Sarah LaPointe Wayne State University For the STAR Collaboration DNP Meeting, October 24, 2008

1. D0 Measurement in Cu+Cu Collisions at √s=200GeV at STAR using the Silicon Inner Tracker (SVT+SSD) Sarah LaPointe Wayne State University For the STAR Collaboration DNP Meeting, October 24, 2008

2. Motivation • 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? DNP - Oakland, October 24, 2008

3. Detection Methods Hadronic Channels Branching Ratio • 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 DNP - Oakland, October 24, 2008

4. 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 STAR: B. I. Abelev et al., Phys. Rev. Lett.98 192301 (2007) PHENIX: A. Adare et al., Phys. Rev. Lett.98 172301 (2007) In principle, single electrons are sensitive to charm and beauty, but relative fraction of b/c uncertain. M. Cacciari et al., Phys. Rev. Lett.95 122001 (2005)

5. 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. DNP - Oakland, October 24, 2008

6. Secondary Vertexing Technique using Silicon 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 ~ 90m DNP - Oakland, October 24, 2008

7. Cu+Cu 200GeV • 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 • In order to obtain a reasonable significance one must suppress the background while retaining the signal. Look at: • Geometrical Cuts • PID cuts, dE/dx DNP - Oakland, October 24, 2008

8. Geometrical Distributions D0 dca to PV Dca daughters ― Pythia D0 ― Cu+Cu Data Pos dca to PV Decay Length

9. 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 DNP - Oakland, October 24, 2008

10. PID using dE/dx • Kaons and pions can be chosen via PID from energy loss in the TPC. • dE/dx is momentum dependent Cut • |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

11. Cu+Cu200GeV results After a some further optimization: • 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 • PID Cuts • |nSigmaPion| < 1.5 • |nSigmaKaon| < 1.5 • For p < 900Mev/c • |nSigmaPion| < 0.75 • |nSigmaKaon| < 0.75 STAR Preliminary • 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 DNP - Oakland, October 24, 2008

12. 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 DNP - Oakland, October 24, 2008

13. Backup Slides DNP - Oakland, October 24, 2008

14. 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). DNP - Oakland, October 24, 2008