Vertex Finding and Pileup in the p+p data
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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

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


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


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


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


What Do We Do With Misidentified Vertices

Vertex Finding Resolution

Counts

Monte Carlo Z Vertex – ppLMV Z Vertex (cm)


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


Events with Reconstructed Vertex(no residual cut)

Total Number of Events

Does Embedding and Real Data Agree?

Efficiency =

DATA

EMBEDDING


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!)


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


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


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


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


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


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.


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


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


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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