On flavourful Easter eggs for New Physics hunger and lepton flavour universality violation

M W helps select Z ' models for b → s ℓ ℓ anomalies

As shown in Allanach et al. (Global fits of third family hypercharge models to neutral current B-anomalies and electroweak precision observables. arXiv:2103.12056), the Third Family Hypercharge ( Y 3 ) Model changes the Standard Model prediction for M W whilst simultaneously explaining anomalies in b → s ℓ ℓ transitions via a heavy Z ' gauge boson which is spawned by a spontaneously broken gauged U ( 1 ) Y 3 symmetry. The 2022 CDF II measurement of M W , which is far from the Standard Model prediction in the statistical sense, somewhat disfavours the Y 3 model. Here, we generalise the gauge charge assignments to the anomaly-free combination s Y 3 + t ( B 3 - L 3 ) and show that incorporating the 2022 CDF II measurement of M W selects a viable domain of integers s and t. For example, s = 1 , t = - 3 yields a p value of .08 in a two-parameter global fit to 277 electroweak and flavour changing b data, much improving a SM p value of 1 × 10 - 6 .

Round table on Standard Model Anomalies

This contribution to “The XVth Quark confinement and the Hadron spectrum conference" covers a description, both theoretical and experimental, of the present status of a set of very different anomalies. The discussion ranges from the long standing b → sℓℓ anomalies, (g − 2) and the new MW anomaly.

The LFU ratio R π in the Standard Model and beyond

We discuss the possibility of performing precise tests of μ / e universality in B → π ℓ + ℓ - decays. We show that in wide regions of the dilepton invariant mass spectrum the ratio between muonic and electronic decay widths can be predicted with high accuracy, both within and beyond the Standard Model. We present numerical expressions which can be used to extract precise information on short-distance dynamics if a deviation from universality is observed in the data.

Combined Explanation of the Z→bb[over ¯] Forward-Backward Asymmetry, the Cabibbo Angle Anomaly, and τ→μνν and b→sℓ^{+}ℓ^{-} Data

In this Letter, we propose a simple model that can provide a combined explanation of the Z→bb[over ¯] forward-backward asymmetry, the Cabibbo angle anomaly (CAA), τ→μνν and b→sℓ^{+}ℓ^{-} data. This model is obtained by extending the standard model (SM) by two heavy vectorlike quarks (an SU(2)_{L} doublet (singlet) with hypercharge -5/6 (-1/3), two new scalars (a neutral and a singly charged one), and a gauged L_{μ}-L_{τ} symmetry. The mixing of the new quarks with the SM ones, after electroweak symmetry breaking, does not only explain Z→bb[over ¯] data, but also generates a lepton flavor universal contribution to b→sℓ^{+}ℓ^{-} transitions. Together with the lepton flavor universality violating effect, generated by loop-induced Z^{'} penguins involving the charged scalar and the heavy quarks, it gives an excellent fit to data (6.1σ better than the SM). Furthermore, the charged scalar (neutral vector) gives a necessarily constructive tree-level (loop) effect in μ→eνν (τ→μνν), which can naturally account for the CAA (Br[τ→μνν]/Br[τ→eνν] and Br[τ→μνν]/Br[μ→eνν]).

Global Fit to Modified Neutrino Couplings and the Cabibbo-Angle Anomaly

Recently, discrepancies of up to 4σ between the different determinations of the Cabibbo angle were observed. In this context, we point out that this "Cabibbo-angle anomaly" can be explained by lepton flavor universality violating new physics in the neutrino sector. However, modified neutrino couplings to standard model gauge bosons also affect many other observables sensitive to lepton flavor universality violation, which have to be taken into account in order to assess the viability of this explanation. Therefore, we perform a model-independent global analysis in a Bayesian approach and find that the tension in the Cabibbo angle is significantly reduced, while the agreement with other data is also mostly improved. In fact, nonzero modifications of electron and muon neutrino couplings are preferred at more than 99.99% C.L. (corresponding to more than 4σ). Still, since constructive effects in the muon sector are necessary, simple models with right-handed neutrinos (whose global fit we update as a by-product) cannot fully explain data, pointing towards more sophisticated new physics models.

