Narrow Results

The current experimental status of the searches for the very rare decays and is discussed. These channels are highly sensitive to various extensions of the Standard Model, especially in the scalar and pseudoscalar sector. The recent, most sensitive measurements from the CDF, ATLAS, CMS and LHCb collaborations are discussed and the combined upper exclusion limit on the branching fractions determined by the LHC experiments is shown to be 4.2×10^-9 for and 0.8×10^-9 for . The implications of these tight bounds on a selected set of New Physics models is sketched.

Semileptonic and leptonic B decays are analyzed in the framework of the relativistic quark model. Special attention is paid to the decays involving τ lepton. It is found that the calculated particular decay branching fractions are consistent with available experimental data within error bars. However, the predicted and recently measured ratios R(D^(*)) of the B→D^(*)τν_τ and B→D^(*)lν_l branching fractions differ by 1.75σ for R(D) and by 2.4σ for R(D*).

Recently, there is a growing interest of a light leptophilic gauge boson $Z^{\prime }$, which might explain the ${(g-2)}_{\mu }$ puzzle. Considering the constraints on the $Z^{\prime }$ coupling $Z\ell {\ell }^{\prime }$, we calculate its contributions to the lepton flavor violation (LFV) decay $Z\to \tau \mu$. We find that this kind of new light gauge boson $Z^{\prime }$ might make the LFV decay $Z\to \tau \mu$ to be probed in future $e^{+}{e}^{-}$ colliders.

Recently, some small hints of lepton flavor violating (LFV) decays have been probed in different colliders and accelerators. Although these processes are forbidden in the Standard Model (SM), observation of LFV decays are considered as good probes to test theory beyond the SM. In this paper, we study some LFV decays, e.g. three-body decay channels of $\mu$ lepton like $\mu \to 3e$, $\mu \to e\nu \bar{\nu }$ and two-body decays of B meson and $\tau$ lepton like $B_{s,d}\to {\mu }^{\pm }{e}^{\mp }$ and $\tau \to e\varphi$ in $Z^{\prime }$ model. We calculate the branching ratio of the above decays considering the effect of both $Z$ and $Z^{\prime }$-mediated flavor-changing neutral-current (FCNC).

Results on $B\to {\mu }^{+}{\mu }^{-}$ decays with the CMS experiment are reported, using 61 fb${}^{-1}$ of data recorded during LHC Run 1 and 2016. With an improved muon identification algorithm and refined unbinned maximum likelihood fitting methods, the decay $B_s^0\to {\mu }^{+}{\mu }^{-}$ is observed with a significance of 5.6 standard deviations. Its branching fraction is measured to be $\overline{{ℬ}}(B_s^0\to {\mu }^{+}{\mu }^{-})=[2.9\pm 0.7(exp)\pm 0.2(\mathrm{\text{frag}})]\times 10^{-9}$, where the first error is the combined statistical and systematic uncertainty and the second error quantifies the uncertainty of the $B_s^0$ and $B^{+}$ fragmentation probability ratio. The $B_s^0\to {\mu }^{+}{\mu }^{-}$ effective lifetime is ${\tau }_{{\mu }^{+}{\mu }^{-}}=1.70_{-0.44}^{+0.61}\mathrm{\text{ps}}$. No evidence for the decay $B^0\to {\mu }^{+}{\mu }^{-}$ is found and an upper limit of ${ℬ}(B^0\to {\mu }^{+}{\mu }^{-})<3.6\times 10^{-10}$ (at 95% confidence level) is determined. All results are consistent with the standard model of particle physics.

The search for the decay η → e⁺e^- was carried out using the WASA detector at the COSY storage ring. Data were taken during a two week experimental run in Sept-Nov 2008 in which about 10⁷ η mesons were collected from the reaction of pp → ppη at energy of 1.4 GeV. Background studies were performed for several different reactions. The performance of the WASA detector for the measurement of electron-positron pairs based on the single Dalitz decay of the η meson was studied. We also identified a statistically significant data sample of other more frequent leptonic decays.

In recent years, B_s →τ⁺τ^- rare decay has attracted a lot of attention since it is very sensitive to the structure of standard model (SM) and potential source of new physics beyond SM. In this paper, we study the effect of both Z and Z′-mediated flavor-changing neutral currents on the B_s →τ⁺τ^- decay. We find the branching ratio B(B_s→τ⁺τ^-) is enhanced relative to SM prediction, which would help to explain the recently observed CP-violation from like-sign dimuon charge asymmetry in the B system.

