1. Charmed Meson measurements using a Silicon Tracker in Au+Au collisions at √S NN = 200 GeV in STAR experiment at RHIC Jaiby Joseph Ajish 11/2/2011

2. Outline Quick summary of my contributions Introduction Why collide nuclei at high energies? RHIC, STAR Physics at RHIC Important observations Heavy quark sector Charm measurement using Silicon Tracker Secondary vertexing Strategy of reconstruction Proof of principle with K s 0 Results Future 2

3. My contributions Charm Analysis  debugging the micro-vertexing code: - QA of the reconstructed parameters, fixing problems with dE/dx cut, and resolution studies.  Detailed Monte Carlo studies for online/offline cut optimization  Productions of Micro DSTs and Pico-DSTs  First observation charmed meson signal in real data (from 2007 Au+Au dataset)  Signal extraction, optimization, fitting, p T binning  Embedding QA, Study of Systematics and Physics Analysis Service Work  acceptance of D-mesons with a prototype design for the HFT upgrade 3

4. Why collide nuclei at high energies? Quantum Chromo Dynamics (QCD) is the theory of strong interactions. QCD provides us with 2 important characteristics of quark-gluon interactions (1) Asymptotic freedom – High energies, weakly interacting quarks and gluons (2) Confinement – No free quarks have been observed Collisions of heavy ions at relativistic speeds creates extreme temperatures/densities: Nuclear Matter Quark-Gluon Plasma (deconfined partonic matter) Lattice QCD predicts the phase transition at: Study the Strong Interaction at high temperatures/densities Understand how matter behaved at the dawn of the Universe Create and study the properties of the Quark-Gluon Plasma (QGP) phase of nuclear matter. Phase Diagram Net Baryon Density T c ~ 150 -170 MeV and ρ ~ 1GeV/fm 3 4

5. RHIC BRAHMS PHOBOS PHENIX STAR AGS TANDEMS 1 km Relativistic Heavy Ion Collider (RHIC) 5

6. RHIC Collisions Collision systems used at RHIC are: Au+Au, Cu+Cu, d+Au and p+p at different energies (7.7 GeV to 500 GeV for p+p) Initial Conditions Initial high Q 2 interactions Partonic matter QGP Hadronization Freeze-Out STAR Detector view of the event 6

7. STAR Detector (in 2007) The tracking system consisted of :  TPC : provides momentum, particle identification  Silicon detectors :  1 layer of silicon strip detectors (SSD) and 3 layers of silicon drift detectors (SVT).  higher spatial resolution : pointing resolution of 250µm in transverse direction (at 1GeV) was achieved (see below). 7

8. Scattered partons propagate through matter radiate energy (~ GeV/fm) in colored medium interaction of parton with partonic matter suppression of high p t particles aka “parton energy loss” or “jet quenching” suppression of angular correlation vacuum QGP Hard Parton Scattering Jets and mini-jets (from hard-scattering of partons)  30 - 50 % of particle production  high p t leading particles  azimuthal correlations Extend into perturbative regime Calculations reliable hadrons q q leading particle leading particle schematic view of jet production Physics @ RHIC New with Heavy Ions

9. Physics @ RHIC Important observations (Light flavors) Partonic Energy Loss Medium created at RHIC has very high opacity In central Au+Au collisions the light hadrons in away-side jets are suppressed. Not the case in p+p and d+Au In addition, a measurement of energy loss of high p T partons using R AB shows significant suppression partons lose energy via gluon radiation 9 Nuclear modification factor R AA → energy loss in partonic mater R AA = (A-A p T spectra)/(p-p p T spectra * “volume”)

10. Physics @ RHIC – Heavy Quark Sector Heavy flavor is produced at the earlier stages of the collision via gluon fusion :  not affected by chiral symmetry restoration (i.e. mass is the same in/out of medium)  production cross-section found to binary scale ideal to probe the medium created in heavy ion collision Theoretical models predicted gluon radiative energy loss for heavy quarks to be smaller than of light quarks, which is not experimentally observed. Measuring collective motion (v 2 ) of charm mesons will indicate whether thermalization is reached in the earlier steps of the collision. There are unresolved charm cross-section discrepancies between STAR and PHENIX 1) Non-photonic electrons (NPE) Method - decayed from charm and beauty hadrons 2) At p T ≥ 6 GeV/c, R AA (NPE) ~ R AA (h ± ) !!! 3) Surprising Results: contradicts pQCD predictions challenges our understanding of the energy loss mechanism Needs Direct measurement of D and B mesons 10

11. Measurement using Silicon Vertex Detector and decay vertex fit ✔ In order to enhance physics capabilities, STAR used a 1-layer silicon strip (SSD) and 3-layer silicon drift (SVT) detectors which are placed inside the TPC. ✔ Full operation in year 2005 and 2007 ✔ Was not designed (thickness, geometry) for charm measurement ✔ Full reconstruction/fit of the decay vertex by combining K and π tracks – Some particle ID capabilities obtained from TPC Poor PID Caveats: 1.Very short lived particles: for a realistic D 0 distribution average decay-length at ~ 1 GeV/c is 60-70 μm 2.Marginal resolution: At ~ 1GeV/c, the resolution achieved with hits on all silicon layers is ~ 250 μm 3.Poor PID: For p > 0.7GeV/c, the K, π bands overlap giving rise to large combinatorial background. dE/dx bands of Kaon, Pion 11

