Exclusive Rare Decays Research Articles

Combined analysis of Bc→Ds(*)μ+μ− and Bc→Ds(*)νν¯ decays within Z′ and leptoquark new physics models

We investigate the exclusive rare semileptonic decays $ B_c \to D_{s}^{(*)}\,(\ell \ell, \nu \bar{\nu})$ induced by neutral current transition $b \to s (\ell \ell,\nu \bar{\nu})$ in the presence of non-universal $Z^{\prime}$, scalar and vector leptoquark new physics models. We constrain the new physics parameter space by using the latest experimental measurements of $R_{K^{(*)}}$, $P_5^{\prime}$, $\mathcal{B}(B_s \to \phi \mu^+ \mu^-)$ and $\mathcal{B}(B_s \to \mu^+ \mu^-)$. Throughout the analysis, we choose to work with the particular new physics scenario $C_9 ^{\mu \mu}(NP)=-C_{10} ^{\mu \mu}(NP)$ where both $Z^{\prime}$, $S_{1/3}^3$ and $U_{-2/3}^3$ leptoquarks satisfy the condition. Using these new coupling parameters we scrutinize the several physical observables such as differential branching fraction, the forward backward asymmetry, the lepton polarization asymmetry, the angular observable $P^{\prime}_5$ and the lepton flavor universal sensitive observables including the ratio of branching ratio $R_{D_s^{(*)}}$ and the few $Q$ parameters in the $B_c \to D_s^{(*)}\,\mu ^+ \mu ^-$ and $B_c \to D_s^{(*)}\, \nu \bar{\nu}$ decay processes.

Exact NLO matching and analyticity in b\u2009\u2192\u2009s\u2113\u2113

Exclusive rare decays mediated by b → sℓℓ transitions receive contributions from four-quark operators that cannot be naively expressed in terms of local form factors. Instead, one needs to calculate a matrix element of a bilocal operator. In certain kinematic regions, this bilocal operator obeys some type of Operator Product Expansion, with coefficients that can be calculated in perturbation theory. We review the formalism and, focusing on the dominant SM operators O1,2, we perform an improved calculation of the NLO matching for the leading dimension-three operators. This calculation is performed completely analytically in the two relevant mass scales (charm-quark mass mc and dilepton squared mass q2), and we pay particular attention to the analytic continuation in the complex q2 plane. This allows for the first time to study the analytic structure of the non-local form factors at NLO, and to calculate the OPE coefficients far below q2 = 0, say {q}^2underset{sim }{<}-10{mathrm{GeV}}^2 . We also provide explicitly the contributions proportional to different charge factors, which obey separate dispersion relations.

Form factors for BK l+l- decay

The exclusive BV(P)l+l- and BV decays, with a vector (V=K*, , …) or pseudo scalar (P=K, , …) meson in the final state are important for the search for flavour-changing new physics. In this paper we study one of the exclusive rare decays of pseudo scalar B mesons: BKl+l- by using a relativistic constituent quark model, including infrared connement. We calculate the relevant form factors of the (BP) transitions in the full kinematical range of momentum transfer. The calculated form factors are used to evaluate differential decay rates and polarization observables. We present numerical results on these observables using the covariant quark model. This model can be viewed as an effective quantum field theory approach based on an interaction Lagrangian of hadrons interacting with their constituent quarks. Universal and reliable predictions for exclusive processes involving both mesons composed from a quark and antiquark and baryons composed from three quarks result from this approach.

Improved Light-Cone QCD Sum Rule Analysis of Rare Decays Λb → Λγ and Λb → Λl+l−

We present a systematic light-cone QCD sum rule study of the exclusive rare radiative decay Λb → Λγ and rare semileptonic decay Λb → Λl+l− within the framework of the standard model. Although some light-cone sum rule (LCSR) studies on these rare processes can be found in different literatures, it is necessary to reanalyze them systematically for the reason that either the baryonic distribution amplitudes are improved or different interpolating currents for the Λb baryon may lead to quite different results. In addition, the rare process Λb → Λγ has not yet been analyzed by LCSR with the Ioffe-type current. Taking all these reasons into account, we perform LCSR calculations of both the processes with two types of interpolating currents. Our calculations show that the choice of the interpolating current for the Λb baryon can affect the predictions significantly, especially for the rare radiative decay process.

Exclusive rare decayb→sγof heavybbaryons

We present an analysis on the exclusive rare radiative decay modes Sigma_b->Sigma gamma and Xi_b->Xi gamma. The transition form factors which parameterize these processes are calculated using QCD light-cone sum rules. The decay widths we predict are Gamma(Sigma_b->Sigma gamma)=(7.21\pm0.04)\times10^{-18}GeV} and Gamma(Xi_b->Xi gamma)=(1.34\pm0.07)\times10^{-16}GeV. The Branching ratio of Xi_b->Xi gamma is predicted to be Br(\Xi_b->Xi gamma)=(3.03\pm0.10)\times10^{-4}.

Study ofB→K0*(1430)ll¯decays

We examine the exclusive rare decays of B{yields}K{sub 0}*(1430)ll with K{sub 0}*(1430) being the p-wave scalar meson and l={nu}, e, {mu}, {tau} in the standard model. The form factors for the B{yields}K{sub 0}* transition matrix elements are evaluated in the light-front quark model. For the decays of B{yields}K{sub 0}*ll, the branching ratios are found to be (11.6,1.63,1.62,0.029)x10{sup -7} with l=({nu},e,{mu},{tau}) and the integrated longitudinal lepton polarization asymmetries (-0.97,-0.95,-0.03) with l=(e,{mu},{tau}), respectively.

