Showing papers on "Meson published in 2006"

Linear confinement and AdS/QCD

Abstract: In a theory with linear confinement, such as QCD, the masses squared m^2 of mesons with high spin S or high radial excitation number n are expected, from semiclassical arguments, to grow linearly with S and n. We show that this behavior can be reproduced within a putative 5-dimensional theory holographically dual to QCD (AdS/QCD). With the assumption that such a dual theory exists and describes highly excited mesons as well, we show that asymptotically linear m^2 spectrum translates into a strong constraint on the INFRARED behavior of that theory. In the simplest model which obeys such a constraint we find m^2 ~ (n+S).

Energy spectra of gamma rays, electrons, and neutrinos produced at proton-proton interactions in the very high energy regime

Abstract: We present new parameterizations of energy spectra of secondary particles, $\ensuremath{\pi}$ mesons, gamma rays, electrons, and neutrinos produced in inelastic proton-proton collisions. The simple analytical approximations based on simulations of proton-proton interactions using the public available SIBYLL code provide very good accuracy for energy distributions of secondary products in the energy range above 100 GeV. Generally, the recommended analytical formulas deviate from the simulated distributions within a few percent over a large range of $x={E}_{i}/{E}_{p}$---the fraction of energy of the incident proton transferred to the secondaries. Finally, we describe an approximate procedure of continuation of calculations towards low energies, down to the threshold of $\ensuremath{\pi}$-meson production.

Infrared properties of QCD from Dyson-Schwinger equations

Abstract: I review recent results on the infrared properties of QCD from Dyson-Schwinger equations. The topics include infrared exponents of one-particle irreducible Green's functions, the fixed point behaviour of the running coupling at zero momentum, the pattern of dynamical quark mass generation and properties of light mesons.

Hadronic spectra and light-front wave functions in holographic QCD.

Abstract: We show how the string amplitude {Phi}(z) defined on the fifth dimension in AdS{sub 5} space can be precisely mapped to the light-front wavefunctions of hadrons in physical spacetime. We find an exact correspondence between the holographic variable z and an impact variable {zeta}, which represents the measure of transverse separation of the constituents within the hadrons. In addition, we derive effective four dimensional Schroedinger equations for the bound states of massless quarks and gluons which exactly reproduce the AdS/CFT results and give a realistic description of the light-quark meson and baryon spectrum as well as the form factors for spacelike Q{sup 2}. Only one parameter which sets the mass scale, {Lambda}{sub QCD}, is introduced.

First measurement of the rho spectral function in high-energy nuclear collisions

Abstract: We report on a precision measurement of low-mass muon pairs in 158 AGeV indium-indium collisions at the CERN SPS. A significant excess of pairs is observed above the yield expected from neutral meson decays. The unprecedented sample size of 360 000 dimuons and the good mass resolution of about 2% allow us to isolate the excess by subtraction of the decay sources. The shape of the resulting mass spectrum is consistent with a dominant contribution from pi+pi--->rho-->µ+µ- annihilation. The associated space-time averaged rho spectral function shows a strong broadening, but essentially no shift in mass. This may rule out theoretical models linking hadron masses directly to the chiral condensate.

Holographic melting and related properties of mesons in a quark-gluon plasma

Abstract: We analyze mesons at finite temperature in a chiral, confining string dual. The temperature dependence of low-spin as well as high-spin meson masses is shown to exhibit a pattern familiar from the lattice. Furthermore, we find the dissociation temperature of mesons as a function of their spin, showing that at a fixed quark mass, mesons with larger spins dissociate at lower temperatures. The Goldstone bosons associated with chiral symmetry breaking are shown to disappear above the chiral symmetry restoration temperature. Finally, we show that holographic considerations imply that large-spin mesons do not experience drag effects when moving through the quark-gluon plasma. They do, however, have a maximum velocity for fixed spin, beyond which they dissociate.

Higher-twist distribution amplitudes of the K meson in QCD.

Abstract: We present a systematic study of twist-3 and twist-4 light-cone distribution amplitudes of the K meson in QCD. The structure of SU(3)-breaking corrections is studied in detail. Non-perturbative input parameters are estimated from QCD sum rules and renormalons. As a by-product, we give a complete reanalysis of the twist-3 and -4 parameters of the π-meson distribution amplitudes; some of the results differ from those usually quoted in the literature.

