Narrow Results

We report on a search for the flavor-changing neutral current decays D⁰ → e⁺e^- and D⁰ → μ⁺μ^-, and the lepton-flavor violating decay D⁰ → e^±μ^∓. The measurement is based on 122 fb^-1 of data collected by the BABAR detector at the PEP-II asymmetric e⁺e^- collider. No evidence is found for any of the decays. The upper limits on the branching fractions, at the 90% confidence level, are 1.2 × 10^-6 for D⁰ → e⁺e^-, 1.3 × 10^-6 for D⁰ → μ⁺μ^-, and 8.1 × 10^-7 for D⁰ → e^±μ^∓.

Hadronic form factors for the rare weak transitions Λ_b → Λ^(*) are calculated using a nonrelativistic quark model. The form factors are extracted in two ways. An analytic extraction using single-component wave functions (SCA) with the quark current being reduced to its nonrelativistic Pauli form is employed in the first method. In the second method, the form factors are extracted numerically using the full quark model wave function (MCN) with the full relativistic form of the quark current. Although there are differences between the two sets of form factors, both sets satisfy the relationships expected from the heavy quark effective theory (HQET). Differential decay rates, branching ratios (BRs) and forward–backward asymmetries (FBAs) are calculated for the dileptonic decays Λ_b → Λ^(*) ℓ⁺ ℓ^-, for transitions to both ground state and excited daughter baryons. Inclusion of the long distance contributions from charmonium resonances significantly enhances the decay rates. In the MCN model the Λ(1600) mode is the dominant mode in the μ channel when charmonium resonances are considered; the Λ(1520) mode is also found to have a comparable BR to that of the ground state in the μ channel.

Processes τ→lγ, τ→lll with l = e, μ and μ(τ) →e (μ) γ are evaluated in the framework of a model based on the extended symmetry gauge SU(3)_c ⊗SU(3)_L ⊗U(1)_Y with a leptonic sector consisting of five triplets. Lepton flavor violating processes are allowed at tree level in this model through the new Z′ gauge boson. We obtained bounds for the mixing angles in the leptonic sector of the model, considering the experimental measurements of the processes from the BELLE and the BABAR collaborations.

We study rare decays , and in NSIs. We calculate the NSIs branching ratios of these decays. There is a strong dependence of these on new physics parameter. We include second and third generation of quarks in the loop for these decays. We show that these provide stringent constraints on , where Q = u, c, t, and , where q = d, s, b. It is also pointed out that for α, β = e, μ.

We study rare decays and in the framework of nonstandard neutrino interactions (NSIs). We calculate branching ratios of these decays. We explore the possibility for second generation of quarks in NSIs just like leptonic contribution in and . We study the dependence of on . We show that there exists a possibility for , also from this reaction and other two reactions, and . Three other processes, and are investigated with s quark in the loop for NSIs instead of d quark. Constraints on and are provided. We point out that constraints for both u and c quark are equal and similarly for d and s quarks the constraints are equal .

Branching ratios, lepton forward–backward asymmetries, and lepton polarization asymmetries for the flavor-changing neutral current (FCNC) dileptonic decays of the ${\Lambda }_b$ baryon to the ground state and a number of excited state $\Lambda$ baryons are calculated using form factors extracted using wave functions from a constituent quark model. The SM branching ratios for the transition to the ground state calculated using these quark model form factors are consistent with the recent measurement reported by the LHCb collaboration. It is shown that the lepton polarization asymmetries are largely insensitive to the transition form factors and, therefore, may also be insensitive to the effects of QCD in the nonperturbative regime. These observables could therefore provide somewhat model independent ways of extracting various combinations of the Wilson coefficients.

The rare semileptonic $B\to K{\mu }^{+}{\mu }^{-}$ and $B\to \varphi {\mu }^{+}{\mu }^{-}$ decay modes mediated by the $b\to s{\mu }^{+}{\mu }^{-}$ transition are studied in this paper. We analyze observables such as the differential branching ratio, lepton polarization asymmetry and forward–backward asymmetry within the standard model in the low $q^2$ region. We then perform an analysis of the same modes within the 331 or $S{U}_C(3)\times S{U}_L(3)\times U_X(1)$ model in which a new heavy $Z^{\prime }$ boson contributes. The effects of the $Z^{\prime }$-mediated flavor-changing neutral current (FCNC) transitions on these modes are investigated by varying the mass $M_{Z^{\prime }}$ and the model parameter $\beta$. The new $Z^{\prime }$ couplings are constrained by flavor observables of $B_s-{\bar{B}}_s$ mixing and branching ratio of $B_s\to {\mu }^{+}{\mu }^{-}$. We find that for some values of these couplings, the observables are quite sensitive to $Z^{\prime }$ effects and deviations from Standard Model can be large.

