D 0 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

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 (R AA ) of the heavy quarks in the partonic medium? – Does the heavy quark experience a collective motion (v 2 ), similar to what is observed for the light quarks? Motivation Sarah LaPointe2DNP - Oakland, October 24, 2008

Detection Methods D 0  K  3.8% D +/-  K   9.2% Semileptonic Channels D 0  e + + anything6.9% D +/-  e +/- + anything 17.2% B 0  e + + anything 10.9% Techniques used to measure open charm Single electrons Reconstruction from hadronic decay Hadronic Channels Branching Ratio Sarah LaPointe3DNP - 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 p T 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. 4 STAR: B. I. Abelev et al., Phys. Rev. Lett (2007) PHENIX: A. Adare et al., Phys. Rev. Lett (2007) M. Cacciari et al., Phys. Rev. Lett (2005)

Direct Hadron Reconstruction D 0 → 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 D 0 decay. Sarah LaPointe5DNP - Oakland, October 24, 2008

The mean lifetime of D mesons is very short. c  for D ± is 318  m c  for D 0 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 Secondary Vertexing Technique using Silicon Sarah LaPointe6DNP - Oakland, October 24, 2008

13M minimum bias events within |PVz| < 20cm Require TPC hits ≥ 7 Initial background (B) estimated to be 6.5x10 9 Initial signal (S) estimated to be 23,000, take from D 0 +D 0 = 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 LaPointe7DNP - Oakland, October 24, 2008

Geometrical Distributions ― Pythia D0 ― Cu+Cu Data D 0 dca to PV Pos dca to PV Decay Length Dca daughters 8

Cu+Cu 200GeV, with Geometrical Cuts Applying the following cuts: TPC hits ≥ 15 SVT hits ≥ 1 If SVT hits =1, SSD hit = 1 p K,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 D 0, one finds that the signal is reduced by a factor 6. Significance = 0.5 Sarah LaPointe9DNP - 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 10

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 By looking at a Pythia sample of D 0, 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 p K,π ≥ 200MeV/c Decay Length 100 μ D0 dca PV ≤ 300μ Daugs DCA PV ≤ 400μ, ≥ 100 μ DCA daughters ≤ 50 μ After a some further optimization: STAR Preliminary Sarah LaPointe 11 DNP - Oakland, October 24, 2008

Background Suppression and Future Plans Further momentum dependent optimization of dE/dx cuts Cosθ decay (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 D 0 +D 0 in y ±1 Increase in SVT hit matching eff., from 40% → 80% D+ Pythia Sarah LaPointe12DNP - Oakland, October 24, 2008

Backup Slides Sarah LaPointe13DNP - Oakland, October 24, 2008

Signals and Background (all candidates) To claim a D 0 signal (S) one looks at the Kπ invariant mass spectrum for excess candidates in a window around the D 0 mass (1.864 GeV/c 2 ) There is also background in that region. I currently estimate the background by fitting a polynomial function outside the window that sits around the D 0 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). Sarah LaPointe14DNP - Oakland, October 24, 2008