Nuclear modification of electron spectra and implications for heavy quark energy loss in Au+Au collisions at sNN=200GeV

Abstract: The PHENIX experiment has measured midrapidity ([FORMULA: SEE TEXT]) transverse momentum spectra ([FORMULA: SEE TEXT]) of electrons as a function of centrality in Au+Au collisions at [FORMULA: SEE TEXT]. Contributions from photon conversions and from light hadron decays, mainly Dalitz decays of pi0 and eta mesons, were removed. The resulting nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons carrying heavy quarks. Nuclear modification factors were determined by comparison to nonphotonic electrons in p+p collisions. A significant suppression of electrons at high pT is observed in central Au+Au collisions, indicating substantial energy loss of heavy quarks.

Elastic J/psi production at HERA

Abstract: Cross sections for elastic production of J/Psi mesons in photoproduction and electroproduction are measured in electron proton collisions at HERA using an integrated luminosity of 55 pb^{-1}. Results are presented for photon virtualities Q^2 up to 80 GeV^2. The dependence on the photon-proton centre of mass energy W_{gamma p} is analysed in the range 40 < \Wgp < 305 GeV in photoproduction and 40 < \Wgp < 160 GeV in electroproduction. The \Wgp dependences of the cross sections do not change significantly with Q^2 and can be described by models based on perturbative QCD. Within such models, the data show a high sensitivity to the gluon density of the proton in the domain of low Bjorken x and low Q^2. Differential cross sections d\sigma/dt, where t is the squared four-momentum transfer at the proton vertex, are measured in the range |t|<1.2 GeV^2 as functions of \Wgp and Q^2. Effective Pomeron trajectories are determined for photoproduction and electroproduction. The J/Psi production and decay angular distributions are consistent with s-channel helicity conservation. The ratio of the cross sections for longitudinally and transversely polarised photons is measured as a function of Q^2 and is found to be described by perturbative QCD based models.

The Unitarity Triangle Fit in the Standard Model and Hadronic Parameters from Lattice QCD: A Reappraisal after the Measurements of Delta m_s and BR(B to tau nu)

Abstract: The recent measurements of the B_s meson mixing amplitude by CDF and of the leptonic branching fraction BR(B to tau nu) by Belle call for an upgraded analysis of the Unitarity Triangle in the Standard Model. Besides improving the previous constraints on the parameters of the CKM matrix, these new measurements, combined with the recent determinations of the angles alpha, beta and gamma from non-leptonic decays, allow, in the Standard Model, a quite accurate extraction of the values of the hadronic matrix elements relevant for K-Kbar and B_{s,d}-B_{s,d}bar mixing and of the leptonic decay constant f_B. These values, obtained ``experimentally'', can then be compared with the theoretical predictions, mainly from lattice QCD. In this paper we upgrade the UT fit, we determine from the data the kaon B-parameter B_Khat, the B^0 mixing amplitude parameters f_Bs B^{1/2}_Bs and xi, the decay constant f_B, and make a comparison of the obtained values with lattice predictions. We also discuss the different determinations of V_ub and show that current data do not favour the value measured in inclusive decays.

Higher-Twist Distribution Amplitudes of the K Meson in QCD

Abstract: We present a systematic study of twist-3 and twist-4 light-cone distribution amplitudes of the K meson in QCD. The structure of SU(3)-breaking corrections is studied in detail. Non-perturbative input parameters are estimated from QCD sum rules and renormalons. As a by-product, we give a complete reanalysis of the twist-3 and -4 parameters of the pi-meson distribution amplitudes; some of the results differ from those usually quoted in the literature.

Mean Field with Tensor Force and Shell Structure of Exotic Nuclei

Abstract: The tensor force is implemented into the mean-field model so that the evolution of nuclear shells can be described for exotic nuclei as well as stable ones. Besides the tensor-force part simulating the meson exchange, the model is an extension of the successful Gogny model. One of the major issues of rare-isotope beam physics is a reduced spin-orbit splitting in neutron-rich exotic nuclei. It will be shown that the effect of the tensor force on this splitting is larger than or about equal to the one due to the neutron skin. We will present predictions for stable and exotic nuclei with comparisons to conventional results and experimental data.

