View source for Physics:CPLEAR experiment

[[File:LEAR experiemental area.jpg|thumb|Low Energy Antiproton Ring experimental area.]]
The '''CPLEAR  experiment''' used the [[Physics:Antiproton|antiproton]] [[Physics:Particle beam|beam]] of the LEAR facility – Low-[[Physics:Energy|Energy]] Antiproton Ring which operated at [[Organization:CERN|CERN]] from 1982 to 1996 – to produce neutral [[Physics:Kaon|kaon]]''s'' through [[Physics:Proton|proton]]-[[Physics:Antiproton|antiproton]] [[Physics:Annihilation|annihilation]] in order to study ''[[Physics:CP violation|CP]]'', ''T'' and ''[[Physics:CPT symmetry|CPT]]'' violation in the neutral kaon system.<ref name=ruf_CPLEAR_website_CERN>
{{cite web
 |url=http://cplear.web.cern.ch/cplear/Welcome.html
 |title=Welcome to the CPLEAR Experiment
 |website=CPLEAR experiment
 |publisher=Thomas Ruf
 |access-date=2018-07-09
 |quote= General Introduction to the experiment }}</ref>

==Background==
According to the theory of the [[Astronomy:Big Bang|Big Bang]], [[Physics:Matter|matter]] and [[Physics:Antimatter|antimatter]] would have existed in the same amount at the beginning of the [[Astronomy:Universe|Universe]]. If this was true, [[Physics:Particle|particle]]''s'' and [[Physics:Antiparticle|antiparticle]]''s'' would have annihilated each other, creating [[Physics:Photon|photon]]''s'', and thus the Universe would have been only compounded by [[Physics:Light|light]] (one particle of matter for 10<sup>18</sup> photons). However, only matter has remained and at a rate of one billion times more particles than expected. What happened then, for the antimatter to disappear in favor of matter? A possible answer to this question is [[Physics:Baryogenesis|baryogenesis]], the hypothetical physical process that took place during the early universe that produced baryonic asymmetry, i.e. the imbalance of [[Physics:Matter|matter]] (baryons) and [[Physics:Antimatter|antimatter]] (antibaryons) in the observed universe. However, baryogenesis is only possible under the following [[Physics:Baryogenesis#GUT Baryogenesis under Sakharov conditions|conditions]] proposed by [[Biography:Andrei Sakharov|Andrei Sakharov]] in 1967:
* [[Physics:Baryon number|Baryon number]] <math>B</math> violation.
* [[Physics:C-symmetry|C-symmetry]] and CP-symmetry violation.
* Interactions out of [[Physics:Thermal equilibrium|thermal equilibrium]].

The first experimental test of [[Physics:CP violation|CP violation]] came in 1964 with the [[Physics:CP violation#Indirect CP violation|Fitch-Cronin experiment]]. The experiment involved particles called neutral [[Physics:Kaon|K-mesons]], which fortuitously have the properties needed to test CP. First, as mesons, they're a combination of a [[Physics:Quark|quark]] and an anti-quark, in this case, [[Physics:Down quark|down]] and [[Physics:Strange quark|antistrange]], or [[Physics:Down quark|anti-down]] and [[Physics:Strange quark|strange]]. Second, the two different particles have different CP values and different [[Physics:Particle decay|decay]] modes: K<sub>1</sub> has CP = +1 and decays into two [[Physics:Pion|pion]]s; K<sub>2</sub> has CP = −1 and decays into three. Because decays with larger changes in mass occur more readily, the K<sub>1</sub> decay happens 100 times faster than the K<sub>2</sub> decay. This means that a sufficiently long beam of neutral Kaons will become arbitrarily pure K<sub>2</sub> after a sufficient amount of time. The Fitch-Cronin experiment exploits this. If all the K<sub>1</sub>s are allowed to decay out of a beam of mixed Kaons, only K<sub>2</sub> decays should be observed. If any K<sub>1</sub> decays are found, it means that a K<sub>2</sub> flipped to a K<sub>1</sub>, and the CP for the particles flipped from −1 to +1, and CP wasn't conserved. The experiment resulted in an excess of 45±9 events around cos(θ) = 1 in the correct mass range for 2-pion decays. This means that for every decay of K<sub>2</sub> into three pions, there are (2.0±0.4)×10-3 decays into two pions. Because of this, neutral K mesons violate CP.<ref name="fitch-cronin">{{cite web |last1=Coleman |first1=Stuart |title=The Fitch-Cronin Experiment |url=http://large.stanford.edu/courses/2008/ph204/coleman1/ |accessdate=27 June 2019}}</ref> The study of the ratio of neutral kaon and neutral anti-kaons production is thus an efficient tool to understand what happened in the early Universe that promoted the production of matter.<ref name="GabathulerPavlopoulos2004" />

