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Rapidity distributions for Lambda and anti-Lambda hyperons in central Pb-Pb collisions at 40, 80 and 158 AGeV and for K 0 s mesons at 158 AGeV are presented. The lambda multiplicities are studied as a function of collision energy together with AGS and RHIC measurements and compared to model predictions. A different energy dependence of the Lambda/pi and anti-Lambda/pi is observed. The anti-Lambda/Lambda ratio shows a steep increase with collision energy. Evidence for a anti-Lambda/anti-p ratio greater than 1 is found at 40 AGeV.
Rapidity distributions for $\Lambda$ and $\bar{\Lambda}$ hyperons in central Pb-Pb collisions at 40, 80 and 158 A$\cdot$GeV and for ${\rm K}_{s}^{0}$ mesons at 158 A$\cdot$GeV are presented. The lambda multiplicities are studied as a function of collision energy together with AGS and RHIC measurements and compared to model predictions. A different energy dependence of the $\Lambda/\pi$ and $\bar{\Lambda}/\pi$ is observed. The $\bar{\Lambda}/\Lambda$ ratio shows a steep increase with collision energy. Evidence for a $\bar{\Lambda}/\bar{\rm p}$ ratio greater than 1 is found at 40 A$\cdot$GeV.
Event-by-event fluctuations of particle ratios in central Pb + Pb collisions at 20 to 158 AGeV
(2004)
In the vicinity of the QCD phase transition, critical fluctuations have been predicted to lead to non-statistical fluctuations of particle ratios, depending on the nature of the phase transition. Recent results of the NA49 energy scan program show a sharp maximum of the ratio of K+ to Pi+ yields in central Pb+Pb collisions at beam energies of 20-30 AGeV. This observation has been interpreted as an indication of a phase transition at low SPS energies. We present first results on event-by-event fluctuations of the kaon to pion and proton to pion ratios at beam energies close to this maximum.
We present the first measurement of fluctuations from event to event in the production of strange particles in collisions of heavy nuclei. The ratio of charged kaons to charged pions is determined for individual central Pb+Pb collisions. After accounting for the fluctuations due to detector resolution and finite number statistics we derive an upper limit on genuine non-statistical fluctuations, perhaps related to a first or second order QCD phase transition. Such fluctuations are shown to be very small.
We present the first large-acceptance measurement of event-wise mean transverse momentum <pt> fluctuations for Au-Au collisions at nucleon-nucleon center-of-momentum collision energy sqrt[sNN] = 130 GeV. The observed nonstatistical <pt> fluctuations substantially exceed in magnitude fluctuations expected from the finite number of particles produced in a typical collision. The r.m.s. fractional width excess of the event-wise <pt> distribution is 13.7±0.1(stat) ±1.3(syst)% relative to a statistical reference, for the 15% most-central collisions and for charged hadrons within pseudorapidity range | eta |<1,2 pi azimuth, and 0.15 <= pt <= 2 GeV/c. The width excess varies smoothly but nonmonotonically with collision centrality and does not display rapid changes with centrality which might indicate the presence of critical fluctuations. The reported <pt> fluctuation excess is qualitatively larger than those observed at lower energies and differs markedly from theoretical expectations. Contributions to <pt> fluctuations from semihard parton scattering in the initial state and dissipation in the bulk colored medium are discussed.
Evidence for an exotic S=-2, Q=-2 baryon resonance in proton-proton collisions at the CERN SPS
(2004)
Results of resonance searches in the Xi - pi -, Xi - pi +, Xi -bar+ pi -, and Xi -bar+ pi + invariant mass spectra in proton-proton collisions at sqrt[s]=17.2 GeV are presented. Evidence is shown for the existence of a narrow Xi - pi - baryon resonance with mass of 1.862±0.002 GeV/c2 and width below the detector resolution of about 0.018 GeV/c2. The significance is estimated to be above 4.2 sigma . This state is a candidate for the hypothetical exotic Xi --3/2 baryon with S=-2, I=3 / 2, and a quark content of (dsdsu-bar). At the same mass, a peak is observed in the Xi - pi + spectrum which is a candidate for the Xi 03/2 member of this isospin quartet with a quark content of (dsusd-bar). The corresponding antibaryon spectra also show enhancements at the same invariant mass.