Averages of b-hadron Properties at the End of 2005

Abstract: This article reports world averages for measurements on b-hadron properties obtained by the Heavy Flavor Averaging Group (HFAG) using the available results as of at the end of 2005. In the averaging, the input parameters used in the various analyses are adjusted (rescaled) to common values, and all known correlations are taken into account. The averages include lifetimes, neutral meson mixing parameters, parameters of semileptonic decays, branching fractions of B decays to final states with open charm, charmonium and no charm, and measurements related to CP asymmetries.

Experimental signature of medium modifications for rho and omega mesons in the 12 GeV p+A reactions.

Abstract: The invariant mass spectra of e{sup +}e{sup -} pairs produced in 12 GeV proton-induced nuclear reactions are measured at the KEK Proton Synchrotron. On the low-mass side of the {omega} meson peak, a significant enhancement over the known hadronic sources has been observed. The mass spectra, including the excess, are well reproduced by a model that takes into account the density dependence of the vector meson mass modification, as theoretically predicted.

The unitarity triangle fit in the standard model and hadronic parameters from lattice QCD: a reappraisal after the measurements of Δms and BR(B→τντ)

Abstract: The recent measurements of the B_s meson mixing amplitude by CDF and of the leptonic branching fraction BR(B to tau nu) by Belle call for an upgraded analysis of the Unitarity Triangle in the Standard Model. Besides improving the previous constraints on the parameters of the CKM matrix, these new measurements, combined with the recent determinations of the angles alpha, beta and gamma from non-leptonic decays, allow, in the Standard Model, a quite accurate extraction of the values of the hadronic matrix elements relevant for K-Kbar and B_{s,d}-B_{s,d}bar mixing and of the leptonic decay constant f_B. These values, obtained "experimentally'', can then be compared with the theoretical predictions, mainly from lattice QCD. In this paper we upgrade the UT fit, we determine from the data the kaon B-parameter B_Khat, the B^0 mixing amplitude parameters f_Bs B^{1/2}_Bs and xi, the decay constant f_B, and make a comparison of the obtained values with lattice predictions. We also discuss the different determinations of V_ub and show that current data do not favour the value measured in inclusive decays.

Study of exclusive semileptonic and nonleptonic decays of B c − in a nonrelativistic quark model

Abstract: We present results for different observables measured in semileptonic and nonleptonic decays of the ${B}_{c}^{\ensuremath{-}}$ meson. The calculations have been done within the framework of a nonrelativistic constituent quark model. In order to check the sensitivity of all our results against the interquark interaction we use five different quark-quark potentials. We obtain form factors, decay widths, and asymmetry parameters for semileptonic ${B}_{c}^{\ensuremath{-}}\ensuremath{\rightarrow}c\overline{c}$ and ${B}_{c}^{\ensuremath{-}}\ensuremath{\rightarrow}\overline{B}$ decays. In the limit of infinite heavy quark mass, our model reproduces the constraints of heavy quark spin symmetry. For the actual heavy quark masses we find nonetheless large corrections to that limiting situation for some form factors. We also analyze exclusive nonleptonic two-meson decay channels within the factorization approximation.

Moments of pseudoscalar meson distribution amplitudes from the lattice

Abstract: Based on lattice simulations with two flavors of dynamical, O(a)-improved Wilson fermions we present results for the first two moments of the distribution amplitudes of pseudoscalar mesons at several values of the valence quark masses. By extrapolating our results to the physical masses of up/down and strange quarks, we find the first two moments of the K{sup +} distribution amplitude and the second moment of the {pi}{sup +} distribution amplitude. We use nonperturbatively determined renormalization coefficients to obtain results in the MS scheme. At a scale of 4 GeV{sup 2} we find a{sub 2}{sup {pi}}=0.201(114) for the second Gegenbauer moment of the pion's distribution amplitude, while for the kaon, a{sub 1}{sup K}=0.0453(9)(29) and a{sub 2}{sup K}=0.175(18)(47)

