In particle physics, flavor-changing neutral currents or flavour-changing neutral currents (FCNCs) are hypothetical interactions that change the flavor of a fermion without altering its electric charge.
Above: Highly suppressed taudecay via flavor-changing neutral current at one-loop order in the Standard Model. Below: Beyond the Standard Model tau decay via flavor-changing neutral current mediated by a new S boson.An example of a hypothetical (i.e., not yet observed) flavor-changing neutral current process in the 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.
If they occur in nature (as reflected by Lagrangian interaction terms), these processes may induce phenomena that have not yet been observed in experiment. Flavor-changing neutral currents may occur in the Standard Model beyond the tree level, but they are highly suppressed by the GIM mechanism. Several collaborations have searched for FCNC.[1][2][3] The TevatronCDF experiment observed evidence of FCNC in the decay of the strange B-meson to phi mesons in 2005.[4]
FCNCs are generically predicted by theories that attempt to go beyond the Standard Model, such as the models of supersymmetry or technicolor. Their suppression is necessary for an agreement with observations, making FCNCs important constraints on model-building.
Exampleedit
Consider a toy model in which an undiscovered bosonS may couple both to the electron as well as the tau ( τ− ) via the term
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 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[5] at the Paul Scherrer Institute near Zürich will search for a similar process, in which an antimuon decays to a photon and an antielectron (a positron). In the Standard Model, such a process proceeds only by emission and re-absorption of a charged W+ , which changes the μ+ into a neutrino on emission and then a positron on re-absorption, and finally emits a photon that carries away any difference in energy, spin, and momentum.
In most cases of interest, the boson involved is not a new boson S but the conventional Z0 boson itself.[6] This can occur if the coupling to weak neutral currents is (slightly) non-universal. The dominant universal coupling to the Z boson does not change flavor, but sub-dominant non-universal contributions can.
FCNCs involving the Z boson for the down-type quarks at zero momentum transfer are usually parameterized by the effective action term
This particular example of FCNC is often studied the most because we have some fairly strong constraints coming from the decay of B0 mesons in 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 |Ubs|. 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 charged currents, because the weak neutral current ( Z0 boson) does not change flavor in the Standard Model proper at the tree level whereas the weak charged currents ( W± bosons) do. New physics in charged current events would be swamped by more numerous W± boson interactions; new physics in the neutral current would not be masked by a large effect due to ordinary Standard Model physics.
^Anderson, S.; et al. (CLEO Collaboration) (2001). "Improved Upper Limits on the Flavor-Changing Neutral Current Decays B→Kℓ+ ℓ− and B→K*(892)ℓ+ ℓ− ". Physical Review Letters. 87 (18): 181803. arXiv:hep-ex/0106060. Bibcode:2001PhRvL..87r1803A. doi:10.1103/PhysRevLett.87.181803. S2CID 119477202.
^CMS Collaboration (2022). "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". Physical Review Letters. 129 (3): 032001. arXiv:2111.02219. doi:10.1103/PhysRevLett.129.032001. PMID 35905365.
^"Hunting for anomalous interactions between the Higgs bosons and the top quarks | CMS Experiment". cms.cern. Retrieved 2021-12-24.
^Acosta, D.; et al. (CDF Collaboration) (2005-07-15). "First Evidence for B0 s→φφ Decay and Measurements of Branching Ratio and A CP for B+ →φK+ ". Physical Review Letters. 95 (3): 031801. arXiv:hep-ex/0502044. Bibcode:2005PhRvL..95c1801A. doi:10.1103/PhysRevLett.95.031801. PMID 16090735. S2CID 40241424.
^FCNCs involving the photon cannot occur at zero momentum transfers, 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 Z boson.
April 11, 2024
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This article may be too technical for most readers to understand Please help improve it to make it understandable to non experts without removing the technical details July 2012 Learn how and when to remove this template message In particle physics flavor changing neutral currents or flavour changing neutral currents FCNCs are hypothetical interactions that change the flavor of a fermion without altering its electric charge Above Highly suppressed tau decay via flavor changing neutral current at one loop order in the Standard Model Below Beyond the Standard Model tau decay via flavor changing neutral current mediated by a new S boson An example of a hypothetical i e not yet observed flavor changing neutral current process in the 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 Contents 1 Details 2 Example 3 See also 4 ReferencesDetails editIf they occur in nature as reflected by Lagrangian interaction terms these processes may induce phenomena that have not yet been observed in experiment Flavor changing neutral currents may occur in the Standard Model beyond the tree level but they are highly suppressed by the GIM mechanism Several collaborations have searched for FCNC 1 2 3 The Tevatron CDF experiment observed evidence of FCNC in the decay of the strange B meson to phi mesons in 2005 4 FCNCs are generically predicted by theories that attempt to go beyond the Standard Model such as the models of supersymmetry or technicolor Their suppression is necessary for an agreement with observations making FCNCs important constraints on model building Example editConsider a toy model in which an undiscovered boson S may couple both to the electron as well as the tau t via the term Sps epst displaystyle S bar psi e psi tau nbsp 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 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 5 at the Paul Scherrer Institute near Zurich will search for a similar process in which an antimuon decays to a photon and an antielectron a positron In the Standard Model such a process proceeds only by emission and re absorption of a charged W which changes the m into a neutrino on emission and then a positron on re absorption and finally emits a photon that carries away any difference in energy spin and momentum In most cases of interest the boson involved is not a new boson S but the conventional Z0 boson itself 6 This can occur if the coupling to weak neutral currents is slightly non universal The dominant universal coupling to the Z boson does not change flavor but sub dominant non universal contributions can FCNCs involving the Z boson for the down type quarks at zero momentum transfer are usually parameterized by the effective action term g2cos 8Wd LaUabgmdLbZm displaystyle frac g 2 cos theta text W overline d L alpha U alpha beta gamma mu d L beta Z mu nbsp This particular example of FCNC is often studied the most because we have some fairly strong constraints coming from the decay of B0 mesons in 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 Ubs 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 charged currents because the weak neutral current Z0 boson does not change flavor in the Standard Model proper at the tree level whereas the weak charged currents W bosons do New physics in charged current events would be swamped by more numerous W boson interactions new physics in the neutral current would not be masked by a large effect due to ordinary Standard Model physics See also editNeutral particle oscillation Penguin diagram Two Higgs doublet modelReferences edit Anderson S et al CLEO Collaboration 2001 Improved Upper Limits on the Flavor Changing Neutral Current Decays B Kℓ ℓ and B K 892 ℓ ℓ Physical Review Letters 87 18 181803 arXiv hep ex 0106060 Bibcode 2001PhRvL 87r1803A doi 10 1103 PhysRevLett 87 181803 S2CID 119477202 CMS Collaboration 2022 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 Physical Review Letters 129 3 032001 arXiv 2111 02219 doi 10 1103 PhysRevLett 129 032001 PMID 35905365 Hunting for anomalous interactions between the Higgs bosons and the top quarks CMS Experiment cms cern Retrieved 2021 12 24 Acosta D et al CDF Collaboration 2005 07 15 First Evidence for B0s ff Decay and Measurements of Branching Ratio and ACP for B fK Physical Review Letters 95 3 031801 arXiv hep ex 0502044 Bibcode 2005PhRvL 95c1801A doi 10 1103 PhysRevLett 95 031801 PMID 16090735 S2CID 40241424 MEG experiment website FCNCs involving the photon cannot occur at zero momentum transfers 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 Z boson Retrieved from https en wikipedia org w index php title Flavor changing neutral current amp oldid 1215979154, wikipedia, wiki, book, books, library,
