Abstract
We investigate the information that can be gained by including flavour data in fits of the Standard Model Effective Field Theory (SMEFT) with the assumption of Minimal Flavour Violation (MFV), allowing — as initial conditions at the high scale — leading terms in spurionic Yukawas only. Starting therefore from a theory with no tree level flavour changing neutral currents at the scale of new physics, we calculate effects in flavour changing processes at one loop, and the resulting constraints on linear combinations of SMEFT coefficients, consistently parameterising the electroweak parameters and the CKM within the SMEFT. By doing a global fit including electroweak, Higgs and low energy precision measurements among others, we show that flavour observables put strong constraints on previously unconstrained operator directions. The addition of flavour data produces four independent constraints at order TeV or above on otherwise flat directions; reducing to three when complete U(3)5 flavour symmetry is assumed. Our findings demonstrate that flavour remains a stringent test for models of new physics, even in the most flavourless scenario.
Highlights
As the particle physics community looks forward to the upcoming LHC run, with the ultimate promise of vastly increased statistics but not significantly increased energy, attention has shifted toward developing an understanding of the subtle effects that new physics beyond the direct reach of the LHC could still have on precision measurements, using Effective Field Theory (EFT) techniques
We investigate the information that can be gained by including flavour data in fits of the Standard Model Effective Field Theory (SMEFT) with the assumption of Minimal Flavour Violation (MFV), allowing — as initial conditions at the high scale — leading terms in spurionic Yukawas only
Our choice of non-flavour observables is motivated by the following considerations, with the aim of getting a fair picture of the new physical information that flavour can add to a global fit: (a) we want to include observables which provide strong and up-to-date constraints on operators involving EW gauge bosons and Higgs fields, and (b) we want to include observables which depend on the same 4-fermion operators that appear at one-loop in flavourchanging neutral currents (FCNCs)
Summary
As the particle physics community looks forward to the upcoming LHC run, with the ultimate promise of vastly increased statistics but not significantly increased energy, attention has shifted toward developing an understanding of the subtle effects that new physics beyond the direct reach of the LHC could still have on precision measurements, using Effective Field Theory (EFT) techniques. As well as LHC measurements, we have precise data from LEP measurements, Standard Model-forbidden process experiments, high-precision measurements like parityviolating electron scattering experiments, and flavour physics experiments Many of these have already been incorporated into EFT fits, but flavour observables have generically been applied only to explicitly flavour-symmetry violating new physics scenarios. Every operator is multiplied by the minimum number of spurionic Yukawa matrices needed to make it formally invariant under the U(3) flavour symmetry, which means that we begin with a theory containing no tree level flavourchanging neutral currents (FCNCs) at the scale of new physics Λ We call these initial conditions at the high scale within the MFV framework ‘leading MFV’ in the following. We present details of our novel treatment of the CKM matrix for the leading MFV SMEFT, analytic results of the Yukawa-weighted operator matching and the relevant matching calculations for processes with final-state neutrino pairs, as well as numerical results for all matching calculations, in the appendices
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