Pentaquarks: review of the experimental evidence

Pentaquarks, namely baryons made by 4 quarks and one antiquark have been predicted and searched for since several decades without success. Theoretical and experimental advances in the last 2 years led to the observation of a number of pentaquark candidates. We review the experimental evidence for pentaquarks as well as their non-observations by some experiments, and discuss to which extend these sometimes contradicting informations may lead to a consistent picture.Comment: Contribution to the International Conference on 'Strangeness in Quark Matter', 15-21 Sept. 2004, Cape Tawn, South Afric

Strangeness from 20 AGeV to 158 AGeV

New results from the energy scan programme of NA49, in particular kaon production at 30 AGeV and phi production at 40 and 80 AGeV are presented. The K+/pi+ ratio shows a pronounced maximum at 30 AGeV; the kaon slope parameters are constant at SPS energies. Both findings support the scenario of a phase transition at about 30 AGeV beam energy. The phi/pi ratio increases smoothly with beam energy, showing an energy dependence similar to K-/pi-. The measured particle yields can be reproduced by a hadron gas model, with chemical freeze-out parameters on a smooth curve in the T-muB plane. The transverse spectra can be understood as resulting from a rapidly expanding, locally equilibrated source. No evidence for an earlier kinetic decoupling of heavy hyperons is found.Comment: Contribution to the proceedings of "Strangeness in Quark Matter 2003" (March 2003, Atlantic Beach NC, USA), to be published in Journal of Physics G. 11 pages, 14 figure

System-size and centrality dependence of charged kaon and pion production in nucleus-nucleus collisions at 40A GeV and158A GeV beam energy

Measurements of charged pion and kaon production are presented in centrality selected Pb+Pb collisions at 40A GeV and 158A GeV beam energy as well as in semi-central C+C and Si+Si interactions at 40A GeV. Transverse mass spectra, rapidity spectra and total yields are determined as a function of centrality. The system-size and centrality dependence of relative strangeness production in nucleus-nucleus collisions at 40A GeV and 158A GeV beam energy are derived from the data presented here and published data for C+C and Si+Si collisions at 158A GeV beam energy. At both energies a steep increase with centrality is observed for small systems followed by a weak rise or even saturation for higher centralities. This behavior is compared to calculations using transport models (UrQMD and HSD), a percolation model and the core-corona approach.Comment: 32 pages, 14 figures, 4 tables, typo table II correcte

Lambda production in central Pb+Pb collisions at CERN-SPS energies

In this paper we present recent results from the NA49 experiment for $\Lambda$ and $\bar{\Lambda}$ hyperons produced in central Pb+Pb collisions at 40, 80 and 158 A$\cdot$GeV. Transverse mass spectra and rapidity distributions for $\Lambda$ are shown for all three energies. The shape of the rapidity distribution becomes flatter with increasing beam energy. The multiplicities at mid-rapidity as well as the total yields are studied as a function of collision energy including AGS measurements. The ratio $\Lambda/\pi$ at mid-rapidity and in 4$\pi$ has a maximum around 40 A$\cdot$GeV. In addition, $\bar{\Lambda}$ rapidity distributions have been measured at 40 and 80 A$\cdot$GeV, which allows to study the $\bar{\Lambda}$/$\Lambda$ ratio.Comment: SQM proceedings. J. Phys. G: Nucl. Part. Phys.: submitte

Antideuteron and deuteron production in midcentral Pb plus Pb collisions at 158A GeV

Production of deuterons and antideuterons was studied by the NA49 experiment in the 23.5% most central Pb + Pb collisions at the top CERN Super Proton Synchroton (SPS) energy of root s(NN) = 17.3 GeV. Invariant yields for (d) over bar and d were measured as a function of centrality in the center-of-mass rapidity range -1.2 < y < -0.6. Results for (d) over bar (d) together with previously published (p) over bar (p) measurements are discussed in the context of the coalescence model. The coalescence parameters B-2 were deduced as a function of transverse momentum p(t) and collision centrality

Proton-Lambda correlations in central Pb plus Pb collisions at root s(NN)=17.3 GeV

The momentum correlation between protons and Lambda particles emitted from central Pb+Pb collisions at root s(NN) = 17.3 GeV was studied by the NA49 experiment at the CERN Super Proton Synchrotron. A clear enhancement is observed for small relative momenta (q(inv) < 0.2 GeV). By fitting a theoretical model, which uses the strong interaction between the proton and the Lambda in a given pair, to the measured data, a value for the effective source size is deduced. Assuming a static Gaussian source distribution, we derive an effective radius parameter of R-G = 3.02 +/- 0.20(stat.)(-0.16)(+0.44)(syst.) fm

Energy dependence of kaon-to-proton ratio fluctuations in central Pb plus Pb collisions from root s(NN)=6.3 to 17.3 GeV

Kaons and protons carry large parts of two conserved quantities: strangeness and baryon number. It is argued that their correlation and thus also fluctuations are sensitive to conditions prevailing at the anticipated parton- hadron phase boundary. Fluctuations of the (K+ + K-)/(p + (p) over bar) and K+/p ratios have been measured for the first time by NA49 in central Pb + Pb collisions at five CERN Super Proton Synchrotron energies between root s(NN) = 6.3 and root s(NN) = 17.3 GeV. Both ratios exhibit a change of sign in sigma(dyn), a measure of nonstatistical fluctuations, around root s(NN) = 8 GeV. Below this energy, sdyn is positive, indicating higher fluctuation compared to a mixed event background sample, while for higher energies, sigma(dyn) is negative, indicating correlated emission of kaons and protons. The results are compared to hadronic transport model calculations which fail to reproduce the energy dependence

Centrality dependence of proton and antiproton spectra in Pb plus Pb collisions at 40A GeV and 158A GeV measured at the CERN Super Proton Synchrotron

The yields of (anti) protons were measured by the NA49 Collaboration in centrality-selected Pb + Pb collisions at 40A and 158A GeV. Particle identification was obtained in the laboratory momentum range from 5 to 63 GeV/c by measuring the energy loss dE/dx in the time projection chamber detector gas. The corresponding rapidity coverage extends 1.6 units from midrapidity into the forward hemisphere. Transverse-mass spectra, the rapidity dependences of the average transverse mass, and rapidity density distributions were studied as a function of collision centrality. The values of the average transverse mass as well as the midrapidity yields of protons normalized to the number of wounded nucleons show only modest centrality dependences. In contrast, the shape of the rapidity distribution changes significantly with collision centrality, especially at 40A GeV. The experimental results are compared to calculations of the hadron-string dynamics and the ultrarelativistic quantum-moleculardynamics transport models