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Register a new userThe role of right-handed neutrinos in $b to c tau bar{ u}$ anomalies
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Motivated by the persistent anomalies reported in the $bto ctaubar{ u}$ data, we perform a general model-independent analysis of these transitions, in the presence of light right-handed neutrinos. We adopt an effective field theory approach and write a low-energy effective Hamiltonian, including all possible dimension-six operators. The corresponding Wilson coefficients are determined through a numerical fit to all available experimental data. In order to work with a manageable set of free parameters, we define eleven well-motivated scenarios, characterized by the different types of new physics that could mediate these transitions, and analyse which options seem to be preferred by the current measurements. The data exhibit a clear preference for new-physics contributions, and good fits to the data are obtained in several cases. However, the current measurement of the longitudinal $D^*$ polarization in $Bto D^*tau bar u$ cannot be easily accommodated within its experimental $1sigma$ range. A general analysis of the three-body $Bto D tau bar u$ and four-body $Bto D^*(to Dpi)tau bar u$ angular distributions is also presented. The accessible angular observables are studied in order to assess their sensitivity to the different new physics scenarios. Experimental information on these distributions would help to disentangle the dynamical origin of the current anomalies.
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We consider the recent LHCb result for $B_cto J/psi tau u$ in conjunction with the existing anomalies in $R(D)$ and $R(D^star)$ within the framework of a right-handed current with enhanced couplings to the third generation. The model predicts a linear relation between the observables and their SM values in terms of two combinations of parameters. The strong constraints from $bto s gamma$ on $W-W^prime$ mixing effectively remove one of the combinations of parameters resulting in an approximate proportionality between all three observables and their SM values. To accommodate the current averages for $R(D)$ and $R(D^star)$, the $W^prime$ mass should be near 1 TeV, and possibly accessible to direct searches at the LHC. In this scenario we find that $R(J/psi)$ is enhanced by about 20% with respect to its SM value and about 1.5$sigma$ below the central value of the LHCb measurement. The predicted $dGamma/dq^2$ distribution for $Bto D(D^star) tau u$ is in agreement with the measurement and the model satisfies the constraint from the $B_c$ lifetime.
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.
We evaluate long-distance electromagnetic (QED) contributions to $bar{B}{}^0 to D^+ tau^{-} bar{ u}_{tau}$ and $B^- to D^0 tau^{-} bar{ u}_{tau}$ relative to $bar{B}{}^0 to D^+ mu^{-} bar{ u}_{mu}$ and $B^- to D^0 mu^{-} bar{ u}_{mu}$, respectively, in the standard model. We point out that the QED corrections to the ratios $R(D^{+})$ and $R(D^{0})$ are not negligible, contrary to the expectation that radiative corrections are almost canceled out in the ratio of the two branching fractions. The reason is that long-distance QED corrections depend on the masses and relative velocities of the daughter particles. We find that theoretical predictions for $R(D^{+})^{tau/mu}$ and $R(D^{0})^{tau/mu}$ can be amplified by $sim4%$ and $sim3%$, respectively, for the soft-photon energy cut in range $20$-$40$ MeV.
We study potential New Physics effects in the $bar B to D^{(*)} tau bar u$ decays. As a particular example of New Physics models we consider the class of leptoquark models and put the constraints on the leptoquark couplings using the recently measured ratios $R(D^{(*)})=BR(bar B to D^{(*)} tau bar u)/BR(bar B to D^{(*)} mu bar u)$. For consistency, some of the constraints are compared with the ones coming from the current experimental bound on $BR(B to X_s u bar u)$. In order to discriminate various New Physics scenarios, we examine the correlations between different observables that can be measured in future.
In this letter we consider that assuming: a) that the only left-handed neutral fermions are the active neutrinos, b) that $B-L$ is a gauge symmetry, and c) that the $L$ assignment is restricted to the integer numbers, the anomaly cancellation imply that at least three right-handed neutrinos must be added to the minimal representation content of the electroweak standard model. However, two types of models arise: i) the usual one where each of the three identical right-handed neutrinos has total lepton number L=1; ii) and the other one in which two of them carry L=4 while the third one carries $L=-5$.
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