Showing papers on "Strangeness published in 2008"

Parton transport and hadronization from the dynamical quasiparticle point of view

Abstract: The hadronization of an expanding partonic fireball is studied within the parton-hadron-string-dynamics (PHSD) approach, which is based on a dynamical quasiparticle model (DQPM) matched to reproduce lattice QCD results in thermodynamic equilibrium. Apart from strong parton interactions, the expansion and development of collective flow is driven by strong gradients in the parton mean fields. An analysis of the elliptic flow ${v}_{2}$ demonstrates a linear correlation with the spatial eccentricity \ensuremath{\epsilon} as in ideal hydrodynamics. The hadronization occurs by quark-antiquark fusion or three-quark/three-antiquark recombination, which is described by covariant transition rates. Since the dynamical quarks become very massive, the formed resonant ``pre-hadronic'' color-dipole states ($q\overline{q}$ or $\mathrm{qqq}$) are of high invariant mass, too, and sequentially decay to the ground-state meson and baryon octets increasing the total entropy. This solves the entropy problem in hadronization in a natural way. The resulting particle ratios turn out to be in line with those from a grand-canonical partition function at temperature $T\ensuremath{\approx}170$ MeV rather independent from the initial temperature and indicate an approximate strangeness equilibration.

Enhanced strange baryon production in Au+Au collisions compared to p+p at sNN=200 GeV

Abstract: We report on the observed differences in production rates of strange and multistrange baryons in Au+Au collisions at √sNN=200 GeV compared to p+p interactions at the same energy. The strange baryon yields in Au+Au collisions, when scaled down by the number of participating nucleons, are enhanced relative to those measured in p+p reactions. The enhancement observed increases with the strangeness content of the baryon, and it increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at the lower collision energy √sNN=17.3 GeV. The previous observations are for the bulk production, while at intermediate pT,1

Strangeness production from SPS to LHC

Abstract: Global strangeness production in relativistic heavy ion collisions at SPS and RHIC is reviewed. Special emphasis is put on the comparison with the statistical model and the canonical suppression mechanism. It is shown that recent RHIC data on strange particle production as a function of centrality can be explained by a superposition of a fully equilibrated hadron gas and particle emission from single independent nucleon–nucleon collisions in the outer corona.

Analyticity constraints on the strangeness changing vector current and applications to τ→Kπντ and τ→Kππντ

Abstract: We discuss matrix elements of the strangeness changing vector current using their relation, due to analyticity, with πK scattering in the P-wave. We take into account experimental phase-shift measurements in the elastic channel as well as results, obtained by the LASS collaboration, on the details of inelastic scattering, which show the dominance of two quasi-two-body channels at medium energies. The associated form factors are shown to be completely determined, up to one flavor symmetry breaking parameter, imposing boundary conditions at t=0 from chiral and flavor symmetries and at t→∞ from QCD. We apply the results to the τ→Kπντ and τ→Kππντ amplitudes and compare the former to recent high statistics results from B factories.

Strangeness -2 and -3 baryons in a constituent quark model.

Abstract: We apply a quark model developed in earlier work to the spectrum of baryons with strangeness -2 and -3. The model describes a number of well-established baryons successfully, and application to cascade baryons allows the quantum numbers of some known states to be deduced. [JLAB-THY-07-728].

Further evidence for formation of a narrow baryon resonance with positive strangeness in K + collisions

Abstract: We have continued our investigation of the charge-exchange reaction K + Xe → K 0 pXein the bubble chamber DIANA. In agreement with our previous results based on part of the present statistics, formation of a narrow pK 0 resonance with mass of 1537 ± 2 MeV/c 2 is observed in the elementary transition K + n → K 0 p on a neutron bound in the Xenon nucleus. Visible width of the peak is consistent with being entirely due to instrumental resolution and allows to place an upper limit on its intrinsic width: < 9 MeV/c 2 . A more precise estimate of the resonance intrinsic width, = 0 .36 ± 0.11 MeV/c 2 , is obtained from the ratio between the numbers of resonant and non-resonant charge-exchange events. The signal is observed in a restricted interval of incident K + momentum, that is consistent with smearing of a

Midrapidity production of secondaries in pp collisions at RHIC and LHC energies in the quark–gluon string model

Abstract: We consider the phenomenological implications of the assumption that baryons are systems of three quarks connected through a gluon string junction. The transfer of baryon number in rapidity space due to the string junction propagation is considered in detail. At high energies this process leads to a significant effect on the net baryon production in hN collisions at midrapidities. The numerical results for midrapidity inclusive densities of different secondaries in the framework of the quark–gluon string model are in reasonable agreement with the experimental data. One universal value, λ≃0.25, for the strangeness suppression parameter correctly describes the yield ratios of Λ/p, Ξ/Λ, and Ω/Ξ. The predictions for pp collisions at LHC energies are also presented.

