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Overview of Major STAR Physics Goals Impacting TPC at Brookhaven National Laboratory

This overview outlines the major physics goals of the STAR experiment at Brookhaven National Laboratory in 2006, focusing on rare probes from high luminosity, g-hadron coincidences for jet tomography, charm and beauty studies, correlation studies across various ranges, and flow analyses. The text delves into momentum and embedding resolution, momentum corrections for resonances, dE/dx resolution for hadrons and electrons, pointing resolution into inner tracking, and challenges related to pileup rejection in different collision scenarios.

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Overview of Major STAR Physics Goals Impacting TPC at Brookhaven National Laboratory

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  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

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