ISMD 2005

NA57 Results

R. Lietava

The University of Birmingham, UK

For the NA57 Collaboration

ISMD 2005

Content

• Introduction• Enhancement at 40 and 160 GeV• Transverse mass spectra -> transverse

dynamics• Rapidity spectra -> longitudinal

dynamics • Nuclear modification factors at 160 GeV• Summary

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NA57 studies the production of strange and multi-strange baryons in heavy ion interactions as a function of

• collision centrality• collision energy

30 cm

5 cm

5 cm

Silicon te

lescope

decay vertices target

- + K- ( BR = 67.8 % )

- + - ( BR = 99.9 % )

- + p ( BR = 63.9 % )

+ + - ( BR = 68.6 % )

s

1M channels

B

NA57 experiment

ISMD 2005

Centrality of the collisions

• Determined from charged particle multiplicity measured in a dedicated Si-strips detector

• Npart and Ncoll from trigger cross section (Glauber + WNM calculations)trigger 60% of Pb-Pb inel

(7.26 b)

Pb-Pb 160 A GeV

0 1 2 3 4

Nch

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Double differential cross section for each particle fitted to: assuming flat/gaussian rapidity distribution in the selected acceptance. Inverse slope of mT distribution ( T = Tapp ) free parameter of the fit.

Yield (average particle multiplicity per event) extrapolated to a common y-pT region:

Strangeness enhancement:

Yields and enhancements

ISMD 2005

systematic

errors:

10% for , 15% for

Most peripheral

class:

<Nwound> = 62 4

[ ]

Significant centrality dependence of strangeness enhancements for all hyperons except for _

Hyperon enhancements at 160 A GeV/c

ISMD 2005

Most peripheral

class:

<Nwound> = 57 4(95 % confidence level)

Enhancements still there at 40 A GeV with the same hierarchy as for higher energy data: E() < E()

Hyperon enhancements at 40 A GeV/c

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enhancements in the most central collisions

(classes 3-4) are larger at 40 GeV than at 160 GeV

steeper increase with centrality at lower energy

Energy dependence of hyperon enhancements

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Transverse mass spectra

G

n

GS Rr

R

rr

)(

rdrT

pI

T

mKmA

mym

N GRtt

TjTT

j

0 01

2sinhcosh

dd

d

)(tanh)( 1 rr

Blast wave description of the spectra:

Sn

2

2Uniform particle density

Ref: E Schnedermann, J Sollfrank and U Heinz, Phys. Rev. C48 (1993) 2462

Hydro picture: mT spectra sensitive to the transverse flow

NA57 papers:F. Antinori et al J. Phys. G: Nucl. Part. Phys. 30 (2004) 823G.E. Bruno et al J. Phys. G: Nucl. Part. Phys. 31 (2005) S127

ISMD 2005

Blast-wave fit to mT spectra

Thermal freeze-out temperature lower at 40 A GeV/c Similar transverse flow

Pb-Pb 160 A GeV/c Pb-Pb 40 A GeV/c

T = 144 ± 7(stat) ± 14(syst) MeV = 0.381 ±0.013(stat) ± 0.012(syst)

T = 118 ± 5(stat) (syst) MeV +11 - 10

0.398 ± 0.010(stat) (syst)+0.011- 0.013

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Centrality dependence of the freeze-out parameters

With increasing centrality:– Transverse flow velocity increases– Freeze-out temperature decreases

Pb-Pb 160 A GeV/c Pb-Pb 40 A GeV/c

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Rapidity distributions in Pb-Pb at 160 A GeV/c

• K0 and deviate significantly from flatness in our limited acceptance• No centrality dependence

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• Tf fixed to 143 MeV (from blast-wave)

• < L > = 0.42 ± 0.03 c

• 2/ndf = 28.2 / 32

Longitudinal dynamics

thermal model

thermal model with longitudinal flow (Bjorken)

)2/tanh(

)(

max

max

max

L

th ydy

dNd

dy

dN

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Central to peripheral nuclear modification factor at sNN=17.3 GeV

(i.e. beam momentum = 158 A GeV/c)

dydpNd

dydpNd

N

NpR

tP

tC

Ccoll

PcolltCP /

/)(

2

2

where “C” and “P” are two centrality classes, in general Central and Peripheral

F. Antinori et al Phys. Lett. B in press, preprint nucl-ex/0507012“Central-to-peripheral nuclear modification factors in Pb-Pb collisions at sNN =

17.3 GeV”

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Nuclear modification factors

Binary scaling

Scaling with Npart

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RCP Comparisons

WA98 vs NA57

For pt < 1.5 GeV (h- -): 0 and h- compatible For pt > 1.5 GeV (h- K-, p): RCP for K0 and h- > RCP for 0