Search for Lepton-Universality Violation in B^{+}→K^{+}ℓ^{+}ℓ^{-} Decays

A measurement of the ratio of branching fractions of the decays B^{+}→K^{+}μ^{+}μ^{-} and B^{+}→K^{+}e^{+}e^{-} is presented. The proton-proton collision data used correspond to an integrated luminosity of 5.0  fb^{-1} recorded with the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV. For the dilepton mass-squared range 1.1<q^{2}<6.0  GeV^{2}/c^{4} the ratio of branching fractions is measured to be R_{K}=0.846_{-0.054}^{+0.060}_{-0.014}^{+0.016}, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of R_{K} to date and is compatible with the standard model at the level of 2.5 standard deviations.

Importance of Loop Effects in Explaining the Accumulated Evidence for New Physics in B Decays with a Vector Leptoquark

In recent years experiments revealed intriguing hints for new physics (NP) in B decays involving b→cτν and b→sℓ^{+}ℓ^{-} transitions at the 4σ and 5σ level, respectively. In addition, there are slight disagreements in b→uτν and b→dμ^{+}μ^{-} observables. While not significant on their own, they point in the same direction. Furthermore, V_{us} extracted from τ decays shows a slight tension (≈2.5σ) with its value determined from Cabibbo-Kobayashi-Maskawa unitarity, and an analysis of BELLE data found an excess in B_{d}→τ^{+}τ^{-}. Concerning NP explanations, the vector leptoquark SU(2) singlet is of special interest since it is the only single particle extension of the standard model which can (in principle) address all the anomalies described above. For this purpose, large couplings to τ leptons are necessary and loop effects, which we calculate herein, become important. Including them in our phenomenological analysis, we find that neither the tension in V_{us} nor the excess in B_{d}→τ^{+}τ^{-} can be fully explained without violating bounds from K→πνν[over ¯]. However, one can account for b→cτν and b→uτν data finding intriguing correlations with B_{q}→τ^{+}τ^{-} and K→πνν[over ¯]. Furthermore, the explanation of b→cτν predicts a positive shift in C_{7} and a negative one in C_{9}, being nicely in agreement with the global fit to b→sℓ^{+}ℓ^{-} data. Finally, we point out that one can fully account for b→cτν and b→sℓ^{+}ℓ^{-} without violating bounds from τ→ϕμ, ϒ→τμ, or b→sτμ processes.

B Meson Anomalies in a Pati-Salam Model within the Randall-Sundrum Background

Lepton number as a fourth color is the intriguing theoretical idea of the famous Pati-Salam (PS) model. While in conventional PS models, the symmetry breaking scale and the mass of the resulting vector leptoquark are stringently constrained by K_{L}→μe and K→πμe, the scale can be lowered to a few TeV by adding vectorlike fermions. Furthermore, in this case, the intriguing hints for lepton flavor universality violation in b→sμ^{+}μ^{-} and b→cτν processes can be addressed. Such a setup is naturally achieved by implementing the PS gauge group in the five-dimensional Randall-Sundrum background. The PS symmetry is broken by boundary conditions on the fifth dimension, and the resulting massive vector leptoquark automatically has the same mass scale as the vectorlike fermions and all other resonances. We consider the phenomenology of this model in the context of the hints for lepton flavor universality violation in semileptonic B decays. Assuming flavor alignment in the down sector, we find that in b→sℓ^{+}ℓ^{-} transitions, the observed deviations from the standard model predictions [including R(K) and R(K^{*})] can be explained with natural values for the free parameters of the model. Even though we find sizable effects in R(D), R(D^{*}), and R(J/Ψ), one cannot account for the current central values in the constrained setup of our minimal model due to the stringent constraints from D-D[over ¯] mixing and τ→3μ.

An empirical model to determine the hadronic resonance contributions to B ¯ 0 → K ¯ ∗ 0 μ + μ - transitions

A method for analysing the hadronic resonance contributions in B ¯ 0 → K ¯ ∗ 0 μ + μ - decays is presented. This method uses an empirical model that relies on measurements of the branching fractions and polarisation amplitudes of final states involving J PC = 1 - - resonances, relative to the short-distance component, across the full dimuon mass spectrum of B ¯ 0 → K ¯ ∗ 0 μ + μ - transitions. The model is in good agreement with existing calculations of hadronic non-local effects. The effect of this contribution to the angular observables is presented and it is demonstrated how the narrow resonances in the q 2 spectrum provide a dramatic enhancement to C P -violating effects in the short-distance amplitude. Finally, a study of the hadronic resonance effects on lepton universality ratios, R K ( ∗ ) , in the presence of new physics is presented.