Exclusive rareB→K*ℓ+ℓ−decays at low recoil: Controlling the long-distance effects

We present a model-independent description of the exclusive rare decays $\overline{B}\ensuremath{\rightarrow}{K}^{*}{e}^{+}{e}^{\ensuremath{-}}$ in the low recoil region (large lepton invariant mass ${q}^{2}\ensuremath{\sim}{m}_{b}^{2}$). In this region the long-distance effects from quark loops can be computed with the help of an operator product expansion in $1/Q$, with $Q={{m}_{b},\sqrt{{q}^{2}}}$. Nonperturbative effects up to and including terms suppressed by $\ensuremath{\Lambda}/Q$ and ${m}_{c}^{2}/{m}_{b}^{2}$ relative to the short-distance amplitude can be included in a model-independent way. Based on these results, we propose an improved method for determining the Cabibbo-Kobayashi-Maskawa matrix element $|{V}_{ub}|$ from a combination of rare and semileptonic $B$ and $D$ decays near the zero recoil point. The residual theoretical uncertainty from long-distance effects in this $|{V}_{ub}|$ determination comes from terms in the operator product expansion of order ${\ensuremath{\alpha}}_{s}(Q)\ensuremath{\Lambda}/{m}_{b}$, ${\ensuremath{\alpha}}_{s}^{2}(Q)$, ${m}_{c}^{4}/{m}_{b}^{4}$, and duality violations, and is estimated to be below 10%.

QCD AND HEAVY HADRON DECAYS

We review recent developments in QCD pertaining to its application to weak decays of heavy hadrons. We concentrate on exclusive rare and nonleptonic B-meson decays, discussing both the theoretical framework and phenomenological issues of current interest.

The exclusive rare decays $$B \rightarrow k{\bar l}l$$ and $$B_c \rightarrow D(D^*){\bar l}l$$ in a relativistic quark model

We study the exclusive rare decay $$B \rightarrow k{\bar l}l$$ . We calculate the relevant form factors within a relativistic constituent quark model, for the first time without employing the impulse approximation. The calculated form factors are used to evaluate differential decay rates and polarization observables. We present results on the q2-dependence of a set of observables with and without long-distance contributions. A similar analysis is done for the exclusive rare decays $$B_c \rightarrow D(D^*){\bar l}l$$ with special emphasis on the cascade decay $$B_c\to D^\ast(\to D\pi){\bar l} l$$ . We derive a four-fold angular decay distribution for this process in terms of helicity amplitudes including lepton mass effects. The four-fold angular decay distribution allows to define a number of physical observables which are amenable to measurement. We compare our results with the results of other studies.

Rare decays B → Xsl+l− and B → K(*)l+l− in SM and beyond

After a brief review of the inclusive and exclusive rare semileptonic decays of B mesons in the standard model (SM), an overview of recent theoretical developments in this field is given. New physics effects on various observables such as branching ratio, backward-forward asymmetry, polarization of lepton and CP violation in the decays are analyzed in models beyond SM (supersymmetric models and two Higgs doublet models).

QCD estimate of the long-distance effect in [formula omitted

We suggest to use operator product expansion and QCD sum rule techniques to estimate the long-distance contribution to the amplitude of the exclusive rare decay B → K ∗γ . In contrast to a phenomenological description in terms of ψ resonances converting into a photon, the virtual charm quark loop interacting with soft gluons is represented by a new quark-gluon-photon operator. The matrix element of this operator is calculated in the same approximation as the matrix element of the leading magnetic penguin operator. The overall correction is found to be small, not more than about 5%.

Search for the decays B d0 → γγ and B s0 → γγ

A search for the decays B d,s 0 → γγ in 2.95 million hadronic Z decays has been performed using the L3 detector at LEP. No candidates are found in the signal region and upper limits have been set on the branching ratios: Br(B d 0→ γ)<3.9×10 −5 and Br(B s 0 → γγ) < 14.8 × 10 −5 at 90% CL. These are the first limits set on these exclusive rare decays.

Semileptonic and exclusive rare B decays

The exclusive rare decay B→K ∗γ takes place in a region of maximum recoil, q 2=0, posing a problem for nonrelativistic quark models which are usually thought to be most reliable at zero recoil. The Bauer-Stech-Wirbel (BSW) model, formulated in the infinite-momentum-frame (IMF) formalism, is designed to work at q 2=0. We show in this model that the ratio relating the decay B→K ∗γ and the q 2-spectrum of the semileptonic decay B→ϱe ν , becomes independent of the wave function in the SU(3) flavor symmetry limit. We show that this feature is also true in relativistic quark models formulated in the IMF or light-cone formalism, if the b quark is infinitely heavy. In fact, these relativistic models, which have a different spin structure from the BSW case, reduce to the BSW model in the heavy b-quark limit. A direct measurement of the q 2-spectrum of the semileptonic decay cam therefore provide accurate information for the exclusive rare decay.

Rare decays B → X d + γ in the standard model

We present an estimate of the inclusive decay rate and photon energy- and hadron mass-spectrum for the CKM-suppressed radiative rare decays B → X d + γ, based on perturbative QCD and a phenomenological model for the B-meson wave function (here X d denotes nonstrange hadrons). Present constraints on | V td| are used to predict BR(B → X d + γ) = (0.8−4) × 10 −5 for the top quark mass in the range 100 ⩽ m t ⩽ 200 GeV. From the inclusive invariant hadron-mass spectrum and using vector meson dominance in the region m(X d) ⩽ 1 GeV, we estimate the branching ratio for the exclusive rare decay mode B → ϱ + γ and the ratio Γ( B → ϱ + γ)/Γ( B → K ∗ + γ) . The importance of measuring these decays in determining the CKM matrix element | V td| is emphasized.