We report measurements of single-particle inclusive spectra and two-particle azimuthal distributions of charged hadrons at high transverse momentum (high pT) in minimum bias and central d+Au collisions at sqrt[sNN]=200 GeV. The inclusive yield is enhanced in d+Au collisions relative to binary-scaled p+p collisions, while the two-particle azimuthal distributions are very similar to those observed in p+p collisions. These results demonstrate that the strong suppression of the inclusive yield and back-to-back correlations at high pT previously observed in central Au+Au collisions are due to final-state interactions with the dense medium generated in such collisions.
Double-differential cross sections have been measured for high-energy p, d, t, 3He, and 4He particles emitted from uranium targets irradiated with 20Ne ions at energies of 250, 400, and 2100 MeV/nucleon and 4He ions at 400 MeV/nucleon. By using the shape and yield of the proton energy spectra, the shape and yield of the d, t, 3He, and 4He energy spectra can be deduced at all measured angles for all incident projectile energies by assuming that they are formed by a coalescence of cascade nucleons, using a model analogous to that of Butler and Pearson, and Schwarzschild and Zupancic-caron.
Relativistic nucleus-nucleus collisions create a "fireball" of strongly interacting matter at high energy density. At very high energy this is suggested to be partonic matter, but at lower energy it should consist of yet unknown hadronic, perhaps coherent degrees of freedom. The freeze-out of this high density state to a hadron gas can tell us about properties of fireball matter. Date (v1): Thu, 19 Dec 2002 12:52:34 GMT (146kb) Date (revised v2): Thu, 16 Jan 2003 15:11:47 GMT (146kb) Date (revised v3): Wed, 14 May 2003 12:49:35 GMT (146kb)
Two-particle correlation functions of negative hadrons over wide phase space, and transverse mass spectra of negative hadrons and deuterons near mid-rapidity have been measured in central Pb+Pb collisions at 158 GeV per nucleon by the NA49 experiment at the CERN SPS. A novel Coulomb correction procedure for the negative two-particle correlations is employed making use of the measured oppositely charged particle correlation. Within an expanding source scenario these results are used to extract the dynamic characteristics of the hadronic source, resolving the ambiguities between the temperature and transverse expansion velocity of the source, that are unavoidable when single and two particle spectra are analysed separately. The source shape, the total duration of the source expansion, the duration of particle emission, the freeze-out temperature and the longitudinal and transverse expansion velocities are deduced.
We analyze hadrochemical freeze-out in central Pb+Pb collisions at CERN SPS and LHC energies. Employing the UrQMD hybrid transport model we study the effects of the final hadron/resonance expansion phase on the hadron multiplicities established at hadronization. The bulk meson yields freeze out directly at hadronization whereas the baryon-antibaryon sector is subject to significant alterations, due to annihilation and regeneration processes. We quantify the latter changes by survival factors for each species which are applied to modify the statistical model predictions for the data. The modified SM analysis recovers the hadronization points, which coincide with the recent lattice QCD predictions of the parton-hadron transition line at finite baryochemical potential.
We compare the reconstructed hadronization conditions in relativistic nuclear collisions in the nucleon–nucleon centre-of-mass energy range 4.7–2760 GeV in terms of temperature and baryon-chemical potential with lattice QCD calculations, by using hadronic multiplicities. We obtain hadronization temperatures and baryon chemical potentials with a fit to measured multiplicities by correcting for the effect of post-hadronization rescattering. The post-hadronization modification factors are calculated by means of a coupled hydrodynamical-transport model simulation under the same conditions of approximate isothermal and isochemical decoupling as assumed in the statistical hadronization model fits to the data. The fit quality is considerably better than without rescattering corrections, as already found in previous work. The curvature of the obtained “true” hadronization pseudo-critical line κ is found to be 0.0048 ± 0.0026, in agreement with lattice QCD estimates; the pseudo-critical temperature at vanishing is found to be 164.3 ± 1.8 MeV.
