2012/7/23 1

Cosmic Ray at STAR

Shuai Yang

Center of Particle Physics and TechnologyUniversity of Science and Technology of China

STAR Regional MeetingWeihai, China, July 23th – 25th, 2012

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Outline Introduction

•Muon charge ratio from cosmic ray EAS •Cosmic ray at STAR

Event and Track selection •Event selection•Track selection•Match muon tracks

Analysis results•Improve of momentum resolution•Reject of gamma conversion event•μ+/μ- vs. Momentum•μ+/μ- vs. Multiplicity

Summary

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Muon charge ratio from cosmic ray EAS

Result of the MC simulation of iron and proton induced showers

• EAS: Extensive Air Shower, A fixed target experiment beyond LHC energy

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Cosmic ray at STAR

Advantage:• Precisely measure muon momentum upto several hundred

GeV/c• Large acceptance to measure muon bundles

x

y

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Event selection

Cosm

ic triggers

Laser run

All E

vents

Reversed Full Field

tracks>=

2 &&

runId cuts

Full Field

• Global tracks >=2• Reject bad runIds• Cosmic trigger Ids & no laser trigger Id

Event Candidates

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Track selection

• 700>Track_flag>0• nHitsFit>=15• P > 3 GeV/c

Muon Track Candidates

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x

y

Match muon tracks Project tracks onto R=220cm plane,

pickup the upstream point For every two tracks with p>3 GeV/c

• Compare R*(pos_phi2-pos_phi1)• Compare pos_z2 – pos_z1• Compare p_phi2-p_phi1 and p_theta2 – p_theta1• p = -p for the track with larger firstPoint_y

For those qualified from above 2D cuts are considered as matched candidates.

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2D cuts to further suppress random combinatorial background

2D correlation for muon matching

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Improve momentum resolutionRefit_1

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•Refit_2： makes a direct helix refit to all the hit points from the paired tracks(still ongoing, not finish yet)

By Prof. Shao

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“Neutral” track

The reason of "neutral" track:• Gamma conversion (low momentum)• The charge sign for one track is wrong(large

momentum)

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Gamma conversion3-10GeV/c

3-10GeV/c

200-1000GeV/c

200-1000GeV/c

The charge sign is redefined from the refit after rejecting the gamma conversion events according the deflection direction

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Results

• μ+/μ- vs. Momentum• μ+/μ- vs. Multiplicity

Proton induced showers: low multiplicity ratio > 1 (~1.3)

Iron induced showers: high multiplicity ratio ~ 1

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Ratio vs. Momentum

FF

FF FF

FF

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Different Multiplicity definition

•MuonpairMult: Selected Muonpair events•MatchMult: flag of global track (0,700) ∈ ; flag of match >0•SingleMuonMult: Selected Muon events•GlobalMult： flag of global track (0,700) ∈ ; nHitsFit>= 15 ; firstpoint.Z() && lastpoint.Z() ∈ [-180,180] •TofMult： BTofHit(BTofHit_mTray[i]<=120)

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MuonpairMult. - the most reliable multiplicity definition

RFF FF

• Decrease with multiplicity?

• Reconstructed Muonpair Multiplicity is too small

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MatchMult.

RFF FF

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SingleMuonMult.

RFF FF

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GlobalMult.

From the ratio vs. four different multiplicity definitions:• Ratio decreases with increasing multiplicity. • Physics reason? Detector reason?

RFF FF

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TofMult.

• Most of the tof hits are not associated with global track.

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Summary

• We can matched muon tracks with good s/b ratio and efficiency

• Combined two muon tracks can improve the momentum resolution

• Can get ratio with very good precision upto several hundred GeV/c

• Can observe ratio decrease with multiplicity, but need to check the reason.