arXiv subjects
Publications and source records attributed to D. Ebert.
The possibility of color ferromagnetism in an SU(2) gauge field model is investigated. The conditions allowing a stable color ferromagnetic state of the quark system in the chromomagnetic field occupying small domains are considered. A phase transition between this state and the color superconducting state is considered. The effect of finite temperature is analyzed.
The masses of the S-wave mesons consisting of the light (u,d,s) quarks are calculated within the constituent quark model. The relativistic Schroedinger-like equation with a confining potential is numerically solved for the complete relativistic q bar q potential including both spin-independent and spin-dependent terms. The obtained masses of the ground-state pi, rho, K, K* and phi mesons and their first radial excitations are in reasonably good overall agreement with experimental data.
The spectrum of meson and diquark excitations of dense cold quark matter is investigated in the framework of a Nambu--Jona-Lasinio type model for light quarks of two flavors. It was found out that a first order phase transition occurs when the chemical potential $μ$ exceeds the critical value $μ_c=350$ MeV. Above $μ_c$, the diquark condensate $\vev{qq}$ forms, breaking the color symmetry of strong interaction. The masses of $π$- and $σ$-mesons are shown to grow with the chemical potential $μ$ in the color-superconducting phase, but the mesons themselves become almost stable particles due to the Mott effect. Moreover, we have found in this phase an abnormal number of three, instead of five, Nambu--Goldstone bosons, together with a color doublet of light stable diquark modes and a color-singlet heavy diquark resonance with the mass $\sim$ 1100 MeV. In the color symmetric phase, \textit{i.e.} for $μ<μ_c$, the splitting between the masses of diquarks and antidiquarks is shown to arise if $μ\ne 0$, contrary to the case of vanishing chemical potential, $μ= 0$, where the mass spectrum of antidiquarks and diquarks is degenerate at the value $\sim$ 700 MeV.
The charmonium mass spectrum is considered in the framework of the constituent quark model with the relativistic treatment of the c quark. The obtained masses are in good agreement with the existing experimental data including the mass of eta_c(2S).
Mass spectra and semileptonic decay rates of doubly heavy baryons are studied in the framework of the relativistic quark model in the quark-diquark approximation.
Semileptonic decays of doubly heavy baryons are studied in the framework of the relativistic quark model. The doubly heavy baryons are treated in the quark-diquark approximation. The transition amplitudes of heavy diquarks bb and bc going respectively to bc and cc are explicitly expressed through the overlap integrals of the diquark wave functions in the whole accessible kinematic range. The relativistic baryon wave functions of the quark-diquark bound system are used for the calculation of the transition matrix elements, the Isgur-Wise function and decay rates in the heavy quark limit.
We consider an extended Nambu--Jona-Lasinio model including both (q \bar q)- and (qq)-interactions with two light-quark flavors in the presence of a single (quark density) chemical potential. In the color superconducting phase of the quark matter the color SU(3) symmetry is spontaneously broken down to SU(2). If the usual counting of Goldstone bosons would apply, five Nambu-Goldstone (NG) bosons corresponding to the five broken color generators should appear in the mass spectrum. Unlike that expectation, we find only three gapless diquark excitations of quark matter. One of them is an SU(2)-singlet, the remaining two form an SU(2)-(anti)doublet and have a quadratic dispersion law in the small momentum limit. These results are in agreement with the Nielsen-Chadha theorem, according to which NG-bosons in Lorentz-noninvariant systems, having a quadratic dispersion law, must be counted differently. The origin of the abnormal number of NG-bosons is shown to be related to a nonvanishing expectation value of the color charge operator Q_8 reflecting the lack of color neutrality of the ground state. Finally, by requiring color neutrality, two massive diquarks are argued to become massless, resulting in a normal number of five NG-bosons with usual linear dispersion laws.
The possibility of a Chern-Simons like term generation in an extended model of QED, in which a Lorentz and CPT non-covariant interaction term for fermions is present, has been investigated at finite temperature and in the presence of a background color magnetic field. To this end, the photon polarization operator in an external constant axial-vector field has been considered. One-loop contributions to its antisymmetric component due to fermions in the linear order of the axial-vector field have been obtained. Moreover, the first nontrivial correction to the induced CS term due to the presence of a weak constant homogeneous color magnetic field has been derived.