== The experiment ==
CPLEAR is a collaboration of about 100 scientists, coming from 17 institutions from 9 different countries. Accepted in 1985, the experiment took data from 1990 until 1996.<ref name=ruf_CPLEAR_website_CERN/> Its main aim was to study CP, ''T'' and ''[[Physics:CPT symmetry|CPT]]'' symmetries in the neutral kaon system.

In addition, CPLEAR performed measurements about quantum [[Physics:Coherence|coherence]] of [[Wave function|wave function]]''s'', Bose-Einstein correlations in multi-[[Physics:Pion|pion]] states, regeneration of the short-lived kaon component in the matter, the [[Physics:EPR paradox|Einstein-Rosen-Podolsky paradox]] using entangled neutral-kaon pair states and the [[Astronomy:Equivalence principle|equivalence principle]] of [[Physics:General relativity|general relativity]].<ref name="PhysicsAtCPLEAR2003">
{{cite report
 |last1=Angelopoulos|first1=A.
 |display-editors=et. al.
 |title=Physics at CPLEAR
 |journal=Physics Reports
 |volume=374
 |year=2003
 |number=3
 |issn=0370-1573}}</ref>

=== Facility description ===

[[File:Cp lear detector.png|thumb|A scheme of the CPLEAR detector.]]

The CPLEAR detector was able to determine the locations, the momenta and the charges of the [[Physics:Tracking (particle physics)|track]]s at the production of the neutral kaon and at its decay, thus visualizing the complete event.

[[Physics:Strangeness|Strangeness]] is not conserved under weak interactions, meaning that under weak interactions a {{SubatomicParticle|Kaon0}} can transform into a {{SubatomicParticle|AntiKaon0}} and vice versa. To study the [[Asymmetry|asymmetries]] between {{SubatomicParticle|Kaon0}} and {{SubatomicParticle|AntiKaon0}} decay rates in the various final states f (f = π<sup>+</sup>π<sup>−</sup>, π<sup>0</sup>π<sup>0</sup>, π<sup>+</sup>π<sup>−</sup>π<sup>0</sup>, π<sup>0</sup>π<sup>0</sup>π<sup>+</sup>, π''l''ν), the CPLEAR collaboration used the fact that the strangeness of kaons is tagged by the charge of the accompanying kaon. Time-reversal invariance would imply that all details of one of the transformations could be deducible from the other one, i.e. the [[Probability|probability]] for a kaon to oscillate into an anti-kaon would be equal to the one for the reverse process. The measurement of these probabilities required the knowledge of the [[Physics:Strangeness|strangeness]] of a kaon at two different times of its life. Since the strangeness of the kaon is given by the [[Physics:Electric charge|charge]] of the accompanying kaon, and thus be known for each [[Physics:Event (particle physics)|event]], it was observed that this symmetry was not respected, thereby proving the ''T'' violation in neutral kaon systems under weak interaction.<ref name="GabathulerPavlopoulos2004" />

