STAR

1
STARs Measurement of Energy and System Size
Dependence of K/p Fluctuations at RHIC
Zubayer Ahammed for the STAR Collaboration
VECC, Kolkata, India
Outline

• Motivation
• Measures
• STAR measurement
• Comparison with SPS
• Comparison with models
• Summary

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Motivation

• QCD predicts QGP phase transition at sufficient
  high temperature/ energy density.
• The nature of this phase transition can be
  reflected in the increased/reduced fluctuations
  in the global observables.
• Kapusta and Mekjian J.I. Kapusta and A. Mekjien,
  Phys. Rev. D33(1986)1304
• suggested that fluctuations in the Kaon to Pion
  ratio can reflect the nature of phase transition.

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K/p at SPS

• Strangeness production is related to
  de-confinement.
• Excitation function of ltKgt/ltpgt shows horn
  around vsNN 7 GeV.
• The excitation function of ltK-gt/ltp-gt is
  monotonic.

C. Blume (NA49), hep-ph/0505137
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K/p Fluctuations measurement
Event-by-event fluctuations in K/p may give
insight into the production mechanism of
kaons. NA49 measured the fluctuations of K/p as
a function of incident energy for central PbPb
collisions from vsNN 6 to 17 GeV using the
observable sdyn. Measure the K/p ratio
event-by-event. K K K- p p p- Produce
histogram of the K/p ratio. Extract the width of
K/p histogram to get sreal Do the same for mixed
event to get smixed
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K/p Fluctuations at SPS

• Define the dynamical fluctuations in terms of sdyn

• Divide by the mean and multiply by 100
• to get

C. Blume (NA49), hep-ph/0505137
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K/p Fluctuations in STAR

• Study AuAu collisions at vsNN 62,130 and 200
  GeV.
• Study CuCu collisions at vsNN62 and 200 GeV.
• Extract the number of K K- and p p-
  event-by-event using energy loss and curvature in
  the STAR TPC
• Select kaons and pions with 0.2 lt pt lt 0.6 GeV/c
  and
• ? lt 1.0
• kaons Ns,K lt 2, Ns,p gt 2
• pions Ns,p lt 2, Ns,K gt 2
• electrons Ns,e lt 1

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STARs K/p Histograms
STAR Preliminary
STAR Preliminary
CuCu 62GeV
CuCu 200 GeV
Data
Data
Mixed
Mixed
0-10
0-10
K/p for data and mixed events.
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Measured sdyn

• Similar at 62 and 200 GeV for CuCu.
• Similar at 62,130 and 200 GeV for AuAu.
• Similar for CuCu and AuAu systems.

• Consistent with NA49.

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Comparison with Statistical Hadronization Model

• Consistent with Torrieri with gq (1.6)
• fitted at higher energy.
• Inconsistent with Torrieri with gq1
• (equilibrium scenario)

AuAu(0-5) CuCu(0-10) NA49(0-6)
Girgio TorreiriNucl-th/0702062(2007)
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Another Measure, ndyn

• The use of sdyn is problematic at smaller
  multiplicities because it involves the
  event-by-event fluctuation of a ratio.
• A better measure is ndyn,Kp.

ndyn introduced to study net charge fluctuations.
(PRC 68, 044905 2003) ndyn,Kp is insensitive
to efficiency. ndyn,Kp properly deals with small
multiplicities.
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Measured ndyn
STAR Preliminary

• Independent of energy for CuCu

CuCu

• 1/N centrality dependence

error stat.sys
a b/ (dN/dh)
STAR Preliminary
AuAu

• Independent of energy for AuAu .
• 1/N centrality dependence.

a b/ (dN/dh)
error stat. only
sys. error for AuAu17.6
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System size dependence
STAR Preliminary
CuCu stat.sys.
AuAu stat.
sys. error for AuAu17.6
Independent of system size.
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Scaled with dN/dh
AuAu stat.
CuCu stat.sys.
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Comparison with SPS results
s2dyn ndyn
NA49 errors only stat. error shown
here Consistent with NA49 within errors.
CuCu stat.sys.
AuAu stat.
sys. error for AuAu17.6
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For Charges Separately
STAR Preliminary CuCu 62 GeV
(C)
(A)
Error stat.sys.
STAR Preliminary CuCu 200 GeV
(B)

• Consistent with zero for same sign
• in data and Hijing .
• Higher in summed charge than same
• and opposite sign in data and Hijing

Error stat.sys.
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Comparison with models

• AMPT, A Multi Phase
• Transport Model

STAR Preliminary

• Hijing hadronic evolution(
• multiple re-scattering)

In agreement with AMPT predictions.
CuCu stat.sys.
AuAu stat.
sys. error for AuAu17.6
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Comparison with Scaled ndyn,Kp
CuCu stat.sys.
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Summary

• We have measured K/p fluctuations on an
  event-by-event basis using the measured sdyn and
  ndyn,Kp.
• The measured fluctuations is found to be
  independent of energy.
• Measured fluctuation is similar for CuCu and
  AuAu system.
• Measured fluctuation strength is consistent with
  SPS results.
• SH model (Torreiri) with fitted gq is consistent
  at higher energy however the Equilibrium scenario
  (gq1) under-predicts at all energies.
• Hijing model over-predicts the measured K/p
  fluctuations.
• AMPT results (Hijing re-scattering) is in
  better agreement with data.