We present the first measurements of charge-dependent correlations on angular difference variables η1 − η2 (pseudorapidity) and φ1 − φ2 (azimuth) for primary charged hadrons with transverse momentum 0.15 <= pt <= 2 GeV/c and |η| <= 1.3 from Au–Au collisions at √sNN = 130 GeV. We observe correlation structures not predicted by theory but consistent with evolution of hadron emission geometry with increasing centrality from one-dimensional fragmentation of color strings along the beam direction to an at least two-dimensional hadronization geometry along the beam and azimuth directions of a hadron-opaque bulk medium.
The multiplicity of hadronic species created in elementary, and in nucleus-nucleus collisions, are known to be well reproduced by the statistical hadronization model, in its canonical and grand-canonical versions.To understand the origin of the implied equilibrium we revisit the hadronization models developed for e+e- annihilation to hadrons which imply spatial color pre-confinement clusters forming at the end of the pQCD evolution, which decays into on-shell hadrons/resonances. The classical ensemble description arises as a consequence of decoherence and phase space dominance during cluster formation, and decay.For A+A collisions we assume that hadronization occurs from similar singlet clusters which will overlap spatially owing to the extreme density. This is imaged in the transition to the grand-canonical ensemble.This transition sets in with increasing A and collision centrality. It can be described by a percolation model.
Recent results on baryon production in relativistic heavy ion collisions show that a revision of the chemical freeze-out conditions is necessary. Particularly, there is evidence that chemical freezeout does not occur at full chemical equilibrium. We present a method to reconstruct original hadronization conditions and show that the newly found points in the T − µB plane are in very good agreement with extrapolations of the lattice QCD critical line.
The large acceptance TPCs of the NA49 spectrometer allow for a systematic multidimensional study of two-particle correlations in different part of phase space. Results from Bertsch-Pratt and Yano-Koonin-Podgoretskii parametrizations are presented differentially in transverse pair momentum and pair rapidity. These studies give an insight into the dynamical space-time evolution of relativistic Pb+Pb collisions, which is dominated by longitudinal expansion.
Transverse mass and rapidity distributions for charged pions, charged kaons, protons, and antiprotons are reported for sqrt[sNN]=200 GeV pp and Au+Au collisions at Relativistic Heary Ion Collider (RHIC). Chemical and kinetic equilibrium model fits to our data reveal strong radial flow and long duration from chemical to kinetic freeze-out in central Au+Au collisions. The chemical freeze-out temperature appears to be independent of initial conditions at RHIC energies.
We report first results on elliptic flow of identified particles at midrapidity in Au+Au collisions at sqrt[sNN] = 130 GeV using the STAR TPC at RHIC. The elliptic flow as a function of transverse momentum and centrality differs significantly for particles of different masses. This dependence can be accounted for in hydrodynamic models, indicating that the system created shows a behavior consistent with collective hydrodynamical flow. The fit to the data with a simple model gives information on the temperature and flow velocities at freeze-out.
The hadronic final state of central Pb+Pb collisions at 20, 30, 40, 80, and 158 AGeV has been measured by the CERN NA49 collaboration. The mean transverse mass of pions and kaons at midrapidity stays nearly constant in this energy range, whereas at lower energies, at the AGS, a steep increase with beam energy was measured. Compared to p+p collisions as well as to model calculations, anomalies in the energy dependence of pion and kaon production at lower SPS energies are observed. These findings can be explained, assuming that the energy density reached in central A+A collisions at lower SPS energies is sufficient to force the hot and dense nuclear matter into a deconfined phase.
The hadronic final state of central Pb+Pb collisions at 20, 30, 40, 80, and 158 AGeV has been measured by the CERN NA49 collaboration. The mean transverse mass of pions and kaons at midrapidity stays nearly constant in this energy range, whereas at lower energies, at the AGS, a steep increase with beam energy was measured. Compared to p+p collisions as well as to model calculations, anomalies in the energy dependence of pion and kaon production at lower SPS energies are observed. These findings can be explained, assuming that the energy density reached in central A+A collisions at lower SPS energies is sufficient to transform the hot and dense nuclear matter into a deconfined phase.
