|pmid=27858422 |arxiv=1604.04406|bibcode=2016PhRvL.117s2501A |s2cid=27564132 }}</ref> [[Physics:Neutral current|neutral current]] interactions,<ref>{{cite journal |author=T2K Collaboration |title=Measurement of the neutrino-oxygen neutral-current interaction cross section by observing nuclear deexcitation gamma rays |journal=Physical Review D |date=31 October 2014 |volume=90 |issue=7 |pages=072012 |doi=10.1103/PhysRevD.90.072012|arxiv=1403.3140|bibcode=2014PhRvD..90g2012A |s2cid=53641707 }}</ref> etc. on different targets such as [[Chemistry:Carbon|carbon]], [[Chemistry:Water|water]] and [[Chemistry:Iron|iron]].<ref>{{cite journal |author=T2K Collaboration |title=Measurement of the muon neutrino charged-current cross sections on water, hydrocarbon and iron, and their ratios, with the T2K on-axis detectors |journal=Progress of Theoretical and Experimental Physics |date=September 2019 |volume=2019 |issue=9 |pages=093C02 |doi=10.1093/ptep/ptz070|arxiv=1904.09611 |bibcode=2019PTEP.2019i3C02A }}</ref>
* The first significant constraint on the ''δ''<sub>CP</sub> parameter, responsible for the [[Physics:CP violation|matter-antimatter asymmetry]] in the neutrino oscillations.<ref name="cpnature" />

''δ''<sub>CP</sub> takes values from -''π'' to ''π'' (i.e. from −180° to 180°) and can be measured by comparing oscillations of neutrinos to those of antineutrinos. The CP symmetry would be conserved, and thus the oscillation probabilities would be the same for neutrinos and antineutrinos, for ''δ''<sub>CP</sub> equal to 0 or ±''π''. T2K provided the first and the strongest yet constraint on ''δ''<sub>CP</sub>, rejecting at the 3σ (99.7%) significance level almost half of the possible values, ruling out the both CP conserving points at the significance level of 95% and giving a strong hint that CP violation may be large in the neutrino sector. The CP violation is one of the [[Physics:Baryogenesis#GUT Baryogenesis under Sakharov conditions|conditions]] proposed by the Russian physicist [[Biography:Andrei Sakharov|Andrei Sakharov]], necessary to [[Physics:Baryogenesis|produce]] the excess of matter with respect to antimatter at the [[Astronomy:Big Bang|early universe]], which forms now our [[Astronomy:Baryon asymmetry|matter-built Universe]]. [[Physics:CP violation|CP violation]] in quark section was confirmed already in 1964,<ref name=CroninFitch1964>{{cite journal | author = J. W. Cronin |author2=V. L. Fitch | year = 1964 | title = Evidence for the 2π decay of the {{PhysicsParticle|K|BR=2|TR=0}} meson | journal = [[Physics:Physical Review Letters|Physical Review Letters]] | volume = 13 | pages = 138–140 | doi = 10.1103/PhysRevLett.13.138| bibcode=1964PhRvL..13..138C | issue = 4|display-authors=0| doi-access = free }}</ref> but it is too small to explain the observed matter-antimatter imbalance. The strong CP violation in the neutrino sector could lead to matter excess production through the process called [[Physics:Leptogenesis|leptogenesis]] and thus such measurement would be important step to understand how the Universe were formed.<ref name="cpnature"/><ref>{{cite news |last1=Cho |first1=Adrian |title=Skewed neutrino behavior could help explain matter's dominion over antimatter |url=https://www.science.org/content/article/skewed-neutrino-behavior-could-help-explain-matter-s-dominion-over-antimatter |access-date=19 April 2020 |work=Science {{!}} AAAS |date=15 April 2020 |language=en |archive-date=20 April 2020 |archive-url=https://web.archive.org/web/20200420204308/https://www.sciencemag.org/news/2020/04/skewed-neutrino-behavior-could-help-explain-matter-s-dominion-over-antimatter |url-status=live }}</ref><ref>{{cite web |url=https://www.bbc.com/news/science-environment-52297058 |title=Biggest cosmic mystery 'step closer' to solution |last=Rincon |first=Paul |date=16 April 2020 |website=BBC News website |access-date=18 April 2020 |archive-date=18 April 2020 |archive-url=https://web.archive.org/web/20200418152823/https://www.bbc.com/news/science-environment-52297058 |url-status=live }}</ref>

The [[Physics:NOvA|NOvA]] experiment is the other neutrino oscillation experiment capable to measure ''δ''<sub>CP</sub> through the comparison between {{SubatomicParticle|Muon neutrino}}→{{SubatomicParticle|Electron neutrino}} and {{SubatomicParticle|Muon antineutrino}}→{{SubatomicParticle|Electron antineutrino}} oscillation channels. NOvA is conducted in the United States and measures accelerator neutrino oscillation at the distance of 810&nbsp;km on the way between beam production place in [[Organization:Fermilab|Fermilab]] and far detector in Ash River, Minnesota. NOvA provided a less precise measurement of ''δ''<sub>CP</sub>, which is in slight tension with the T2K result. The T2K best-fit point lies in the region disfavoured by NOvA at the confidence level of 90%. There are ongoing works to obtain a joint fit to data from both experiments to quantify consistency between them.<ref>{{cite conference|last=Himmel|first=Alex|collaboration=NOvA Collaboration|title=New Oscillation Results from the NOvA Experiment|date=2 July 2020|book-title=Neutrino2020|url=https://indico.fnal.gov/event/43209/timetable/#194-new-oscillation-results-fr|doi=10.5281/zenodo.3959581|conference=|access-date=30 September 2021|archive-date=5 March 2021|archive-url=https://web.archive.org/web/20210305050206/https://indico.fnal.gov/event/43209/timetable/#194-new-oscillation-results-fr|url-status=live}}</ref><ref>{{cite journal | author=NOvA Collaboration |title=Improved measurement of neutrino oscillation parameters by the NOvA experiment |journal=Physical Review D |year=2022 |volume=106 |issue=3 |page=032004 |doi=10.1103/PhysRevD.106.032004 | arxiv=2108.08219|s2cid=237195004 }}</ref>

