Searching for BSM physics in Yukawa couplings and flavour symmetries

Abstract

In the framework of the Standard Model Effective Field Theory, we compare the lower bounds on the scale of new physics possibly contributing to the $$ f\overline{f}h $$ f f ¯ h effective couplings, obtained from the measurements of different observables, under the assumption that the Wilson coefficients of the relevant dim 6 operators respect certain flavour structure: either the Minimal Flavour Violation (MFV) ansatz or a flavour symmetry, often invoked to explain the observed pattern of fermion masses and mixings. We perform a global analysis of the bounds following from the limits on the diagonal couplings measured in the Higgs boson production and decays at the LHC experiments. Another set of bounds is obtained from the limits on non-diagonal couplings constrained by the variety of flavour changing neutral current (FCNC) and radiative decay processes. With the present precision of the LHC data, the FCNC data give stronger bounds on the scale of new physics than the collider data (obviously, for the MFV ansatz only collider data are relevant): once the Wilson coefficients respect some flavour structure, the obtained bounds are in the TeV range. In the quark case, these limits are compatible with a few percent deviations from the SM Yukawa couplings and only mildly more stringent than those obtained from the available collider data. For leptons, instead, the FCNC bounds are stronger and then a signal in the near future collider data would mean the violation of the flavour symmetry or indicate the presence of additional beyond the Standard Model contributions, affecting the flavour observables, that leads to cancellations.

Dynamical Minimal Flavour Violating inverse seesaw

“…In an effective field theory approach, the Wilson coefficients of higher dimensional operators are made invariant under the flavour symmetry by inserting powers of the spurions, providing suppressions in terms of fermion masses and mixing. 1 The result is that, while the bounds on the NP scale in the absence of any flavour symmetry would be of hundreds of TeVs [19,20], in the M(L)FV case the NP scale can be at the TeV scale as proven in several contexts [8][9][10][11][12][13][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. Despite this success, M(L)FV is not a flavour model, as masses and mixing can only be described and not predicted: in other words, the background values of the spurions are simply assumed as working hypotheses.…”

After the Higgs boson discovery: a turning point in particle physics

“…The pattern of Wilson coefficients is strongly constrained by the flavour symmetry and it is linked to the fermion masses. It turns out that the flavour bounds on the BSM physics scale become as low as the collider bounds [29]! Another consequence of the flavour symmetry is that the bounds on κ~s become equal within each sector, of leptons, down quarks and up quarks (figure 2).…”

The Low‐Scale Seesaw Solution to the MW$M_W$ and (g−2)μ$(g-2)_\umu$ Anomalies

“…Using these collider data, Ref. [45–47] performed a global fit obtaining a bound on a combination of $$\kappa _\umu$$ and $$\tilde{\kappa }_\umu$$, $$$$\begin{equation} 0.36 \lesssim \kappa ^2_\umu +\tilde{\kappa }^2_\umu \lesssim 1.85\,, \end{equation}$$$$that, given the matching in Equation (), translates into a bound on the ratio $$\widetilde{m}^2_R/\widetilde{M}^2_\psi$$, $$$$\begin{equation} 0.6 \lesssim \kappa _\umu \lesssim 1.36\quad \Longrightarrow \quad \dfrac{\widetilde{m}^2_R}{\widetilde{M}^2_\psi }\lesssim 0.27\,. \end{equation}$$$$…”