Flavour-changing neutral current (FCNC) processes are absent in the SM at the tree level and, thus, are are suppressed in comparison to the charged current processes. Due to this FCNC, can be used as
an excellent probe of New Physics, which can considerably alter the predictions of the SM.
The rare processes impose very important constraints on Beyond-the-SM (BSM) physics.
Peng4BSM@LO uses MATHEMATICA
and FeynArts/FeynCalc
to evaluate different contributions to the coefficient functions \(N^0_{L,R}\), \(E^{0,2}_{L,R}\) and \(M^1_{L,R}\) of the operators
$$
H_{eff} \ni \left(\bar F_a^\prime P_{L,R}F_bS_c\right)N^{0,c}_{a,b},
$$
with external scalar boson, and
$$
H_{eff} \ni
\left(\overline F_a^\prime \gamma^\mu P_{L,R}F_b\right)\left[\left(E^{0,c}_{a,b}\right)_{L,R}
g^{\mu\nu}
\left(g^{\mu\nu}q^2-q^\mu q^\nu\right)\left(E^{2,c}_{a,b}\right)_{L,R}
\right]V^c_\nu
$$
and
$$
H_{eff} \ni
\left(\overline F_a^\prime \sigma^{\mu\nu} P_{L,R}F_bq_\mu V^c_\nu\right)\left(M^{1,c}_{a,b}\right)_{L,R},
$$
with external vector field from penguin diagrams in the SM and Models Beyond the SM.
Here \(S\) is a neutral scalar boson, \(q_\mu\) is the outgoing momentum of a neutral vector boson \(V\) entering the operator, \(F,~F^\prime\) are fermions of different families, \(a,~b,~c\) are some, e.g., color indices and \(P_{L,R}=(1/2)(1\mp\gamma^5)\) are the projection operators.
From the form factors one can easily extract the corresponding Wilson coefficients.
Peng4BSMatLO.m - main package file
test_Peng4BSMatLO.nb - example of usage