H. Helstrup

Strangeness enhancement at mid-rapidity in Pb Pb collisions at 158-A-GeV/c

We study the influence of lexical subsumption on the semantic proximity of terms. By studying the semantic relation between lexically subsumed terms in an existing controlled vocabulary and those in a corpus, we formulate hypotheses on the underlying specific relations. These hypotheses enable us to propose an ordering of term variants found in the corpus, by probability of decreasing semantic proximity. Results from our experiment show that surface indices like the presence of proper names, compounds and the number of added words in a term variant are criteria for ordering the variants of the same term. The ranking obtained will enable us to form more semantically-cohesive clusters. Applications targeted are text mining, question-answering and science and technology watch.

Production of multistrange baryons and antibaryons in sulphur-tungsten interactions at 200 GeV/c per nucleon

Abstract Production of Ξ − and Ξ − has been observed for the first time in heavy ion interactions by the WA85 Experiment. Multistrange baryon and antibaryon production is expected to be a useful probe in the search for quark-gluon plasma formation. We describe the procedure used to select these cascade candidates and show that Ξ − and Ξ − decays can be identified. The ratio of Ξ Ξ production, corrected for geometrical acceptances and reconstruction efficiencies, is 0.39 ± 0.07 for sulphur-tungsten interactions in the central rapidity interval 2.3 Y lab p T >1.1 GeV/ c .

Production of strange and multistrange hadrons in nucleus—nucleus collisions at the SPS

Abstract We report the main results from experiment WA97 on the production of strange and multistrange hadrons at midrapidity in Pb+Pb, p+Pb and p+Be collisions at 158 A GeV/ c . A comparison with some of the available models is presented.

Strangeness enhancements at central rapidity in 40 A GeV/c Pb-Pb collisions

Results are presented on the production of K0S, hyperons and antihyperons in Pb–Pb and p–Be interactions at 40 GeV/c per nucleon. The enhancement pattern follows the same hierarchy as seen in the higher energy data—the enhancement increases with the strangeness content of the hyperons and with the centrality of collision. The centrality dependence of the Pb–Pb yields and enhancements is steeper at 40 than at 158 A GeV/c. The energy dependence of strangeness enhancements at mid-rapidity is discussed.

LAMBDA , XI AND OMEGA PRODUCTION AT MID-RAPIDITY IN PB-PB AND P-PB COLLISIONS AT 158 A GEV/C

Λ, Ξ and Ω yields and transverse mass spectra measured at central rapidity in Pb-Pb and p-Pb collisions at 158 A GeV/c are presented. The yields in Pb-Pb interactions are given as a function of the collision centrality and compared with those obtained from p-Pb collisions. The dependence of hyperon yields on the centrality of the impact and on the strangeness content of the hyperon are discussed.

Observation of Ω− and Ω− in sulphur-tungsten interactions at 200 GeV/c per nucleon

Abstract Multistrange baryon and antibaryon production is suggested to be a useful probe in the search for Quark-Gluon Plasma formation. We report the detection of an Ω − + Ω − signal in central S + W interactions at 200 A GeV/c and measure the ratio Ω − Ω − = 0.57±0.41 at central rapidity and p T >1.6 GeV/ c .

Construction and characterization of a 117 cm/sup 2/ silicon pixel detector

A silicon pixel detector, developed in RD19, and consisting of 4 planes, /spl sim/30 cm/sup 2/ each, is operating for the first time in the lead ion experiment WA97 at CERN. The 288 CMOS readout chips are bump-bonded to 48 Si detector matrices, assembled in 8 identical arrays. The total number of pixel cells is nearly 300000 and each cell, 75 /spl mu/m/spl times/500 /spl mu/m, contains a complete signal processing chain. Overall dead area is less than 3%. >

Central-to-peripheral nuclear modification factors in Pb-Pb collisions at √sNN = 17.3 GeV

We present central-to-peripheral nuclear modification factors, R_CP, for the p_T distributions of K^0_S, Lambda, Anti-Lambda, and negatively charged particles, measured at central rapidity in Pb-Pb collisions at top SPS energy. The data cover the 55% most central fraction of the inelastic cross section. The K^0_S and Lambda R_CP(p_T) are similar in shape to those measured at sqrt{s_NN} = 200 GeV at RHIC, though they are larger in absolute value. We have compared our K^0_S R_CP data to a theoretical calculation. The prediction overestimates the data at p_T \approx 3-4 GeV/c, unless sizeable parton energy loss is included in the calculation.

Centrality dependence of the expansion dynamics in Pb Pb collisions at 158-A-GeV/c

Two-particle correlation functions of negatively charged hadrons from Pb-Pb collisions at 158 GeV/c per nucleon have been measured by the WA97 experiment at the CERN SPS. A Coulomb correction procedure that assumes an expanding source has been implemented. Within the framework of an expanding thermalized source model the size and dynamical state of the collision fireball at freeze-out have been reconstructed as a function of the centrality of the collision. Less central collisions exhibit a different dynamics than central ones: both transverse and longitudinal expansion velocities are slower, the expansion duration is shorter and the system freezes out showing smaller dimensions and higher temperature.Two-particle correlation functions of negatively charged hadrons from Pb-Pb collisions at 158?GeV?c-1/nucleon have been measured by the WA97 experiment at the CERN SPS. A Coulomb correction procedure that assumes an expanding source has been implemented. Within the framework of an expanding thermalized source model the size and dynamical state of the collision fireball at freeze-out have been reconstructed as a function of the centrality of the collision. Less central collisions exhibit a different dynamics than central ones: both transverse and longitudinal expansion velocities are slower, the expansion duration is shorter and the system freezes out, showing smaller dimensions and higher temperature.

Determination of the event centrality in the WA97 and NA57 experiments

The procedure employed by the WA97 and NA57 experiments to determine the number of wounded nucleons in different Pb + Pb centrality classes is discussed. We will also compare different centrality scales and their influence on the measurement of the centrality dependence of strange particle yields.