Post on 01-Jan-2016
D0 Measurement in Cu+Cu Collisions at √s=200GeV at STAR using the Silicon Inner Tracker (SVT+SSD)
Sarah LaPointeWayne State UniversityFor the STAR CollaborationDNP Meeting, October 24, 2008
• The hot, high density partonic matter produced at RHIC is mostly composed of light quarks.
• Heavy quarks are also produced at RHIC • The most likely production mechanism for charm is gluon fusion in
parton-parton hard scattering.• Thus, charm production occurs in the early stage of the collision,
making charm an attractive probe of the initial conditions.
• Questions:– What is the energy loss (RAA) of the heavy quarks in the partonic
medium?– Does the heavy quark experience a collective motion (v2), similar to
what is observed for the light quarks?
Motivation
Sarah LaPointe 2DNP - Oakland, October 24, 2008
Detection Methods
• D0 K 3.8%• D+/- K 9.2%
Semileptonic Channels• D0 e+ + anything 6.9%• D+/- e+/- + anything 17.2%• B0 e+ + anything 10.9%
Techniques used to measure open charm
• Single electrons• Reconstruction from hadronic
decay
Hadronic ChannelsBranching Ratio
Sarah LaPointe 3DNP - Oakland, October 24, 2008
Single Electrons
STAR measures single electrons in the TPC, TOF and EMC. The combination of all three detectors gives us a pT spectrum from 200 MeV – 10 GeV
Disadvantages:• Missing decay products• Need to reliably subtract photonic electrons and non-photonic background electrons• Challenging to disentangle D and B decays
In principle, single electrons are sensitive to charm and beauty, but relative fraction of b/c uncertain.
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STAR: B. I. Abelev et al., Phys. Rev. Lett. 98 192301 (2007) PHENIX: A. Adare et al., Phys. Rev. Lett. 98 172301 (2007)
M. Cacciari et al., Phys. Rev. Lett. 95 122001 (2005)
Direct Hadron Reconstruction• D0 → K-π+
• Invariant mass analysis
• Visualization of decay vertex not possible because the decay occurs before the tracking detectors
• Reconstruction via combinatorial methods
Oppositely charged tracks are paired and projected toward the primary vertex The two trajectories are compared to see if they cross at some point If so, the tracks are considered candidates for D0 decay.
Sarah LaPointe 5DNP - Oakland, October 24, 2008
The mean lifetime of D mesons is very short.
c for D± is 318 m c for D0 is 126 m Most of the D decay vertices are within a millimeter of the primary vertex.
What is necessary in order to perform direct reconstruction of the D mesons?
• Impact parameter resolution need to be comparable to the decay length
• TPC alone ~ 1.8mm
• TPC+SVT+SSD ~ 200μm (@ 1GeV/c)
• Event vertex resolution
• Currently the TPC yields ~ 2mm
• SVT+SSD yields ~ 90m
Secondary Vertexing Technique using Silicon
Sarah LaPointe 6DNP - Oakland, October 24, 2008
• 13M minimum bias events within |PVz| < 20cm• Require TPC hits ≥ 7• Initial background (B) estimated to be 6.5x109
• Initial signal (S) estimated to be 23,000, take from• D0+D0 = 0.45 in y ±1, 3.8% branching ratio, 80% primary tracking
efficiency, 40% Si hit matching efficiency in CuCu.
• Significance can be defined as • The initial S and B give a significance of 0.2
Cu+Cu 200GeV
• In order to obtain a reasonable significance one must suppress the background while retaining the signal. Look at:• Geometrical Cuts• PID cuts, dE/dx
Sarah LaPointe 7DNP - Oakland, October 24, 2008
BS
S
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Geometrical Distributions
― Pythia D0― Cu+Cu Data
D0 dca to PV
Pos dca to PV Decay Length
Dca daughters
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Cu+Cu 200GeV, with Geometrical Cuts
Applying the following cuts:
• TPC hits ≥ 15• SVT hits ≥ 1• If SVT hits =1, SSD hit = 1• pK,π ≥ 200MeV/c• Decay Length < 500μ• D0 dca PV ≤ 300μ• DCA daughters ≤ 400μ• Daugs DCA PV ≤ 400• |nSigmaPion| < 2.5• |nSigmaKaon| < 2.5
• Cu+Cu background is reduced by a factor of 270. • By looking at a Pythia sample of D0, one finds that the
signal is reduced by a factor 6.• Significance = 0.5
Sarah LaPointe 9DNP - Oakland, October 24, 2008
PID using dE/dx• Kaons and pions can be chosen
via PID from energy loss in the TPC.
• dE/dx is momentum dependent
• |nSigmaPion| < 1.5• |nSigmaKaon| < 1.5• For daughter track with p <
900MeV/c • |nSigmaPion| < 0.75• |nSigmaKaon| < 0.75
• Background is reduced by a factor 340.
• Signal reduction can not be estimated because Pythia contains no dE/dx information
• At best the significance = 0.7
Cut
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Cu+Cu 200GeV results
PID Cuts• |nSigmaPion| < 1.5• |nSigmaKaon| < 1.5• For p < 900Mev/c
• |nSigmaPion| < 0.75• |nSigmaKaon| < 0.75
• CuCu background is reduced by a factor of 6225. • By looking at a Pythia sample of D0, one finds that the signal is reduced by a
factor 77.• Significance = 2.0
• TPC hits ≥ 25• SVT hits ≥ 1• If SVT = 1, SSD hit =1• pK,π ≥ 200MeV/c• Decay Length < 500μ, > 100μ• D0 dca PV ≤ 300μ• Daugs DCA PV ≤ 400μ, ≥ 100μ• DCA daughters ≤ 50μ
After a some further optimization:
STAR Preliminary
Sarah LaPointe11
DNP - Oakland, October 24, 2008
Background Suppression and Future Plans• Further momentum dependent optimization of dE/dx cuts
• Cosθdecay (successful cut for D* in p+p)
• Previously this cut biased my sample• The cause of the bias may be due to the method I use to define my bkg.• Further investigation of this in needed.
• Reconstruction of D± from K-π+π+
• B.R. 9.5%
• Mean lifetime ~ 318 μm
• AuAu Data • 75M minimum bias events• 2 D0+D0 in y ±1
• Increase in SVT hit matching eff., from 40% → 80%
D+ Pythia
Sarah LaPointe 12DNP - Oakland, October 24, 2008
Backup Slides
Sarah LaPointe 13DNP - Oakland, October 24, 2008
Signals and Background (all candidates)
• To claim a D0 signal (S) one looks at the Kπ invariant mass spectrum for excess candidates in a window around the D0 mass (1.864 GeV/c2)
• There is also background in that region. I currently estimate the background by fitting a polynomial function outside the window that sits around the D0 peak.
• It is reasonable, for the CuCu data set to require the statistical significance, σ to be greater than 4 (currently the mixed event technique gives σ=4.3).
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BS
S
Sarah LaPointe 14DNP - Oakland, October 24, 2008