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Inparticle physics,flavor-changing neutral currents orflavour-changing neutral currents (FCNCs) are a class of hypothetical interactions betweenelementary particles. These interactions would change a particle'sflavor (i.e., its type, such as astrange quark changing into adown quark) without changing itselectric charge.

In theStandard Model of particle physics, the dominant "tree-level" interactions cannot produce FCNCs. This is described by theGIM mechanism. However, FCNCs can occur through more complex, higher-order processes (so-called "loop diagrams"), but these are extremely rare. Because FCNCs are so heavily suppressed in the Standard Model, physicists consider them a "zero-background" phenomenon. Any clear observation of an FCNC would be a strong indicator ofphysics beyond the Standard Model.

Experiments at particle colliders, such as theLarge Hadron Collider, and dedicated searches, like theMEG experiment, actively look for evidence of FCNCs. So far, the results have been largely consistent with the predictions of the Standard Model. The lack of observed FCNCs places important constraints on the development of new theories and models in physics.

Flavor-changing neutral currents are interactions predicted by some theories, and their potential existence is studied through theLagrangian terms that would describe them. If they occur in nature, these processes may induce phenomena that have not yet been observed in experiment. While FCNCs may occur in theStandard Model beyond thetree level, they are highly suppressed by theGIM mechanism. Several collaborations have searched for FCNC.^[1][2][3] TheTevatronCDF 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 ofsupersymmetry ortechnicolor. Their suppression is necessary for an agreement with observations, making FCNCs important constraints on model-building.

Consider atoy model in which an undiscoveredbosonS may couple both to theelectron as well as thetau (τ⁻
) via the term

${\displaystyle S{\overline{\psi }}_e{\psi }_{\tau }}$

Since the electron and the tau have equal charges, the electric charge ofS clearly must vanish to respect the conservation of electric charge. AFeynman diagram withS as the intermediate particle is able to convert a tau into an electron (plus some neutral decay products of theS).

TheMEG experiment^[5] at thePaul Scherrer Institute nearZürich will search for a similar process, in which anantimuon decays to aphoton and an antielectron (apositron). In the Standard Model, such a process proceeds only by emission and re-absorption of a chargedW⁺
, which changes theμ⁺
into aneutrino on emission and then apositron on re-absorption, and finally emits aphoton that carries away any difference inenergy, spin, andmomentum.

In most cases of interest, the boson involved is not a new bosonS but the conventionalZ⁰
 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 theZ boson for the down-typequarks at zero momentum transfer are usually parameterized by theeffective action term

${\displaystyle \frac{g}{2\cos {\theta }_{\text{W}}}{\overline{d}}_{L\alpha }{U}_{\alpha \beta }{\gamma }^{\mu }{d}_{L\beta }{Z}_{\mu }}$

This particular example of FCNC is often studied the most because we have some fairly strong constraints coming from the decay ofB⁰
 mesons inBelle andBaBar. The off-diagonal entries ofU parameterizes the FCNCs and current constraints restrict them to be less than one part in a thousand for |U_bs|. 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 tocharged currents, because theweak 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 numerousW^±
 boson interactions; new physics in the neutral current would not be masked by a large effect due to ordinary Standard Model physics.

• Neutral particle oscillation
• Penguin diagram
• Two-Higgs-doublet model

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• Physics beyond the Standard Model
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