Future upgrades of T2K is expected to provide more precise measurements of Δ''m''{{su|b=23|p=2}} and ''θ''<sub>23</sub> parameters, cross-section measurements which will extend our understanding of neutrino interactions and thus improve theoretical models used in neutrino generators, as well as further constrain on the ''δ''<sub>CP</sub> phase and confirmation if the CP symmetry is conserved or violated in the neutrino oscillation at the 3σ significance level in the T2K-II and 5σ in the Hyper-Kamiokande experiment.<ref name="upgradeproposal">{{cite arXiv |author=T2K Collaboration |title=Proposal for an Extended Run of T2K to 20E21 POT |date=13 September 2016 |class=hep-ex |eprint=1609.04111}}</ref><ref name="1805.04163">{{cite arXiv |author=Hyper-Kamiokande Collaboration |title=Hyper-Kamiokande Design Report |date=28 November 2018 |class=physics.ins-det |eprint=1805.04163}}</ref>

==Neutrino beam==

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| caption1 = Bird's-eye view of the entire facility
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T2K uses a muon neutrino or muon antineutrino [[Physics:Accelerator neutrino|beam]] produced at the [[Organization:J-PARC|J-PARC]] facility using a proton beam gradually accelerated to 30 GeV by a system of three [[Physics:Particle accelerator|accelerators]]: first to 400 MeV energy by the Linac linear accelerator, then up to 3 GeV by the RCS (Rapid Cycle Synchrotron), and finally up to 30 GeV by the MR [[Physics:Synchrotron|synchrotron]] (Main Ring). [[Physics:Proton|Proton]]s collide with a [[Chemistry:Graphite|graphite]] target, producing [[Physics:Meson|meson]]s, mainly [[Physics:Pion|pion]]s and [[Physics:Kaon|kaon]]s, which are then focused by a set of three [[Physics:Magnetic horn|magnetic horn]]s and directed into a tunnel called the decay volume. Depending on the horns polarity, either positive or negative particles are focused. Positive pions and kaons decay mainly into {{SubatomicParticle|Muon+}} and {{SubatomicParticle|Muon neutrino}}, forming a muon neutrino beam, while negative pions and kaons decay mainly into {{SubatomicParticle|Muon-}} and {{SubatomicParticle|Muon antineutrino}}, forming a muon antineutrino beam. All remaining [[Physics:Hadron|hadron]]s and charged [[Physics:Lepton|lepton]]s are stopped by a 75-ton block of graphite (so-called beam dump) and in the ground, while neutrinos travel underground towards the far detector.<ref name="t2knim"/>

===Off-axis beam===

T2K is the first experiment in which the concept of off-axis [[Physics:Accelerator neutrino|neutrino beam]] was realized. The neutrino beam at J-PARC is designed so that it can be directed 2 to 3 [[Degree (angle)|degrees]] away from the [[Astronomy:Super-Kamiokande|Super-Kamiokande]] far detector and one of the near detectors, ND280. The average energy of neutrinos decreases with the deviation from the beam axis. The off-axis angle was chosen to 2.5° to maximize the probability of oscillation at a distance corresponding to the far detector, which for {{convert|295|km}} is maximal for around 600&nbsp;MeV neutrinos. In this neutrino energy range, the dominant type of neutrino interactions are [[Physics:Charged current|charged current]] quasielastic interactions, for which it is possible to reconstruct the energy of the interacting neutrino only on the basis of the momentum and direction of the produced charged lepton. The higher neutrino energies are suppressed by the off-axis configuration, decreasing the number of interactions with meson production, which are background in the oscillation analysis in the T2K experiment.<ref name="t2knim"/><ref name="t2kflux">{{cite journal |author=T2K Collaboration |title=T2K neutrino flux prediction |journal=Physical Review D |date=2 January 2013 |volume=87 |issue=1 |pages=012001 |doi=10.1103/physrevd.87.012001|arxiv=1211.0469|bibcode=2013PhRvD..87a2001A |s2cid=55114627 }}</ref>

==Near detectors==

The near detector complex<ref name="t2knim"/> is located at a distance of {{convert|280|m}} from the graphite target. Its purpose is to measure the neutrino flux before oscillations and to study neutrino interactions. The system consists of three main detectors:
* INGRID detector (Interactive Neutrino GRID) located on the axis of the neutrino beam,
* ND280 detector located 2.5° away from the beam axis, i.e. at the same angle as the far detector.
* WAGASCI-BabyMIND (WAter Grid SCIntillator Detector – prototype Magnetized Iron Neutrino Detector) is a magnetised neutrino detector located at 1.5° off-axis angle, built to explore the energy spectrum variation with the off-axis angle and cross-sections at higher average neutrino energy.<ref name="babymind">{{cite journal|display-authors=1 |last1=Antonova |first1=M. |last2=Asfandiyarov |first2=R. |title=Baby MIND: A magnetised spectrometer for the WAGASCI experiment |arxiv=1704.08079 |date=April 2017 |journal=Prospects in Neutrino Physics | issue=NUPHYS2016-HALLSJO, FERMILAB-CONF-17-270-APC}}</ref><ref name="wagasci">{{cite journal|display-authors=1 |last1=Ovsiannikova |first1=T |last2=Antonova |first2=M |title=The new experiment WAGASCI for water to hydrocarbon neutrino cross section measurement using the J-PARC beam |journal=Journal of Physics: Conference Series |date=5 February 2016 |volume=675 |issue=1 |pages=012030 |doi=10.1088/1742-6596/675/1/012030|bibcode=2016JPhCS.675a2030O |doi-access=free |hdl=2433/235156 |hdl-access=free }}</ref>

===Signal readout===

[[File:Scintillator.svg|thumb|Principle of operation of a scintillator in the T2K near detectors]]
Except for the Time Projection Chambers in ND280, the entire active material (enabling particle tracking) of the near detectors is [[Chemistry:Plastic|plastic]] [[Physics:Scintillator|scintillator]]. The light produced by traversing charged particles in the plastic scintillator bars and planes is collected by [[Physics:Wavelength shifter|wavelength-shifting]] [[Engineering:Optical fiber|fibres]] and detected by Hamamatsu [[Physics:Silicon photomultiplier|Multi-pixel photon counters]] located at one or both ends of the fibres. Scintillator bars are organised into layers, where bars in two neighbouring layers are perpendicular to each other providing together 3D information about the traversing particles.<ref name="t2knim"/>

