Event by Event Fluctuations in the NA49 Experiment

1
Event -by- Event Fluctuations in the NA49
Experiment

• P. Christakoglou, A. Petridis, M. Vassiliou
• Athens University
• For the NA49 Collaboration
• SPHIC06, Catania
  27-29 Sept. 2006

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Outline

• Introduction
• Energy - Centrality dependence of e-by-e
  fluctuations
• particle ratio
• average transverse momentum
• multiplicity
• net electric charge
• System size, rapidity and energy dependence
• electric charge correlations
• Conclusions

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Introduction

• Event-by-Event measurements offer the
  possibility for studying the QGP phase transition
  and the nature of the QGP matter.
• PbPb collisions at top SPS energy
• Initial energy density exceeds the critical value
  predicted by lattice QCD
• Strong collective behavior
• Proposed signatures for deconfinement observed
• Signatures not specific for deconfinement
• NA49 energy scan 20, 30, 40, 80, 158
  GeV/nucleon
• To search for structure in the energy dependence
  of hadron production characteristics indicating
  the onset of deconfinement
• To search for fluctuations which might occur if
  distinct phases coexisted in the early stage of
  the reactions or if hadrons froze out close to
  the critical point.

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The NA49 Detector
Target 20cm liquid H2 or Pb foil VCAL detects
projectile spectators ?p/p2 7 (0.3) 10-4
(GeV/c)-1 (VTPC-1, VTPCMTPC)
dE/dx resolution 3 6 TOF resolution 60 ps
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PbPb collision at 158A GeV
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Particle Ratio Strangeness Fluctuations

• Since the number of d.o.f in the QGP is lower
  than in a hadron gas ,non-Statistical
  fluctuations of the ?/p ratio could signal
  fluctuations in the fraction of deconfined matter
  at the early stage of the collision.
• NA49 performed measurements on particle
  ratios in single PbPb collisions at all
  available beam energies.

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Acceptance
40 GeV
160 GeV
20 GeV
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The Event-by-Event K/p ratio
Beam Energy
s2dyn s2data s2mixed
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The K/p ratio Energy dependence
STAR

• Data Increased fluctuation signal at lower
  beam energies
• UrQMD ?/p fluctuation signal independent of beam
  energy

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The Event-by-Event (p p)/p ratio
Beam Energy
The distribution of the E-by-E p/p ratio is
narrower for data than mixed events. Effect of
baryon resonance decay ?
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The (p p)/p ratio Energy dependence
NA49 preliminary
p/p fluctuation signal in UrQMD closely matches
the energy dependence observed in the data
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Event-by-event fluctuations of ltpTgt

• The average transverse momentum in the event is
  related to the Temperature and the Radial flow in
  the fireball, which might fluctuate with the
  initial energy density and the fraction of
  deconfined matter at the early stage of the
  collision.
• The fluctuation measure

To remove the influence of volume fluctuations,
due to variations of the impact parameter of the
collision
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Event-by-event fluctuations of ltpTgt
1.1 lt yp lt 2.6

• distributions of ltpTgt similar
• for real and mixed events
• non-statistical (dynamical)
• fluctuations are small, lt few

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Centrality and Energy dependence
158A GeV
1.1 lt yp lt 2.6
1.1 lt yp lt 2.6
NA49 preliminary
increase for peripheral collisions, no increase
at lower energy
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Multiplicity Fluctuations

• Fluctuations of the charged particle
  multiplicity may be related to fluctuations in
  the initial entropy and the fraction of
  deconfined matter produced in the initial stage
  of the collision

• Used measure of fluctuations scaled variance
• 1 for Poissonian distribution

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Controlling Centrality
AProj
Veto Calorimeter
EVeto(AProj -NPProj)Ekin
NPProj

• Veto calorimeter -gt projectile spectators, number
  of projectile participants NPProj
• Target spectators not measured in NA49

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Multiplicity Fluctuations
1.1 lt y lt 2.6
For non central PbPb coll. the distributions
are wider than the Poissonian one (independent
particle production)
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Centrality dependence (I)

• Var(n)/ltngt increases with decreasing centrality
• Approximate scaling in NPProj/AProj

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Results II
Centrality dependence (II)
HSD, UrQMD V. Konchakovskyi et al. Phys. Rev.
C 73 (2006) 034902 HIJINGM. Gyulassy, X. N.
Wang Comput. Phys. Commun. 83 (1994) 307
Simulation performed byM. Rybczynski

