arXiv subjects

D. Bečirević

Publications and source records attributed to D. Bečirević.

6 recordsLinked to original sources

arXiv2026

Charmonium radiative transitions to dileptons from lattice QCD: The case of $h_c \to η_c \ell^+\ell^-$ and $χ_{c1} \to J/ψ\,\ell^+\ell^-$

We present a lattice QCD study of dilepton production in charmonium transitions, specifically focusing on the $1^{+-} \to 0^{-+}$ and $1^{++} \to 1^{--}$ processes: $h_c \to η_c \ell^+ \ell^-$ and $χ_{c1} \to J/ψ\ell^+ \ell^-$, where $\ell = e, μ$. The relevant hadronic matrix elements are computed using gauge field configurations generated by the Extended Twisted Mass Collaboration with $N_f = 2+1+1$ dynamical Wilson--Clover twisted-mass fermions at four lattice spacings. Simulations are performed at physical dynamical $u$, $d$, $s$, and $c$ quark masses, except for the coarsest lattice, where the lightest sea quark mass corresponds to a slightly heavier pion mass. A controlled continuum extrapolation is carried out. In the continuum limit for the $h_c$ decays, we obtain $Γ(h_c \to η_c e^+ e^-) = 5.45(19)~\mathrm{keV}$, and $Γ(h_c \to η_c μ^+ μ^-) = 0.635(22)~\mathrm{keV}$. For the $χ_{c1}$ decays, we find: $Γ(χ_{c1} \to J/ψe^+ e^-)= 2.869(90)~\mathrm{keV}$, and $Γ(χ_{c1} \to J/ψμ^+ μ^-) = 0.1993(72)~\mathrm{keV}$. Our results for the $χ_{c1}$ decays show good compatibility with experimental data. However, our prediction for the $h_c \to η_c e^+ e^- $ decay rate is approximately $3σ$ larger than the BESIII result. We also present predictions for the differential decay widths as functions of the dilepton invariant mass, $q^2$, and for angular observables sensitive to longitudinal transition form factors, which are inaccessible in radiative decays with real photon emission. These results constitute the first fully dynamical lattice QCD predictions for dilepton decay rates in $h_c$ and $χ_{c1}$ charmonium transitions, including their differential distributions and angular observables. They provide benchmark predictions for future experimental studies.

hep-lat↗

arXiv2026

Interpreting the results on exclusive $c\rightarrow sμν$ modes

We examine the $q^2$-binned distributions of the $D\to Kμν$ decay rate and of the corresponding forward-backward asymmetry which were recently measured by BESIII, showing a mild deviation from the Standard Model predictions. We point out that the proposed solution to remedy the discrepancy by turning on a complex-valued New Physics coupling is in tension with the constraints deduced from the LHC bounds on the high-$p_T$ tail of the relevant Drell-Yan process. We then show that there are several plausible scenarios that are compatible with both the measured low-energy and high-energy constraints but the selected couplings appear to be too small to be observed in the measurements of integrated observables, except for possibly the $q^2$-binned distribution of the angular observables relevant to $D_s\to ϕ(\to KK) μν$ or $Λ_c\to Λ(\to p π) μν$ modes.

hep-ph↗

arXiv2025

Lattice QCD study of the $χ_{c1}\to J/ψ\, γ$ decay

We present the results of our lattice QCD computation of the electric ($E_{1}$) and magnetic $(M_{2})$ form factors relevant to the $χ_{c1}\to J/ψ\,γ$ decay by using the gauge field configurations produced by the Extended Twisted Mass Collaboration with $N_{f}=2+1+1$ dynamical Wilson-Clover twisted mass fermions at four different lattice spacings with physical dynamical $u$ , $d$, $s$ and $c$ quark masses (except for the coarsest lattice for which the lightest sea quark corresponds to a pion with $m_π\simeq 175~\mathrm{MeV}$). In the continuum limit, we obtain $Γ( χ_{c1}\to J/ψ γ) = 0.3265(79)~\mathrm{MeV}$, which agrees to $(1÷2) σ$ with the experimental results and disagrees with a previous (unquenched) lattice QCD calculation. Our result for the magnetic quadrupole fractional transition amplitude, $a_{2} =M_{2}/\sqrt{E_{1}^{2}+M_{2}^{2}} = -0.0666(22)$, is in agreement with the experiment and represents an improvement by a factor of about $30$ with respect to the only existing (quenched) lattice QCD result.

hep-lat↗

arXiv2025

Lattice QCD determination of the radiative decay rates $h_{c}\to η_{c}\, γ$ and $h_{b}\to η_{b}\, γ$