===INGRID detector===

The main purpose of the INGRID detector is the monitoring of the direction and intensity of the beam on a daily basis by direct detection of neutrino interactions. The INGRID detector consists of 16 identical modules arranged in the shape of a cross, 7 in a vertical and 7 in a horizontal arm, plus 2 modules outside the cross. The height and width of the arms are {{convert|10|m}}. A single module consists of alternating layers of iron and a plastic scintillator. An additional 4 veto<ref group=note>Veto is a part of a detector where no activity should be registered to accept an event. Such requirement allows constraining the number of background events in a selected sample; here the background from particles produced outside of the detector.</ref> layers of the scintillator surround the module on the sides to distinguish particles entering from the outside from those produced by interactions inside the module. The total mass of iron in one module is 7.1 tons and constitutes 96% of the module weight. On the neutrino beam axis, in the middle of the cross between the vertical and horizontal arm, there is an additional module built only from layers of the plastic scintillator (Proton Module) with a mass of 0.55 tons. Its purpose is to register quasielastic interactions and compare the obtained results with the simulations.<ref name="t2knim"/>

===ND280 detector===

{{multiple image
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The ND280 detector is used to measure the flux, energy spectrum and electron neutrino beam pollution for the same off-axis angle as for the far detector. ND280 also investigates various types of muon and electron neutrino and antineutrino interactions. All this allows estimating the expected number and type of interactions in the far detector, reducing the systematic error in the neutrino oscillations analysis associated with models of neutrino interactions and flux.<ref name="t2knim"/>

ND280 is composed of the set of inner sub-detectors: Pi-Zero detector and a tracker with 2 Fine-Grained Detectors interleaved with 3 Time Projection Chambers, placed inside of a metal frame called a basket. The basket is surrounded by the electromagnetic calorimeter and a magnet recycled from the [[Physics:UA1 experiment|UA1 experiment]] producing 0.2 T uniform horizontal [[Physics:Magnetic field|magnetic field]] and instrumented with scintillator planes constituting the Side Muon Range Detector.<ref name="t2knim"/>

====Pi-Zero detector====

[[File:Nd280 p0d layers structure.png|180px|thumb|Pi-Zero detector scheme.]]
The Pi-Zero ({{SubatomicParticle|Pion0}}) Detector (P0D) contains 40 plastic scintillator module planes, which in the central part are interleaved with 2.8&nbsp;cm thick bags fillable of water and thick brass sheets, and in two peripheral regions scintillator modules are sandwiched with lead sheets. By comparison of the amount of interaction between modes with and without water in the bags, it is possible to extract the number of neutrino interactions occurring on the water – the target material inside the far detector Super-Kamiokande.
The size of the entire active P0D volume is around 2.1 m × 2.2 m × 2.4 m (X×Y×Z) and its mass with and without water is 15.8 and 12.9 tons respectively.

The main goal of the Pi-Zero Detector is a measurement of neutral [[Physics:Pion|pion]]s production in [[Physics:Neutral current|neutral current]] neutrino interactions on water:
: {{SubatomicParticle|Muon neutrino}} + N → {{SubatomicParticle|Muon neutrino}} + N’ + {{SubatomicParticle|Pion0}}
This reaction can mimic electron neutrino interactions because photons from {{SubatomicParticle|Pion0}} decay can be mis-reconstructed as an electron in the Super-Kamiokande detector, thus this reaction can mimic electron neutrino interactions and constitute an important background in electron neutrino appearance measurement.<ref name="t2knim"/><ref>{{cite journal |first1=S|last1=Assylbekov|first2=B E|last2=Berger|display-authors=1 |title=The T2K ND280 off-axis pi–zero detector |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |date=September 2012 |volume=686 |pages=48–63 |doi=10.1016/j.nima.2012.05.028|arxiv=1111.5030|bibcode=2012NIMPA.686...48A|s2cid=118606685}}</ref>

====Time projection chambers====
[[File:TPC_for_T2K.jpg|thumb|TPC for the ND280 Upgrade of T2K experiment in Japan]]
Three [[Physics:Time projection chamber|Time Projection Chambers]] (TPCs) are gas-tight rectangular boxes, with a cathode plane in the centre and readout [[Physics:MicroMegas detector|MicroMegas]] modules at both sides parallel to the cathode. TPCs are filled with [[Chemistry:Argon|argon]]-based drift gas under atmospheric pressure. Charged particles crossing TPC ionise the gas along their track. The ionisation electrons drift from the cathode to the sides of the TPC, where they are detected by the MicroMegas providing a 3D image of a path of the traversing charged particle. Y and Z coordinates are based on the position of the detected ionisation electrons on the MicroMegas modules, and X coordinate is based on the electrons drift time. In the magnetic field, the curvature of this path allows to determine [[Physics:Electric charge|charge]] and [[Physics:Momentum|momentum]] of the particle, and the amount of the ionisation electrons per unit distance is used to identify particles based on the [[Physics:Bethe formula|Bethe-Bloch formula]].<ref name="t2knim"/><ref>{{cite journal |author=T2K ND280 TPC collaboration |title=Time projection chambers for the T2K near detectors |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |date=May 2011 |volume=637 |issue=1 |pages=25–46 |doi=10.1016/j.nima.2011.02.036|arxiv=1012.0865|bibcode=2011NIMPA.637...25A }}</ref>

====Fine-grained detectors====

Two Fine-Grained Detectors (FGDs) are placed after the first and second TPCs. Together the FGDs and TPCs make up the tracker of ND280. The FGDs provide the active target mass for the neutrino interactions and are able to measure the short tracks of proton recoil. The first FGD is composed of scintillator layers only, while the second FGD is composed of alternating layers of scintillator and water. The second FGD is partially composed of water because the Super-Kamiokande detector is water-based. Cross sections on carbon and on the water can be determined from a comparison of neutrino interactions in the two FGDs.<ref name="t2knim"/><ref>{{cite journal |author=T2K ND280 FGD Collaboration |title=The T2K fine-grained detectors |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |date=December 2012 |volume=696 |pages=1–31 |doi=10.1016/j.nima.2012.08.020 |arxiv=1204.3666|bibcode=2012NIMPA.696....1A }}</ref>

