 and  results from NA57 in Pb-Pb collisions at 160 A GeV/c at the CERN SPS Giuseppe E. Bruno Università di Bari and INFN, Bari, Italy On behalf of the NA57 Collaboration AthensCERNPrague BariKosiceRome BergenLegnaroSalerno BirminghamOsloSt. Petersburg BratislavaPaduaStrasbourg CataniaParisUtrecht

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 2 Contents  Introduction  Pb-Pb at 160 A GeV/c –  results m T spectra yields –  preliminary results: m T spectra yields  Outlook

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 3 Introduction * QGP prediction: strangeness enhancement in Pb-Pb w.r.t. p-A increases with strangeness  WA97 results : (J. Phys. G: Nucl. Part. Phys. 27 (2001) ) –Strangeness enhancement grows with the strangeness of the produced hadrons: up to a factor 15 for  –WA97 results are not reproduced by purely hadronic collision models  NA57 Search for the onset of enhancement: * Extend centrality range below N wound 100 (J. Phys G: Nucl. Part. Phys. 27 (2001) ) * Lower beam momentum from 160 to 40 GeV/c

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 4   + anti-particles,   S and negatives (mainly pions)  p   BR = 63.9%      BR =      p        BR = 67.8%    p   K 0 S      BR = 68.6% 30 cm 5 cm    B Particle reconstruction Silicon pixel telescope  compact part)

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 5   and  + signals Mass spectraAcceptance window FWHM  8 MeV 79 %

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 6  and  signals Mass spectraAcceptance window FWHM  5 MeV 86 %

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 7  Aim: Study of the residual combinatorial background: combine events to search for “fake”  s in the mixed events mixed events reconstructed as real ones to estimate the residual background  Residual background negligible for  and  :   background:  4 % for both   and   Background study  0.3 % for  1.0 % for 

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 8 Corrections for acceptance and reconstruction efficiency  For each measured hyperon a weight is computed by tracing it through the experimental apparatus by GEANT and by reconstructing as for real data  Complete ’98 sample of  weighted  A fraction of ’98  weighted ( cpu time expensive ) – 1/200 for  – 1/25 for 

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 9  and  transverse mass spectra

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 10  and  transverse mass spectra for the five centrality classes IV III II I 0 IV III II I 0

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 11  Inverse slopes compatible for particles and their antiparticles  Slight increase of  and  slopes with centrality  Inverse slopes in agreement with WA97 -- 303 ± 11  321 ± 23  287 ± 6  284 ± 6 Inverse slopes T (MeV)  0258 ± ± 18 I261 ± ± 10 II276 ± ± 10 III313 ± ± 13 IV310 ± ± 14 Preliminary results Full centrality range: 0-IV most central  most peripheral 

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 12  and preliminary  corrected yields  The enhancement is confirmed by NA57  NA57 yields tend to be larger by up to 20-30% for  + and   Extended checks completed on  w.r.t. WA97

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 13  and preliminary  corrected yields  Yield per N wound rises from  N wound  = 62  121 for   - and  + : factor 2.6 for  + (3.5  effect)  Steep onset of  + enhancement ?  Similar behaviour between  - and 

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 14 Outlook  Pb-Pb 160 A GeV/c data  doubling of the statistics (2000 data taking)     and    inverse slopes and yields  Low energy beam data  results from Pb-Pb at 40 A GeV/c  results from p-Be at 40 GeV/c