Particle production in central Pb+Pb collisions was studied with the NA49 large acceptance spectrometer at the CERN SPS at beam energies of 20, 30, 40, 80, and 158 GeV per nucleon. A change of the energy dependence is observed around 30A GeV for the yields of pions and strange particles as well as for the shapes of the transverse mass spectra. At present only a reaction scenario with onset of deconfinement is able to reproduce the measurements.
A data-driven method was applied to Au+Au collisions at √sNN = 200 GeV made with the STAR detector at RHIC to isolate pseudorapidity distance η-dependent and η-independent correlations by using two- and four-particle azimuthal cumulant measurements. We identified a η-independent component of the correlation, which is dominated by anisotropic flow and flow fluctuations. It was also found to be independent of η within the measured range of pseudorapidity |η| < 1. In 20–30% central Au+Au collisions, the relative flow fluctuation was found to be 34%±2%(stat.)±3%(sys.) for particles with transverse momentum pT less than 2 GeV/c. The η-dependent part, attributed to nonflow correlations, is found to be 5% ± 2%(sys.) relative to the flow of the measured second harmonic cumulant at |η| > 0.7.
We report on a polarization measurement of inclusive J/ψ mesons in the di-electron decay channel at mid-rapidity at 2 < pT < 6 GeV/c in p + p collisions at √s = 200 GeV. Data were taken with the STAR detector at RHIC. The J/ψ polarization measurement should help to distinguish between different models of the J/ψ production mechanism since they predict different pT dependences of the J/ψ polarization. In this analysis, J/ψ polarization is studied in the helicity frame. The polarization parameter λθ measured at RHIC becomes smaller towards high pT , indicating more longitudinal J/ψ polarization as pT increases. The result is compared with predictions of presently available models.
The short-lived K(892)* resonance provides an efficient tool to probe properties of the hot and dense medium produced in relativistic heavy-ion collisions. We report measurements of K* in sqrt[sNN]=200GeV Au+Au and p+p collisions reconstructed via its hadronic decay channels K(892)*0-->K pi and K(892)*±-->K0S pi ± using the STAR detector at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. The K*0 mass has been studied as a function of pT in minimum bias p+p and central Au+Au collisions. The K*pT spectra for minimum bias p+p interactions and for Au+Au collisions in different centralities are presented. The K*/K yield ratios for all centralities in Au+Au collisions are found to be significantly lower than the ratio in minimum bias p+p collisions, indicating the importance of hadronic interactions between chemical and kinetic freeze-outs. A significant nonzero K*0 elliptic flow (v2) is observed in Au+Au collisions and is compared to the K0S and Lambda v2. The nuclear modification factor of K* at intermediate pT is similar to that of K0S but different from Lambda . This establishes a baryon-meson effect over a mass effect in the particle production at intermediate pT (2<pT <= 4GeV/c).
We report the first observation of K*(892)0--> pi K in relativistic heavy ion collisions. The transverse momentum spectrum of (K*0+K*0)/2 from central Au+Au collisions at sqrt[sNN]=130 GeV is presented. The ratios of the K*0 yield derived from these data to the yields of negative hadrons, charged kaons, and phi mesons have been measured in central and minimum bias collisions and compared with model predictions and comparable e+e-, pp, and p-barp results. The data indicate no dramatic reduction of K*0 production in relativistic heavy ion collisions despite expected losses due to rescattering effects.
Mid-rapidity transverse mass spectra and multiplicity densities of charged and neutral kaons are reported for Au + Au collisions at √sNN = 130 GeV at RHIC. The spectra are exponential in transverse mass, with an inverse slope of about 280 MeV in central collisions. The multiplicity densities for these particles scale with the negative hadron pseudo-rapidity density. The charged kaon to pion ratios are K+/π− = 0.161± 0.002(stat) ± 0.024(syst) and K−/π− = 0.146± 0.002(stat) ± 0.022(syst) for the most central collisions. The K+/π− ratio is lower than the same ratio observed at the SPS while the K−/π− is higher than the SPS result. The ratios are enhanced by about 50% relative to p + p and p¯ + p collision data at similar energies.