====Electromagnetic Calorimeter====

The Electromagnetic Calorimeter (ECal) surrounds the inner detectors (P0D, TPCs, FGDs) and consists of scintillator layers sandwiched with lead absorber sheets. Its role is to detect neutral particles, especially photons, and measure their energy and direction, as well as to detect charged particles providing additional information relevant for their identification.<ref name="t2knim"/><ref>{{cite journal |author=T2K UK Collaboration |title=The electromagnetic calorimeter for the T2K near detector ND280 |journal=Journal of Instrumentation |date=17 October 2013 |volume=8 |issue=10 |pages=P10019 |doi=10.1088/1748-0221/8/10/P10019 |arxiv=1308.3445|bibcode=2013JInst...8P0019A |s2cid=55993395 }}</ref>

====Side Muon Range Detector====

The Side Muon Range Detector (SMRD) consists of scintillator modules which are inserted into the gaps in the magnet. The SMRD records muons escaping the inner parts of the detector at large angles with respect to the beam direction. The remaining types of particles (except for neutrinos) are mostly stopped in the calorimeter. SMRD can also act as a [[Physics:Trigger (particle physics)|trigger]] for cosmic rays. Finally, it can help identify beam interactions in the surrounding walls and in the magnet itself.<ref name="t2knim"/><ref>{{cite journal |first1=S|last1=Aoki|first2=G|last2=Barr|display-authors=1|title=The T2K Side Muon Range Detector (SMRD) |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |date=January 2013 |volume=698 |pages=135–146 |doi=10.1016/j.nima.2012.10.001|arxiv=1206.3553|bibcode=2013NIMPA.698..135A|s2cid=119280423}}</ref>

===WAGASCI-BabyMIND===

[[File:Wagasci and ND280 neutrino flux.png|thumb|The predicted T2K neutrino flux at the site of the WAGASCI-BabyMIND (red line) and of the ND280 (black line) detectors]]
WAGASCI-BabyMIND is a new detector located next to the INGRID and ND280 detectors, devoted to [[Physics:Neutrino|neutrino]] interaction studies. It provided the first neutrino beam data using a full detector setup during the 2019/2020 winter run.<ref name="babymind"/><ref name="wagasci"/>

The WAGASCI-BabyMIND consists of several sub-detectors:
* Two new [[Chemistry:Water|water]]-[[Physics:Scintillator|scintillator]] detectors (WAGASCI, WAter-Grid-SCIntillator-Detector) that act as the main water targets and particle trackers. The 3D grid-like structure of scintillator bars creates hollow cavities filled with water. 
Thanks to such a structure, a high water to scintillator mass ratio was obtained (80% H<sub>2</sub>O + 20% CH) and the acceptance is high and approximately constant in all directions.<ref name="babymind"/><ref name="wagasci"/>
* One Proton Module, the same as in the INGRID detector, made of plain [[Chemistry:Plastic|plastic]] [[Physics:Scintillator|scintillator]] (CH) bars, that acts as the main CH target and particle tracker.<ref name="babymind"/><ref name="wagasci"/>
* Two WallMRD (Wall Muon Range Detector) that are non-magnetized muon spectrometers to detect side going muons. They are made of passive [[Chemistry:Iron|iron]] planes intertwined with active scintillator planes.<ref name="babymind"/><ref name="wagasci"/>
* One BabyMIND (prototype Magnetized Iron Neutrino Detector) that is a magnetized muon spectrometer to detect forward-going muons. BabyMIND sports an original configuration of scintillation modules intertwined with magnetized [[Chemistry:Ferrite (iron)|ferrite]] modules like a sandwich. The modules can be rearranged easily to adapt the magnetic field to the particular needs of the experiment. The magnetic field is created only inside the ferrite so it is very power efficient compared to magnets that have to magnetize empty spaces around them like the ND280 one. However, the magnetic field is not homogeneous over the travel volume of the muons, and this poses a still open challenge for momentum reconstruction.<ref name="babymind"/>
All the active material in the detectors is made up of plastic scintillator and is read as explained in section Signal readout.<ref name="babymind"/><ref name="wagasci"/>

The main goal of the WAGASCI-BabyMIND detector is a reduction of the systematic error in the T2K [[Physics:Neutrino oscillation|oscillation]] analysis, which will be achieved thanks to its complementarity with respect to the ND280 detector:
* Different target material between ND280 (80% CH + 20% H<sub>2</sub>O) and SK (pure H<sub>2</sub>O) forces us to rely on cross-section models to disentangle the H<sub>2</sub>O cross-section estimate from the CH one. The fraction of water in WAGASCI water-scintillator modules is 80% enabling a measurement of the charged-current neutrino cross-section ratio between water (H<sub>2</sub>O) and plastic (CH) with 3% accuracy.<ref name="babymind"/><ref name="wagasci"/>
* The new detector will provide measurements of various charged-current neutrino interaction channels with high precision, lower momentum threshold and full angular acceptance. These will constrain flux and cross-section models uncertainties for the particles produced at high angles. These assets will also facilitate detection of low momentum hadrons produced in the interaction of the neutrino with bounded states of 2 nucleons or through reinteractions inside the target nucleus of particles produced by the neutrino, and thus better modelling of such interactions in the far detector.<ref name="babymind"/><ref name="wagasci"/>
* Location at the same distance of 280 meters from the graphite target as ND280 and INGRID detectors, but at a different off-axis angle of 1.5 degrees, causes that the energy spectrum of the neutrino beam is peaked around different energies for each of the off-axis angles corresponding to the detectors. [[Linear combination|Combination]] of measurements from these detectors will provide an improved constraint on the neutrino cross-sections as a function of their energy.<ref name="babymind"/><ref name="wagasci"/>

