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)
Present address GSI, Darmstadt
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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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Search for QCD Phase Transition
The production of strangeness may be related to
the onset of deconfinement Excitation function
of ltKgt/lt?gt shows structure around sNN1/2 7
GeV The excitation function of ltK-gt/lt?-gt is
smooth
C. Blume (NA49), hep-ph/0505137
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Fluctuations in ltKgt/lt?gt

• Event-by-event fluctuations in ltKgt/lt?gt may give
  insight into the production mechanism of Ks
• NA49 measured dynamical fluctuation in K/p ratio
  by comparing the width of distributions from data
  and mixed events

PbPb at 158 A GeV (top 5 central)
S. V. Afanasiev et al. for NA 49 Collaboration
PRL 86 (2001)
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Fluctuations in ltKgt/lt?gt
NA49 has also measured thefluctuation of
ltKgt/lt??gtas a function of incidentenergy for
central PbPbcollisions from 6 to 17GeV using
the observable ?dyn
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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Potential 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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Dynamical fluctuation 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)

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Phase space distributions of identified K and p
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Dynamical Fluctuation (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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sdyn Kp for different center of mass energies
Data not corrected for efficiency
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Excitation Function for ?dyn
We extracted ?dyn for AuAu, 0 - 5 most central,
for 20, 62, 130, and 200 GeV and compared the
results with the NA49-SPS results

• Possibilities
• Related to K/? horn
• Increase of resonances decaying intoK ? with
  increasing incident energy
• Dilution of correlations as multiplicity
  increases with energy

Statistical errors only
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Better Measure

• The use of ?dyn is problematic because it
  involves event-by-event fluctuations of a ratio
• A better measure is ?dyn,K?
• Insensitive to efficiency
• Properly deals with small multiplicities
• Centrality studies
• We will use ?dyn,K? for our systematic studies of
  K/? fluctuations

First proposed by Pruneau, Gavin and Voloshin PRC
66 (2002) Used in STAR Net Charge fluctuation
paper PRC 68 (2003)
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Determination of collision centrality
Based on multiplicity of charged particles in the
STAR TPC (hlt0.5)
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Centrality dependence of dynamical fluctuation
for different collision energies
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Summary

• Excitation function of ?dyn remains flat at RHIC
  energies.
• Study of the excitation function and centrality
  dependence of K/?? fluctuations requires a robust
  variable like ?dyn,Kp .
• ndyn,Kp is positive and it decreases with
  increasing centrality of collisions for 62 and
  200 GeV centre of mass energies.
• We are looking into the detailed systematics of
  20 and 130 GeV dataset to decrease the errors.

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STAR Collaboration