View source for Physics:Flavor-changing neutral current

[[Image:Tau-decay-fcnc.png|thumb|right|
''Above'': Highly suppressed [[Physics:Tau (particle)|tau]] [[Physics:Particle decay|decay]] via '''flavor-changing neutral current''' at one-loop order in the [[Physics:Standard Model|Standard Model]].<br/>
''Below'': [[Physics:Beyond the Standard Model|Beyond the Standard Model]] tau decay via flavor-changing neutral current mediated by a new ''S'' boson.]]
[[Image:FCNC.svg|frame|right|An example of a hypothetical (i.e., not yet observed) flavor-changing neutral current process in the [[Physics:Minimal Supersymmetric Standard Model|Minimal Supersymmetric Standard Model]]. A strange quark emits a bino, turning into a sdown-type squark, which then emits a Z boson and reabsorbs the bino, turning into a down quark.  If the MSSM squark masses are flavor-violating, such a process can occur.]]

In [[Physics:Particle physics|particle physics]], '''flavor-changing neutral currents''' or '''flavour-changing neutral currents''' ('''FCNCs''') are hypothetical interactions that change the flavor of a [[Physics:Fermion|fermion]] without altering its [[Physics:Electric charge|electric charge]].

==Details==
If they occur in nature (as reflected by [[Physics:Lagrangian (field theory)|Lagrangian]] interaction terms), these processes may induce phenomena that have not yet been observed in experiment. Flavor-changing neutral currents may occur in the [[Physics:Standard Model|Standard Model]] beyond the [[Physics:Feynman diagram|tree level]], but they are highly suppressed by the [[Physics:GIM mechanism|GIM mechanism]]. Several collaborations have searched for FCNC.<ref>{{cite journal |last=Anderson |first=S. |display-authors=0|collaboration=CLEO Collaboration |year=2001 |title=Improved Upper Limits on the Flavor-Changing Neutral Current Decays B→Kℓ{{su|p=+}}ℓ{{su|p=−}} and B→K*(892)ℓ{{su|p=+}}ℓ{{su|p=−}}|journal=[[Physics:Physical Review Letters|Physical Review Letters]] |volume=87 |issue=18 |pages=181803 |doi=10.1103/PhysRevLett.87.181803 |bibcode=2001PhRvL..87r1803A |arxiv=hep-ex/0106060|s2cid=119477202 }}</ref><ref>{{cite journal|last=CMS Collaboration|title=Search for Flavor-Changing Neutral Current Interactions of the Top Quark and Higgs Boson in Final States with Two Photons in Proton-Proton Collisions at s=13 TeV|journal=Physical Review Letters |year=2022 |volume=129 |issue=3 |page=032001 |doi=10.1103/PhysRevLett.129.032001 | pmid=35905365 |arxiv=2111.02219}}</ref><ref>{{Cite web|title=Hunting for anomalous interactions between the Higgs bosons and the top quarks {{!}} CMS Experiment|url=https://cms.cern/news/hunting-anomalous-interactions-between-higgs-bosons-and-top-quarks|access-date=2021-12-24|website=cms.cern}}</ref>  The [[Physics:Tevatron|Tevatron]] [[Physics:Collider Detector at Fermilab|CDF experiment]] observed evidence of FCNC in the decay of the strange B-meson to phi mesons in 2005.<ref>{{Cite journal|title = First Evidence for B{{su|p=0|b=s}}→φφ Decay and Measurements of Branching Ratio and A{{su|b=CP}} for B{{su|p=+}}→φK{{su|p=+}}|journal = [[Physics:Physical Review Letters|Physical Review Letters]]|date = 2005-07-15|pages = 031801|volume = 95|issue = 3|doi = 10.1103/PhysRevLett.95.031801 |last1=Acosta |first1=D. |pmid = 16090735|collaboration=CDF Collaboration |display-authors=0 |arxiv = hep-ex/0502044 |bibcode = 2005PhRvL..95c1801A|s2cid = 40241424}}</ref>

FCNCs are generically predicted by theories that attempt to go beyond the Standard Model, such as the models of [[Physics:Supersymmetry|supersymmetry]] or [[Physics:Technicolor|technicolor]]. Their suppression is necessary for an agreement with observations, making FCNCs important constraints on model-building.

