Showing papers on "Meson published in 1996"

Hyperon rich matter in neutron stars

Abstract: We study the equation of state of hyperon-rich matter in neutron stars using an extended relativistic mean-field model. We take special care of the recently proposed nonlinear behavior of the vector field providing a much better agreement with Dirac-Br\"uckner calculations. The hyperon-hyperon interaction is also implemented by introducing additional meson exchanges. With these new terms we avoid the instability found at high densities while keeping the excellent description for finite nuclear systems. We also demonstrate within the mean-field approach that the presence of hyperons inside neutron stars on one hand and the hyperon-hyperon interactions on the other hand make the onset of kaon condensation less favorable. \textcopyright{} 1996 The American Physical Society.

Phenomenology of Heavy Meson Chiral Lagrangians

Abstract: The approximate symmetries of Quantum ChromoDynamics in the infinite heavy quark ($Q=c,b$) mass limit ($m_Q \to \infty$) and in the chiral limit for the light quarks ($m_q \to 0,\;q=\,u,\,d,\,s$) can be used together to build up an effective chiral lagrangian for heavy and light mesons describing strong interactions among effective meson fields as well as their couplings to electromagnetic and weak currents, including the relevant symmetry breaking terms. The effective theory includes heavy ($Q \bar q$) mesons of both negative and positive parity, light pseudoscalars, as well as light vector mesons. We summarize the estimates for the parameters entering the effective lagrangian and discuss in particular some phenomenologically important couplings, such as $g_{B^* B \pi}$. The hyperfine splitting of heavy mesons is discussed in detail. The effective lagrangian allows for the possibility to describe consistently weak couplings of heavy ($B,\, D$) to light ($\pi,\, \rho, \, K^*,\,$ etc.) mesons. The method has however its own limitations, due to the requirement that the light meson momenta should be small, and we discuss how such limitations can be circumvented through reasonable ansatz on the form factors. Flavour conserving (e. g. $B^* \to B\, \gamma$) and flavour changing (e. g. $B \to K^* \, \gamma$) radiative decays provide another field of applications of effective lagrangians; they are discussed together with their phenomenological implications. Finally we analyze effective lagrangians describing heavy charmonium- like ($\bar Q Q$) mesons and their strong and electromagnetic interactions. The role of approximate heavy quark symmetries for this case and the phenomenological tests of these models are also discussed.

Is f 0 (1500) a Scalar Glueball

Abstract: Following the discovery of two new scalar mesons, f0(1370) and f0(1500) at the Low Energy Antiproton Ring at CERN, we argue that the observed properties of this pair are incompatible with them both being QQ mesons. We show instead that f0(1500) is compatible with the ground state glueball expected around 1500 MeV mixed with the nearby states of the 0 ++ Q ¯

rho meson light-cone distribution amplitudes of leading twist reexamined.

Abstract: We give a complete re-analysis of the leading twist quark-antiquark light-cone distribution amplitudes of longitudinal and transverse ρ mesons. We derive Wandzura-Wilczek type relations between different distributions and update the coefficients in their conformal expansion using QCD sum rules including next-to-leading order radiative corrections. We find that the distribution amplitudes of quarks inside longitudinally and transversely polarized ρ mesons have a similar shape, which is in contradiction to previous analyses.

Higher Quarkonia

Abstract: We discriminate gluonic hadrons from conventional $q\bar q$ states by surveying radial and orbital excitations of all I=0 and I=1 $n\bar n$ systems anticipated up to 21 GeV We give detailed predictions of their quasi-two-body branching fractions and identify characteristic decay modes that can isolate quarkonia Several of the ``missing mesons'' with L$_{q\bar q}=2$ and L$_{q\bar q}=3$ are predicted to decay dominantly into certain S+P and S+D modes, and should appear in experimental searches for hybrids in the same mass region We also consider the topical issues of whether some of the recently discovered or controversial meson resonances, including glueball and hybrid candidates, can be accommodated as quarkonia

