D 0 Measurement in Cu+Cu Collisions at √s=200GeV at STAR using the Silicon Inner Tracker (SVT+SSD)...

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


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.


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


• 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


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


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


Backup Slides


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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D 0 Measurement in Cu+Cu Collisions at √s=200GeV at STAR using the Silicon Inner Tracker (SVT+SSD)...

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