Semileptonic and nonleptonic decays of the B_c meson are studied in the framework of the relativistic quark model. The heavy quark expansion in inverse powers of the heavy (b,c) quark mass is used to simplify calculations while the light final quarks in the B and D mesons are treated relativistically. The decay form factors are explicitly expressed through the overlap integrals of the meson wave functions in the whole accessible kinematical range. The obtained results are compared with the predictions of other approaches.
Semileptonic and nonleptonic decays of the B_c meson to charmonium and D mesons are studied in the framework of the relativistic quark model. The decay form factors are explicitly expressed through the overlap integrals of the meson wave functions in the whole accessible kinematical range. The relativistic meson wave functions are used for the calculation of the decay rates. The obtained results are compared with the predictions of other approaches.
Semileptonic and nonleptonic decays of the B_c meson to B_s and B mesons, caused by the c\to s,d quark transitions, are studied in the framework of the relativistic quark model. The heavy quark expansion in inverse powers of the active c and spectator \bar b quark is used to simplify calculations while the final s and d quarks in the B_s and B mesons are treated relativistically. The decay form factors are explicitly expressed through the overlap integrals of the meson wave functions in the whole accessible kinematical range. The obtained results are compared with the predictions of other approaches.
The relation between the hyperfine structure and leptonic decay rates of heavy quarkonia is considered with the account of relativistic and radiative corrections. The calculated decay rates agree well with the available experimental data, while the predicted eta_c(2S) mass is significantly smaller than the value measured recently by the Belle and BaBar Collaborations.
The influence of a constant homogeneous external magnetic field $H$ on the formation and stability of quark droplets is investigated within a simple Nambu -- Jona-Lasinio model by using a thermodynamic approach. For a vanishing magnetic field stable quark droplets, which are schematically the bags of massless quarks, are allowed to exist only at $G>G_{bag}$, where $G$ is the quark coupling constant, $G_{bag}=1.37G_{crit}$, and $G_{crit}$ is the value of the coupling constant above which chiral symmetry is spontaneously broken down. On the other hand, a nonvanishing external magnetic field can induce the stability of quark droplets so that they may exist even at $G<G_{bag}$. In this case, depending on the value of $H$, quark droplets are composed either of massive or massless quarks.
The hyperfine splitting in heavy quarkonia is considered in its relation to the leptonic decay rates with the account of relativistic and radiative corrections. The calculated decay rates agree well with the available experimental data, while the predicted η_c(2S) mass is significantly smaller than the value measured recently by the Belle Collaboration.
The mass spectra and electromagnetic decay rates of charmonium, bottomonium and B_c mesons are comprehensively investigated in the relativistic quark model. The presence of only heavy quarks allows the expansion in powers of their velocities. All relativistic corrections of order v^2/c^2, including retardation effects and one-loop radiative corrections, are systematically taken into account in the computations of the mass spectra. The obtained wave functions are used for the calculation of radiative magnetic dipole (M1) and electric dipole (E1) transitions. It is found that relativistic effects play a substantial role. Their account and the proper choice of the Lorentz structure of the quark-antiquark interaction in a meson is crucial for bringing theoretical predictions in accord with experimental data. A detailed comparison of the calculated decay rates and branching fractions with available experimental data for radiative decays of charmonium and bottomonium is presented. The possibilities to observe the currently missing spin-singlet S and P states as well as D states in bottomonium are discussed. The results for B_c masses and decays are compared with other quark model predictions.
Two-photon decay rates of pseudoscalar, scalar and tensor states of charmonium and bottomonium are calculated in the framework of the relativistic quark model. Both relativistic effects and one-loop radiative corrections are taken into account. The obtained results are compared with other theoretical predictions and available experimental data.
The one-loop corrections to the bottomonium mass spectrum due to the finite charm mass are evaluated in the framework of the relativistic quark model. The obtained corrections are compared with the results of perturbative QCD.
Nucleon structure and the origin and nature of the nuclear force are investigated in the context of a QCD-based effective field theory and the path-integral method of hadronization. We start from a microscopic model of quarks and diquarks where the gluons have been integrated out. In particular, we use the chiral Nambu-Jona-Lasinio model to describe quark dynamics and assume that the nucleon can be conceived as a quark-diquark relativistic bound state. The hadronization method is then used to rewrite the problem in terms of the physical meson and nucleon degrees of freedom. Next, by employing a loop and derivative expansion of the resulting quark/diquark determinants, we arrive at an effective chiral meson-nucleon Lagrangian. Nucleon properties such as mass, coupling constants, electromagnetic radii, anomalous magnetic moments, and form factors are derived using a theory of at most two free parameters.