Chiral expansion of baryon masses and $\sigma$-terms

Abstract: We analyze the octet baryon masses and the pion/kaon--nucleon $\sigma$--terms in the framework of heavy baryon chiral perturbation theory. We include {\it all} terms up-to-and-including quadratic order in the light quark masses, $m_q$. We develop a consistent scheme to estimate low--energy constants related to scalar--isoscalar operators in the framework of resonance exchange involving one--loop graphs. The pertinent low--energy constants can only be estimated up to some finite coefficients. Including contributions from loop graphs with intermediate spin--3/2 decuplet and spin--1/2 octet states and from tree graphs including scalar mesons, we use the octet baryon masses and the pion--nucleon $\sigma$--term to fix all but one of these coefficients. Physical results are insensitive to this remaining parameter. It is also demonstrated that two--loop corrections only modify some of the subleading low--energy constants. We find for the baryon mass in the chiral limit, $m_0 = 770 \pm 110$ MeV. While the corrections of order $m_q^2$ are small for the nucleon, they are still sizeable for the $\Lambda$, the $\Sigma$ and the $\Xi$. Therefore a definitive statement about the convergence of three--flavor baryon chiral perturbation can not yet be made. The strangeness content of the nucleon is $y = 0.21 \pm 0.20$. We also estimate the kaon--nucleon $\sigma$--terms and some two--loops contributions to the nucleon mass.

Effective action for the chiral quark-meson model.

Abstract: The scale dependence of an effective average action for mesons and quarks is described by an exact nonperturbative flow equation. The running of couplings follows from a truncation of this equation. For the dimensionless scalar and Yukawa couplings it proceeds according to renormalization-group-improved one-loop \ensuremath{\beta} functions including mass threshold functions. The running scalar mass term becomes negative for small scales and induces spontaneous chiral symmetry breaking. We argue that for strong Yukawa coupling between quarks and mesons the low momentum physics is essentially determined by infrared fixed points. This allows us to establish relations between various parameters related to the meson potential. The results for ${\mathit{f}}_{\mathrm{\ensuremath{\pi}}}$ and 〈\ensuremath{\psi}\ifmmode\bar\else\textasciimacron\fi{}\ensuremath{\psi}〉 are not very sensitive to the poorly known details of the quark-meson effective action at scales where the mesonic bound states form. For realistic constituent quark masses we find ${\mathit{f}}_{\mathrm{\ensuremath{\pi}}}$ around 100 MeV. \textcopyright{} 1996 The American Physical Society.

Aspects of chiral pion-nucleon physics

Abstract: The next-to-leading order chiral pion-nucleon Lagrangian contains seven finite low-energy constants. Two can be fixed from the nucleon anomalous magnetic moments and another one from the quark mass contribution to the neutron-proton mass splitting. We find a set of nine observables, which to one loop order do only depend on the remaining four dimension two couplings. These are then determined from a best fit. We also show that their values can be understood in terms of resonance exchange related to $\Delta$ excitation as well as vector and scalar meson exchange. In particular, we discuss the role of the fictitious scalar-isoscalar meson. We also investigate the chiral expansion of the two P-wave scattering volumes $P_1^-$ and $P_2^+$ as well as the isovector S-wave effective range parameter $b^-$. The one-loop calculation is in good agreement with the data. The difference $P_1^- - P_2^+$ signals chiral loop effects in the $\pi N$ P-waves. The calculated D- and F-wave threshold parameters compare well with the empirical values.

Perturbative QCD analysis of B meson decays.

Abstract: The resummation of large QCD radiative corrections, including leading and next-to-leading logarithms, in the pion electromagnetic form factor is reviewed. A similar formalism is applied to exclusive processes involving heavy mesons, and leads to Sudakov suppression for the semileptonic decay {ital B}{r_arrow}{pi}{ital l}{nu}. It is found that, with the inclusion of Sudakov effects, a perturbative QCD analysis of this decay is reliable for the energy fraction of the pion above 0.4. Combining predictions from soft pion theorems we estimate that the upper limit of the matrix element {parallel}{ital V}{sub {ital ub}}{parallel} is 2.8{endash}4.8{times}10{sup {minus}3} from different models of the {ital B} meson wave function. {copyright} {ital 1996 The American Physical Society.}

Hard diffractive electroproduction of vector mesons in QCD.

