Charm diffusion in a pion gas implementing unitarity, chiral and heavy quark symmetries

We compute the charm drag and diffusion coefficients in a hot pion gas, such as is formed in a heavy ion collision after the system cools sufficiently to transit into the hadron phase. We fully exploit heavy quark effective theory (with both D and D^∗ mesons as elementary degrees of freedom during the collision) and chiral perturbation theory, and employ standard unitarization to reach higher temperatures. We find that a certain friction and shear diffusion coefficients are almost p²-independent at a fixed temperature which simplifies phenomenological analysis.At the higher end of reliability of our calculation, , we report a charm relaxation length , in agreement with the model estimate of He, Fries and Rapp.The momentum of a 1 GeV charm quark decreases about 50 MeV per fermi when crossing the hadron phase.     

Two body non-leptonic decaysB →Dπ with heavy quark and chiral symmetry

We have analysed the two body non-leptonic charmed-meson decays of heavyB meson based on the factorization assumption. The transition matrix elements and the corresponding decay widths are calculated in the heavy quark and chiral symmetry limit and the Isgur-Wise function present in the expression is determined by the wave function model of Aliet al. The results obtained are quite interesting and agree reasonably well with the experimental data.     

Self-energy of heavy quark

We demonstrate that to calculate the selfenergy of a heavy quark in the heavy quark limit (or the heavy fermion limit in what is called the Baryon Chiral Perturbation Theory), the use of standard dimensional regularization provides only the quantum limit: opposite to the heavy quark (or classical) limit that one wishes to obtain. We thus devised a standard ultraviolet/infrared regularization procedure in calculating the one- and two-loop contributions to the heavy quark self-energy in this limit. Then the one-loop result is shown to provide the standard classical Coulomb self-energy of a static colour source that is linearly proportional to the ultraviolet cutoff Λ. All the two-loop contributions are found to be proportional to Λ In (Λ/γ) where γ is the infrared cutoff. Often only the contribution from the bubble (light quarks, gluon and ghost) insertion to the gluon propagator has been considered as theO(α _s ) correction to the Coulomb energy to this order. Our result shows that other contributions are of the same magnitude, thus have to be taken into account.     

Nonlinear realization of chiral symmetries and localizability

We prove that the nonlinear realization ofS U(n)×S U(n) (n3) is uniquely determined by the requirement that the Lagrangian, with a minimal number of derivatives of those fields parametrizing the adjoint representation of the diagonalS U(n) subgroup, is localizable in the sense of Jaffe.     

Broken chiral symmetries and light composite fermions

We elaborate on a mechanism for obtaining naturally light composite fermions in a strongly interacting gauge theory. A simple toy model is used to illustrate the basic ideas.     

Photoproduction of heavy quark flavours

Within QCD the photoproduction of new heavy flavours proceeds via photon-gluon amalgamation. The production cross sections of new particles (charm, top, bottom, …) are calculated. Photon and lepton beams are better suited than hadron beams for producing the new particles.     

Roper resonances in chiral quark models

We derive a method to calculate the multi-channel K -matrix applicable to a broad class of models in which mesons linearly couple to the quark core. The method is used to calculate pion scattering amplitudes in the energy region of low-lying P11 and P33 resonances. A good agreement with experimental data is achieved if in addition to the elastic channel we include the and N ( channels where the -meson models the correlated two-pion decay. We solve the integral equation for the K -matrix in the approximation of separable kernels; it yields a sizable increase of the widths of the (1232) and the N(1440) resonances compared to the bare quark values.     

H-particle in a chiral quark model

In this paper we discuss the binding energy of the H-particle using a chiral quark model, where pion exchange plays an important role to reproduce the mass difference between the nucleon and Δ resonance. Since the main source for the bound H-particle is believed to be the color magnetic interaction, which gives the nucleon and Δ mass difference, it is very interesting to investigate whether the chiral quark model gives rise to the bound H-particle or not. We employ an extended resonating group method in order to take into account the possibility of a change of baryon wave functions when two baryons interact with each other. We found that a change of baryon size together with the Hamiltonian which consists of gluon, pseudoscalar meson and sigma meson exchange potentials gives rise to the bound H-particle. The binding energy is found to be about 25 MeV in a hybrid chiral quark model. Differences between the ordinary gluon dominant model and chiral quark models are also investigated. It is found that a pure chiral model has no bound state when the widely used sigma-quark coupling strength is employed.     

Radiative decays of heavy mesons with heavy quark symmetry

The radiative decays of heavy mesons are considered in heavy quark symmetry formalism. Introducing the interpolating fields for the heavy mesons we obtain the transition matrix element and the corresponding decay widths in the heavy quark symmetry limits. The Isgur-Wise function present in the expression is determined by ISGW quark model. The results obtained agree reasonably well with other estimations.     

Spin dependence of heavy quark fragmentation

We propose that the non-perturbative part of the fragmentation function describing the transition from a heavy quark to a heavy meson is proportional to the square of the produced meson wave function at the origin, taking into account hyperfine interactions. We analyze the effects of this proposal on the number of pseudoscalar mesons compared to the number of vector mesons produced and find a good agreement with experimental data. Finally, we discuss further experimental checks for our hypothesis.     

Radiative decays of heavy baryons with heavy quark symmetry

The decay widths for the radiative decays of heavy baryons are calculated in the heavy quark effective theory. Introducing the interpolating fields for heavy baryons we obtain the transition matrix elements and the corresponding decay widths. Considering theSU(6) flavor-spin wave functions for heavy baryons, the coupling constants are calculated in the nonrelativistic quark model. Since the masses of the heavy baryons are not available, we have taken the predicted bag model masses. We find our results are quite different from that of the heavy quark bag model calculations.