The neutral kaons are initially produced in the annihilation channels
* {{SubatomicParticle|Antiproton}}p → π<sup>+</sup> {{SubatomicParticle|Kaon-}} {{SubatomicParticle|Kaon0}}
* {{SubatomicParticle|Antiproton}}p → π<sup>−</sup> {{SubatomicParticle|Kaon+}} {{SubatomicParticle|AntiKaon0}}<ref name="GabathulerPavlopoulos2004">
{{cite journal
 |last1=Gabathuler|first1=E.
 |last2=Pavlopoulos|first2=P.
 |title=The CPLEAR experiment
 |journal=Physics Reports
 |volume=403-404
 |year=2004
 |pages=303–321
 |doi=10.1016/j.physrep.2004.08.020|bibcode=2004PhR...403..303G
 }}</ref>
which happen when the 10<sup>6</sup> anti-protons per second beam coming from the LEAR facility is stopped by a highly-pressurized [[Chemistry:Hydrogen|hydrogen]] [[Physics:Gas|gas]] target. The low [[Physics:Momentum|momentum]] of the antiprotons and the high [[Physics:Pressure|pressure]] allowed to keep the size of the stopping region small in the [[Physics:Particle detector|detector]].<ref name="AngelopoulosApostolakis2003">
{{cite journal
 |last1=Angelopoulos|first1=A.
 |last2=Apostolakis|first2=A.
 |last3=Aslanides|first3=E.
 |title=Physics at CPLEAR
 |journal=Physics Reports
 |volume=374
 |issue=3
 |year=2003
 |pages=165–270
 |issn=0370-1573
 |doi=10.1016/S0370-1573(02)00367-8|bibcode=2003PhR...374..165A
 }}</ref> Since the proton-antiproton reaction happens at rest, the particles are produced [[Isotropy|isotropically]], and as a consequence, the detector has to have a near-4π symmetry. The whole detector was embedded in a 3.6&nbsp;m long and 2&nbsp;m diameter warm solenoidal [[Physics:Magnet|magnet]] providing a 0.44&nbsp;T uniform [[Physics:Magnetic field|magnetic field]].<ref name="GabathulerPavlopoulos2004" />

The antiprotons were stopped using a pressurized hydrogen gas target. A hydrogen gas target was used instead of liquid hydrogen to minimize the amount of matter in the decay volume. The target initially had a radius of 7&nbsp;cm and subjected to a pressure of 16 bar. Changed in 1994, its radius became equal to 1.1&nbsp;cm, under a 27 bar pressure.<ref name="GabathulerPavlopoulos2004" />

=== Layout of the detector ===

[[File:The CPLEAR detector.png|thumb|The CPLEAR detector|upright=1.8]]

The detector had to fulfill the specific requirements of the experiment and thus had to be able to: 
* do an efficient kaon identification
* select the annihilation channels mentioned in Facility description among the very large number of multi-pions annihilation channels
* distinguish between the different neutral-kaon decay channels
* measure the decay proper time
* acquire a large number of statistics, and for this, it had to have both a high rate capability and a large geometrical coverage<ref name="GabathulerPavlopoulos2004" />

Cylindrical tracking detectors together with a solenoid field were used to determine the charge signs, momenta and positions of the charged particles. They were followed by the particle identification detector (PID) whose role was to identify the charged kaon. It was compounded by a [[Physics:Cherenkov detector|Cherenkov detector]], which carried out the kaon-pion separation; and [[Physics:Scintillator|scintillator]]''s'', measuring the energy loss and the [[Physics:Time of flight|time of flight]] of the charged particles. It was also used for the [[Physics:Electron|electron]]-pion separation. The detection of photons produced in π<sup>0</sup> decays was performed by ECAL, an outermost lead/gas sampling calorimeter, complementary to the PID by separating pions and electrons at higher momenta. Finally, hardwired processors (HWK) were used to analyze and select the events in a few microseconds, deleting the unwanted ones, by providing a full event reconstruction with sufficient precision.<ref name="GabathulerPavlopoulos2004" />

== References ==
<references />

== External links ==

* [https://inspirehep.net/experiments/1109103 CPLEAR] experiment record on [[Physics:INSPIRE-HEP|INSPIRE-HEP]]
* [http://cplear.web.cern.ch/Welcome.html CPLEAR] website
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[[Category:Particle experiments]]

{{Sourceattribution|CPLEAR experiment}}