STAR Overview of major STAR pA and AA

ó STAR Overview of major STAR p+A and A+A physics goals as they impact the TPC James Dunlop Brookhaven National Laboratory October 7, 2006 Dunlop TPC Review, 10/6/2006 1

ó STAR in the RHIC II Era • Rare probes from high luminosity – g-hadron coincidences for jet emission tomography – Charm and beauty: partonic collectivity and lessstrongly 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 p. T: coalescence, “Mach cones”, etc. – Particle-identified flow Dunlop TPC Review, 10/6/2006 2

ó STAR How high in p. T? STAR Preliminary With 10 ub-1 equivalent, factor 3000 expected at RHIC II (but g s down from 10 to 15 Ge. V/c) • • • Current: statistically limited to ~12 Ge. V/c No trigger: with DAQ 1000 might get to 15 Ge. V/c in untriggered probes g-hadron coincidences: expecting g up to 15 Ge. V/c BEMC electrons for B: hadron contamination limits electron identification to ~10 -15 Ge. V/c, depending on performance of the preshower Target: good reconstruction for p. T up to ~ 15 Ge. V/c Dunlop TPC Review, 10/6/2006 3

ó STAR Momentum Resolution: Sagitta • Sagitta Formula: 1. 9 e-4 * L 2/p. T (for full field) • Implication: ~½ cm sagitta at 12 Ge. V (primaries) – twice as small for globals (without inner tracking) due to L 2 Dunlop TPC Review, 10/6/2006 4

ó STAR • • • Momentum Resolution: Embedding (no distortion) ~ primaries 0. 5%/Ge. V 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 V 0 decays; however, efficiency of adding points at low radius low for longlived hyperons like L Dunlop TPC Review, 10/6/2006 5

ó STAR Positive vs. Negative: momentum shifts Distortions in sagitta lead to opposite-signed shifts in p. T for positives vs. negatives • Combined with steeply falling spectrum, changes positive/negative ratio • Rough size: 0. 1%/Ge. V shift (50 um) leads to ~20% drop at 10 Ge. V – 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 Ge. V/c Somewhat alleviated by summing positives and negatives, but limits physics reach to charge-independent observables Dunlop TPC Review, 10/6/2006 6

ó STAR Momentum resolution: resonances • Limiting resolution on resonance widths is dp. T • Maximize significance, reduce effect of background by minimizing width – E. g. f desire to keep width < 10 Me. V, requires 2% dp. T/p. T • For small S/B, significance of signal essentially scales with resolution: 2 x worse resolution, significance down by factor sqrt(2) Dunlop TPC Review, 10/6/2006 7

ó STAR Side note on Upsilon RHIC CDF resolution m+m 0. 85% @ 4. 9 Ge. V Muons STAR resolution e+e 3% @ 4. 9 Ge. V+ Brem, • Preferable to separate Upsilonium states: sequential dissociation • Current limitation is Bremsstrahlung in inner material • Even without Brem (muons? ), really hard: – 0. 5%/Ge. V implies ~200 Me. V resolution on the peak – 2 S-1 S = 560 Me. V, 3 S-2 S= 330 Me. V; somewhat marginal Dunlop TPC Review, 10/6/2006 8

ó STAR Momentum resolution: corrections to spectra Half field: ~10% dpt/pt for primaries at 5. 5 Ge. V, – Correction factor for bin smearing ~25%, limiting systematic – Depends on steepness of spectrum, somewhat less at higher p. T • Requirement: better than ~10%; at 15 Ge. V, full field, requires ~400 um resolution on primary sagitta (~sqrt(2) increase relative to undistorted case) Dunlop TPC Review, 10/6/2006 9

ó STAR d. E/dx resolution: hadrons p p • Purity of pion selection depends on d. E/dx window – E. g. 4. 5 Ge. V/c, d. E/dx resolution 8%, 80% efficiency at 10% contamination • Contamination already an issue in protons (~10 -20% systematic); worsening d. E/dx resolution will rapidly make this worse Dunlop TPC Review, 10/6/2006 10

ó STAR d. E/dx resolution: electrons BEMC • Major contamination rejector is the TPC d. E/dx • EMC: Currently ~20+/-5% at 8 Ge. V with 50% efficiency; limiting factor (no preshower as of yet) • Worsening of d. E/dx resolution will rapidly make this worse Dunlop TPC Review, 10/6/2006 11

ó STAR 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 Dunlop TPC Review, 10/6/2006 12

ó Pileup STAR • Pileup levels (+/- 40 us = 80 us): – Current (Cu+Cu): 40 k. Hz * 80 us =~ 3 extra events – Expected RHIC 2 (Au+Au): 90 k. Hz * 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? Dunlop TPC Review, 10/6/2006 13

ó STAR Conclusions Requirement: pt resolution not worsen by more than sqrt(2), and preferably do better – In heavy ion physics, p. T ~ 15 Ge. V/c is likely the highest p. T 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 L 2; may, however, take some of the onus off the most-distorted inner points in determining the sagitta Requirement: d. E/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 Dunlop TPC Review, 10/6/2006 14