==Super-Kamiokande==
{{main|Astronomy:Super-Kamiokande}}

[[File:Superkamiokande electron muon discriminator.png|thumb|Detection of electrons and muons in the Super-Kamiokande detector]]
Super-Kamiokande detector is located 1000 m underground in the Mozumi Mine, under Mount Ikeno in the Kamioka area of Hida city. It is a [[Chemistry:Stainless steel|stainless steel]] [[Cylinder|cylindrical]] tank of about 40 m height and diameter, filled with 50,000 tons of [[Chemistry:Water|water]] and instrumented with around 13,000 [[Physics:Photomultiplier tube|photomultiplier tube]]s (PMT). It detects a [[Cone|cone]] of [[Physics:Cherenkov radiation|Cherenkov light]] emitted by charged particles moving in water faster than light in this medium.
Its goal is to measure [[Physics:Muon|muon]]s and [[Physics:Electron|electron]]s produced in [[Physics:Charged current|charged current]] quasielastic interactions (CCQE) of {{SubatomicParticle|Muon neutrino}} and {{SubatomicParticle|Electron neutrino}}, respectively. Due to relatively large mass, muons usually do not change their direction and thus produce a well-defined cone of Cherenkov light observed by PMTs as a clear, sharp ring. In contrast, electrons, because of smaller mass, are more susceptible to scattering and almost always produce electromagnetic [[Physics:Particle shower|showers]], observed by PMTs as a ring with fuzzy edges. Neutrino energy is calculated based on the direction and energy of a charged [[Physics:Lepton|lepton]] produced in the CCQE interaction. In this way, {{SubatomicParticle|Muon neutrino}} and {{SubatomicParticle|Electron neutrino}} spectra are determined, leading to the measurement of the [[Physics:Neutrino oscillation|oscillation]] parameters relevant for muon neutrino disappearance and electron neutrino appearance.<ref name="t2knim"/><ref>{{cite journal |author=The Super-Kamiokande Collaboration |title=The Super-Kamiokande detector |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |date=April 2003 |volume=501 |issue=2–3 |pages=418–462 |doi=10.1016/S0168-9002(03)00425-X|bibcode=2003NIMPA.501..418F }}</ref>

==History==

T2K is a successor of the KEK to Kamioka (K2K) experiment, which ran from 1999 till 2004. In the K2K experiment, an [[Physics:Accelerator neutrino|accelerator beam]] of muon neutrinos was produced at [[Organization:KEK|KEK]] facility in Tsukuba ({{wipe|Japan}}) and sent towards the [[Astronomy:Super-Kamiokande|Super-Kamiokande]] detector, located 250&nbsp;km away. The K2K experiment results confirmed at the confidence level of 99.9985% (4.3 [[Standard deviation|σ]]) the [[Physics:Neutrino oscillation|disappearance]] of the [[Physics:Muon neutrino|muon neutrino]]s and were consistent with the previous measurements of oscillation parameters measured by the Super-Kamiokande detector for [[Physics:Neutrino#Atmospheric|atmospheric neutrino]]s.<ref>{{cite book |last1=Oyama |first1=Yuichi |title=Nuclear Science and Safety in Europe |chapter=Results from K2K and status of T2K |series=NATO Security through Science Series |year=2006 |pages=113–124 |doi=10.1007/978-1-4020-4965-1_9 |arxiv=hep-ex/0512041|isbn=978-1-4020-4963-7 |s2cid=117059077 }}</ref><ref>{{cite journal |author=K2K Collaboration |title=Measurement of neutrino oscillation by the K2K experiment |journal=Physical Review D |date=12 October 2006 |volume=74 |issue=7 |pages=072003 |doi=10.1103/PhysRevD.74.072003|arxiv=hep-ex/0606032|bibcode=2006PhRvD..74g2003A |s2cid=22053653 }}</ref>

The construction of the neutrino beamline started in 2004 and it was successfully commissioned in 2009. Construction of the entire INGRID detector and majority of the ND280 detector (without barrel part of the electromagnetic calorimeter) was completed in 2009. The missing part of the calorimeter was installed in the fall of 2010. T2K far detector is the large Super-Kamiokande detector, which has been running since 1996 and studying proton lifetime and oscillations of [[Physics:Neutrino#Atmospheric|atmospheric]], [[Astronomy:Solar neutrino|solar]] and [[Physics:Accelerator neutrino|accelerator]] neutrinos.<ref name="t2knim"/>

T2K experiment started to take neutrino data for a physics analysis in January 2010, initially with an incomplete ND280 detector, and starting from November 2010 with the full setup. The data taking was interrupted for one year by the Great Tohoku Earthquake in March 2011. The proton beam power, and thus the neutrino beam intensity, was constantly growing, reaching by February 2020 the power of 515&nbsp;kW and a total number of accumulated protons on target of 3.6×10<sup>21</sup> protons<ref>{{cite web |title=T2K experiment official page – T2K Run 10 |url=https://t2k-experiment.org/2020/02/t2k-run-10-ended-with-record-beam-power/ |access-date=2020-03-31 |archive-date=2020-05-03 |archive-url=https://web.archive.org/web/20200503142816/https://t2k-experiment.org/2020/02/t2k-run-10-ended-with-record-beam-power/ |url-status=live }}</ref> with 55% of data in neutrino-mode and 45% in antineutrino-mode.<ref name="panic2021"/>{{rp|12}}

==Future plans==

The T2K experiment operated in the current form until 2020. In 2021 the first data run with [[Chemistry:Gadolinium|gadolinium]] loaded into the Super-Kamiokande far detector was taken.<ref name="panic2021"/>{{rp|12}} In 2021–2022 a major upgrade of the neutrino beamline and the ND280 near detector will be performed. From 2023 till 2026 neutrino data will be taken within the second phase of the T2K experiment (T2K-II).<ref name="lomonosov2021">{{cite web |last1=Kudenko |first1=Yury |title=Physics and status of SuperFGDdetector for T2K experiment |url=https://lomcon.ru/files/20LomCon/presentations/Presenteations/19/lomonosov2021_kudenko.pdf |website=The 20th Lomonosov Conference on Elementary Particle Physics |date=August 19–25, 2021 |pages=17–18 |access-date=2021-09-29 |archive-date=2021-09-29 |archive-url=https://web.archive.org/web/20210929150951/https://lomcon.ru/files/20LomCon/presentations/Presenteations/19/lomonosov2021_kudenko.pdf |url-status=live }}</ref><ref name="upgradeproposal"/> In 2027, the successor of the T2K experiment – the Hyper-Kamiokande (HK) experiment – will be launched with the new, 250,000-ton water [[Physics:Cherenkov radiation|Cherenkov]] far detector – the [[Astronomy:Hyper-Kamiokande|Hyper-Kamiokande]] detector.<ref name="panic2021"/>{{rp|20}}<ref>{{cite journal |author=Hyper-Kamiokande Proto-Collaboration |title=Physics Potential of a Long Baseline Neutrino Oscillation Experiment Using J-PARC Neutrino Beam and Hyper-Kamiokande |journal=Progress of Theoretical and Experimental Physics |date=19 May 2015 |volume=2015 |issue=5 |pages=53C02–0 |doi=10.1093/ptep/ptv061 |arxiv=1502.05199 |bibcode=2015PTEP.2015e3C02A |issn=2050-3911}}</ref><ref name="hyperk">{{cite arXiv |author=Hyper-Kamiokande Proto-Collaboration |title=Hyper-Kamiokande Design Report |date=28 November 2018 |class=physics.ins-det |eprint=1805.04163}}</ref> The building of an additional Intermediate Water Cherenkov detector at a distance of around {{convert|2|km}} is also considered for the HK experiment.<ref name="hyperk"/>