Lambda and Antilambda reconstruction in central Pb+Pb collisions using a time projection chamber
(1997)
The large acceptance time projection chambers of the NA49 experiment are used to record the trajectory of charged particles from Pb + Pb collisions at 158 GeV per nucleon. Neutral strange hadrons have been reconstructed from their charged decay products. To obtain distributions of Λ, and Ks0 in discrete bins of rapidity, y, and transverse momentum, pT, calculations have been performed to determine the acceptance of the detector and the efficiency of the reconstruction software as a function of both variables. The lifetime distributions obtained give values of cτ = 7.8 ± 0.6 cm for Λ and cτ = 2.5 ± 0.3 cm for Ks0, consistent with data book values.
Production of Lambda and Antilambda hyperons was measured in central Pb-Pb collisions at 40, 80, and 158 A GeV beam energy on a fixed target. Transverse mass spectra and rapidity distributions are given for all three energies. The Lambda/pi ratio at mid-rapidity and in full phase space shows a pronounced maximum between the highest AGS and 40 A GeV SPS energies, whereas the anti-Lambda}/pi ratio exhibits a monotonic increase. PACS numbers: 25.75.-q
Lambda 's produced in central collisions of 40Ar+KC1 at 1.8-GeV/u incident energy were detected in a streamer chamber by their charged-particle decay. For central collisions with impact parameters b<2.4 fm the Lambda production cross section is 7.6±2.2 mb. A calculation in which Lambda production occurs in the early stage of the collision qualitatively reproduces the results but underestimates the transverse momenta. An average Lambda polarization of -0.10±0.05 is observed. PACS numbers: 25.70 Bc
Dihadron angular correlations in d + Au collisions at √sNN = 200 GeV are reported as a function of the measured zero-degree calorimeter neutral energy and the forward charged hadron multiplicity in the Au-beam direction. A finite correlated yield is observed at large relative pseudorapidity (η) on the near side (i.e. relative azimuth φ ∼ 0). This correlated yield as a function of η appears to scale with the dominant, primarily jet-related, away-side (φ ∼ π) yield. The Fourier coefficients of the φ correlation, Vn = (cosnφ), have a strong η dependence. In addition, it is found that V1 is approximately inversely proportional to the mid-rapidity event multiplicity, while V2 is independent of it with similar magnitude in the forward (d-going) and backward (Au-going) directions.
We report the first measurement of inclusive antiproton production at midrapidity in Au+Au collisions at sqrt[sNN] = 130 GeV by the STAR experiment at RHIC. The antiproton transverse mass distributions in the measured transverse momentum range of 0.25<pperp<0.95 GeV/c are found to fall less steeply for more central collisions. The extrapolated antiproton rapidity density is found to scale approximately with the negative hadron multiplicity density.
Transverse energy ( ET ) distributions have been measured for Au+Au collisions at sqrt[sNN ]=200 GeV by the STAR Collaboration at RHIC. ET is constructed from its hadronic and electromagnetic components, which have been measured separately. ET production for the most central collisions is well described by several theoretical models whose common feature is large energy density achieved early in the fireball evolution. The magnitude and centrality dependence of ET per charged particle agrees well with measurements at lower collision energy, indicating that the growth in ET for larger collision energy results from the growth in particle production. The electromagnetic fraction of the total ET is consistent with a final state dominated by mesons and independent of centrality.
We report results on the ratio of midrapidity antiproton-to-proton yields in Au+Au collisions at sqrt[sNN] = 130 GeV per nucleon pair as measured by the STAR experiment at RHIC. Within the rapidity and transverse momentum range of | y|<0.5 and 0.4<pt<1.0 GeV/c, the ratio is essentially independent of either transverse momentum or rapidity, with an average of 0.65±0.01(stat)±0.07(syst) for minimum bias collisions. Within errors, no strong centrality dependence is observed. The results indicate that at this RHIC energy, although the p-p-bar pair production becomes important at midrapidity, a significant excess of baryons over antibaryons is still present.
We report the first measurement of strange ( Lambda ) and antistrange ( Lambda -bar) baryon production from sqrt[sNN]=130 GeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). Rapidity density and transverse mass distributions at midrapidity are presented as a function of centrality. The yield of Lambda and Lambda -bar hyperons is found to be approximately proportional to the number of negative hadrons. The production of Lambda -bar hyperons relative to negative hadrons increases very rapidly with transverse momentum. The magnitude of the increase cannot be described by existing hadronic string fragmentation models alone.
