The Balance Function: Experimental Studies in STAR Marguerite Belt Tonjes, G.D. Westfall, A.M. Vander Molen, the STAR collaboration

STAR STAR Solenoidal Tracker At RHIC large acceptance detector with near 4  coverage, study hadrons, jets, event-by-event, high p t particles, strangeness.

Selecting Good Data Use Au+Au 130GeV events from Year1 running, summer, 2000 Physics events: –Central trigger –Minimum bias trigger -75cm ≤ vertex z ≤ 75cm -1cm ≤ vertex x ≤ 1cm, -1cm ≤ vertex y ≤ 1cm

Selecting good TPC tracks Collaboration-wide selection cuts |  | < GeV/c ≤ p t ≤ 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( I meas /I hyp(  ) ) –I meas = measured truncated mean for track –I hyp(  ) = 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 - )|

Conclusions and future analysis More statistics are needed, and are being analyzed We see no difference between central and peripheral events in HIJING HNeed to study beam pipe/detector effects for simulations HStudy p t sensitivity HLook at pp simulations

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