AdS-QCD quark-antiquark potential, meson spectrum and tetraquarks

Abstract: The AdS/QCD correspondence predicts the structure of the quark–antiquark potential in the static limit. We use this piece of information together with the Salpeter equation (Schrodinger equation with relativistic kinematics) and a short range hyperfine splitting potential to determine quark masses and the quark potential parameters from the meson spectrum. The agreement between theory and experimental data is satisfactory, provided one considers only mesons comprising at least one heavy quark. We use the same potential (in the one-gluon-exchange approximation) and these data to estimate the constituent diquark masses. Using these results as input we compute tetraquark masses using a diquark–antidiquark model. The masses of the states X(3872) or Y(3940) are predicted rather accurately. We also compute tetraquark masses with open charm and strangeness. Our result is that tetraquark candidates such as D s (2317), D s (2457) or X(2632) can hardly be interpreted as diquark–antidiquark states within the present approach.

$D_sD^*$ molecule as an axial meson

Abstract: We use QCD sum rules to study the possible existence of a $D_s\bar{D}^*+ D_s^*\bar{D}$ molecule with the quantum number $J^P=1^+$. We consider the contributions of condensates up to dimension eight and work at leading order in $\alpha_s$. We obtain $m_{D_sD^*}=(3.96\pm0.10) \GeV$ around 100 MeV above the mass of the meson X(3872). The proposed state is a natural generalized state to the strangeness sector of the X(3872), which was also found to be consistent with a multiquark state from a previous QCD sum rule analysis.

Effect of strangeness for neutrino (antineutrino) scattering in the quasi-elastic region

Abstract: We present the neutral- and charged-current reactions by incident neutrino (antineutrino) scattering on the nucleon and on the {sup 12}C target in the quasi-elastic region within the framework of a relativistic single-particle model. The incident energies at 500 MeV and 1 GeV are used for the scattering. Effects of strangeness are studied thoroughly on the cross sections, the ratios between the neutral- and charged-current reactions, and the asymmetries by incident neutrino and antineutrino. We find that there exists some cancellation of the strange quark contributions between the knocked-out protons and neutrons in the neutrino (antineutrino)-nucleus scattering. The effect of strangeness is exhibited more strongly on the asymmetry than on the ratio or cross section. On a difference of the asymmetry, the effect of strangeness appears strongly, but on a summation of the asymmetry, the effect almost disappears in the low and middle kinetic energies of the knocked-out nucleon.

Form Factors and Dyson-Schwinger Equations

Abstract: A synopsis exemplifying the employment of Dyson-Schwinger equations in the calculation and explanation of hadron electromagnetic form factors and related phenomena. In particular the contribution: presents the pion form factor computed simultaneously at spacelike and timelike momenta; reports aspects of the evolution of the nucleon and Delta masses with current-quark mass and the correlation of their mass difference with that between scalar and axial-vector diquarks; describes an estimate of the s-quark content of a dressed u-quark and its impact on the nucleon's strangeness magnetic moment; and comments upon the domain within which a pseudoscalar meson cloud can materially contribute to hadron form factors.

Spin and flavor strange quark content of the nucleon

Abstract: Several spin and flavor dependent parameters characterizing the strangeness content of the nucleon have been calculated in the chiral constituent quark model with configuration mixing ({chi}CQM{sub config}) which is known to provide a satisfactory explanation of the ''proton spin crisis'' and related issues. In particular, we have calculated the strange spin polarization {delta}s, the strangeness contribution to the weak axial vector couplings {delta}{sub 8} etc., strangeness contribution to the magnetic moments {mu}(p){sup s} etc., the strange quark flavor fraction f{sub s}, the strangeness dependent quark flavor ratios (2s/u+d) and (2s/u+d) etc. Our results are consistent with the recent experimental observations.

Recent lattice results on finite temperature and density QCD, part I

Abstract: We discuss recent progress made studies of bulk thermodynamics of strongly interacting matter through lattice simulations of QCD with an almost physical light and strange quark mass spectrum. We present results on the QCD equation of state at vanishing and non-vanishing quark chemical potential and show first results on baryon number and strangeness fluctuations, which might be measured in event-by-event fluctuations in low energy runs at RHIC as well as at FAIR.

