Search Results (6)

There is evidence that the properties of hadrons are modified in a nuclear medium. Information about the medium modifications of the internal structure of hadrons is fundamental for the study of dense nuclear matter and high-energy processes, including heavy-ion and nucleus–nucleus collisions. At the moment, however, empirical information about medium modifications of hadrons is limited; therefore, theoretical studies are essential for progress in the field. In the present work, we review theoretical studies of the electromagnetic and axial form factors of octet baryons in symmetric nuclear matter. The calculations are based on a model that takes into account the degrees of freedom revealed in experimental studies of low and intermediate square transfer momentum $q^2=-Q^2$: valence quarks and meson cloud excitations of baryon cores. The formalism combines a covariant constituent quark model, developed for a free space (vacuum) with the quark–meson coupling model for extension to the nuclear medium. We conclude that the nuclear medium modifies the baryon properties differently according to the flavor content of the baryons and the medium density. The effects of the medium increase with density and are stronger (quenched or enhanced) for light baryons than for heavy baryons. In particular, the in-medium neutrino–nucleon and antineutrino–nucleon cross-sections are reduced compared to the values in free space. The proposed formalism can be extended to densities above the normal nuclear density and applied to neutrino–hyperon and antineutrino–hyperon scattering in dense nuclear matter. Full article

The aim of this text is to present the covariant confined quark model (CCQM) and review its applications in the decays of B mesons. We do so in the context of existing experimental measurements and theoretical results of other authors, which we also review. The physics principles are, in detail, exposed for the CCQM; the other results (theoretical and experimental) are surveyed in an enumerative way with comments. We proceed by considering, successively, three categories of decay processes: leptonic, semileptonic and non-leptonic. Full article

We review the existing results on the exotic XYZ states and their decays obtained within the confined covariant quark model. This dynamical approach is based on a non-local Lagrangian of hadrons with quarks, has built-in quark confinement, and is suited well for the description of different multiquark states, including the four quark ones. We focus our analysis on the various decay modes of five exotic states, $X\left(3872\right)$ , $Z_c\left(3900\right)$ , $Y\left(4260\right)$ , $Z_b\left(10610\right)$ , and $Z_b^{\prime }\left(10650\right)$ , aiming to clarify their internal quark structures. By considering mostly branching fractions and decay widths using the molecular-type or the tetraquark-type interpolating currents, conclusions about the nature of these particles are drawn: the molecular structure is favored for $Z_c\left(3900\right)$ , $Z_b\left(10610\right)$ , and $Z_b^{\prime }\left(10650\right)$ and the tetraquark for $X\left(3872\right)$ and $Y\left(4260\right)$ . Full article

Measurements of the branching fractions of the semileptonic decays $B\to D^{(*)}\tau {\overline{\nu }}_{\tau }$ and $B_c\to J/\psi \tau {\overline{\nu }}_{\tau }$ systematically exceed the Standard Model predictions, pointing to possible signals of new physics that can violate lepton flavor universality. The unknown origin of new physics realized in these channels can be probed using a general effective Hamiltonian constructed from four-fermion operators and the corresponding Wilson coefficients. Previously, constraints on these Wilson coefficients were obtained mainly from the experimental data for the branching fractions. Meanwhile, polarization observables were only theoretically studied. The situation has changed with more experimental data having become available, particularly those regarding the polarization of the tau and the $D^{*}$ meson. In this study, we discuss the implications of the new data on the overall picture. We then include them in an updated fit of the Wilson coefficients using all hadronic form factors from our covariant constituent quark model. The use of our form factors provides an analysis independent of those in the literature. Several new-physics scenarios are studied with the corresponding theoretical predictions provided, which are useful for future experimental studies. In particular, we find that under the one-dominant-operator assumption, no operator survives at $1\sigma$ . Moreover, the scalar operators ${\mathcal{O}}_{S_L}$ and ${\mathcal{O}}_{S_R}$ are ruled out at $2\sigma$ if one uses the constraint $\mathcal{B}(B_c\to \tau {\nu }_{\tau })\le 10\%$ , while the more relaxed constraint $\mathcal{B}(B_c\to \tau {\nu }_{\tau })\le 30\%$ still allows these operators at $2\sigma$ , but only minimally. The inclusion of the new data for the $D^{*}$ polarization fraction $F_L^{D^{*}}$ reduces the likelihood of the right-handed vector operator ${\mathcal{O}}_{V_R}$ and significantly constrains the tensor operator ${\mathcal{O}}_{T_L}$ . Specifically, the $F_L^{D^{*}}$ alone rules out ${\mathcal{O}}_{T_L}$ at $1\sigma$ . Finally, we show that the longitudinal polarization $P_L^{\tau }$ of the tau in the decays $B\to D^{*}\tau {\overline{\nu }}_{\tau }$ and $B_c\to J/\psi \tau {\overline{\nu }}_{\tau }$ is extremely sensitive to the tensor operator. Within the $2\sigma$ allowed region, the best-fit value $T_L=0.04+i0.17$ predicts $P_L^{\tau }(D^{*})=-0.33$ and $P_L^{\tau }(J/\psi )=-0.34$ , which are at about 33% larger than the Standard Model (SM) prediction $P_L^{\tau }(D^{*})=-0.50$ and $P_L^{\tau }(J/\psi )=-0.51$ . Full article

In this lecture, we provide a basic introduction into the topic of charmed baryons and their nonleptonic two-body decays. Some features of the baryon weak decays on the quark level are discussed in detail in the framework of effective field theory. The calculation of the matrix elements of the four-quark operators arising in the effective theory proceeds by using the covariant constituent quark model. The model allows one to evaluate not only the factorizing tree-level diagrams but also more complicated diagrams with the internal W–exchange. The technique required for such calculation is discussed in some detail. Finally, the numerical results are presented, and comparison of the contributions coming from the W–exchange diagrams with those from the tree-level are carefully performed. Full article

The recent discovery of double charm baryon states by the LHCb Collaborarion and their high precision mass determination calls for a comprehensive analysis of the nonleptonic decays of double and single heavy baryons. Nonleptonic baryon decays play an important role in particle phenomenology since they allow for studying the interplay of long and short distance dynamics of the Standard Model (SM). Furthermore, they allow one to search for New Physics effects beyond the SM. We review recent progress in experimental and theoretical studies of the nonleptonic decays of heavy baryons with a focus on double charm baryon states and their decays. In particular, we discuss new ideas proposed by the present authors to calculate the W-exchange matrix elements of the nonleptonic decays of double heavy baryons. An important ingredient in our approach is the compositeness condition of Salam and Weinberg, and an effective implementation of infrared confinement both of which allow one to describe the nonperturbative structure of baryons composed of light and heavy quarks. Furthermore, we discuss an ab initio calculational method for the treatment of the so-called W-exchange diagrams generated by $W^{\pm }$ boson exchange between quarks. We found that the $W^{\pm }$ -exchange contributions are not suppressed in comparison with the tree-level (factorizing) diagrams and must be taken into account in the evaluation of matrix elements. Moreover, there are decay processes such as the doubly Cabibbo-suppressed decay ${\mathrm{\Xi }}_c^{+}\to p\varphi$ recently observed by the LHCb Collaboration, which is contributed to only by one single W-exchange diagram. Full article