Explaining the hints for lepton flavour universality violation with three S2 leptoquark generations

Abstract

Leptoquarks are prime candidates for explaining the intriguing hints for lepton flavour universality violation. In particular, the SU(2)L doublet of scalar leptoquarks S2 is capable of providing an explanation for the tensions between the measurements and the Standard Model predictions in (g − 2)μ, b → sℓ+ℓ− and b → cτν processes, as well as in non-resonant di-electron production. However, in the minimal setup with a single leptoquark generation, a common explanation for all these issues is not possible as this would lead to unacceptably large charged lepton flavour violation. We therefore propose a model with three generations of S2, each coupling exclusively to a single lepton flavour, i.e. a model extending the Standard Model particle content by an electroquark, a muoquark and a tauquark. We show that after taking into account other constraints, such as those originating from electroweak precision observables and ∆F = 2 processes, it is possible to provide a combined explanation for all these hints of lepton flavour universality violation. Moreover, we find that the presence of the tauquark can generate a dimension-six $$ {\mathcal{O}}_9^U $$ O 9 U operator via off-shell photon penguin diagrams, which, together with the muoquark contribution, further improves the global fit to b → sℓ+ℓ− data.

Standard model anomalies: lepton flavour non-universality, g − 2 and W-mass

“…In particular, the combination S 1 & S 3 , which is not in conflict with µ → eγ, is excluded by the interplay between K 0 L → eµ, Z → e + e − and Z → µ + µ − data. This recent study [117] shows, however, that with three generations of a weak doublet scalar LQ S 2 it is possible to accommodate simultaneously all of the anomalies while being compatible with the other experimental data. The deviation in (g − 2) for the muon is addressed by loops, as shown in figure 7, containing the LQ and the top quark.…”

“…More recently, it has been further shown [117][118][119][120]206] that leptoquarks can also accommodate the deviations in (g − 2) . In the following, we discuss how LQs can explain the anomalies in b → s + − transitions and in (g−2) .…”

“…These are bosonic particles carrying both quark and lepton numbers naturally coupling to a lepton and a quark. Leptoquark (LQ) induced explanations of the flavor anomalies have been extensively discussed in the literature [32,[100][101][102][103][104][105][106][107][108][109][110][111][112][113][114][115][116][117].…”

Scalar leptoquarks at the LHC and flavour anomalies: a comparison of pair-production modes at NLO-QCD

“…in superstring models [8], R-parity violating supersymmetric scenarios [9,10], composite models [11][12][13][14][15], neutrino mass models [16][17][18][19][20][21], or as mediators in simplified models of dark matter [22][23][24][25][26][27]. Scalar leptoquarks may also offer an explanation [28][29][30][31][32][33][34][35][36][37][38][39] to the anomalies appearing in B-physics observables [40][41][42][43][44][45][46][47], as well as to the discrepancy between theoretical predictions [48] and experimental measurements [49,50] related to the anomalous magnetic moment of the JHEP11(2022)006 muon. In this context, most interesting scenarios feature large Yukawa couplings between leptons, quarks, and leptoquarks.…”

Drell-Yan Tails Beyond the Standard Model

“…In particular, the latter prevent an explanation of the b → cτ ν anomalies via only left-handed couplings in this scenario. Note, also, that LHC constraints turn out to be practically irrelevant, at the EFT level, since the contributions to pp → τ τ are CKM suppressed for the scalar/tensor operators, and absent for the particular combination of C [121,136,[141][142][143][144]. In other words, an imaginary part of the scalar/tensor coefficients is needed to simultaneously explain the deviations observed in R D and R D * , as shown in Fig.…”