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The Balance Function:Experimental Studies in STAR
Marguerite Belt Tonjes,
G.D. Westfall, A.M. Vander Molen, the STAR collaboration
STARSTARSSolenoidal TTracker AAt RRHIC
• large acceptance detector with near 4 coverage, study hadrons, jets, event-by-event, high pt particles, strangeness.
Selecting Good Data• Use Au+Au 130GeV
events from Year1 running, summer, 2000
• Physics events:– Central trigger
– Minimum bias trigger
• -75cm ≤ vertexz ≤ 75cm
• -1cm ≤ vertexx ≤ 1cm,
• -1cm ≤ vertexy ≤ 1cm
Selecting good TPC tracks
Collaboration-wide selection cuts• | | < 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 > 0.52
PID
• Compare measured dE/dx with Bethe-Bloch calculation of dE/dx for , K, p, and e.
• Particles within 2 of predicted ionization are tagged.
PID cuts• ZPid = ln( Imeas/Ihyp())
– Imeas = measured truncated mean for track
– Ihyp() = expected ionization for hypothesis
• : p≤0.7 GeV/c, and |ZPidp|≤0.2• K: p≤0.8 GeV/c, |ZpidK|≤0.2, |ZPid|>0.2, |ZPidp|
>0.2• P: p≤ 1 GeV/c, |ZPidp|≤0.2, |ZPid|>0.2, |ZPidK|>0.2• E: |ZPide|≤0.2, |ZPid|>0.2, |ZPidK|>0.2, |ZPidp|>0.2
Calculating the Balance Function
• For pairs:– B = 1/2 { + - /N+ + - +/N- - ++/N+ - - -/N-}
– +- = Histogram of | y(+) - y(-) |, 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.– +- = Histogram of | (particle+)-(particle-)|
-0.1
0
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B(( )) - y HIJING central
( )y
pairs
: 80,000 HIJING central events
HIJING has || <1.3
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B(( ))/ - y binsize HIJING central
( )y
pairs
: 80,000 HIJING central events
|HIJING has | <0.8
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B(()) - HIJING central
()
All charged particle pairs
HIJING central: 80,000 events
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
Thanks to
• Gary Westfall, Skip Vander Molen, Scott Pratt for much discussion of the analysis
• The STAR Collaboration
• DOE
• NSF