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Register a new userAddendum to Impact of polarization observables and $B_cto tau u$ on new physics explanations of the $bto c tau u$ anomaly
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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.
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The combined analysis of the BaBar, Belle, and LHCb data on $Bto Dtau u$, $Bto D^*tau u$ and $B_cto J/Psitau u$ decay observables shows evidence of physics beyond the Standard Model (SM). In this article, we study all the one- and two-dimensional scenarios which can be generated by adding a single new particle to the SM. We put special emphasis on the model-discriminating power of $F_L(D^*)$ and of the $tau$ polarizations, and especially on the constraint from the branching fraction ${rm BR}(B_ctotau u)$. We critically review this constraint and do not support the aggressive limit of ${rm BR}(B_ctotau u)<10%$ used in some analyses. While the impact of $F_L(D^*)$ is currently still limited, the ${rm BR}(B_ctotau u)$ constraint has a significant impact: depending on whether one uses a limit of $60%$, $30%$ or $10%$, the pull for new physics (NP) in scalar operators changes drastically. More specifically, for a conservative $60%$ limit a scenario with scalar operators gives the best fit to data, while for an aggressive $10%$ limit this scenario is strongly disfavored and the best fit is obtained in a scenario in which only a left-handed vector operator is generated. We find a sum rule for the branching ratios of $Bto Dtau u$, $Bto D^*tau u$ and $Lambda_bto Lambda_ctau u$ which holds for any NP contribution to the Wilson coefficients. This sum rule entails an enhancement of ${rm BR}(Lambda_bto Lambda_ctau u)$ over its SM prediction by $(24pm 6)%$ for the current $mathcal{R}(D^{(*)})$ data.
We develop a rigorous, semi-analytical method for maximizing any $bto ctau u$ observable in the full 20-real-dimensional parameter space of the dimension 6 effective Hamiltonian, given some fixed values of $R_{D^{(*)}}$. We apply our method to find the maximum allowed values of $F^L_{D^*}$ and $R_{J/psi}$, two observables which have both come out higher than their SM predictions in recent measurements by the Belle and LHCb collaborations. While the measurements still have large error bars, they add to the existing $R_{D^{(*)}}$ anomaly, and it is worthwhile to consider NP explanations. It has been shown that none of the existing, minimal models in the literature can explain the observed values of $F^L_{D^*}$ and $R_{J/psi}$. Using our method, we will generalize beyond the minimal models and show that there is no combination of dimension 6 Wilson operators that can come within $1sigma$ of the observed $R_{J/psi}$ value. By contrast, we will show that the observed value of $F^L_{D^*}$ can be achieved, but only with sizable contributions from tensor and mixed-chirality vector Wilson coefficients.
The meson decays $Bto Dtau u$ and $Bto D^* tau u$ are sensitive probes of the $bto ctau u$ transition. In this work we present a complete framework to obtain the maximum information on the physics of $Bto D^{(*)}tau u$ with polarized $tau$ leptons and unpolarized $D^{(*)}$ mesons. Focusing on the hadronic decays $tauto pi u$ and $tautorho u$, we show how to extract seven $tau$ asymmetries from a fully differential analysis of the final-state kinematics. At Belle II with $50~text{ab}^{-1}$ of data, these asymmetries could potentially be measured with percent level statistical uncertainty. This would open a new window into possible new physics contributions in $bto ctau u$ and would allow us to decipher its Lorentz and gauge structure.
At Moriond 2019, Belle collaboration has announced new measurements on the flavour ratios $R_D - R_{D^*}$ which are consistent with their Standard Model predictions within $1.2sigma$. After inclusion of these measurements, the global tension in $R_D - R_{D^*}$ has reduced from $4.1sigma$ to $3.1sigma$ which is still significant. The measurements of these ratios indicate towards the violation of lepton flavor universality in $brightarrow c,l,bar{ u}$ decay. Assuming new physics in $brightarrow c,tau,bar{ u}$ transition, we have done a global fit to all available data in this sector to identify the allowed new physics solutions. We find that there are seven allowed new physics solutions which can account for all measurements in $brightarrow c,tau,bar{ u}$ transition. We show that a simultaneous measurement of the $tau$ polarization fraction and forward-backward asymmetry in $Brightarrow D,tau,bar{ u}$, the zero crossing point of forward backward asymmetry in $Brightarrow D^*taubar{ u}$ and the branching ratio of $B_crightarrow tau,bar{ u}$ decay can distinguish these seven new physics solutions if they can be measured with a required precision.
The recent measurements on $R_D$, $R_{D^*}$ and $R_{J/psi}$ by three pioneering experiments, BaBar, Belle and LHCb, indicate that the notion of lepton flavour universality is violated in the weak charged-current processes, mediated through $b to c ell bar u_ell$ transitions. These intriguing results, which delineate a tension with their standard model predictions at the level of $(2-3)sigma$ have triggered many new physics propositions in recent times, and are generally attributed to the possible implication of new physics in $ b to c tau bar u$ transition. This, in turn, opens up another avenue, i.e., $ b to u tau bar u$ processes, to look for new physics. Since these processes are doubly Cabibbo suppressed, the impact of new physics could be significant enough, leading to sizeable effects in some of the observables. In this work, we investigate in detail the role of new physics in $B to (pi,rho,omega)tau bar u$ and $B_s to (K,K^*) tau bar u$ processes considering a model independent approach. In particular, we focus on the standard observables like branching fraction, lepton flavour non-universality (LNU) parameter, forward-backward asymmetry and polarization asymmetries. We find significant deviations in some of these observables, which can be explored by the currently running experiments LHCb and Belle-II. We also briefly comment on the impact of scalar leptoquark $R_2(3,2,7/6)$ and vector leptoquark $U_1(3,1,2/3)$ on these decay modes.
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