Dilepton production from ρ-mesons in a quark-gluon plasma

Assuming that ρ-mesons exist in a quark-gluon plasma at temperatures close to the QCD phase transition, we calculate the dilepton production rate from q-ˉq annihilation via a ρ-meson state using Vector Meson Dominance. The result is compared to the rates from direct q-ˉq annihilation and from π⁺-π⁻ annihilation. Furthermore we discuss the suppression of low mass dileptons if the quarks assume an effective mass in the quark-gluon plasma. Received: 8 September 1999 / Revised version: 22 October 1999     

The K * Kπ and ρππ couplings in QCD

The light cone QCD sum rules are derived for the K ^* Kπ coupling g _K ^* Kπ and the ρππ coupling g _ρππ. The contribution from the excited states and the continuum is subtracted cleanly through the double Borel transform with respect to the two external momenta, p ₁ ², p ₂ ²= (p−q)². Our result g _K ^* Kπ= (8.7 ± 0.5) and g _ρππ= (11.5 ± 0.8) is in good agreement with the experimental value. Received: 31 July 1998 / Revised version: 20 November 1998     

Hadron diffractive processes: the structure of soft pomeron and colour screening

On the basis of the experimental data on diffractive processes in πp, pp and pˉp collisions at intermediate, moderately high and high energies, we restore the scattering amplitude related to the t-channel exchange by vacuum quantum numbers by taking account of the diffractive s-channel rescatterings. At intermediate and moderately high energies, the t-channel exchange amplitude turns, with a good accuracy, into an effective pomeron which renders the results of the additive quark model. At superhigh energies the scattering amplitude provides a Froissart-type behaviour, with an asymptotic universality of cross sections such as σ^tot _πp/σ^tot _pp→ 1 at s→∞. The quark structure of hadrons being taken into account at the level of constituent quarks, the cross sections of pion and proton (antiproton) in the impact parameter space of quarks, σ_π(r _1⊥, r _2⊥; s) and σ_p(r _1⊥, r _2⊥, r _3⊥; s), are found as functions of s. These cross sections implicate the phenomenon of colour screening: they tend to zero at |r _i⊥−r _k⊥|→ 0. The effective colour screening radius for pion (proton) is found for different s. The predictions for the diffractive cross sections at superhigh energies are presented. Received: 15 December 1998     

Investigation of Binding Energy and Potential in Mesons

Truly by looking through the analytical model of constituent quarks and further theimportance of the effects of relativity on quark dynamics in studying the interior structure of mesons,in this research we have strived to have a much more precise modeling for quark interior structure.Certainly by observing the constituent model of quarks, at first we consider the mesons as two-body system,then we place the considered calculated Potential, which is a function of location and spin,in Schrödinger's equation. Next we will solve the mentioned equation in analytical method. Moving on this solution,we will import the spin and isospin interactions as perturbation in our problem,and finally by using these solutions we can obtain both binding energy and wave function for bound state and excited states of meson.Eventually, by applying these calculations in the next and last step we will compare our data about meson'sbinding energy and masses with others results.     

A coupled-channel lattice study of the resonance-like structure Zc(3900)

In this exploratory study, near-threshold scattering of D and \begin{document}$\bar{D}^*$\end{document} meson is investigated using lattice QCD with \begin{document}$N_f=2+1+1$\end{document} twisted mass fermion configurations. The calculation is performed in the coupled-channel Lüscher finite-size formalism. The study focuses on the channel with \begin{document}$I^G(J^{PC})=1^+(1^{+-})$\end{document} where the resonance-like structure \begin{document}$Z_c(3900)$\end{document} was discovered. We first identify the two most relevant channels and the lattice study is performed in the two-channel scattering model. Combined with the two-channel Ross-Shaw theory, scattering parameters are extracted from the energy levels by solving the generalized eigenvalue problem. Our results for the scattering length parameters suggest that for the particular lattice parameters that we studied, the best fit parameters do not correspond to the peak in the elastic scattering cross-section near the threshold. Furthermore, in the zero-range Ross-Shaw theory, the scenario of a narrow resonance close to the threshold is disfavored beyond the 3\begin{document}$\sigma$\end{document} level.     

