L and X results from NA57 in Pb-Pb collisions at 160 A GeV/c at the CERN SPS

1. L and X 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

2. Contents • Introduction • Pb-Pb at 160 A GeV/c • X results • mT spectra • yields • L preliminary results: • mT spectra • yields • Outlook 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) 375-381) • Strangeness enhancement grows with the strangeness of the produced hadrons: up to a factor 15 for W ! • WA97 results are not reproduced by purely hadronic collision models • NA57Search for the onset of enhancement: • Extend centrality range below Nwound 100 (J. Phys G: Nucl. Part. Phys. 27 (2001) 391-396) • Lower beam momentum from 160 to 40 GeV/c Giuseppe E. Bruno -- Università di Bari and INFN

4. Silicon pixel telescope (compact part) 5 cm 5 cm 30 cm L p- B X- Particle reconstruction • L, X, W + anti-particles , K0S and negatives (mainly pions) L --> p + p- BR = 63.9% X---> L + p-BR = 99.89% p + p- W---> L + K-BR = 67.8%  p + p- K0S--> p+ + p-BR = 68.6% Giuseppe E. Bruno -- Università di Bari and INFN

5. 79 % X- and X+ signals Mass spectra Acceptance window FWHM = 8 MeV Giuseppe E. Bruno -- Università di Bari and INFN

6. 86 %  and L signals Mass spectra Acceptance window FWHM = 5 MeV Giuseppe E. Bruno -- Università di Bari and INFN

7. L Background study • 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 L and L: • X background:  4 % for both X- and X+ • 0.3 % for L • 1.0 % for L 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 X weighted • A fraction of ’98 L weighted (cpu time expensive) • 1/200 for L • 1/25 for L Giuseppe E. Bruno -- Università di Bari and INFN

9. X and L transverse mass spectra Giuseppe E. Bruno -- Università di Bari and INFN

10. IV IV III III II II I I 0 0 L and L transverse mass spectra for the five centrality classes Giuseppe E. Bruno -- Università di Bari and INFN

11. Inverse slopes T (MeV) Preliminary results Full centrality range: 0-IV most peripheral  most central  • Inverse slopes compatible for particles and their antiparticles • Slight increase of L and L slopes with centrality • Inverse slopes in agreement with WA97 Giuseppe E. Bruno -- Università di Bari and INFN

12. w.r.t. WA97 • The enhancement is confirmed by NA57 • NA57 yields tend to be larger by up to 20-30% for X+ and L • Extended checks completed on X X and preliminary L corrected yields Giuseppe E. Bruno -- Università di Bari and INFN

13. Yield per Nwound rises from <Nwound> = 62  121 for L, X- and X+: factor 2.6 for X+ (3.5 s effect) • Steep onset of X+enhancement? • Similar behaviour between X-and L X and preliminary L corrected yields Giuseppe E. Bruno -- Università di Bari and INFN

14. Outlook • Pb-Pb 160 A GeV/c data • doubling of the statistics (2000 data taking) • W-and W+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 Giuseppe E. Bruno -- Università di Bari and INFN