• String hadronic models shown (UrQMD, HSD, HIJING)
  do not reproduce data on multiplicity fluctuations

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Net Charge Fluctuations

• Net charge fluctuations will be smaller in QGP
  (due to fractional charges of quarks) than
  in gas of hadrons
• The Fluctuation Measure

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Net Charge Fluctuations
?Fq Fq - Fq,gcc
QGP signature possibly erased by hadronisation
(Bialas) or the effect of resonance decays
(Zaranek)
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Charge Correlations -The Balance Function
Bass,Danielewicz,Pratt PRL 85,2689(2000)

• oppositely charged particles created
• at the same point in space time
• particles get separated in rapidity by
• thermal motion (rescattering) and
• developing collective flow
• early produced pairs are separated more
• in rapidity than late produced pairs
• separation d? quantified by the
• balance function

delayed hadronisation
narrowing of balance function predicted as
signature of first order phase
transition
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System size and centrality dependence - vsNN
17.2 GeV

• The width takes its maximum value for pp
  interactions.
• Data show a strong system size and centrality
  dependence.
• Neither HIJING nor shuffled data show any sign
  of system size or centrality dependence.

C. Alt et al. NA49 collaboration, Phys.Rev.
C71, 034903 (2005).
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Comparison NA49 STAR

• NA49 data show a strong centrality dependence of
  the order of (17 3).

• STAR data show also a strong centrality
  dependence of the order of (14 2).

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Rapidity dependence - vsNN 17.2 GeV
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Energy dependence
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Conclusions

• NA49 has studied the e-by-e fluctuations
• The K/p ratio fluctuation signal increases at
  lower SPS energies.
• The fluctuations of ltPTgt show a system size
  dependence with a maximum for peripheral PbPb
  collisions.
• Similar dependence is found for the scaled
  variance of the multiplicity
• of neg. charged particles (possible
  correlation ltPTgt - multiplicity).
• Measurements of fluctuations of the net electric
  charge do not show
• a reduction as had been proposed for
  QGP production .
• The range of charge correlations (BF) decreases
  in central PbPb
• collisions. The degree of reduction
  exhibits a smooth energy
• dependence
• Further experimental studies
• Identify fluctuation signals of the onset of
  deconfinement
• Search for the existence of the critical point
  of QCD.
• LoI NA49-future Collaboration
• CERN-SPSC-2006-001,CERN-SPSC-235
• Study of hadron production in
  collisions of protons and nuclei

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The NA49 collaboration
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BACK UP
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Event-by-Event estimationof the particle ratios
One Event
Event Ensemble
ltdE/dxgt
ltdE/dxgt
log(ptot GeV)
log(ptot GeV)


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Extracting the Fluctuation Signal
Process the relative widths of the distributions
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Net Charge Fluctuations Energy dependence

• The predicted large suppression of the net-charge
  fluctuations is not observed!

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Charge Correlations -The Balance Function

• The Balance function is defined as a
  correlation in y of oppositely charged particles,
  minus the correlation of same charged particles,
  normalized to the total number of particles.

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vsNN 8.8 GeV RAPIDITY DEPENDENCE
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Rapidity dependence - vsNN 17.2 GeV
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Energy dependence of average transverse mass

• Increase of for final state
  particles (p,?,p)
• slows sharply at the lowest SPS
  energy

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Energy dependence pion yields

• Increase of ratio with energy
• gets steeper in the SPS range
• Pion deficit changes to enhancement compared to
  pp
• SMES statistical model of the
• early stage

Increase of initial d.o.f. between AGS and SPS
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Energy dependence ratio of strange hadrons to
pions

• Strangeness to pion ratio peaks
• sharply at the SPS
• The peak is not seen in pp
• collisions
• SMES reproduces the data
• Suggests ONSET of
• deconfinement at SPS

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Rapidity Spectra
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Global view Phase diagram
Statistical model describes yields from AGS to
RHIC energies T of hadrochemical freeze out
increases
decreases
hadrochemical freeze out points at SPS energies
approach the phase boundary
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Energy dependence of antibaryon/baryon ratios
The effect of the increasing baryon density is
seen in the strong decrease of antibaryon/baryon
ratios towards lower SPS energies.