===T2K-II===

Phase II of the T2K experiment is expected to start at the beginning of 2023 and last until 2026, following by the commencement of the HK experiment. The physics goals of T2K-II are a measurement of the [[Physics:Neutrino oscillation|oscillation]] parameters ''θ''<sub>23</sub> and Δ''m''{{su|b=23|p=2}} with a precision of 1.7° and 1%, respectively, as well as a confirmation at the level of 3 [[Standard deviation|σ]] or more of the matter-antimatter asymmetry in the neutrino sector in a wide range of possible true values of ''δ''<sub>CP</sub> – the parameter responsible for the [[Physics:CP violation|CP]] (matter-antimatter) asymmetry. Achievement of these goals requires the reduction of statistical and systematic errors. Thus a significant upgrade of the beamline and the ND280 detector, doping of SK water with [[Chemistry:Gadolinium|gadolinium]] to allow {{SubatomicParticle|Neutrino}}/{{SubatomicParticle|Antieutrino}} discrimination in the far detector, as well as improvements in the software and analysis methods will be done.<ref name="upgradeproposal"/>

====Beam upgrade====

The beam upgrade plan requires one year long shut down of the [[Organization:J-PARC|J-PARC]] Main Ring [[Physics:Particle accelerator|accelerator]] in 2021, followed by a constant gradual increase of the [[Physics:Proton|proton]] [[Physics:Particle beam|beam]] power until the start of the HK experiment. The beam power should reach 750&nbsp;kW in 2022 and then grow to 1.3 MW by 2029.<ref name="beamupgradetdr">{{cite arXiv |author=T2K Collaboration and J-PARC Neutrino Facility Group |title=J-PARC Neutrino Beamline Upgrade Technical Design Report |date=14 August 2019 |class=physics.ins-det |eprint=1908.05141}}</ref>

In February 2020, the proton beam power reached 515&nbsp;kW with 2.7x10<sup>14</sup> protons per pulse and with 2.48 seconds between pulses (so-called repetition cycle). To reach 750&nbsp;kW, the repetition cycle will be reduced to 1.32 s with 2.0x10<sup>14</sup> protons per pulse, while for 1.3 MW the repetition cycle has to be further decreased to 1.16 s and the number of protons per pulse has to increase to 3.2x10<sup>14</sup>. In addition to increasing the primary proton beam power, the current in the [[Physics:Magnetic horn|horns]] focusing secondary particles ([[Physics:Pion|pion]]s, [[Physics:Kaon|kaon]]s, etc.) with a chosen [[Physics:Electric charge|electric charge]] will also be increased from 250 kA to 320 kA. This will increase the amount of right-sign neutrinos (neutrinos in the neutrino mode beam and anti-neutrinos in the anti-neutrino mode beam) by 10%, and reduce the amount of wrong-sign neutrinos (anti-neutrinos in the neutrino-mode beam and neutrinos in the anti-neutrino mode beam) by around 5&nbsp;-&nbsp;10%.<ref name="beamupgradetdr"/><ref name="beamupgradeprogramme">{{cite journal |last1=Friend |first1=M |title=J-PARC accelerator and neutrino beamline upgrade programme |journal=Journal of Physics: Conference Series |date=September 2017 |volume=888 |issue=1 |pages=012042 |doi=10.1088/1742-6596/888/1/012042 |bibcode=2017JPhCS.888a2042F |language=en |issn=1742-6588|doi-access=free }}</ref>

Reduction of the repetition cycle will require a series of hardware upgrades, including a major upgrade of the Main Ring power supplies and a minor upgrade of the focusing horn power supplies, all of which will be installed during the long shutdown in 2021. Increasing the horn current will require using an additional (third) horn power supply. Meanwhile, the higher proton beam power demands enhancement of the [[Physics:Cooling|cooling]] capacity of the secondary beamline components such as the [[Chemistry:Graphite|graphite]] target, the magnetic horns and the beam dump, as well as disposal of a larger amount of irradiated cooling water.<ref name="beamupgradetdr"/><ref name="beamupgradeprogramme"/>

====ND280 Upgrade====

[[File:Nd280upgrade scheme.png|thumb|Scheme of the inner part of the ND280 detector after planned upgrade]]
The current design of the ND280 detector is optimized for the detection and reconstruction of forward-going [[Physics:Lepton|lepton]]s ([[Physics:Muon|muon]]s and [[Physics:Electron|electron]]s), but it also has a number of limitations, like low reconstruction efficiency of particles produced almost perpendicular and backward with respect to the direction of the interacting [[Physics:Neutrino|neutrino]], as well as too high momentum threshold to reconstruct a large part of produced pions and knocked-out nucleons (protons and neutrons). In Charged Current Quasi-Elastic (CCQE) interactions, the dominating interaction in the ND280 near detector, kinematics of produced lepton is enough in the reconstruction of the incoming neutrino energy. However, other types of neutrino interactions in which additional particles ([[Physics:Pion|pion]]s, [[Physics:Kaon|kaon]]s, [[Physics:Nucleon|nucleon]]s) were lost, may be mis-reconstructed as CCQE and introduce a [[Bias (statistics)|bias]] in the reconstructed neutrino energy spectrum. Thus, it is essential to optimize the detector to be sensitive to additional particles and [[Physics:Nuclear structure|nuclear effects]].

Three main measures need to be taken to address these issues:
* The detector needs to efficiently detect the nucleons in the final state of neutrino interactions. For this, the detection thresholds need to be lowered.
* High-angle and backwards-going tracks must be well-reconstructed. This is achieved by increasing the angular acceptance and the efficiency of the discrimination between backward from forward going tracks using timing information.
* Finally, the total fiducial volume (the mass available for neutrino interactions) of the tracker part of the ND280 detector, characterised with a better reconstruction ability, needs to be enlarged in order to increase the rate of neutrino interactions.

