and results from NA57 in Pb-Pb collisions at 160 A GeV/c

at the CERN SPSGiuseppe E. Bruno

Università di Bari and INFN, Bari, ItalyOn behalf of the NA57 Collaboration

Athens CERN Prague

Bari Kosice Rome

Bergen Legnaro Salerno

Birmingham Oslo St. Petersburg

Bratislava Padua Strasbourg

Catania Paris Utrecht

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 2

Contents

Introduction

Pb-Pb at 160 A GeV/c– results

• mT spectra

• yields

– preliminary results:• mT 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) 375-381)

– 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 Nwound 100

(J. Phys G: Nucl. Part. Phys. 27 (2001) 391-396)

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

K0S BR = 68.6%

30 cm

5 cm

5 cm

B

Particle reconstruction Silicon pixeltelescope

compact part)

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 5

and + signalsMass spectra Acceptance window

FWHM 8 MeV

79 %

March 16-23, 2002 Giuseppe E. Bruno -- Università di Bari and INFN 6

and signalsMass spectra Acceptance 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

T

mm

dm

dN TT

T

exp

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)

0 258 ± 19 274 ± 18

I 261 ± 11 258 ± 10

II 276 ± 11 279 ± 10

III 313 ± 13 307 ± 13

IV 310 ± 15 309 ± 14

Preliminary resultsFull 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 Nwound rises from Nwound = 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 datadoubling of the statistics (2000 data taking)

andinverse slopes and yields

Low energy beam dataresults from Pb-Pb at 40 A GeV/cresults from p-Be at 40 GeV/c