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-)|

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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

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: 80,000 HIJING central events

|HIJING has | <0.8

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B(()) - HIJING central

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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