na57 results
DESCRIPTION
NA57 Results. R. Lietava The University of Birmingham, UK For the NA57 Collaboration. Content. Introduction Enhancement at 40 and 160 GeV Transverse mass spectra -> transverse dynamics Rapidity spectra -> longitudinal dynamics Nuclear modification factors at 160 GeV Summary. - PowerPoint PPT PresentationTRANSCRIPT
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NA57 Results
R. Lietava
The University of Birmingham, UK
For the NA57 Collaboration
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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
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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
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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
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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
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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)