We present the first measurement of midrapidity vector meson phi production in Au+Au collisions at RHIC (sqrt[sNN]=130 GeV) from the STAR detector. For the 11% highest multiplicity collisions, the slope parameter from an exponential fit to the transverse mass distribution is T=379±50(stat)±45(syst) MeV, the yield dN/dy=5.73±0.37(stat)±0.69(syst) per event, and the ratio N phi /Nh- is found to be 0.021±0.001(stat)±0.004(syst). The measured ratio N phi /Nh- and T for the phi meson at midrapidity do not change for the selected multiplicity bins.
The minimum-bias multiplicity distribution and the transverse momentum and pseudorapidity distributions for central collisions have been measured for negative hadrons ( h-) in Au+Au interactions at sqrt[sNN] = 130 GeV. The multiplicity density at midrapidity for the 5% most central interactions is dNh-/d eta | eta = 0 = 280±1(stat)±20(syst), an increase per participant of 38% relative to pp-bar collisions at the same energy. The mean transverse momentum is 0.508±0.012 GeV/c and is larger than in central Pb+Pb collisions at lower energies. The scaling of the h- yield per participant is a strong function of pperp. The pseudorapidity distribution is almost constant within | eta |<1.
System size dependence of multiplicity fluctuations of charged particles produced in nuclear collisions at 158 A GeV was studied in the NA49 CERN experiment. Results indicate a non-monotonic dependence of the scaled variance of the multiplicity distribution with a maximum for semi-peripheral Pb+Pb interactions with number of projectile participants of about 35. This effect is not observed in a string-hadronic model of nuclear collision HIJING.
The transverse mass spectra and midrapidity yields for Xi s and Omega s are presented. For the 10% most central collisions, the Xi -bar+/h- ratio increases from the Super Proton Synchrotron to the Relativistic Heavy Ion Collider energies while the Xi -/h- stays approximately constant. A hydrodynamically inspired model fit to the Xi spectra, which assumes a thermalized source, seems to indicate that these multistrange particles experience a significant transverse flow effect, but are emitted when the system is hotter and the flow is smaller than values obtained from a combined fit to pi , K, p, and Lambda s.
A non-monotonic energy dependence of the K + / pi + ratio with a sharp maximum close to 30 A GeV is observed in central Pb+Pb collisions. Within a statistical model of the early stage, this is interpreted as a sign of the phase transition to a QGP, which causes a sharp change in the energy dependence of the strangeness to entropy ratio. This observation naturally motivates us to study the production of multistrange hyperons (Xi, Omega) as a function of the beam energy. Furthermore it was suggested that the kinematic freeze-out of Omega takes place directly at QGP hadronization. If this is indeed the case, the transverse momentum spectra of the Omega directly reflect the transverse expansion velocity of a hadronizing QGP. In this report we show preliminary NA49 results on Omega - and Omega + production in central Pb+Pb collisions at 40 and 158 A GeV and compare them to measurements of Xi - and Xi + production in central Pb+Pb collisions at 30, 40, 80 and 158 A GeV.
The balance function is a new observable based on the principle that charge is locally conserved when particles are pair produced. Balance functions have been measured for charged particle pairs and identified charged pion pairs in Au+Au collisions at sqrt[sNN]=130 GeV at the Relativistic Heavy Ion Collider using STAR. Balance functions for peripheral collisions have widths consistent with model predictions based on a superposition of nucleon-nucleon scattering. Widths in central collisions are smaller, consistent with trends predicted by models incorporating late hadronization.
We present the results of charged particle fluctuations measurements in Au+Au collisions at sqrt[sNN ]=130 GeV using the STAR detector. Dynamical fluctuations measurements are presented for inclusive charged particle multiplicities as well as for identified charged pions, kaons, and protons. The net charge dynamical fluctuations are found to be large and negative providing clear evidence that positive and negative charged particle production is correlated within the pseudorapidity range investigated. Correlations are smaller than expected based on model-dependent predictions for a resonance gas or a quark-gluon gas which undergoes fast hadronization and freeze-out. Qualitative agreement is found with comparable scaled p+p measurements and a heavy ion jet interaction generation model calculation based on independent particle collisions, although a small deviation from the 1/N scaling dependence expected from this model is observed.
