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Register a new userAre we overlooking Lepton Flavour Universal New Physics in $bto sellell$ ?
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The deviations with respect to the Standard Model (SM) that are currently observed in $b to s ellell$ transitions (the so-called flavour anomalies) can be interpreted in terms of different New Physics (NP) scenarios within a model-independent effective approach. We reconsider the determination of NP in global fits from a different perspective by removing one implicit hypothesis of current analyses, namely that NP is only Lepton-Flavour Universality Violating (LFUV). We examine the roles played by LFUV NP and Lepton-Flavour Universal (LFU) NP altogether, providing new directions to identify the possible theory beyond the SM responsible for the anomalies observed. New patterns of NP emerge due to the possibility of allowing at the same time large LFUV and LFU NP contributions to $C_{10mu}$, which provides a different mechanism to obey the constraint from the $B_s tomu^+mu^-$ branching ratio. In this landscape of NP, we discuss how to discriminate among these scenarios in the short term thanks to current and forthcoming observables. While the update of $R_K$ will be a major milestone to confirm the NP origin of the flavour anomalies, additional observables, in particular the LFUV angular observable $Q_5$, turn out to be central to assess the precise NP scenario responsible for the observed anomalies.
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We investigate new-physics contributions to $bto s ellell$ transitions in the context of an effective field theory extension of the Standard Model, including operator mixing at one loop. We identify the few scenarios where a single Wilson coefficient, $C/Lambda^2 sim 1/{rm TeV}^2$, induces a substantial shift in the lepton flavour universality ratios $R_K$ and $R_{K^*}$ at one loop, while evading $Z$-pole precision tests, collider bounds, and other flavour constraints. Good fits to the present data are achieved by a left-handed current operator with quark-flavour indices $(2,2)$ or $(3,3)$, hitherto overlooked. Interestingly, the running of the Standard Model Yukawa matrices gives the dominant effect for these scenarios. We match the favoured effective-theory scenarios to minimal, single-mediator models, which are subject to additional stringent constraints. Notably, we recognise three viable instances of a leptoquark with one coupling to fermions only. If the anomalies were confirmed, it appears that one-loop explanations have good prospects of being directly tested at the LHC.
We perform a model-independent global fit to $bto sell^+ell^-$ observables to confirm existing New Physics (NP) patterns (or scenarios) and to identify new ones emerging from the inclusion of the updated LHCb and Belle measurements of $R_K$ and $R_{K^*}$, respectively. Our analysis, updating Refs. [1,2] and including these new data, suggests the presence of right-handed couplings encoded in the Wilson coefficients ${cal C}_{9mu}$ and ${cal C}_{10mu}$. It also strengthens our earlier observation that a lepton flavour universality violating (LFUV) left-handed lepton coupling (${cal C}_{9mu}^{rm V}=-{cal C}_{10mu}^{rm V}$), often preferred from the model building point of view, accommodates the data better if lepton-flavour universal (LFU) NP is allowed, in particular in ${cal C}_{9}^{rm U}$. Furthermore, this scenario with LFU NP provides a simple and model-independent connection to the $bto ctau u$ anomalies, showing a preference of $approx 7,sigma$ with respect to the SM. It may also explain why fits to the whole set of $bto sell^+ell^-$ data or to the subset of LFUV data exhibit stronger preferences for different NP scenarios. Finally, motivated by $Z^prime$ models with vector-like quarks, we propose four new scenarios with LFU and LFUV NP contributions that give a very good fit to data. We provide also an addendum collecting our updated results after including the data for the $Bto K^*mumu$ angular distribution released in 2020 by the LHCb collaboration.
The recent measurements of $R_K$, $B_stomu^+mu^-$, a set of CP-averaged angular observables for the $B^0to K^{*0}mu^+mu^-$ decay, and its isospin partner $B^+to K^{*+}mu^+mu^-$ by the LHCb Collaboration, consistently hint at lepton universality violation in the $bto sellell$ transitions. The so-called $B$ anamolies can be best explained in five one-dimensional scenarios, i.e, $delta C_9^{mu}$, $delta C_{10}^{mu}$, $delta C_L^{mu}$, $delta C_9^{mu}=delta C_{10}^{muprime}$, and $delta C_9^{mu}=-delta C_9^{muprime}$, as demonstrated in recent model independent anlayses~cite{Alok:2019ufo,Alguero:2021anc,Geng:2021nhg,Altmannshofer:2021qrr}. In this work we explore how these scenarios can be distinguished from each other. We show that the combinations of four angular asymmetries $A_i$~$(i=3,4,5,9)$ together with the ratio $R_6$ first proposed in~cite{Jager:2014rwa} can discriminate the five new physics scenarios in proper intervals of $q^2$ and with future high-precision measurements.
We revisit electroweak radiative corrections to Standard Model Effective Field Theory (SMEFT) operators which are relevant for the $B$-meson semileptonic decays. The one-loop matching formulae onto the low-energy effective field theory are provided without imposing any flavor symmetry. The on-shell conditions are applied especially in dealing with quark-flavor mixings. Also, the gauge independence is shown explicitly in the $R_xi$ gauge.
LHCb found hints for physics beyond the Standard Model (SM) in $Bto K^*mu^+mu^-$, $R(K)$ and $B_stophimu^+mu^-$. These intriguing hints for NP have recently been confirmed by the LHCb measurement of $R(K^*)$ giving a combined significance for NP above the $5,sigma$ level. In addition, the BABAR, BELLE and LHCb results for $Bto D^{(*)}tau u$ also point towards lepton flavour universality (LFU) violating new physics (NP). Furthermore, there is the long-standing discrepancy between the measurement and the theory prediction of the anomalous magnetic moment of the muon ($a_mu$) at the $3,sigma$ level. Concerning NP effects, $bto smu^+mu^-$ data can be naturally explained with a new neutral gauge bosons, i.e. a $Z^prime$ but also with heavy new scalars and fermions contributing via box diagrams. Another promising solution to $bto smu^+mu^-$, which can also explain $Bto D^{(*)}tau u$, are leptoquarks. Interestingly, leptoquarks provide also a viable explanation of $a_mu$ which can be tested via correlated effects in $Ztomu^+mu^-$ at future colliders. Considering leptoquark models, we show that an explanation of $Bto D^{(*)}tau u$ predicts an enhancement of $bto stau^+tau^-$ processes by around three orders of magnitude compared to the SM. In case of a simultaneous explanation of $Bto D^{(*)}tau u$ and $bto smu^+mu^-$ data, sizable effects in $bto staumu$ processes are predicted.
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