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Register a new userNew physics in $bto sellell$ transitions at one loop
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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.
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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.
$B$ decays proceeding via $bto cell u$ transitions with $ell=e$ or $mu$ are tree-level processes in the Standard Model. They are used to measure the CKM element $V_{cb}$, as such forming an important ingredient in the determination of e.g. the unitarity triangle; hence the question to which extent they can be affected by new physics contributions is important, specifically given the long-standing tension between $V_{cb}$ determinations from inclusive and exclusive decays and the significant hints for lepton flavour universality violation in $bto ctau u$ and $bto sellell$ decays. We perform a comprehensive model-independent analysis of new physics in $bto cell u$, considering all combinations of scalar, vector and tensor interactions occuring in single-mediator scenarios. We include for the first time differential distributions of $Bto D^*ell u$ angular observables for this purpose. We show that these are valuable in constraining non-standard interactions. Specifically, the zero-recoil endpoint of the $Bto Dell u$ spectrum is extremely sensitive to scalar currents, while the maximum-recoil endpoint of the $Bto D^*ell u$ spectrum with transversely polarized $D^*$ is extremely sensitive to tensor currents. We also quantify the room for $e$-$mu$ universality violation in $bto cell u$ transitions, predicted by some models suggested to solve the $bto ctau u$ anomalies, from a global fit to $Bto Dell u$ and $Bto D^*ell u$ for the first time. Specific new physics models, corresponding to all possible tree-level mediators, are also discussed. As a side effect, we present $V_{cb}$ determinations from exclusive $B$ decays, both with frequentist and Bayesian statistics, leading to compatible results. The entire numerical analysis is based on open source code, allowing it to be easily adapted once new data or new form factors become available.
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 present results of global fits of all relevant experimental data on rare $b to s$ decays. We observe significant tensions between the Standard Model predictions and the data. After critically reviewing the possible sources of theoretical uncertainties, we find that within the Standard Model, the tensions could be explained if there are unaccounted hadronic effects much larger than our estimates. Assuming hadronic uncertainties are estimated in a sufficiently conservative way, we discuss the implications of the experimental results on new physics, both model independently as well as in the context of the minimal supersymmetric standard model and models with flavour-changing $Z$ bosons. We discuss in detail the violation of lepton flavour universality as hinted by the current data and make predictions for additional lepton flavour universality tests that can be performed in the future. We find that the ratio of the forward-backward asymmetries in $B to K^* mu^+mu^-$ and $B to K^* e^+e^-$ at low dilepton invariant mass is a particularly sensitive probe of lepton flavour universality and allows to distinguish between different new physics scenarios that give the best description of the current data.
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.
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