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Register a new userRevisiting the new-physics interpretation of the $bto ctau u$ data
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We revisit the status of the new-physics interpretations of the anomalies in semileptonic $B$ decays in light of the new data reported by Belle on the lepton-universality ratios $R_{D^{(*)}}$ using the semileptonic tag and on the longitudinal polarization of the $D^*$ in $Bto D^*tau u$, $F_L^{D^*}$. The preferred solutions involve new left-handed currents or tensor contributions. Interpretations with pure right-handed currents are disfavored by the LHC data, while pure scalar models are disfavored by the upper limits derived either from the LHC or from the $B_c$ lifetime. The observable $F_L^{D^*}$ also gives an important constraint leading to the exclusion of large regions of parameter space. Finally, we investigate the sensitivity of different observables to the various scenarios and conclude that a measurement of the tau polarization in the decay mode $Bto Dtau u$ would effectively discriminate among them.
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$B$ 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.
In this addendum to arXiv:1811.09603 we update our results including the recent measurement of ${cal R}(D)$ and ${cal R}(D^*)$ by the Belle collaboration: ${cal R}(D)_{rm Belle} = 0.307pm0.037pm0.016$ and ${cal R}(D^*)_{rm Belle}=0.283pm0.018pm0.014$, resulting in the new HFLAV fit result ${cal R}(D) = {0.340pm0.027 pm 0.013}$, ${cal R}(D^*) = {0.295pm0.011 pm 0.008 }$, exhibiting a $3.1,sigma$ tension with the Standard Model. We present the new fit results and update all figures, including the relevant new collider constraints. The updated prediction for ${cal R}(Lambda_c)$ from our sum rule reads ${cal R}(Lambda_c)= mathcal{R}_{rm SM}(Lambda_c) left( 1.15 pm 0.04 right) = 0.38 pm 0.01 pm 0.01$. We also comment on theoretical predictions for the fragmentation function $f_c$ of $bto B_c$ and their implication on the constraint from $B_{u/c}totau u$ data.
In the Standard Model (SM), the rare transitions where a bottom quark decays into a strange quark and a pair of light leptons exhibit a potential sensitivity to physics beyond the SM. In addition, the SM embeds Lepton Flavour Universality (LFU), leading to almost identical probabilities for muon and electron modes. The LHCb collaboration discovered a set of deviations from the SM expectations in decays to muons and also in ratios assessing LFU. Other experiments (Belle, ATLAS, CMS) found consistent measurements, albeit with large error bars. We perform a global fit to all available $bto sell^+ell^-$ data ($ell=e,mu$) in a model-independent way allowing for different patterns of New Physics. For the first time, the NP hypothesis is preferred over the SM by $5,sigma$ in a general case when NP can enter SM-like operators and their chirally-flipped partners. LFU violation is favoured with respect to LFU at the 3-4$,sigma$ level. We discuss the impact of LFU-violating New Physics on the observable $P_5^prime$ from $B to K^*mu^+mu^-$ and we compare our estimate for long-distance charm contributions with an empirical model recently proposed by a group of LHCb experimentalists. Finally, we discuss NP models able to describe this consistent pattern of deviations.
Motivated by deviations with respect to Standard Model predictions in $bto sell^+ell^-$ decays, we evaluate the global significance of the new physics hypothesis in this system, including the look-elsewhere effect for the first time. We estimate the trial-factor with psuedo-experiments and find that it can be as large as seven. We calculate the global significance for the new physics hypothesis by considering the most general description of a non-standard $bto sell^+ell^-$ amplitude of short-distance origin. Theoretical uncertainties are treated in a highly conservative way by absorbing the corresponding effects into a redefinition of the Standard Model amplitude. Using the most recent measurements of LHCb, ATLAS and CMS, we obtain the global significance to be $3.9$ standard deviations.
In recent years, intriguing hints for the violation of Lepton Flavour Universality (LFU) have been accumulated in semileptonic $B$ decays, both in the neutral-current transitions $bto sell^+ell^-$ (i.e., $R_K$ and $R_{K^*}$) and the charged-current transitions $bto cell^-bar u_ell$ (i.e., $R_D$, $R_{D^*}$ and $R_{J/psi}$). LHCb has reported deviations from the Standard Model (SM) expectations in $bto smu^+mu^-$ 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. Furthermore, hints for LFU violation in $R_{D^{(*)}}$ and $R_{J/psi}$ point at large deviations from the SM in processes involving tau leptons. Together, these hints for NP motivate the possibility of huge LFU-violating effects in $bto stau^+tau^-$ transitions. In this article we predict the branching ratios of $Bto Ktau^+tau^-$, $Bto K^{*}tau^+tau^-$ and $B_sto phi tau^+tau^-$ taking into account NP effects in the Wilson coefficients $C_{9()}^{tautau}$ and $C_{10()}^{tautau}$. Assuming a common NP explanation of $R_{D^{}}$ , $R_{D^{(*)}}$ and $R_{J/psi}$, we show that a very large enhancement of $bto stau^+tau^-$ processes, of around three orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of $B_sto tau^+tau^-$, $B_sto phi tau^+tau^-$ and $Bto K^{(*)}tau^+tau^-$ under these assumptions are in the observable range for LHCb and Belle II.
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