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Vertex Finding and Pileup in the p+p data

Vertex Finding and Pileup in the p+p data. Jon Gans Yale University STAR Collaboration. Detector Setup Vertexing Bias – p+p and Au+Au Influence of Bias on the Data New Methodology to Remove Bias (Works for me, but probably not for you) Remarks on pileup rejection Contamination Outlook.

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Vertex Finding and Pileup in the p+p data

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  1. Vertex Finding and Pileup in the p+p data Jon Gans Yale University STAR Collaboration • Detector Setup • Vertexing Bias – p+p and Au+Au • Influence of Bias on the Data • New Methodology to Remove Bias • (Works for me, but probably not for you) • Remarks on pileup rejection • Contamination • Outlook

  2. Time Projection Chamber: 2.1 meter drift over 40 μs |η| < 1.5 , Full Azimuth Central Trigger Barrel (Scintillator): Trigger For Au+Au (not for p+p) 6° x 1 m – 240 Cover Entire Barrel Sample 11 (Δt 110ns) times per trigger Beam Beam Counters: +/- 3meters , 3.5 < | η| < 5.5 Use for p+p Trigger (Coincidence) 70cm Vertex Resolution 420 CM Must Use TPC Tracks For Vertexing

  3. Vertex Efficiency with Embedding Vertex Efficiency RECO 60 Total Bunches in Ring ( I only show 5) 55 Are Filled With Protons 5 are empty 213 ns between each bunch Since the empty bunches are only in the detector for short period of time, upstream, beam gas and pileup events are sampled. Empty Bunch Event

  4. Vertex Finding Efficiency Events with abs(ppLMVz – mcz) < 1cm Efficiency = Total Number of Events Vertex Finder Correct ~ 74% Why don’t we use these numbers to correct data? Loses More Low Multiplicity Events than High Multiplicity Events I Haven’t Told you the Whole Truth Raw Multiplicity

  5. What Do We Do With Misidentified Vertices Vertex Finding Resolution Counts Monte Carlo Z Vertex – ppLMV Z Vertex (cm)

  6. Vertex Finding Contamination Events with abs(ppLMVz – mcz) > 1cm Contamination = Total Number of Events Contamination ~ 11% Contamination .2 Lost Vertices ~ 14% .4 .1 .2 0 0 Raw Multiplicity Raw Multiplicity

  7. Events with Reconstructed Vertex(no residual cut) Total Number of Events Does Embedding and Real Data Agree? Efficiency = DATA EMBEDDING

  8. Spectra Using Primary Vertex Analysis Method STAR / UA1 RAW Vertex Corrected STAR / UA1 Correction For η Acceptance: (STAR abs(η) < .5, UA1 abs(η) < 2.5 pt GeV/c No Efficiency or Acceptance Corrections (which would move spectra up anyways!)

  9. Consequences of Multiplicity Bias Mean pt scales with multiplicity UA1 – Phys Lett B 366 (1996) 434 Mean pt GeV/c 0.2 0.6 Momentum bias tied to vertex finding bias. 0 25 Multiplicity Vertex Finder looses low multiplicity events => Spectra is shifted to higher pt

  10. Events with Reconstructed Vertex(no residual cut) Total Number of Events What about Au+Au data? Thanks to P. Fachini Efficiency = HIJING + TRS Data fitPoints < 15 HIJING + TRS Data No fitpoints cut

  11. More on Au+Au HIJING Studies Thanks to P. Fachini Wrong Vertex HIJING + TRS No Vertex HIJING + TRS Vertex Found HIJING + TRS • # good global tracks: • # fit points  25 • flag  0 • # good global tracks: • # fit points  25 • flag  0 • # good global tracks: • # fit points  25 • flag  0 Shapes Similar to proton+proton Embedding

  12. Determining Spectra Without Vertexing Beamline position is well known in STAR. For events with a found Vertex: Parameterize X and Y position of Vertex versus Z STAR Preliminary -1.6 1.2 Y Vertex Position (cm) -200 0 200 Z Vertex Position (cm) x(z) = -0.304233 cm + -0.00065732 * z y(z) = 0.407226 cm + 0.00012396 * z 1) Project Track To Beam Line Check for 1cm DCA or less

  13. 1 0.5 0 0 Azimuth 180 360 Match Track To CTB To Reject Pileup Track Matches CTB Hit CTB Match Eff. = Track Projects To CTB Good Measure Of Pileup Lower Matching Efficiency => Higher Pileup STAR Preliminary STAR Preliminary Z Position -2 2 0 180 360 Azimuth Lower Luminosity (Early in Run) Higher Luminosity (Late in Run) Note: 5-10% Matching Rate When CTBs Rotated by 90 degrees

  14. Same Studies with SVT Matched Tracks Lower Matching Efficiency => Higher Pileup 1 STAR Preliminary STAR Preliminary Z Position .5 -2 2 0 0 180 360 Azimuth Azimuth 0 180 360 Lower Luminosity (Early in Run) Higher Luminosity (Late in Run) Note: 5-10% Matching Rate When CTBs Rotated by 90 degrees Appears that SVT Matched Tracks Suffer Less Pileup!! Can we use the SVT for pilup rejection? Very Preliminary Result, needs more attention.

  15. Contamination ~ 8% STAR Preliminary ΔZ Drawbacks to No Vertex Method Z position of vertex unknown From Higher Multiplicity Events Get Z position STAR Preliminary Due To Added Material outside +/- 30 cm (Inner Vertex Detector Electronics and Cooling) Acceptance and Efficiency Corrections Difficult -100cm -30 cm 30cm 100 cm

  16. 1/nEvents 1/pt d((h+ + h-)/2)/dηdpt BLACK – UA1 C. Albajar et al., Nucl. Phys. B335, 261 (1990) RED – STAR Uncorrected Spectra STAR / UA1 Correction For η Acceptance: (STAR abs(η) < .5, UA1 abs(η) < 2.5 STAR / UA1 pt GeV/c NEVER Trust A Log Plot UNCORRECTED Spectra STAR Preliminary

  17. Future and Outlook Investigating methods to constrain Z Vertex Position. JG and P. Fachini investigating methods to accomplish this. -60 -40 -20 0 20 40 60 Mean z Projection - mcVertz Efficiency and Acceptance Corrections currently being studied in pp ITTF Vertex Finder

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