The Nambu–Jona-Lasinio model and its development

Abstract: The historical development of the Nambu–Jona-Lasinio (NJL) model is briefly reviewed. The SU(2) × SU(2) and U(3) × U(3) local quark NJL models are considered. The mechanisms responsible for spontaneous breaking of chiral symmetry and vector dominance are exhibited. The local NJL model is adequate in describing the mass spectrum and the strong and electroweak decay modes of the four ground-state meson nonets: pseudoscalar, scalar, vector, and axial-vector. The applicability of the model to mesons in a hot dense medium is discussed. It is shown that solving problems related to the description of meson radial excitations and quark confinement requires the nonlocal extension of the NJL model. The primary emphasis of this review is on the methods that are used in various versions of the NJL model. The reader is referred to the cited works for what these models predict in low-energy hadron physics.

Comprehensive interpretation of thermal dileptons measured at the CERN super proton synchrotron.

Abstract: Employing thermal dilepton rates based on a medium-modified electromagnetic correlation function we show that recent dimuon spectra of the NA60 Collaboration in central In-In collisions at the CERN-SPS can be understood in terms of radiation from a hot and dense hadronic medium. Earlier calculated in-medium rho-meson spectral functions provide an accurate description of the data up to dimuon invariant masses of about M approximately or equal to 0.9 GeV, with good sensitivity to the predicted rho-meson line shape, identifying baryon-induced modifications as the prevalent ones. A reliable evaluation of the contribution enables the study of further medium effects: at masses M>0.9 GeV, 4-pion type annihilation accounts for the experimentally observed excess (possibly augmented by effects of "chiral mixing"), while predictions for thermal emission from in-medium omega and phi mesons may be tested in the future.

Dense Hadronic Matter in Holographic QCD

Abstract: We provide a method to study hadronic matter at finite density in the context of the Sakai-Sugimoto model. We introduce the baryon chemical potential through the external $U(1)_v$ in the induced (DBI plus CS) action on the D8-probe-brane, where baryons are skyrmions. Vector dominance is manifest at finite density. We derive the baryon density effect on the energy density, the dispersion relations of pion and vector mesons at large $N_c$. The energy density asymptotes to a constant at large density suggesting that dense matter at large $N_c$ freezes, with the pion velocity dropping to zero. Holographic dense matter enforces exactly the tenets of vector dominance, and screens efficiently vector mesons. At the freezing point the $\rho-\pi\pi$ coupling vanishes with a finite rho mass of about 20% its vacuum value.

D Mesons in Nuclear Matter: A DN Coupled-Channel Equations Approach

Abstract: A set of coupled two-body scattering equations is solved for the DN system embedded in an isosymmetric nuclear matter. The in-medium behavior of charmed D mesons, (D + ,D 0 ), is investigated from the self-consistent solution within this scheme. The effective meson-baryon Lagrangian in charm quantum number one sector, the key ingredient in the present study, is adopted from a recent model by Hofmann and Lutz that has aimed at combining the charmed meson degree of freedom in a consistent manner with chiral unitary models. After a critical examination, the original model is modified in several important aspects, such as the method of regularization, to be more consistent and practical for our objective. The resultant interaction is used to reproduce � ¯

Chiral Lagrangian with heavy quark-diquark symmetry

Abstract: We construct a chiral Lagrangian for doubly heavy baryons and heavy mesons that is invariant under heavy quark-diquark symmetry at leading order and includes the leading O(1/mQ) symmetry violating operators. The theory is used to predict the electromagnetic decay width of the J = 3 member of the ground state doubly heavy baryon doublet. Numerical estimates are provided for doubly charm baryons. We also calculate chiral corrections to doubly heavy baryon masses and strong decay widths of low lying excited doubly heavy baryons.

Covariant theory of particle-vibrational coupling and its effect on the single-particle spectrum

Abstract: The relativistic mean field (RMF) approach describing the motion of independent particles in effective meson fields is extended by a microscopic theory of particle vibrational coupling. It leads to an energy dependence of the relativistic mass operator in the Dyson equation for the single-particle propagator. This equation is solved in the shell-model of Dirac states. As a result of the dynamics of particle-vibrational coupling we observe a noticeable increase of the level density near the Fermi surface. The shifts of the single-particle levels in the odd nuclei surrounding $^{208}\mathrm{Pb}$ and the corresponding distributions of the single-particle strength are discussed and compared with experimental data.

