2012/7/231 Cosmic Ray at STAR Shuai Yang Center of Particle Physics and Technology University of Science and Technology of China STAR Regional Meeting Weihai, China, July 23 th – 25 th, 2012
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 2 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
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 3 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
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 4 Cosmic ray at STAR Advantage: Precisely measure muon momentum upto several hundred GeV/c Large acceptance to measure muon bundles x y
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 5 Event selection Cosmic triggers Laser run All Events Reversed Full Field tra ck s> =2 & & ru nI d cu ts Full Field Global tracks >=2 Reject bad runIds Cosmic trigger Ids & no laser trigger Id Event Candidates
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 6 Track selection 700>Track_flag>0 nHitsFit>=15 P > 3 GeV/c Muon Track Candidates
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 7 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.
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 8 2D cuts to further suppress random combinatorial background 2D correlation for muon matching
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 9 Improve momentum resolution Refit_1
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 10 Refit_2 : makes a direct helix refit to all the hit points from the paired tracks(still ongoing, not finish yet) By Prof. Shao
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 11 “Neutral” track The reason of "neutral" track: Gamma conversion (low momentum) The charge sign for one track is wrong (large momentum)
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 12 Gamma conversion 3-10GeV/c GeV/c The charge sign is redefined from the refit after rejecting the gamma conversion events according the deflection direction
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 13 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
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 14 Ratio vs. Momentum FF
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 15 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)
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 16 MuonpairMult. - the most reliable multiplicity definition RFF FF Decrease with multiplicity? Reconstructed Muonpair Multiplicity is too small
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 17 MatchMult. RFF FF
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 18 SingleMuonMult. RFF FF
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 19 GlobalMult. From the ratio vs. four different multiplicity definitions: Ratio decreases with increasing multiplicity. Physics reason? Detector reason? RFF FF
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 20 TofMult. Most of the tof hits are not associated with global track.
2012/7/23 Shuai Yang, STAR Regional Meeting, Weihai 21 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.