==Example==
Consider a [[Physics:Toy model|toy model]] in which an undiscovered [[Physics:Boson|boson]] ''S'' may couple both to the [[Physics:Electron|electron]] as well as the [[Physics:Tau (particle)|tau ({{math|{{SubatomicParticle|tau}}}})]] via the term
:<math>S\bar\psi_e\psi_\tau</math>
Since the electron and the tau have equal charges, the electric charge of ''S'' clearly must vanish to respect the conservation of electric charge. A [[Physics:Feynman diagram|Feynman diagram]] with ''S'' as the intermediate particle is able to convert a tau into an electron (plus some neutral decay products of the ''S'').

The MEG experiment<ref>{{cite web |url=http://meg.web.psi.ch |title=MEG experiment website}}</ref> at the Paul Scherrer Institute near Zurich will search for a similar process, in which an [[Physics:Muon|antimuon]] decays to a [[Physics:Photon|photon]] and an antielectron (a [[Physics:Positron|positron]]). In the Standard Model, such a process proceeds only by emission and re-absorption of a charged [[Physics:W and Z bosons|{{SubatomicParticle|W boson+}}]], which changes the {{SubatomicParticle|muon+}} into a [[Physics:Neutrino|neutrino]] on emission and then a [[Physics:Positron|positron]] on re-absorption, and finally emits a [[Physics:Photon|photon]] that carries away any difference in [[Physics:Conservation of energy|energy]], spin, and [[Physics:Conservation of momentum|momentum]].

In most cases of interest, the boson involved is not a new boson ''S'' but the conventional [[Physics:W and Z bosons|{{SubatomicParticle|Z boson0}}&nbsp;boson]] itself.<ref>FCNCs involving the photon cannot occur at zero [[Physics:Momentum transfer|momentum transfer]]s, because of the unbroken electromagnetic gauge symmetry; as such, FCNCs involving the photon at a non-zero momentum transfer are relatively suppressed compared to FCNCs involving the {{SubatomicParticle|Z boson}}&nbsp;boson.</ref> This can occur if the coupling to weak neutral currents is (slightly) non-universal. The dominant universal coupling to the Z&nbsp;boson does not change flavor, but sub-dominant non-universal contributions can.

FCNCs involving the {{SubatomicParticle|Z boson}}&nbsp;boson for the down-type [[Physics:Quark|quark]]s at zero momentum transfer are usually parameterized by the [[Physics:Effective action|effective action]] term
:<math>\frac{g}{2\cos \theta_\text{W}}\overline{d}_{L\alpha}U_{\alpha\beta}\gamma^\mu d_{L\beta}Z_\mu</math>

This particular example of FCNC is often studied the most because we have some fairly strong constraints coming from the decay of [[Physics:B meson|{{SubatomicParticle|B0}}&nbsp;mesons]] in [[Physics:Belle experiment|Belle]] and BaBar. The off-diagonal entries of ''U'' parameterizes the FCNCs and current constraints restrict them to be less than one part in a thousand for |''U''<sub>bs</sub>|. The contribution coming from the one-loop Standard Model corrections are actually dominant, but the experiments are precise enough to measure slight deviations from the Standard Model prediction.

Experiments tend to focus on flavor-changing neutral currents as opposed to [[Physics:Charged current|charged current]]s, because the weak neutral current ({{SubatomicParticle|Z boson0}}&nbsp;boson) does not change flavor in the Standard Model proper at the tree level whereas the weak charged currents ({{SubatomicParticle|W boson+-}}&nbsp;bosons) do. New physics in charged current events would be swamped by more numerous {{SubatomicParticle|W boson+-}}&nbsp;boson interactions; new physics in the neutral current would not be masked by a large effect due to ordinary Standard Model physics.

==See also==
*[[Physics:Neutral current|Neutral current]]
*[[Physics:Neutral particle oscillation|Neutral particle oscillation]]
*[[Penguin diagram]]
*[[Physics:Two-Higgs-doublet model|Two-Higgs-doublet model]]

==References==
{{reflist|25em}}

[[Category:Standard Model]]
[[Category:Physics beyond the Standard Model]]

{{Sourceattribution|Flavor-changing neutral current}}