STAR vs NA57

RCP at SPS > RCP at RHIC Relative pattern similar at SPS and RHIC

STAR, PRL92 (2004) 052302WA98, EPJC23 (2002) 225

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Summary• Strangeness enhancements measured at 40

GeV/c with the same hierarchy as at 160 GeV/c

• Blast wave analysis of transverse and longitudinal spectra:– Centrality dependence of freeze out temperature and

expanding velocity observed at both energies

– < L > < >

• Nuclear modification factor Rcp:– Similar pattern as at RHIC, lower in magnitude– Interpretation: initial state effect (PDF) versus final state

effect (parton energy loss,recombination) systematic theoretical study required

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Back up slides

Prague

CERN

Athens

Paris - CdF , Strasbourg

Bari, Catania, Padua, Rome, Salerno

Utrecht

Bergen, Oslo

St. Petersburg

Bratislava, Košice Birmingham

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The NA57 Collaboration

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Layout of the NA57 experiment at CERN SPS

Target: 1% Pb , 8% Be

Scintillator Petals: centrality trigger

MSD: Multiplicity Silicon Detector

Tracking device: silicon pixel planes (5 x 5 cm2 ) pixel size: 50500 m2

Lever arm: pixel planes & double side strips

  

1

M c

hann

els

Pb-

Pb

only

System Beam energy Sample size

Pb-Pb 160 A GeV (230+230) x 106 evts

Pb-Pb 40 A GeV 240 x 106 evts

p-Be 40 A GeV (60+110) x 106 evts

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Centrality determination - from charged particle multiplicity measurementNwound - from trigger cross section (Glauber model calculations)

<Nwound> <Nbin>

bin % of tot 160 40 160 40

0 42-56 % 62 57 77 81

1 25-42% 121 119 191 203

2 12-25 % 209 208 395 416

3 5-12 % 290 292 614 644

4 0-5 % 349 346 789 807

Pb-Pb 160 A GeV

0 1 2 3 4

Nch

Centrality of the collision

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Acceptance windows in Pb-Pb at 158 A GeV/c

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Sample invariant mass spectra

Residual combinatorial background (few %)subtracted or included in the systematics

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Hyperon yield measurements

• Data corrected for acceptance and also for detector and reconstruction efficiency by Monte Carlo simulation

• In the acceptance window:– Yield

(i.e. particle per event)

– Transverse mass spectra (Tapp)

• Extrapolation to a common window:– one unit of rapidity about ycm

– full range of pT

20

2 mpm TT

app

TT

T T

mAm

my

Nexp

dd

d2

5.0

5.0

2

0 dd

ddd

CM

CM

y

y mT

Textr ym

NmyY

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Enhancements w.r.t. number of binary collisions at 160 A GeV/c

<Nbin>

10

1

10

1

<Yield> / <Nbin>

(<Yield> / <Nbin>)p-Be

Enhancement =

• Going from p-Be to Pb-Pb and yields scale faster than <Nbin>

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Centrality dependence (i)

No centrality dependence in our limited acceptance

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Centrality dependence (ii)

No centrality dependence in our limited acceptance

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Few details about the analysis

• Full collected statistics (two years of data taking in nice agreement)

• New centrality classes

• Rcp for unweighted spectra

• Dedicated analysis for particle selection at high pT

• Residual combinatorials estimated with event mixing and subtracted

• Paper submitted to PLB and soon available as pre-print

Comparisons with theoretical pre-dictions

• X N Wang: perturbative-QCD-based calculation with and without in-medium parton energy loss

– Initial dNg/dy scaled down from RHIC according to the observed decrease in dNch/dy ( factor 2)

– Initial-state partonic kT broadening tuned on the original Cronin effect data

Ref: X N Wang PRC 68 (2001) 064910; PLB 595 (2004) 165; private communication

• Cross-checked using different model: PQM (BDMPS-based)– medium density determined from RHIC data and extrapolated down to SPS energy; Cronin

enhancement includedRef: Dainese, Loizides, Paic, EPJC38 (2005) 495; private communication

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Comparison NA57-NA49Particle yields per participant

NA57 NA49*

40 GeV 5% 7%

160 GeV (K)

5% 7%

160 GeV (, )

12% 10%

*Refs: Physical Review C 66, 054902 (2002), arXiv:nucl-ex/0311024, Phys. Lett. B 538 (2002), 275.

about 30% systematics on the absolute value of the

yields (under investigation) but …

Similar centrality regions:

For NA49: K0 = 0.5*(K++K-)s

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Comparison NA57-NA49Particle ratios

*Refs: Physical Review C 66, 054902 (2002), arXiv:nucl-ex/0311024, arXiv:nucl-ex/0305021, arXiv:nucl-ex/0311029.

… particle ratios compatible within errors (no impact on relative yields)