We scrutinize recent QCD spectral sum rules (QSSR) results to lowest order (LO) predicting the masses of the $BK$ molecule and $(su)(\overline{bd})$ four-quark states. We improve these results by adding NLO and N2LO corrections to the PT contributions giving a more precise meaning on the $b$-quark mass definition used in the analysis. We extract our optimal predictions using Laplace sum rule (LSR) within the standard stability criteria versus the changes of the external free parameters ($\tau$-sum rule variable, $t_c$ continuum threshold and subtraction constant $\mu$). The smallness of the higher order PT corrections justifies (a posteriori) the LO order results ⊕ the uses of the ambiguous heavy quark mass to that order. However, our predicted spectra in the range $(5173\sim 5226)$ MeV, summarized in Table 7, for exotic hadrons built with four different flavors $(buds)$, do not support some previous interpretations of the D0 candidate,¹$X(5568)$, as a pure molecule or a four-quark state. If experimentally confirmed, it could result from their mixing with an angle: $sin2𝜃\approx 0.15$. One can also scan the region $(2327\sim 2444)$ MeV (where the $D_{s0}^{\ast }(2317)$ might be a good candidate) and the one $(5173\sim 5226)$ MeV for detecting these $(cuds)$ and $(buds)$ unmixed exotic hadrons (if any) via, eventually, their radiative or $\pi$+hadrons decays.

In this paper, the general structure of leptonic decay constants of vector mesons is evaluated in the framework of the Bethe–Salpeter equation under the Covariant Instantaneous Ansatz (CIA), which is a Lorentz-invariant generalization of Instantaneous Approximation (IA). The numerical values of f_V in this CIA framework are on the low side in comparison to recent calculations of these quantities. However, the overall asymptotic behavior of f_V is in conformity with QCD predictions.

We have employed the framework of Bethe–Salpeter equation under covariant instantaneous ansatz to calculate leptonic decay constants of unequal mass pseudoscalar mesons like π^±, K, D, D_S and B, and radiative decay constants of neutral pseudoscalar mesons like π⁰ and η_c into two photons. In the Dirac structure of hadronic Bethe–Salpeter wave function, the covariants are incorporated from their complete set in accordance with a recently proposed power counting rule. The contribution of both leading order and next-to-leading order Dirac covariants to decay constants are studied. The results are found to improve and hence validating the power counting rule which provides a practical means of incorporating Dirac covariants in the Bethe–Salpeter wave function for a hadron.

We first review recent measurements of D → μν and D_s → μν, τν from CLEO-c. These decays are used to determine leptonic decay constants f_D and f_Ds which may be compared to modern Lattice QCD (LQCD) calculations. Such cross-checks can provide confidence in Lattice results for B_(s) meson decay constants which are needed to fully exploit data used to extract the CKM matrix elements V_ub and V_cb. We also comment briefly on the outlook for further progress at BESIII.

We present a selection of recent charm results from the BESIII Collaboration. The topics include measurements of strong phase in D⁰ → Kπ decay and y_CP, decay constant f_D⁺ measurement from D⁺ → μ⁺ν, form-factors measurement in D⁰ → K⁻e⁺ν, π⁻e⁺ν, Dalitz plot analysis of , search for D⁰ → γγ rare decay and D_s decays.

Using the data sample taken at the center-of-mass energies 3.773, 4.18 and 4.6 GeV by the BESIII detector, BESIII colloboration studied the semileptonic and leptonic charmed hadron decays very well in recent years. The studies mainly contains the precision measurements of decay constants, form factors and CKM matrix elements. The properties of a₀(980), f₀(500), f₀(980) and K₁(1270)⁰ have also been extracted. BESIII also test the LFU at a very high precision.

Selected recent results on measurements of form factors by the BaBar Collaboration are reviewed, including e⁺e^- → η^(')γ, leptonic and semileptonic charm decays from data collected at or near the ϒ(4S) resonance.

Purely leptonic decays of the B meson are important predictions of the Standard Model, but have not been observed. We present the results of recent BaBar searches for the decays B⁺ → ℓ⁺ν_ℓ where ℓ = e, μ, τ. We also present the results of a search for the rare radiative leptonic decay B⁰ → ℓ⁺ℓ^-γ, where ℓ = e, μ. We find no evidence for these decays and proceed to set upper limits on their branching ratios.