A simple model is proposed for the emission of nucleons with velocities intermediate between those of the target and projectile. In this model, the nucleons which are mutually swept out from the target and projectile form a hot quasiequilibrated fireball which decays as an ideal gas. The overall features of the proton-inclusive spectra from 250- and 400-MeV/nucleon 20Ne ions and 400-MeV/nucleon 4He ions interacting with uranium are fitted without any adjustable parameters.
Observation of an exotic S = -2, Q = -2 baryon resonance in proton-proton collisions at the CERN SPS
(2003)
Results of resonance searches in the Xi- pi-, Xi- pi+, antiXi+ pi- and antiXi+ pi+ invariant mass spectra in proton-proton collisions at sqrt s=17.2 GeV are presented. Evidence is shown for the existence of a narrow Xi- pi- baryon resonance with mass of 1.862+/-0.002 GeV/c^2 and width below the detector resolution of about 0.018 GeV/c^2. The significance is estimated to be 4.0 sigma. This state is a candidate for the hypothetical exotic Xi_(3/2)^-- baryon with S = -2, I = 3/2 and a quark content of (d s d s ubar). At the same mass a peak is observed in the Xi- pi+ spectrum which is a candidate for the Xi_(3/2)^0 member of this isospin quartet with a quark content of (d s u s dbar). The corresponding antibaryon spectra also show enhancements at the same invariant mass.
Observation of enhanced subthreshold K+ production in central collisions between heavy nuclei
(1994)
In the very heavy collision system 197Au+197Au the K+ production process was studied as a function of impact parameter at 1 GeV/nucleon, a beam energy well below the free N-N threshold. The K+ multiplicity increases more than linearly with the number of participant nucleons and the K+/ pi + ratio rises significantly when going from peripheral to central collisions. The measured K+ double differential cross section is enhanced by a factor of 6 compared to microscopic transport calculations if secondary processes (Delta N-->K Lambda N and Delta Delta -->K Lambda N) are ignored.
Transverse momentum event-by-event fluctuations are studied within the string-hadronic model of high energy nuclear collisions, LUCIAE. Data on non-statistical pT fluctuations in p+p interactions are reproduced. Fluctuations of similar magnitude are predicted for nucleus-nucleus collisions, in contradiction to the preliminary NA49 results. The introduction of a string clustering mechanism (Firecracker Model) leads to a further, significant increase of pT fluctuations for nucleus-nucleus collisions. Secondary hadronic interactions, as implemented in LUCIAE, cause only a small reduction of pT fluctuations.
Midrapidity open charm spectra from direct reconstruction of D0(D0-bar)-->K± pi ± in d+Au collisions and indirect electron-positron measurements via charm semileptonic decays in p+p and d+Au collisions at sqrt[sNN]=200 GeV are reported. The D0(D0-bar) spectrum covers a transverse momentum (pT) range of 0.1<pT<3 GeV/c, whereas the electron spectra cover a range of 1<pT<4 GeV/c. The electron spectra show approximate binary collision scaling between p+p and d+Au collisions. From these two independent analyses, the differential cross section per nucleon-nucleon binary interaction at midrapidity for open charm production from d+Au collisions at BNL RHIC is d sigma NNcc-bar/dy=0.30±0.04(stat)±0.09(syst) mb. The results are compared to theoretical calculations. Implications for charmonium results in A+A collisions are discussed.
Nuclear resonance fluorescence experiments with linearly polarized bremsstrahlung were performed to determine parities of strong dipole transitions in 40Ar. A total of 14 transitions—ten of them previously unknown—in the energy range from 4.7 to 10.2 MeV could be identified. From this experiment it is evident that the main dipole strength to bound states is due to E1 excitations. An upper limit of B(M1) [up arrow] <0.5 µN2 was found for individual magnetic dipole excitations in 40Ar in the energy region below neutron threshold.