The Upgrade of the ND280 detector (ND280 Upgrade) addresses these requirements by replacing a part of the P0D sub-detector with three types of new sub-detectors. The existing downstream part, consisting of two Fine-Grained scintillation Detectors (FGDs) and three Time Projection Chambers (TPCs), will maintain their sandwiched structure and continue to detect forward going leptons and high momentum hadrons. The upstream part which now hosts the P0D sub-detector will be replaced by three novel sub-detectors: a scintillating 3D target (Super Fine-Grained Detector or SuperFGD), two new TPCs on top and below the SuperFGD (High-Angle TPCs or HATPCs), and six Time-of-Flight (TOF) detectors surrounding the new structure. Each of these sub-detectors is briefly described below.<ref name="nd280upgradetdr">{{cite arXiv |author=T2K Collaboration|title=T2K ND280 Upgrade – Technical Design Report |date=11 January 2019 |class=physics.ins-det |eprint=1901.03750}}</ref> The installation of the new sub-detectors into ND280 will be done in 2022.<ref name="lomonosov2021"/><ref name="nd280upgradetdradd">{{cite journal |author=The T2K ND280 Upgrade Working Group |title=NP07: ND280 Upgrade project |date=June 19, 2020 |url=https://cds.cern.ch/record/2713578 |journal=CERN Scientific Committee Paper |volume=CERN-SPSC-2020-008. SPSC-SR-267 |access-date=September 29, 2021 |archive-date=September 29, 2021 |archive-url=https://web.archive.org/web/20210929150951/https://cds.cern.ch/record/2713578 |url-status=live }}</ref>{{rp|18}}

=====SuperFGD=====

The SuperFGD is a {{convert|2|x|2|x|0.5|m}} detector consisting of approximately 2 million 1&nbsp;cm<sup>3</sup> [[Physics:Scintillator|scintillating]] [[Chemistry:Polystyrene|polystyrene]] [[Cube|cube]]s. The cubes are woven with a series of [[Engineering:Optical fiber|optical fibres]] designed to detect the light emitted by the particles produced during the interactions in the target. Unlike the current FGDs, the SuperFGD has a three-fold projective 2D readouts providing a quasi-3D readout. This readout configuration increases the detection of short tracks almost uniformly in all directions. Due to its geometry and coupled with the TOF and the HATPCs, the SuperFGD has the capability to detect fast-neutrons, which could be useful in the reconstruction of the antineutrino energy.<ref name="nd280upgradetdr"/>

=====HATPC=====

The High Angle Time Projection Chambers (HATPCs) will surround the SuperFGD in the plane perpendicular to the incoming neutrino beam. Their design is similar to that of the existing TPCs, as they both use the [[Physics:MicroMegas detector|MicroMegas]] modules technology for track reconstruction. The main novel feature of the HATPCs, aside from their high angle coverage, is the use of the resistive MicroMegas technology. The latter consists of applying a layer of [[Physics:Electrical resistance and conductance|resistive]] material to increase the charge-sharing capabilities of the MicroMegas modules. This reduces the number of readout channels and allows for a spatial resolution which is as good as the one in the current TPCs.<ref name="nd280upgradetdr"/>

=====TOF=====

The six Time-of-Flight (TOF) detectors surrounding the HATPCs and SuperFGD are a series of [[Chemistry:Plastic|plastic]] [[Physics:Scintillator|scintillator]] layers designed to identify the particle direction sense through the measurement of the [[Physics:Time of flight|time of flight]] for each crossing track with a timing resolution of the order of 600 ps. The capability to determine track direction sense has been proven in the actual ND280 to be critical to reduce background generated outside the active inner detectors.<ref name="nd280upgradetdr"/>

=====Impact on Neutrino Oscillation Physics=====

The impact the ND280 Upgrade will have on the analyses at T2K is two-fold. Firstly, an increase in statistics thanks to the 2 ton SuperFGD target will allow to nearly double the amount of data in certain samples. Secondly and more relevant, the new configuration will allow for better detection of additional final state particles: high angle particles thanks to the increased angular acceptance, and less energetic particles because of lower detection thresholds. This detector acceptance improvement is important to cover almost the same phase space available at the far detector (SK). In addition, final state particles will allow probing nuclear effects which are essential for constraining the systematic effects of the oscillation analysis. It is an important step as well in the transition to using semi-inclusive or exclusive models in neutrino oscillation physics, as opposed to current inclusive models which use only the final state lepton in their predictions.<ref name="nd280upgradetdr"/>

