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Register a new userLepton mass effects in exclusive semileptonic $B_c$-meson decays
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In this work, we discuss exclusive semileptonic $B_c$-meson decays: $B_cto eta_c(J/psi)l u$ and $B_cto D(D^*)l u$ in the framework of the relativistic independent quark(RIQ) model based on an average flavor independent confining potential in equally mixed scalar-vector harmonic form. We calculate the invariant form factors representing decay amplitudes from the overlapping integrals of meson wave functions derivable in the RIQ model. To evaluate the lepton mass effects in the semileptonic decays, we first study the $q^2$-dependence of the form factors in the accessible kinematic range of $q^2$ involved in the decay process in its $e^-$ and $tau^-$ mode separately. Similar studies on helicity amplitudes, $q^2-$spectra for different helicity contributions, and total $q^2$-spectra for each decay process are carried out separately in their $e^-$ and $tau^-$ modes. We predict the decay rates/ branching fractions, forward-backward asymmetry, and the asymmetry parameter in reasonable agreement with other model predictions, which can hopefully be tested in future experiments at the Tevatron and LHC. We also predict the observable $R$ which corresponds to the ratio of branching fractions for the decay process in its $e^-$ mode to its corresponding value in the $tau^-$ mode. Our results are comparable to another standard model(SM) predictions which highlight the failure of the lepton flavor universality hinting at new physics beyond SM for the explanation of the observed deviation of observable $R$ value from the corresponding SM predictions.
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We study the semileptonic decays of $B_c$ meson to S-wave charmonium states in the framework of relativistic independent quark model based on an average flavor-independent confining potential $U(r)$ in the scalar-vector harmonic form $U(r)=frac{1}{2}(1+gamma^0)(ar^2+V_0)$, where ($a$, $V_0$) are the potential parameters.The form factors for $B_c^+to eta_c /psi e^+ u_e$ transitions are studied in the physical kinematic range. Our predicted branching ratios (BR) for transitions to ground state charmonia are found comparatively large $sim $ $10^{-2}$, compared to those for transitions to radially excited 2S and 3S states. Like all other mpdel predictions, our predicted BR are obtained in the hierarchy: BR($B_c^+to eta_c /psi (3S)$) $<$ BR($B_c^+to eta_c/ psi (2S)$) $<$ BR($B_c^+to eta_c /psi (1S)$). The longitudinal ($Gamma_L$) and transverse polarization ($Gamma_T$) for $B_c to psi(ns)$ decay modes are predicted in the small and large $q^2$ - region as well as in the whole physical region. The ratios for such transitions are obtained $frac {Gamma_L}{Gamma_T} < 1$ throughout the kinematic range which means the $B_c^+$ meson transitions to vector meson charmonium states take place predominantly in transverse polarization mode. The theoretical predictions on these transitions could be tested in the on-going and forthcoming experiments at LHCb.
B meson semileptonic decays are a crucial tool in our studies of the quark mixing parameters Vcb and Vub. The interplay between experimental and theoretical challenges to achieve precision in the determination of these fundamental parameters is discussed.
We present a general study on exclusive semileptonic decays of heavy (B, D, B_s) to light (pi, rho, K, K^*) mesons in the framework of effective field theory of heavy quark. Transition matrix elements of these decays can be systematically characterized by a set of wave functions which are independent of the heavy quark mass except for the implicit scale dependence. Form factors for all these decays are calculated consistently within the effective theory framework using the light cone sum rule method at the leading order of 1/m_Q expansion. The branching ratios of these decays are evaluated, and the heavy and light flavor symmetry breaking effects are investigated. We also give comparison of our results and the predictions from other approaches, among which are the relations proposed recently in the framework of large energy effective theory.
In the semileptonic decays of heavy mesons and baryons the lepton-mass dependence factors out in the quadratic $cos^2theta$ coefficient of the differential $costheta$ distribution. We call the corresponding normalized coefficient the convexity parameter. This observation opens the path to a test of lepton universality in semileptonic heavy meson and baryon decays that is independent of form-factor effects. By projecting out the quadratic rate coefficient, dividing out the lepton-mass-dependent factor and restricting the phase space integration to the $tau$ lepton phase space, one can define optimized partial rates which, in the Standard Model, are the same for all three $(e,mu,tau)$ modes in a given semileptonic decay process. We discuss how the identity is spoiled by New Physics effects. We discuss semileptonic heavy meson decays such as $bar{B}^0 to D^{(ast)+} ell^- bar u_ell$ and $B_c^- to J/psi (eta_c)ell^- bar u_ell$, and semileptonic heavy baryon decays such as $Lambda_b to Lambda_c ell^- bar u_ell$ for each $ell=e,mu,tau$.
In this paper, we calculate the next-to-leading order (NLO) quantum chromodynamics (QCD) corrections to the exclusive processes $B_c^+to chi_{cJ}(h_c)pi^+$ in the framework of the nonrelativistic QCD (NRQCD) factorization formalism. The results show that NLO QCD corrections markedly enhance the branching ratios with $K$ factors of about 2.5. In combination with the study of $B_c^+to J/psipi^+$, we find that the NLO NRQCD prediction for the ratio of branching fractions $frac{mathcal{B}(B_c^+to chi_{c0}pi^+)}{mathcal{B}(B_c^+to J/psi pi^+)}$ is then compatible with the experimental measurement.
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