We study the rare semileptonic $B\to \pi {\mu }^{+}{\mu }^{-}$ and $B\to \rho {\mu }^{+}{\mu }^{-}$ decay modes mediated by the quark level $b\to d{\mu }^{+}{\mu }^{-}$ transition in this paper. We analyze observables such as the differential branching ratio, lepton polarization asymmetry and forward–backward asymmetry within the standard model in the large hadronic recoil region. We then analyze these observables within the 331 or $S{U}_C(3)\times S{U}_L(3)\times U_X(1)$ model in which a new heavy $Z^{\prime }$ boson contributes. The effects of the $Z^{\prime }$-mediated flavor changing neutral current (FCNC) transitions on these modes are examined by varying the mass $M_{Z^{\prime }}$ and the model parameter $\beta$. The new $Z^{\prime }$ couplings are constrained by flavor observables of $B_d-{\bar{B}}_d$ mixing. For the obtained parameter space, we observe $Z^{\prime }$ sensitivities of our observables of interest and deviations from standard model can range from moderate to large depending on the $\beta$ value.

We calculate bounds for the branching ratio of the $B_s^0\to \tau e$ decay, for the first time, in the context of flavor changing neutral currents mediated by a $Z^{\prime }$ gauge boson, which can arise from five extended models. In this sense, by using experimental measurements on the $B\to X_s\gamma$ decay and the $B_s^0\to {\mu }^{+}{\mu }^{-}$ process, we look for constraints of the $Z^{\prime }bs$ coupling, where the more restrictive bound is offered by the last one. On the other hand, by employing the experimental restriction on the $\tau \to eeē$ decay, the strength of the $Z^{\prime }\tau e$ coupling is estimated. Our analysis is based on the more recent experimental results on searches for the $Z^{\prime }$ gauge boson in ATLAS and CMS detectors. In addition, we revisited the $B_s^0\to \tau \mu ,\mu e$ meson decays by using different approaches not previously reported. The strengths of the $Z^{\prime }\tau \mu$ and $Z^{\prime }\mu e$ couplings were estimated by employing experimental restrictions on the $\tau \to \mu \mu \bar{\mu }$ decay and the $\mu -e$ conversion rate, respectively. Thus, we predict the following upper bounds: $B_s^0\to \tau e\sim 10^{-6}$, $B_s^0\to \tau \mu \sim 10^{-6}$ and $B_s^0\to \mu e\sim 10^{-11}$.

Rare decays of hyperons involving di-neutrinos in the final state $\mathrm{\Lambda }\to n\bar{\nu }\nu$, ${\mathrm{\Sigma }}^{+}\to p\bar{\nu }\nu$, ${\mathrm{\Xi }}^0\to \mathrm{\Lambda }\bar{\nu }\nu$, ${\mathrm{\Xi }}^0\to {\mathrm{\Sigma }}^0\bar{\nu }\nu$, ${\mathrm{\Xi }}^{-}\to {\mathrm{\Sigma }}^{-}\bar{\nu }\nu$, ${\mathrm{\Omega }}^{-}\to {\mathrm{\Xi }}^{-}\bar{\nu }\nu$ in the Standard Model and beyond are studied. It is claimed that the branching ratios (Brs) calculated in this work are larger than the previously calculated values. For the search of NP the nonstandard neutrino interactions (NSI) are explored and constrains on NSI free parameters ${𝜖}_{\tau \tau }^{uL}$ are found. Stringent bounds on ${𝜖}_{\tau \tau }^{uL}$ of $O(10^{-2})$ are obtained. It is shown that Brs could be closed to BES-III, if the constraints are $O(0.3)$. We predict that these reactions with the current value of parameter of $O(1)$ will be in sensitivity of BES-III. To check the validity of our approach we also calculate Brs of these reactions by using 331 model and finding them consistent with previously available values of the same model.