Is there a non-Standard-Model contribution in non-leptonic b ? s decays?

Abstract: Precision measurements of branching fractions and CP asymmetries in non-leptonic b → s decays reveal certain ``puzzles'' when compared with Standard Model expectations based on a global fit of the CKM triangle and general theoretical expectations. Without reference to a particular model, we investigate to what extent the (small) discrepancies observed in B → J/ψK, B → K and B → Kπ may constrain new physics in b → sq operators. In particular, we compare on a quantitative level the relative impact of different quark flavours q = c, s, u, d.

Form factors and other measures of strangeness in the nucleon

Abstract: We discuss the phenomenology of strange-quark dynamics in the nucleon, based on experimental and theoretical results for electroweak form factors and for parton densities. In particular, we construct a model for the generalized parton distribution that relates the asymmetry s(x)-s(x) between the longitudinal momentum distributions of strange quarks and antiquarks with the form factor F{sub 1}{sup s}(t), which describes the distribution of strangeness in transverse position space.

Strangeness and glue in the nucleon from lattice QCD

Abstract: We study the strangeness contribution to nucleon matrix elements using Nf=2+1 dynamical clover fermion configurations generated by the CP-PACS/JLQCD collaboration. In order to evaluate the disconnected insertion (DI), we use the Z(4) stochastic method, along with unbiased subtraction from the hopping parameter expansion which reduces the off-diagonal noises in the stochastic method. Furthermore, we find that using many nucleon sources for each configuration is effective in improving the signal. Our results for the quark contribution to the first moment _q in the DI, and the strangeness magnetic moment show that the statistical errors are under control with these techniques. We also study the gluonic contribution to the nucleon using the overlap operator to construct the gauge field tensor, F_{mu,nu}. The application to the calculation of first moment, _G, gives a good signal in quenched lattice QCD.

Ambiguities of neutrino (antineutrino) scattering on the nucleon due to the uncertainties of relevant strangeness form factors

Abstract: Strange quark contributions to the neutrino (antineutrino) scattering on the nucleon level are investigated in the quasi-elastic region. Incident energy is used at 500 MeV for the scattering. All of the physical observables by the scattering are investigated within available experimental and theoretical results for strangeness form factors of the nucleon. Specifically, a newly combined datum of the parity violating electron scattering and the neutrino scattering is exploited for the strangeness form factors. Feasible quantities to be explored for the strangeness contents are discussed for the application to the neutrino–nucleus scattering.

Present and future challenges for quark matter theorists

Abstract: We review some theoretical challenges in the quest for understanding quark matter. Several contributions to the Strangeness in Quark Matter 2007 Conference indicated problems to be solved and methods to be developed further. This summary gives an individual review of questions held to be important by the author.

Three-body hadronic structure of low-lying 1/2 + Σ and Λ resonances ⋆

Abstract: We discuss the dynamical generation of some low-lying 1/2+ Σ’s and Λ’s in two-meson onebaryon systems. These systems have been constructed by adding a pion in the S-wave to the \( \bar K \) N pair and its coupled channels, where the 1/2− Λ(1405)-resonance gets dynamically generated. We solve Faddeev equations in the coupled-channel approach to calculate the T-matrix for these systems as a function of the total energy and the invariant mass of one of the meson-baryon pairs. This squared T-matrix shows peaks at the energies very close to the masses of the strangeness −1, 1/2+ resonances listed in the particle data book.

phi and ω - ρ production in d C, d U, S U and Pb Pb at SPS energies

Abstract: Nuclear dependence of 0 and ω - ρ production is studied from heavy nuclei and deuteron-induced reactions. In addition to the ratio Φ/(ω + p), closely related to the strangeness saturation factor γ s in transverse mass (M T ) domains, absolute cross-sections and multiplicities are also presented. The Φ multiplicities for central Pb-Pb collisions, as obtained in the most recent NA50 measurement, lead to an updated version of the Φ puzzle, where reduced discrepancy between experiments opens a new space for a systematic difference between the leptonic and hadronic decay channels.

