Search Results (17 total)

Neutral pion transverse momentum spectra were measured in p+C and p+Pb collisions at sqrt[s_NN]=17.4  GeV at midrapidity (2.3≲η_lab≲3.0) over the range 0.7≲p_T≲3.5  GeV/c. The spectra are compared to π⁰ spectra measured in Pb+Pb collisions at sqrt[s_NN]=17.3  GeV in the same experiment. For a wide range of Pb+Pb centralities (N_part≲300), the yield of π⁰’s with p_T≳2  GeV/c is larger than or consistent with the p+C or p+Pb yields scaled with the number of nucleon-nucleon collisions (N_coll), while for central Pb+Pb collisions with N_part≳350, the π⁰ yield is suppressed.

Two-particle correlations of direct photons were measured in central ²⁰⁸Pb+²⁰⁸Pb collisions at 158A   GeV. The invariant interferometric radii were extracted for 100<K_T<300   MeV/c and compared to radii extracted from charged pion correlations. The yield of soft direct photons, K_T<300   MeV/c, was extracted from the correlation strength and compared to theoretical calculations.

Results on the study of localized fluctuations in the multiplicity of charged particles and photons produced in 158A GeV/c Pb+Pb collisions are presented for varying centralities. The charged versus neutral particle multiplicity correlations in common phase space regions of varying azimuthal sizes are analyzed by two different methods. Various types of mixed events are constructed to probe fluctuations arising from different sources. The measured results are compared to those from simulations and from mixed events. The comparison indicates the presence of nonstatistical fluctuations in both the charged particle and photon multiplicities in limited azimuthal regions. However, no correlated charged-neutral fluctuations, a possible signature of formation of disoriented chiral condensates, are observed. An upper limit on the production of disoriented chiral condensates is set.

Several hadronic observables have been studied in central 158A GeV Pb+Pb collisions using data measured by the WA98 experiment at CERN: single π^- and K^- production, as well as two- and three-pion interferometry. The Wiedemann-Heinz hydrodynamical model has been fitted to the pion spectrum, giving an estimate of the temperature and transverse flow velocity. Bose-Einstein correlations between two identified π^- have been analyzed as a function of k_T, using two different parametrizations. The results indicate that the source does not have a strictly boost invariant expansion or spend time in a long-lived intermediate phase. A comparison between data and a hydrodynamical based simulation shows very good agreement for the radii parameters as a function of k_T. The pion phase-space density at freeze-out has been measured, and agrees well with the Tomás̆ik-Heinz model. A large pion chemical potential close to the condensation limit of m_π seems to be excluded. The three-pion Bose-Einstein interferometry shows a substantial contribution of the genuine three-pion correlation, but not quite as large as expected for a fully chaotic and symmetric source.

Event-by-event fluctuations in the multiplicities of charged particles and photons, and the total transverse energy in 158A GeV Pb+Pb collisions are studied for a wide range of centralities. For narrow centrality bins the multiplicity and transverse energy distributions are found to be near perfect Gaussians. The effect of detector acceptance on the multiplicity fluctuations has been studied and demonstrated to follow statistical considerations. The centrality dependence of the charged particle multiplicity fluctuations in the measured data has been found to agree reasonably well with those obtained from a participant model. However, for photons the multiplicity fluctuations have been found to be lower compared to those obtained from a participant model. The multiplicity and transverse energy fluctuations have also been compared to those obtained from the VENUS event generator.

Localized fluctuations in the multiplicity of charged particles and photons produced in central 158A GeV/c Pb+Pb collisions are studied. The charged versus neutral correlations in common η-φ phase space regions of varying azimuthal size are analyzed by two different methods. The analysis provides a model-independent demonstration of nonstatistical fluctuations in both charged particle and photon multiplicities in limited azimuthal regions. However, no correlated charge-neutral fluctuations are observed, contrary to expectations for the production of a disoriented chiral condensate. The result is not explained by the widely used VENUS model.

A measurement of direct photon production in ²⁰⁸Pb+²⁰⁸Pb collisions at 158A GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval 0.5<p_T<4 GeV/c. A significant direct photon signal, compared to statistical and systematical errors, is seen at p_T>1.5 GeV/c. The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions. It could be significant for diagnosis of quark-gluon-plasma formation.

Three-particle correlations have been measured for identified π^- from central 158A GeV Pb+Pb collisions by the WA98 experiment at CERN. A substantial contribution of the genuine three-body correlation has been found as expected for a mainly chaotic and symmetric source.