We present the results of our lattice QCD computation of the hadronic matrix elements relevant to the $h_{c}\to η_{c}γ$ and $h_{b}\to η_{b}γ$ decays by using the gauge configurations produced by the Extended Twisted Mass Collaboration with $N_{f}=2+1+1$ dynamical Wilson-Clover twisted mass fermions at five different lattice spacings with physical dynamical $u$ , $d$, $s$ and $c$ quark masses (except for the the coarsest lattice for which the lightest sea quark corresponds to a pion with $m_π\simeq 175~\mathrm{MeV}$). While the hadronic matrix element for $h_{c}\to η_{c}γ$ is obtained directly, the one relevant to $h_{b}\toη_{b}γ$ is reached by working with heavy quark masses $m^{(n)}_{H} = λ^{n-1} m_{c}$, with $λ\sim 1.24$ and $n=1,2, \ldots ,6$, and then extrapolated to $m_{b}$ by several judicious ansätze. In the continuum limit we obtain $Γ( h_{c}\to η_{c} γ) = 0.604(24)~\mathrm{MeV}$, which is by a factor of $2.3$ more accurate than the previous lattice estimates, and in good agreement with the experimental measurement. In the $b$-quark case we obtain $Γ( h_{b}\to η_{b} γ) =46.0(4.8)~\mathrm{keV}$.

hep-lat↗

arXiv2024

Lattice QCD determination of the normalization of the leading-twist photon distribution amplitude and susceptibility of the quark condensate

The normalization of the leading-twist photon distribution amplitude (DA), $f_γ^{\perp}$, is an important ingredient in the study of exclusive processes involving the photon emission by means of QCD sum-rules. In this paper we determine the up- , down- and strange-quark contribution to $f_γ^{\perp}$ by exploiting its relation to the zero-momentum two-point correlation function of the electromagnetic current $J_{\rm em}^μ$ and the electric component of the tensor current $T^{μν}$. To that end we employ the gauge ensembles obtained by using $N_{f}=2+1+1$ Wilson-Clover twisted-mass quark flavors, generated by the Extended Twisted Mass (ETM) Collaboration, and after adding all sources of systematic uncertainties, we obtain a total error of $1.5\%$ and $3.5\%$, respectively, for the light- ($u$ and $d$) and strange-quark contribution to $f_γ^{\perp}(2~{\rm GeV})$ in the $\overline{\mathrm{MS}}$ scheme, thus improving their accuracy by a factor of $2.3$ and $2.8$, respectively. For the strange-quark contribution $f_{γ,s}^{\perp}(2~{\rm GeV})$, we observe a discrepancy with respect to previous lattice calculations. By combining our result with the world average lattice value of the chiral condensate, we obtain for the susceptibility of the quark condensate $χ_d^{\overline{\mathrm{MS}}} (2\, {\rm GeV}) \simeq χ_u^{\overline{\mathrm{MS}}} (2\, {\rm GeV}) =2.17(12)~{\rm GeV^{-2}}$.

hep-lat↗

arXiv2020

Heavy baryon wave functions, Bakamjian-Thomas approach to form factors, and observables in ${Λ_b \to Λ_c\left({1 \over 2}^\pm \right) \ell \overlineν}$ transitions

Motivated by the calculation of observables in the decays $Λ_b \to Λ_c\left({1 \over 2}^\pm \right) \ell \overlineν$, as tests of Lepton Flavor Universality, we present a calculation of form factors in the quark model. Our scheme combines a spectroscopic model, providing the internal wave functions, and the Bakamjian-Thomas relativistic formalism to deduce the wave functions in motion and current matrix elements, that amount in the heavy quark limit to the Isgur-Wise (IW) function. For baryons we meet difficulties using standard spectroscopic models, leading us to propose a simple phenomenological model : a Q-pointlike-diquark model, non-relativistic with harmonic oscillator forces, with a reasonable low-lying spectrum and good slope of the IW function. We extract this slope from Lattice data and find $ρ_Λ^2 \sim 2$. We are not able to reproduce the right $ρ_Λ^2$ when using standard linear + Coulomb potential models, both with three quarks $Qqq$ or in a Q-pointlike-diquark picture. These difficulties seem to derive from the high sensitivity of $ρ_Λ^2$ to the structure of the light quark subsystem. After fixing the parameters of our interim model to yield correct spectrum and $ρ_Λ^2$, we compute observables. Bjorken sum rule shows that the inelastic IW function is large, and therefore $Λ_b \to Λ_c \left({1 \over 2}^-, {3 \over 2}^- \right) \ell \overlineν$ could be studied at LHCb. Some observables in the $τ$ case present zeroes for specific values of $q^2$ that could be tests of the Standard Model.

hep-ph↗