Holographic mesons in various dimensions

Abstract: Following the analysis of [1], we calculate the spectrum of fluctuations of a probe Dk-brane in the background of N Dp-branes, for k = p,p+2,p+4 and p<5. The result corresponds to the mesonic spectrum of a (p+1)-dimensional super-Yang-Mills (SYM) theory coupled to `dynamical quarks', i.e. fields in the fundamental representation — the latter are confined to a defect for k = p and p+2. We find a universal behaviour where the spectrum is discrete and the mesons are deeply bound. The mass gap and spectrum are set by the scale M ~ mq/geff(mq), where mq is the mass of the fundamental fields and geff(mq) is the effective coupling evaluated at the quark mass, i.e. geff2(mq) = gYM2N mqp−3. We consider the evolution of the meson spectra into the far infrared of three-dimensional SYM, where the gravity dual lifts to M-theory. We also argue that the mass scale appearing in the meson spectra is dictated by holography.

Inclusive spectra in charmless semileptonic B decays by dressed gluon exponentiation

Abstract: The triple differential spectrum in Xul is computed by Dressed Gluon Exponentiation (DGE). In this framework the on-shell calculation, converted into hadronic variables, can be directly used as an approximation to the meson decay spectrum, without involving a leading–power non-perturbative function. Sudakov resummation for the fully differential Xul width is formulated in moment space, where moments are defined using the ratio between the lightcone momentum components of the partonic jet p+/p− and the hard scale is p−. In these variables the correspondence with the Xsγ case is transparent. The Sudakov exponent is known to next–to–next–to–leading logarithmic accuracy. Further constraints are put on its Borel sum using the cancellation of the leading renormalon ambiguity and the absence of the next–to–leading one, which was proven in the large–β0 limit and assumed here to be general. Based on the resummed spectrum, matched to the fully differential NLO result, we calculate the event fraction associated with experimental cuts on the hadronic mass (or the small lightcone component) as well as on the lepton energy. Finally, we extract |Vub| from recent measurements by Belle and analyze the theoretical uncertainty.

Scalar glueball, scalar quarkonia, and their mixing

Abstract: The isosinglet scalar mesons f{sub 0}(1710), f{sub 0}(1500), f{sub 0}(1370) and their mixing are studied. We employ two recent lattice results as the starting point; one is the isovector scalar meson a{sub 0}(1450) which displays an unusual property of being nearly independent of quark mass for quark masses smaller than that of the strange, and the other is the scalar glueball mass at 1710 MeV in the quenched approximation. In the SU(3) symmetry limit, f{sub 0}(1500) turns out to be a pure SU(3) octet and is degenerate with a{sub 0}(1450), while f{sub 0}(1370) is mainly an SU(3) singlet with a slight mixing with the scalar glueball which is the primary component of f{sub 0}(1710). These features remain essentially unchanged even when SU(3) breaking is taken into account. We discuss the sources of SU(3) breaking and their consequences on flavor-dependent decays of these mesons. The observed enhancement of {omega}f{sub 0}(1710) production over {phi}f{sub 0}(1710) in hadronic J/{psi} decays and the copious f{sub 0}(1710) production in radiative J/{psi} decays lend further support to the prominent glueball nature of f{sub 0}(1710)

Common Suppression Pattern of η and π0 Mesons at High Transverse Momentum in Au + Au Collisions at √sNN = 200GeV

Abstract: Inclusive transverse momentum spectra of eta mesons have been measured within p(T)=2-10 GeV/c at midrapidity by the PHENIX experiment in Au+Au collisions at root s(NN) = 200 GeV. In central Au+Au the eta yields are significantly suppressed compared to peripheral Au+Au, d+Au, and p+p yields scaled by the corresponding number of nucleon-nucleon collisions. The magnitude, centrality, and p(T) dependence of the suppression is common, within errors, for eta and pi(0). The ratio of eta to pi(0) spectra at high p(T) amounts to 0.40 < R-eta/pi(0)< 0.48 for the three systems, in agreement with the world average measured in hadronic and nuclear reactions and, at large scaled momentum, in e(+)e(-) collisions.