Three-body hadronic structure of low-lying $1/2^+$ $\Sigma$ and $\Lambda$ resonances

Abstract: We discuss the dynamical generation of some low-lying $1/2^+$ $\Sigma$'s and $\Lambda$'s in two-meson one-baryon systems. These systems have been constructed by adding a pion in $S$-wave to the $\bar{K} N$ pair and its coupled channels, where the $1/2^-$ $\Lambda$(1405)-resonance gets dynamically generated. We solve Faddeev equations in the coupled-channel approach to calculate the $T$-matrix for these systems as a function of the total energy and the invariant mass of one of the meson-baryon pairs. This squared $T$-matrix shows peaks at the energies very close to the masses of the strangeness -1, $1/2^+$ resonances listed in the particle data book.

The Compressed Baryonic Matter experiment at FAIR: physics of strangeness and charm, status of preparations

Abstract: The Compressed Baryonic Matter (CBM) experiment at the future accelerator centre FAIR is being designed to investigate heavy-ion collisions in fixed-target mode at beam energies from 10 to 45 GeV/nucleon. The goal of the research program is to explore the QCD phase diagram in the region of highest baryon densities. The detector has to measure hadronic, leptonic and photonic observables with large acceptance. The interaction rates will be as high as 10 MHz in order to measure extremely rare probes like charm near threshold. The CBM detector concept, the physics programme with respect to heavy flavours and the results of feasibility studies are presented.

Doubly strange hypernuclei at PANDA

Abstract: The FAIR project aims to investigate several fields of physics: QCD, nuclear structure, astrophysics, atomic physics, plasma physics and their applications. In particular, the high-energy storage ring (HESR) will allow high-precision measurements in the antiproton momentum range from 1.5 to 15 GeV/c. Inside HESR, the PANDA experiment will study the charm and strangeness physics, the form factor in the time-like region and other topics like the crossed-channel Compton scattering. The study of the strangeness will be focused onto the doubly strange systems (hypernuclei and hyperatoms) produced with a new technique starting from the antiproton-nucleus reaction.

Influence of heavy-quark recombination on the nucleon strangeness asymmetry

Abstract: The nucleon strange and antistrange distribution asymmetry is an important issue in the study of the nucleon structure. In this work, we show that the heavy- quark recombination processes from a perturbative QCD picture can give a sizable influence on the measurement of the nucleon strangeness asymmetry from charged- current charm production processes, such as the CCFR and NuTeV dimuon measurements. When the influence of heavy- quark recombination is considered, a positive effective delta S(HR)(-) should be added to the initially extracted strangeness asymmetry S(-) equivalent to integral dx [s (x ) - (S) over bar (x)] x, supporting the strangeness asymmetry S(-) being positive, which is helpful to explain the NuTeV anomaly within the framework of the standard model.

Strangeness and bulk freeze-out properties at RHIC

Abstract: Identified charged kaon, pion and proton spectra and ratios from and 62.4 GeV Cu+Cu collisions are studied with a hydro-motivated blast-wave and a statistical model framework in order to explore the strangeness production at RHIC and characterize the bulk freeze-out properties of the created system. The spectra are measured at mid-rapidity (|y| < 0.1) over the transverse momentum range of 0.25 < pT < 1.2 GeV/c with particle identification derived from the ionization energy loss in the STAR time projection chamber. The multi-dimensional systematic study of π±, K±, p and production in Cu+Cu, Au+Au, d+Au and p+p collisions is used to discuss the energy, system size and inferred energy density dependence of freeze-out parameters and strangeness production. The new data from Cu+Cu collisions bridge the gap between the smaller d+Au and larger Au+Au systems, allowing a detailed study of the onset of strangeness equilibration at RHIC.

Rapidity-dependent chemical potentials in a statistical approach

Abstract: We present a single-freeze-out model with thermal and geometric parameters dependent on the position within the fireball and use it to describe the rapidity distribution and transverse-momentum spectra of pions, kaons, protons and antiprotons measured at RHIC at √SNN = 200 GeV by BRAHMS. THERMINATOR is used to perform the necessary simulation, which includes all resonance decays. The result of the fit to the data is the expected growth of the baryon and strange chemical potentials with the spatial rapidity α ∥ . The value of the baryon chemical potential at α∥ ∼ 3 is about 200 MeV, i.e. it lies in the range of the highest SPS energies. The chosen geometry of the fireball has a decreasing transverse size as the magnitude of α∥ is increased, which also corresponds to decreasing transverse flow. The strange chemical potential obtained from the fit to the K + /K - ratio is such that the local strangeness density in the fireball is compatible with zero. The resulting rapidity distribution of net protons are described qualitatively within the statistical approach. As a result of our study, the knowledge of the 'topography' of the fireball is acquired, allowing for other analyses and predictions.