Neutral pion production in central 158A GeV ²⁰⁸Pb+ ²⁰⁸Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron. The π⁰ transverse mass spectrum has been analyzed in terms of a thermal model with hydrodynamic expansion. The high accuracy and large kinematic coverage of the measurement allow one to limit previously noted ambiguities in the extracted freeze-out parameters. The results are shown to be sensitive to the shape of the velocity distribution at freeze-out.

The NA44 Collaboration has measured charged kaon and pion distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon, respectively. The inverse slopes of kaons, are larger than those of pions. The difference in the inverse slopes of pions, kaons, and protons, all measured in our spectrometer, increases with system size and is consistent with the buildup of collective flow for larger systems. The target dependence of both the yields and inverse slopes is stronger for the sulphur beam, suggesting the increased importance of secondary rescattering for SA reactions. The rapidity density dN/dy of both K⁺ and K^- increases more rapidly with system size than for π⁺ in a similar rapidity region. This trend continues with increasing centrality, and according to RQMD, it is caused by secondary reactions between mesons and baryons. The K^-/K⁺ ratio falls with increasing system size but more slowly than the p̅ /p ratio. The π^-/π⁺ ratio is close to unity for all systems. From pBe to SPb the K⁺/p ratio decreases while K^-/p̅ increases and sqrt[(K⁺⋅K^-)/(p⋅p̅ )] stays constant. These data suggest that as larger nuclei collide, the resulting system has a larger transverse expansion and baryon density and an increasing fraction of strange quarks.

The production of neutral pions in 158A GeV ²⁰⁸Pb+²⁰⁸Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron (SPS). Transverse momentum spectra are studied for the range 0.3≤m_T-m₀≤4.0 GeV/c. The results for central collisions are compared to various models. The centrality dependence of the neutral pion spectral shape and yield is investigated. An invariance of the spectral shape and a simple scaling of the yield with the number of participating nucleons is observed for centralities with greater than about 30 participating nucleons. This is most naturally explained by assuming an equilibrated system.

Experiment NA44 has measured proton and antiproton distributions at midrapidity in sulphur and proton collisions with nuclear targets at 200 and 450 GeV/c per nucleon respectively. The inverse slopes of transverse mass distributions increase with system size for both protons and antiprotons but are slightly lower for antiprotons. This could happen if antiprotons are annihilated in the nuclear medium. The antiproton yield increases with system size and centrality and is largest at midrapidity. The proton yield also increases with system size and centrality, but decreases from backward rapidity to midrapidity. The stopping of protons at these energies lies between the full stopping and nuclear transparency scenarios. The data are in reasonable agreement with RQMD predictions except for the antiproton yields from sulphur-nucleus collisions.

For a set of N identical massive boson wave packets with optimal initial quantum mechanical localization, we calculate the Hanbury-Brown–Twiss (HBT) two-particle correlation function. Our result provides an algorithm for calculating one-particle spectra and two-particle correlations from an arbitrary phase space occupation (q_i,p_i,t_i)_i=1,N as, e.g., returned by event generators. It is a microscopic derivation of the result of the coherent state formalism, providing explicit finite multiplicity corrections. Both the one- and two-particle spectra depend explicitly on the initial wave packet width σ which parametrizes the quantum mechanical wave packet localization. They provide upper and lower bounds which suggest that a realistic value for σ has the order of the Compton wavelength.

Inclusive photons of low transverse momenta have been measured in 200 GeV/nucleon ³²S+Au collisions at the CERN SPS. Data were taken in the WA93 experiment using a small acceptance BGO detector with longitudinal segmentation. The results are compared to WA80 measurements for the same system and results from hadron decay calculations. An excess of soft photons over the expectations from neutral meson decays is observed.

Transverse mass spectra of pions, kaons, and protons from the symmetric heavy-ion collisions 200 A GeV S+S and 158 A GeV Pb+Pb, measured in the NA44 focusing spectrometer at CERN, are presented. The mass dependence of the slope parameters provides evidence of collective transverse flow from expansion of the system in heavy-ion induced central collisions.

First results of the m_T dependence of π⁺π⁺ and K⁺K⁺ correlations from S+Pb collisions at 200 GeV /c per nucleon measured by the focusing spectrometer of the NA44 experiment at CERN are presented. Multidimensional fits characterize the pion and kaon emission volume. The pion radius parameter decreases with increasing p_T. Furthermore, the pion and kaon radii show a common 1/sqrt[m_T] dependence. This behavior can be interpreted as a result of a strong momentum-position correlation arising from collective flow.