Abstract: Hard diffractive electroproduction of longitudinally polarized vector mesons is calculated within the leading {alpha}{sub {ital s}}ln({ital Q}{sup 2}/{Lambda}{sub QCD}{sup 2}) approximation of QCD using the leading order parton densities within the nucleon. Novel QCD features of the production of excited states and of the restoration of flavor symmetry are analyzed. At the onset of the asymptotic regime, our analysis finds an important role of quark Fermi motion within the diffractively produced vector mesons, and we suggest to use this effect to measure the high momentum tail of the wave function of the vector mesons. We deduce a kinematical boundary for the region of applicability of the decomposition of the hard amplitudes over powers of {ital Q}{sup 2} and/or a limit on the increase of the cross sections of hard processes at small {ital x}, and briefly analyze its consequences. We also estimate the nuclear attenuation of the diffractive electroproduction of vector mesons and compare with estimates of the shadowing of the longitudinal structure function. {copyright} {ital 1996 The American Physical Society.}

Medium effects in high energy heavy-ion collisions

Abstract: The change of hadron properties in dense matter based on various theoretical approaches are reviewed. Incorporating these medium effects in the relativistic transport model, which treats consistently the change of hadron masses and energies in dense matter via the scalar and vector fields, heavy-ion collisions at energies available from SIS/GSI, AGS/BNL and SPS/CERN are studied. This model is seen to provide satisfactory explanations for the observed enhancement of kaon, antikaon and antiproton yields as well as soft pions in the transverse direction from the SIS experiments. In the AGS heavy-ion experiments, it can account for the enhanced ratio, the difference in the slope parameters of the and transverse kinetic energy spectra and the lower apparent temperature of antiprotons than that of protons. This model also provides possible explanations for the observed enhancement of low-mass dileptons, phi mesons and antilambdas in heavy-ion collisions at SPS energies. Furthermore, the change of hadron properties in hot dense matter leads to new signatures of the quark - gluon plasma to hadronic matter transition in future ultrarelativistic heavy-ion collisions at RHIC/BNL.

Bounds on the light quark masses

Abstract: The corrections to the current algebra mass formulae for the pseudoscalar mesons are analyzed by means of a simultaneous exansion in powers of the light quark masses and powers of 1/N A set of mass formulae is derived, including an inequality, which leads to bounds for ratios of quark masses

Extended NJL Model for light and heavy mesons without q-qbar thresholds

Abstract: We consider the NJL model as an effective quark theory to describe the interaction which is responsible for the quark flavor dynamics at intermediate energies. In addition to the usual ultraviolet cut-off which is necessary since the model is non-renormalizable, we also introduce an infrared cut-off which drops off the unknown confinement part of the quark interaction, which is believed to be less important for the flavor dynamics. The infrared cut-off eliminates all q-qbar thresholds, which plague the application of the usual NJL model beyond low-energy pion physics. We apply this two-cut-off prescription to the extended NJL model with chiral and heavy quark symmetries proposed recently by us. We find a satisfactoring description even of the heavy mesons with spin/parity J/P = (0+, 1+). Furthermore, the shape-parameters of the Isgur-Wise function are studied as a function of the residual heavy meson mass.

Simple description of pi pi scattering to 1 GeV.

Abstract: Motivated by the 1/Nc expansion, we present a simple model of �� scattering as a sum of a current-algebra contact term and resonant pole exchanges. The model preserves crossing symmetry as well as unitarity up to 1.2 GeV . Key features include chiral dynamics, vector meson dominance, a broad low energy scalar (�) meson and a Ramsauer-Townsend mechanism for the understanding of the 980 MeV region. We discuss in detail the regularization (corresponding to rescattering effects) necessary to make all these nice features work.

The $B_c$ Meson Lifetime

Abstract: We investigate the total inclusive decay rate of the (ground state) $B_c$ meson within the framework of an operator product expansion in inverse powers of the heavy quark masses and subsequent matching onto nonrelativistic QCD. The expansion is organized as a series in the strong coupling and in powers of the heavy quark velocities in the $B_c$, reflecting the nonrelativistic nature of a heavy-heavy bound state. In this aspect the character of the expansion differs from the more familiar case of heavy-light mesons. The framework incorporates systematically corrections to the leading $b$- and $c$-quark decays due to binding effects, as well as contributions from weak annihilation and Pauli interference. Based on this approach we find for the $B_c$ meson lifetime $\tau_{B_c}=(0.4 - 0.7)\,$ps, the dominant mechanism being the decay of the charm constituent.

Orbitally excited and hybrid mesons from the lattice.

