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Register a new userExploring the role of new physics in $b to u tau bar u$ decays
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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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Recent experimental results for the ratios of the branching fractions of the decays $bar{B} to D^{(*)} tau bar u$ and $bar{B} to D^{(*)} mu bar u$ came as a surprise and lead to a discussion of possibility of testing New Physics beyond the Standard Model through these modes. We show that these decay channels can provide us with good constraints on New Physics and several New Physics cases are favored by the present experimental data. In order to discriminate various New Physics scenarios, we examine the $q^2$ distributions and estimate the sensitivity of this potential measurement at the SuperKEKB/Belle II experiment.
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.
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.
At present, the measurements of $R_{D^{(*)}}$ and $R_{J/psi}$ hint at new physics (NP) in $b to c tau^- {bar u}$ decays. The angular distribution of ${bar B} to D^* (to D pi) , tau^{-} {bar u}_tau$ would be useful for getting information about the NP, but it cannot be measured. The reason is that the three-momentum ${vec p}_tau$ cannot be determined precisely since the decay products of the $tau^-$ include an undetected $ u_tau$. In this paper, we construct a measurable angular distribution by considering the additional decay $tau^- to pi^- u_tau$. The full process is ${bar B} to D^* (to D pi) , tau^{-} (to pi^- u_tau) {bar u}_tau$, which includes three final-state particles whose three-momenta can be measured: $D$, $pi$, $pi^-$. The magnitudes and relative phases of all the NP parameters can be extracted from a fit to this angular distribution. One can measure CP-violating angular asymmmetries. If one integrates over some of the five kinematic parameters parametrizing the angular distribution, one obtains (i) familiar observables such as the $q^2$ distribution and the $D^*$ polarization, and (ii) new observables associated with the $pi^-$ emitted in the $tau$ decay: the forward-backward asymmetry of the $pi^-$ and the CP-violating triple-product asymmetry.
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.
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