Overview of major STAR p+A and A+A physics goals as they impact the TPC

1. Overview of major STAR p+A and A+A physics goals as they impact the TPC James Dunlop Brookhaven National Laboratory October 7, 2006

2. STAR in the RHIC II Era • Rare probes from high luminosity • g-hadron coincidences for jet emission tomography • Charm and beauty: partonic collectivity and less-strongly interacting “grey” tomographic probes • Charm- and Bottom-onium dissociation • Correlations across full range in h, especially for the FMS; bulk of these studies can be done with EMC’s • Bulk studies of the medium • Particle-identified multiparticle correlations at intermediate pT: coalescence, “Mach cones”, etc. • Particle-identified flow

3. How high in pT? STAR Preliminary • Current: statistically limited to ~12 GeV/c • No trigger: with DAQ1000 might get to 15 GeV/c in untriggered probes • g-hadron coincidences: expecting g up to 15 GeV/c • BEMC electrons for B: hadron contamination limits electron identification to ~10-15 GeV/c, depending on performance of the preshower • Target: good reconstruction for pT up to ~ 15 GeV/c With 10 ub-1 equivalent, factor 3000 expected at RHIC II (but g s down from 10 to 15 GeV/c)

4. Momentum Resolution: Sagitta • Sagitta Formula: 1.9e-4 * L2/pT (for full field) • Implication: ~½ cm sagitta at 12 GeV (primaries) • twice as small for globals (without inner tracking) due to L2

5. Momentum Resolution: Embedding • Embedding (no distortion) ~ primaries 0.5%/GeV from position resolution • Implies ~300 um resolution on sagitta (12*0.005*0.5 cm = 0.03 cm) • Global resolution is improvable with addition of microvertex detectors for V0 decays; however, efficiency of adding points at low radius low for long-lived hyperons like L

6. Positive vs. Negative: momentum shifts Distortions in sagitta lead to opposite-signed shifts in pT for positives vs. negatives • Combined with steeply falling spectrum, changes positive/negative ratio • Rough size: 0.1%/GeV shift (50 um) leads to ~20% drop at 10 GeV • Depends on how steeply falling the spectrum is; this is for pions, larger for protons • Current limiting systematic on e.g. pbar/p: ~20% at 7 GeV/c • Somewhat alleviated by summing positives and negatives, but limits physics reach to charge-independent observables

7. Momentum resolution: resonances • Limiting resolution on resonance widths is dpT • Maximize significance, reduce effect of background by minimizing width • E.g. f desire to keep width < 10 MeV, requires 2% dpT/pT • For small S/B, significance of signal essentially scales with resolution: 2x worse resolution, significance down by factor sqrt(2)

8. Side note on Upsilon RHIC • Preferable to separate Upsilonium states: sequential dissociation • Current limitation is Bremsstrahlung in inner material • Even without Brem (muons?), really hard: • 0.5%/GeV implies ~200 MeV resolution on the peak • 2S-1S = 560 MeV, 3S-2S= 330 MeV; somewhat marginal CDF resolution m+m 0.85% @ 4.9 GeV Muons STAR resolution e+e 3% @ 4.9 GeV+ Brem,

9. Momentum resolution: corrections to spectra Half field: ~10% dpt/pt for primaries at 5.5 GeV, • Correction factor for bin smearing ~25%, limiting systematic • Depends on steepness of spectrum, somewhat less at higher pT • Requirement: better than ~10%; at 15 GeV, full field, requires ~400 um resolution on primary sagitta (~sqrt(2) increase relative to undistorted case)

10. dE/dx resolution: hadrons p • Purity of pion selection depends on dE/dx window • E.g. 4.5 GeV/c, dE/dx resolution 8%, 80% efficiency at 10% contamination • Contamination already an issue in protons (~10-20% systematic); worsening dE/dx resolution will rapidly make this worse p

11. dE/dx resolution: electrons BEMC • Major contamination rejector is the TPC dE/dx • EMC: Currently ~20+/-5% at 8 GeV with 50% efficiency; limiting factor (no preshower as of yet) • Worsening of dE/dx resolution will rapidly make this worse

12. Pointing resolution into inner tracking • TPC is only necessary as a pointing apparatus into the SSD or other inner tracking detector • Figure of merit is occupancy in the radius of confusion; not currently considered to be a limiting requirement (e.g. 1 mm fine) • Would like to minimize issue of systematic offsets to avoid calibration delays

13. Pileup • Pileup levels (+/- 40 us = 80 us): • Current (Cu+Cu): 40 kHz * 80 us =~ 3 extra events • Expected RHIC 2 (Au+Au): 90 kHz * 80 us =~6 extra events • Already using existing detectors to reject pileup: EMC’s and CTB • CTB does not work in a heavy-ion environment: occupancy too high • BEMC solves the problem in Cu+Cu for ~70% most central, could probably be extended down to lower multiplicities with some work • EMC occupancy an issue in central Au+Au: loss of rejection power • TOF with low occupancy (10% in central collisions) and tight timing cuts will solve this • p+p • Some current issues with minbias collisions due to low multiplicity, could be solved with work and full coverage of the BEMC • At RHIC 2: 10 MHz * 80 us =~ 800 extra events • TPC Occupancy slightly higher than central Au+Au event; 2*Npart/2 = 700 • Multiple collisions per bunch; timing of TOF will be critical?

14. Conclusions Requirement: pt resolution not worsen by more than sqrt(2), and preferably do better • In heavy ion physics, pT ~ 15 GeV/c is likely the highest pT of interest in the foreseeable future: sqrt(2) would give you dpt/pt ~10%, at which point smearing corrections will start to be significant • Statistics-starved resonance studies (D, f) lose significance as sqrt(resolution); Upsilon in the muon channel would benefit from any improvement (only ~2 s or so at best) and would be badly impacted by any worsening of the resolution • Inner tracking probably does not buy much, due to L2; may, however, take some of the onus off the most-distorted inner points in determining the sagitta Requirement: dE/dx resolution not worsen from current (~8%) Limiting factor in e-h and p-p separation Pileup likely solved by the TOF in ions; p+p has different issues Pointing resolution to SSD/IST likely not a large issue Systematic distortions may continue to be a time-limiter in calibrations