Abstract: We discuss in general the construction of gauge-invariant nonlocal meson operators on the lattice. We use such operators to study the {ital P}- and {ital D}-wave mesons as well as hybrid mesons in quenched QCD, with quark masses near the strange quark mass. The resulting spectra are compared with experiment for the orbital excitations. For the states produced by gluonic excitations (hybrid mesons) we find evidence of mixing for nonexotic quantum numbers. We give predictions for masses of the spin-exotic hybrid mesons with {ital J}{sup {ital PC}}=1{sup {minus}+}, 0{sup +{minus}}, and 2{sup +{minus}}. {copyright} {ital 1996 The American Physical Society.}

Color dipole phenomenology of diffractive electroproduction of light vector mesons at HERA

Abstract: We develop the color dipole phenomenology of diffractive photo- and electroproduction $\gamma^{*}\,N\rightarrow V(V')\,N$ of light vector mesons ($V(1S) = \phi^0, \omega^0, \rho^0$) and their radial excitations ($V'(2S) = \phi', \omega', \rho'$). The node of the radial wave function of the $2S$ states in conjunction with the energy dependence of the color dipole cross section is shown to lead to a strikingly different $Q^2$ and $ u$ dependence of diffractive production of the $V(1S)$ and $V'(2S)$ vector mesons. We discuss the restoration of flavor symmetry and universality properties of production of different vector mesons as a function of $Q^{2}+m_{V}^{2}$. The color dipole model predictions for the $\rho^{0}$ and $\phi^{0}$ production are in good agreement with the experimental data from the EMC, NMC, ZEUS and H1 collaborations. We present the first direct evaluation of the dipole cross section from these data.

The Xi (2220) revisited: Strong decays of the 1(3) F2 1(3) F4 s anti-s mesons

Abstract: We calculate the decay widths of the 1{sup 3}{ital F}{sub 2} and 1{sup 3}{ital F}{sub 4} {ital s{bar s}} mesons and compare them to the measured properties of the {xi}(2220) [now known as the {ital f}{sub 4}(2220)]. Including previously neglected decay modes we find that the width of the {sup 3}{ital F}{sub 2} state {ital s{bar s}} meson is much larger than previously believed, making this explanation unlikely. On the other hand the predicted width of the {sup 3}{ital F}{sub 4} state, although broader than the observed width, is consistent within the uncertainties of the model. This interpretation predicts large partial widths to {ital KK{sup {asterisk}}}(892) and {ital K}{sup {asterisk}}(892){ital K}{sup {asterisk}}(892) final states which should be looked for. A second possibility that would account for the different properties of the {xi}(2220) seen in different experiments is that two hadronic states exist at this mass. The first would be a broader {sup 3}{ital F}{sub 4} {ital s{bar s}} state which is seen in hadron beam experiments while the second would be a narrow state with high glue content seen in the gluon rich {ital J}/{psi} radiative decay. Further experimental results are needed to sort this out. {copyright} {ital 1996 Themore » American Physical Society.}« less

Variation of hadron masses in finite nuclei

Abstract: The quark-meson coupling model, based on a mean-field description of nonoverlapping nucleon bags bound by the self-consistent exchange of $\ensuremath{\sigma}$, $\ensuremath{\omega}$, and $\ensuremath{\rho}$ mesons, is extended to investigate the change of hadron properties in finite nuclei. Relativistic Hartree equations for spherical nuclei have been derived from a relativistic quark model of the structure of bound nucleons and mesons. Using this unified, self-consistent description of both infinite nuclear matter and finite nuclei, we investigate the properties of some closed-shell nuclei and study the changes in the hadron masses of the nonstrange vector mesons, the hyperons, and the nucleon in those nuclei. We find a new, simple scaling relation for the changes of the hadron masses, which can be described in terms of the number of nonstrange quarks in the hadron and the value of the scalar mean field in a nucleus.

COSY- 11, an internal experimental facility for threshold measurements

Abstract: The COSY-11 installation is an internal experiment at the cooler synchrotron and storage ring COSY Julich. It has been designed for full geometrical acceptance close to threshold for meson production studies, especially in the 1 GeV/c2 mass range. The experimental setup makes use of a regular C-type COSY dipole magnet, following a cluster target, to separate reaction products from the beam and to analyze their momenta, thus allowing the observation of charged reaction products at small angles with beam energies close to threshold. Resonances will be identified by missing mass reconstructions from measured four-momenta of two outgoing protons in the predominantly studied pp → ppX reaction. In addition, charged mesons either produced directly or from decays of X will be detected. The different components of the experimental facility are presented.