====SK-Gd====

The third element to be improved within T2K – phase II is the introduction of [[Chemistry:Gadolinium|gadolinium]] into the [[Physics:T2K experiment#Super-Kamiokande|Super-Kamiokande]], which so far was filled with ultra-pure water. SK is not able to measure the [[Physics:Electric charge|charge]] of the registered particle. That means it is not possible to distinguish neutrino from antineutrino interaction based on a charge of produced [[Physics:Lepton|lepton]] (e.g. {{SubatomicParticle|Muon-}} is produced by {{SubatomicParticle|Muon neutrino}} while {{SubatomicParticle|Muon+}} by {{SubatomicParticle|Muon antineutrino}}). In (anti)neutrino-nucleus interactions, apart from a charged lepton production, a [[Physics:Nucleon|nucleon]] is usually realised from the [[Physics:Atomic nucleus|nucleus]]. Because of [[Physics:Charge conservation|charge conservation]], for neutrinos it is mostly a proton and for antineutrinos – a neutron:
: {{SubatomicParticle|Lepton neutrino}} + {{SubatomicParticle|Neutron}} → {{SubatomicParticle|Lepton-}} + {{SubatomicParticle|Proton}}
: {{SubatomicParticle|Lepton antineutrino}} + {{SubatomicParticle|Proton}} → {{SubatomicParticle|Lepton+}} + {{SubatomicParticle|Neutron}}.<ref>{{cite journal |last1=Formaggio |first1=J. A. |last2=Zeller |first2=G. P.|title=From eV to EeV: Neutrino cross sections across energy scales |journal=Reviews of Modern Physics |date=24 September 2012 |volume=84 |issue=3 |pages=1307–1341 |doi=10.1103/RevModPhys.84.1307|arxiv=1305.7513 |bibcode=2012RvMP...84.1307F |hdl=1721.1/75437 |s2cid=54087013 }}</ref>{{rp|23}}
Cherenkov energy threshold (minimal total energy of a charged particle to produce [[Physics:Cherenkov radiation|Cherenkov light]]) is proportional to the particle mass, and in water it equals 0.8 MeV for electrons, 160 MeV for muons and 1400 MeV for protons.<ref>{{cite journal |last1=Scholberg |first1=Kate |title=Supernova Neutrino Detection in Water Cherenkov Detectors |journal=Journal of Physics: Conference Series |date=10 August 2011 |volume=309 |issue=1 |pages=012028 |doi=10.1088/1742-6596/309/1/012028|bibcode=2011JPhCS.309a2028S |s2cid=121098709 |doi-access=free }}</ref> Thus, protons released in neutrino interactions often fall below the threshold and remain undetected. Neutron, as a neutral particle, does not produce Cherenkov light. However, it can be [[Physics:Neutron capture|absorbed]] by another nucleus, which goes into an [[Physics:Excited state|excited state]] and during deexcitation produced [[Physics:Gamma ray|gamma ray]]s. High energy photons (for gadolinium their total energy is about 8 MeV) [[Physics:Compton scattering|scatter electrons]] from an atom and/or [[Physics:Pair production#Photon to electron and positron|produce electron-positron pairs]], which then produce Cherenkov light. Gadolinium is a naturally occurring element with the highest cross-section on the capture of neutrons at thermal energy. For 25 meV neutrons, the cross-section for gadolinium is about 10<sup>5</sup> times higher than for [[Chemistry:Hydrogen|hydrogen]]. The fraction of neutrons that will be captured in SK is 50% for 0.01% Gd concentration and 90% for 0.1% concentration – the planned final Gd concentration in SK. The signal from neutron capture is delayed by a fraction of a millisecond (the time the neutron travels across the water before the capture plus the time when Gd remains in the excited state) with respect to the charged lepton signal and usually appears within a distance of 50&nbsp;cm (the distance travelled by the neutron before the capture) from the neutrino interaction point. Such a double flash event (the first flash from the charged lepton, the second flash from the Gd deexcitation photons) is a signature of an antineutrino interaction.<ref name='skgdnews'>{{cite web |title=Super-Kamiokande Official Website |url=http://www-sk.icrr.u-tokyo.ac.jp/sk/news/2020/08/sk-gd-detail-e.html |website=www-sk.icrr.u-tokyo.ac.jp |access-date=2021-10-07 |archive-date=2021-10-07 |archive-url=https://web.archive.org/web/20211007142738/http://www-sk.icrr.u-tokyo.ac.jp/sk/news/2020/08/sk-gd-detail-e.html |url-status=live }}</ref><ref name='skgd'>{{cite journal |author=The Super-Kamiokande Collaboration |title=First gadolinium loading to Super-Kamiokande |journal=Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment |year=2022 |volume=1027 |page=166248 |doi=10.1016/j.nima.2021.166248 |arxiv=2109.00360|bibcode=2022NIMPA102766248A |s2cid=237372721 }}</ref>

The first loading of 13 tons of Gd<sub>2</sub>(SO<sub>4</sub>)<sub>3</sub>·8{{H2O}} (gadolinium(III) sulfate [[Chemistry:Hydrate|octahydrate]]) into SK water was done in  July–August 2020 and lead to a 0.011% concentration of Gd. T2K collected its first data with Gd in SK in March–April 2021. Usage of gadolinium-doped water will allow studying [[Astronomy:Diffuse supernova neutrino background|remote supernova neutrinos]], for which {{SubatomicParticle|Electron antineutrino}}'s are the most reactive in SK but were yet indistinguishable from neutrinos from other sources. It will also improve the detector performance for [[Astronomy:Supernova|supernova]] explosions in [[Astronomy:Milky Way|our galaxy]] and study better matter-antimatter differences in [[Physics:Accelerator neutrino|accelerator neutrino]] oscillations.<ref name='skgdnews'/><ref name='skgd'/>

===Hyper-Kamiokande experiment===
{{main|Astronomy:Hyper-Kamiokande}}

The successor of the T2K experiment, the [[Astronomy:Hyper-Kamiokande|Hyper-Kamiokande]] (HK) experiment, will use the upgraded system of the currently used accelerator and neutrino beamline and upgraded set of the near detector. Apart from that, a new far detector, the [[Astronomy:Hyper-Kamiokande#Hyper-Kamiokande Far Detector|Hyper-Kamiokande detector]], and possibly also a new [[Astronomy:Hyper-Kamiokande#Intermediate Detector|intermediate detector]] will be built. Part of the beam related upgrade works and the upgrade of the ND280 detector will be performed yet before the start of phase II of the T2K experiment. The HK experiment is expected to start operation around the year 2027.<ref name="panic2021"/>{{rp|20}}<ref name="hyperk"/><ref name="hyperkstart">{{cite news|url=http://www.j-parc.jp/c/en/topics/2020/02/12000416.html|title=The Hyper-Kamiokande project is officially approved.|date=12 February 2020|access-date=1 April 2020|archive-date=27 September 2020|archive-url=https://web.archive.org/web/20200927061633/https://j-parc.jp/c/en/topics/2020/02/12000416.html|url-status=live}}</ref><ref>{{cite journal |author=Hyper-Kamiokande Proto-Collaboration |title=Physics potential of a long-baseline neutrino oscillation experiment using a J-PARC neutrino beam and Hyper-Kamiokande |journal=Progress of Theoretical and Experimental Physics |date=19 May 2015 |volume=2015 |issue=5 |pages=53C02–0 |doi=10.1093/ptep/ptv061 |arxiv=1502.05199|bibcode=2015PTEP.2015e3C02A }}</ref>

==See also==
* [[Astronomy:Kamioka Observatory|Kamioka Observatory]]

==Notes==
{{reflist|group=note}}

==References==
{{reflist}}

==External links==
{{Commons|T2K}}
* [http://www.t2k-experiment.org/ T2K Experiment Official Website]
* [http://www-sk.icrr.u-tokyo.ac.jp/sk/index-e.html Super-Kamiokande Official Website]
* [http://www-sk.icrr.u-tokyo.ac.jp/realtimemonitor/ Super-Kamiokande Realtime Monitor]
* [https://www.youtube.com/watch?v=tBrFrdSneZg Neutrino physics – The T2K experiment – YouTube]
* [https://www.youtube.com/watch?v=cs02i8TIphs Inside Japan's Big Physics | Part one: Super Kamiokande – YouTube]

{{Neutrino detectors}}

[[Category:Accelerator neutrino experiments]]
[[Category:CERN experiments]]

{{Sourceattribution|T2K experiment}}