Event By Event Fluctuation

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Event By Event Fluctuation in K/p ratio at RHIC
Supriya Das VECC, Kolkata (for STAR
Collaboration)
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Outline

• Physics Motivation
• Possibility at STAR
• Analysis Methods and results
• Summary

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Physics Motivation

• Statistical fluctuations
• Arise due to the measurement process itself
  such as finite number statistics

Dynamical fluctuations
Reflect the dynamics and responses of the
system (density fluctuations etc.)

• K/p ratio shows interesting
• behavior as function of
• collision energy.
• It would be interesting to
• see what happens to the
• fluctuation of the ratio.

Why K/p ?
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Similar work at SPS
PbPb at 158 A GeV (top 5 central)
PbPb at 20, 30, 40, 80, 158 A GeV
sdyn 2.80.5
S. V. Afanasiev et al. for NA 49 Collaboration
PRL 86 (2001)
C. Roland QM 2004
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Relativistic Heavy Ion Collider
1 km
v 0.99995?c 186,000 miles/sec
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Solenoidal Tracker At RHIC
Tracking Detectors TPC, FTPC, SVT Calorimeters
BEMC, EEMC, ZDC Trigger Detectors CTB, BBC,
MWPC Photon Detector PMD
Others TOF, FPD
Goal to track all the hadrons (and photons) in
each collision
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One collision seen by STAR TPC
Momentum determined by track curvature in
magnetic field
and by direction relative to beam
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Possibility at STAR

• Large acceptance detector ( 2 units of h with
  full f symmetry)
• Good K to p separation (for a wide momentum
  range)
• from Time Projection Chamber

M. Anderson et al. for STAR Collaboration,
NIMA499 (2003)
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Phase space distributions of identified K and p
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Analysis Method I sdyn

• Kaons and Pions are counted event by event and
  the
• ratio is plotted for a large number of
  events.
• Same ratio is calculated from mixed events and
  distribution
• of that is obtained.
• Width of the distribution (s) is the measure of
  fluctuation
• Dynamical fluctuation, sdyn is calculated as
• sdyn ?(sdata2
  smixed2)

Statistical fluctuation
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Analysis Method I (contd)
Mixed Event

• To get the measure of the statistical
  fluctuation from
• Finite number statistics
• Experimental resolution
• Correlations if any

• Mixed track pool constructed by randomly picking
  up one track
• from each event.
• Same cuts applied on the mixed tracks before
  applying the particle
• identification.

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Results (Method I)
STAR Preliminary
Supriya Das for STAR Collaboration
nucl-ex/0503023
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Analysis Method II ndynamic
First proposed by Pruneau, Gavin and Voloshin PRC
66 (2002) Used in STAR Net Charge fluctuation
paper PRC 68 (2003)
Independent of detection efficiency
M is the multiplicity in a particular centrality
bin
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Determination of collision centrality
Based on multiplicity of charged particles in the
STAR TPC (hlt0.75)
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Results
STAR Preliminary
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Results (contd.)
AuAu _at_ 200 GeV
STAR Preliminary
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Excitation function for dynamic fluctuation
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Summary

• The event by event fluctuation in strange to
  non-strange ratio has
• been studied for AuAu collisions at all
  center of momentum
• energies available at RHIC.
• sdyn shows a saturation at RHIC energies.
• ndyn,Kp is positive and it decreases with
  increasing centrality of
• collisions for all center of momentum
  energies.
• ndyn for different charge species are negative
  and approaches zero
• for central collisions.
• Comparison with results from various models
  (HIJING, UrQMD)
• are being done.

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STAR Collaboration
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Back Up
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Momentum Measurement
B0.5 T
p
Radius R
Trajectory is a helix in 3D a circle in the
transverse plane
Collision Vertex
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Physics Motivation
The strange to non-strange particle ratio for
same multiplicity is expected to give large
fluctuation as a signal of phase
transition. This fluctuation in strangeness is
expected to survive through the mixed phase.
Dynamical fluctuation in strangeness production
To study the fluctuation with Variation with
centrality Variation with CM energy
This analysis has been presented in EbyE PWG
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Event, Track selection and Particle
Identification
AuAu data at 200 GeV (prod gt P02gd)

• Events selected with -25 cm lt z-vertex lt 25 cm
• Primary tracks taken from MuDST, the following
  cuts
• have been applied,
• DCA lt 3 cm
• Nhits lt 15 (for both Kaons and Pions)
• -1 lt h lt 1
• 100 MeV lt Pt lt 600 MeV
• Particle identification applied based on nSigma
  cut i.e.
• Kaons nSigmaKaon lt 2 nSigmaPion gt 2
  Pions nSigmaPion lt 2 nSigmaKaon gt 2

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Results (Method II)
J. Adams et al. for STAR collaboration PRC 68
(2003)
Centrality at STAR

• decreases monotonically with
• increasing centrality

Magnitude of ndynamic
Strength of dynamical fluctuation
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Dynamical Net charge Fluctuations

• Sensitive to net charge
• Insensitive to volume fluctuations
• Statistical Limit Independent particle
  production.
• Dynamical Fluctuations.

Measured Quantity