Searching for New Physics with b→sτ^{+}τ^{-} Processes

In recent years, intriguing hints for the violation of lepton flavor universality (LFU) have been accumulated in semileptonic B decays, both in the charged-current transitions b→cℓ^{-}ν[over ¯]_{ℓ} (i.e., R_{D}, R_{D^{*}}, and R_{J/ψ}) and the neutral-current transitions b→sℓ^{+}ℓ^{-} (i.e., R_{K} and R_{K^{*}}). Hints for LFU violation in R_{D^{(*)}} and R_{J/ψ} point at large deviations from the standard model (SM) in processes involving tau leptons. Moreover, LHCb has reported deviations from the SM expectations in b→sμ^{+}μ^{-} processes as well as in the ratios R_{K} and R_{K^{*}}, which together point at new physics (NP) affecting muons with a high significance. These hints for NP suggest the possibility of huge LFU-violating effects in b→sτ^{+}τ^{-} transitions. In this Letter, we predict the branching ratios of B→Kτ^{+}τ^{-}, B→K^{*}τ^{+}τ^{-}, and B_{s}→ϕτ^{+}τ^{-}, taking into account NP effects in the Wilson coefficients C_{9(^{'})}^{ττ} and C_{10(^{'})}^{ττ}. Assuming a common NP explanation of R_{D}, R_{D^{(*)}}, and R_{J/ψ}, we show that a very large enhancement of b→sτ^{+}τ^{-} processes, of around 3 orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of B_{s}→τ^{+}τ^{-}, B_{s}→ϕτ^{+}τ^{-}, and B→K^{(*)}τ^{+}τ^{-} under these assumptions are in the observable range for LHCb and Belle II.

Anomaly-free models for flavour anomalies

We explore the constraints imposed by the cancellation of triangle anomalies on models in which the flavour anomalies reported by LHCb and other experiments are due to an extra U(1) ' gauge boson Z ' . We assume universal and rational U(1) ' charges for the first two generations of left-handed quarks and of right-handed up-type quarks but allow different charges for their third-generation counterparts. If the right-handed charges vanish, cancellation of the triangle anomalies requires all the quark U(1) ' charges to vanish, if there are either no exotic fermions or there is only one Standard Model singlet dark matter (DM) fermion. There are non-trivial anomaly-free models with more than one such 'dark' fermion, or with a single DM fermion if right-handed up-type quarks have non-zero U(1) ' charges. In some of the latter models the U(1) ' couplings of the first- and second-generation quarks all vanish, weakening the LHC Z ' constraint, and in some other models the DM particle has purely axial couplings, weakening the direct DM scattering constraint. We also consider models in which anomalies are cancelled via extra vector-like leptons, showing how the prospective LHC Z ' constraint may be weakened because the Z ' → μ + μ - branching ratio is suppressed relative to other decay modes.

On flavourful Easter eggs for New Physics hunger and lepton flavour universality violation

Within the standard approach of effective field theory of weak interactions for Δ B = 1 transitions, we look for possibly unexpected subtle New Physics effects, here dubbed "flavourful Easter eggs". We perform a Bayesian global fit using the publicly available HEPfit package, taking into account state-of-the-art experimental information concerning these processes, including the suggestive measurements from LHCb of R K and R K ∗ , the latter available only very recently. We parametrise New Physics contributions to b → s transitions in terms of shifts of Wilson coefficients of the electromagnetic dipole and semileptonic operators, assuming CP-conserving effects, but allowing in general for violation of lepton flavour universality. We show how optimistic/conservative hadronic estimates can impact quantitatively the size of New Physics extracted from the fit. With a conservative approach to hadronic uncertainties we find nonzero New Physics contributions to Wilson coefficients at the level of ∼ 3 σ , depending on the model chosen. Furthermore, given the interplay between hadronic contributions and New Physics effects in the leptonic vector current, a scenario with nonstandard leptonic axial currents is comparable to the more widely advocated one with New Physics in the leptonic vector current.