Charmed Meson Production in Deep Inelastice+A Scattering at Small-x

Based on the nuclear effects considered in the Glauber-Gribov approach, charmed meson production in high energy deep inelastic e+A scattering is investigated in the color dipole formalism. Using the Peterson fragmentation function and the KLR-AdS/CFT color dipole model, which reasonably well describes the HERA data for the inclusive structure functions at small Bjorken-x, we present the predictive results for the D-meson transverse spectra at EIC and LHeC energies. The theoretical results indicate that the $x$-independence phenomena at very small-x predicted by the KLR-AdS/CFT model are not seen in our calculation.     

Study on the Effects of the Light CP-odd Higgs via the Leptonic Decays of Pseudoscalar Mesons

To explain the anomalously large decay rate of Σ⁺→p+μ⁺μ^-, it was proposed that a new mechanism where a light CP-odd pseudoscalar boson of m_A1⁰=214.3 MeV makes a crucial contribution. Later, some authors have studied the transition π⁰→ e⁺e^- and r → γA₁⁰ in terms of the same mechanism and their result indicates that with the suggested mass one cannot fit the data. This discrepancy might be caused by experimental error of Σ⁺→ p+μ⁺μ^- because there were only a few events. Whether the mechanism is a reasonable one motivates us to investigate the transitions π⁰→ e⁺e^-;η(η')→ μ⁺μ^-; η_c→ μ⁺μ^-; η_b→τ⁺τ^- within the same framework. It is noted that for π⁰→ e⁺e^-, the standard model (SM) prediction is smaller than the data, whereas the experimental central value of η→μ⁺μ^- is also above the SM prediction. It means that there should be extra contributions from other mechanisms and the contribution of A₁⁰ may be a possible one. Theoretically calculating the branching ratios of the concerned modes, we would check if we can obtain a universal mass for A₁⁰ which reconcile the theoretical predictions and data for all the modes. Unfortunately, we find that it is impossible to have such a mass with the same coupling |gl|. Therefore we conclude that the phenomenology does not favor such a light A₁⁰, even though a small window is still open.     

η mesons in hot magnetized nuclear matter

\begin{document}$\eta N$\end{document} interactions are investigated in hot magnetized asymmetric nuclear matter using the chiral SU(3) model and chiral perturbation theory (ChPT). In the chiral model, the in-medium properties of η-mesons are calculated using medium modified scalar densities under the influence of an external magnetic field. Further, in a combined chiral model and ChPT approach, off-shell contributions of the \begin{document}$\eta N$\end{document} interactions are evaluated from the ChPT effective \begin{document}$\eta N$\end{document} Lagrangian, and the in-medium effect of scalar densities are incorporated from the chiral SU(3) model. We find that the magnetic field has a significant effect on the in-medium mass and optical potential of η mesons, and we observe a deeper mass-shift in the combined chiral model and ChPT approach than in the solo chiral SU(3) model. In both approaches, no additional mass-shift is observed due to the uncharged nature of η mesons in the presence of a magnetic field.     

Charm mixing from BABAR

We present recent results from BABAR experiment for D0-D0 mixing measurements. Mixing parameters can be measured in different ways using different D0 decay modes, here we discuss the most sensitive analyses such as DO→K+π- where we had the first evidence of charm mixing, the measurement of the ratio of lifetimes of the decays DO→K+K-and DO→π- relative to D0→K-π+, the time dependent Dalitz plot analysis of D0→K+π-π0.New limits on CP-violating time-integrated asymmetries in D0→K+K- and D0→π+π- are also discussed. The analyses presented are based on 384 fb-1 data collected with the BABAR detector at the PEP-Ⅱ asymmetric B Factory.