The Balance Function: Experimental Studies in STAR

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The Balance FunctionExperimental Studies in STAR

• Marguerite Belt Tonjes,
• G.D. Westfall, A.M. Vander Molen, the STAR
  collaboration

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STAR

• Solenoidal Tracker At RHIC
• large acceptance detector with near 4? coverage,
  study hadrons, jets, event-by-event, high pt
  particles, strangeness.

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Selecting Good Data

• Use AuAu 130GeV events from Year1 running,
  summer, 2000
• Physics events
• Central trigger
• Minimum bias trigger
• -75cm vertexz 75cm
• -1cm vertexx 1cm,
• -1cm vertexy 1cm

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Selecting good TPC tracks

• Collaboration-wide selection cuts
• h lt 1.3
• 0.1 GeV/c pt 2 GeV/c
• DCA 1 cm (distance of closest approach to the
  primary vertex ? choose tracks from the vertex)
• Fit points 15
• Fit points/possible points gt 0.52

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PID

• Compare measured dE/dx with Bethe-Bloch
  calculation of dE/dx for p, K, p, and e.
• Particles within 2s of predicted ionization are
  tagged.

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PID cuts

• ZPidp ln( Imeas/Ihyp(p))
• Imeas measured truncated mean for track
• Ihyp(p) expected ionization for p hypothesis
• p p0.7 GeV/c, and ZPidp0.2
• K p0.8 GeV/c, ZpidK0.2, ZPidpgt0.2,
  ZPidpgt0.2
• P p 1 GeV/c, ZPidp0.2, ZPidpgt0.2,
  ZPidKgt0.2
• E ZPide0.2, ZPidpgt0.2, ZPidKgt0.2,
  ZPidpgt0.2

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Calculating the Balance Function

• For p pairs
• B 1/2 D - /N D- /N- - D/N - D- -/N-
• D- Histogram of y(p) - y(p-) , for all
  possible pairs within an event. This histogram is
  summed over all events.
• For charged particle pairs, we sum over all
  charged particle pairs.
• D- Histogram of h(particle)-h(particle-)

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Conclusions and future analysis

• More statistics are needed, and are being
  analyzed
• We see no difference between central and
  peripheral events in HIJING
• Need to study beam pipe/detector effects for
  simulations
• Study pt sensitivity
• Look at pp simulations

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Thanks to

• Gary Westfall, Skip Vander Molen, Scott Pratt for
  much discussion of the analysis
• The STAR Collaboration
• DOE
• NSF