NA 57 Hyperon yields in PbPb collisions Latest

NA 57 Hyperon yields in Pb-Pb collisions Latest results from the WA 97 and NA 57 collaborations Roman Lietava University of Birmingham, United Kingdom on behalf of the WA 97 and NA 57 collaborations ICHEP 02, Amsterdam 24 th-31 st July 2002

NA 57 Talk content • Physics motivation • Hyperon yields - centrality dependence New: yields at 160 A Ge. V/c from NA 57 - energy dependence New: and yields at 40 A Ge. V/c from NA 57 Comparison with STAR • Conclusions • Outlook ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

NA 57 Physics motivation ØEnhancement of strange baryons : QGP signature (PRL 42, 1982) E= Y Nwound WA 97 Pb-Pb 160 AGe. V 40% of in (SQM 98) Pb-Pb p-Pb Y – strange particle yield N wound – number of wounded nucleons in collision ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

NA 57 Study of hyperon enhancement § § Dependence on the centrality of collision Dependence on energy of colliding system WA 97 experiment: Energy: 160 A Ge. V/c. Systems: p-Be; p-Pb; Pb-Pb – 40% of in, , Nwound > 100. NA 57 experiment: Energy: 160 A Ge. V/c. Systems: Pb-Pb – 60% of in, Nwound > 50 Energy: 40 A Ge. V/c. Systems: p-Be; Pb-Pb ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

NA 57 Collisions centrality Centrality selection -- charged part. multiplicity (MSD) Nwound from cross sections for each class (Glauber model) n n Pb-Pb @ 160 A Ge. V 0 I II IV 160 Ge. V: 56 % most central collisions in 5 bins (from 0 to IV) Most peripheral bin: <Nwound> = 62 4 ICHEP 02, Amsterdam Pb-Pb @ 40 A Ge. V 40 Ge. V: 25% most central events (corresponding to bins II+IV @160 Ge. V) <Nwound> = 262 17 R. Lietava, University of Birmingham, United Kingdom

Centrality dependence: WA 97 @160 A Ge. V/c p. T 0, y-ycm 0. 5 NA 57 p. T 0, y-ycm 0. 5 Hierarchy of enhancements En(W) > En(X) > En(L) up to 15 times for W from p-Be to Pb-Pb ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

NA 57 hyperon yields vs WA 97 (160 A Ge. V/c) NA 57 confirms the enhancements observed by WA 97 (black symbols) ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

Centrality dependence: NA 57@160 A Ge. V/c NA 57 black bars – systematic errors All particle yields per participant, but +, decrease with centrality (power law? ) Significant change of + behaviour in the most peripheral bin: yield rises from <Nwound> = 62 to <Nwound> = 121 by a factor 2. 6 (3. 5 effect) ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

NA 57 Energy dependence of the yields § and yields at 40 and 160 Ge. V/c (NA 57) § and yields from SPS to RHIC ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

NA 57 and yields @ 40 and 160 Ge. V/c p. T > 0, y-ycm < 0. 5 Energy dependence in the 25% most central sin for both energies Black bars systematic errors § from 160 Ge. V to 40 Ge. V: - about the same, - down by a factor 3, + and + down by a factor 5 Larger baryon density ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

and yields from SPS to RHIC NA 57 @ 40 Ge. V 160 Ge. V NA 57 @ STAR @ 130 Ge. V 40 Ge. V 160 Ge. V STAR @ 130 Ge. V QM 2002 STAR results from: - J. Castillo in SQM 2001 for L and X - B. Hippolyte, Ph. D thesis, Universite de Strasbourg, for W Most central events: 12% for NA 57 (bins III+IV) 14% for STAR § from NA 57 to STAR energy: both - and - yields increase slower than their anti-particles at RHIC energy about same production rate for hyperon and anti-hyperon ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

Conclusions n Enhancements: n n NA 57 confirms the enhancement observed by WA 97 for L, and their antiparticles at 160 A Ge. V/c. Hierarchy of enhancements En( ) > En(L) up to 15 times for from p-Be to Pb-Pb. Yields per participant decrease with centrality (power law? ) in Pb-Pb @ 160 A Ge. V/c. and yields indicates a larger baryon density @ 40 w. r. t. 160 Ge. V/c. ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom

NA 57 Outlook n Pb-Pb @ 160 A Ge. V/c: n n n Pb-Pb @ 40 A Ge. V/c: n n n Increase statistics for - and + by a factor 2 (2000 data set). Negatives. L and L yields. Negatives. p-Be @ 40 A Ge. V/c: n n L, L and - ( +? ) yields. Negatives. Enhancements in Pb-Pb relative to p-Be @ 40 Ge. V/c. ICHEP 02, Amsterdam R. Lietava, University of Birmingham, United Kingdom