12. D 0 Decay Topology ✿ Full reconstruction/fit of the decay vertex ✿ Introduction/Use of full track error matrix for best error estimates ✿ Optimization of cuts based on MC studies ✿ Better resolution in secondary vertex position is achieved with the fit method compared to usual helix swimming methods. 12 Mean of the difference reconstructed -MC Rms of the difference reconstructed -MC Reco - MC [cm]

13. Reconstructed Quantities (example) (MC Data (pure D 0) Events) Invariant Mass Reconstructed pT Resolution Decay Vertex Resolution ✿ Resolution : Inv Mass ~ 13 MeV (0.7%, after a gauss fit) Trans. Momentum ~ 17 MeV Decay Vertex Coordinates ~ 220 μm (transverse) ~ 200 μm (z-direction) ✿ The reconstructed parameters behave as expected with the current detector resolution. 13

14. 14 After cut  Test with K 0 s decay reconstruction : K 0 S  π + π - (BR = 69.2%) ; c  = 2.68 cm ; Mass = 0.497 MeV/c 2  Signed decay length : – an excess can be observed on the positive side of the decay length distribution, indicating the presence of long-lived decays. – use the decay length significance L/  L to improve the signal. – more appropriate because of the momentum dependence of the decay length. Proof of principle with K 0 s Before cut  After using a cut S L > 10, a clear peak at the K 0 S mass is observed. background Signal+background

15. D 0 +D 0 bar Signal (in 2007 Data) 24 Million Au+Au @ 200 GeV/c events are used for this analysis. 3 rd degree polynomial fit is used for background estimation. Kinematic fit yields an improved signal of 10-σ for combined D 0 +D 0 bar signal. Signal remains stable as cuts are varied. Pol3 + gaus Pol3 Gaussian Mean = 1864.19 ± 10 MeV 15 gaus fit

16. Invariant Mass of D 0 and D 0 bar separately D 0 bar/D 0 Ratio ~ 1.18 ± 0.24 Statistical thermal models predict vanishing baryonic chemical potential (μ B ) at RHIC energies ( ) The D 0 bar/D 0 ratio obtained here is compatible with unity indicating a vanishing μ B. 16

17. Attempts to extract physics ✿ Uncorrected pT spectra: ✿ A normalized p T Spectra corrected for acceptance and efficiency is used to: - extract total charm cross-section, freeze out parameters etc. - calculate energy loss R AA ✿ At this time, we do not have a proper embedding sample to do corrections - sample has too few Silicon hits 17 Some of the results with a polynomial background estimate seem to be inconsistent. A robust background estimation method needed to see if the peak observed was an artifact – a “same sign” background subtraction method was performed ✿ The ratio of central to MB yield in |y|<0.5, scaled by the number of binary collisions: ✿ A ratio 1 is expected ✿ results from polynomial fit background is inconsistent with the binary collision scaling of charm.

18. 18 The fact that about a third of the SVT/SSD system was dead during Run-7, combined with the marginal resolution of the previous generation silicon detector and combinatorial background limits our efforts. A final effort to measure the signal using a multivariate analysis is in progress. Same cuts are used to produce this picture that were used in the polynomial fit case An explanation for the non-consistent physics results

19. Ongoing Analysis with Multivariate Analysis (TMVA) ✿ TMVA is a ROOT integrated machine learning technique. It uses classifiers to discriminate signal from background. ✿ We used the Boosted Decision Tree (BDT) classifier ✿ Training samples for signal (pure D 0 ) and background (`same sign’) are provided. It will produce a classifier output with weight files for signal and background. ✿ After training, testing can be done with Data sample (MC Embedding/Real) MC D 0 Embedding 2007 Au+Au Data (1-2% of available data) ✿ Preliminary results looks promising, work in progress to run over the whole data – which will be the final phase of this analysis. 19

20. Recent charm measurements with Time Of Flight (TOF) Detector STAR Time Of Flight (TOF) detector provides better particle ID – measure particle velocity β dE/dx + TOF offers excellent K, π separation up to p ~ 1.5 - 2 GeV/c New results use ~ 250 Million Au+Au Events from year 2010 and p+p events from 2009 Corrected p T Spectrum and R AA in AuAu ✿ Charm cross section shows scaling with number of binary collisions indicating charm production via initial hard scattering ✿ Suppression of charmed meson observed around ~ 4 GeV/c D 0 and D* in p+p Charm Cross Section 20

21. Future Heavy Flavor Tracker (HFT) STAR is undergoing a detector upgrade for the unambiguous measurement of charm – The Heavy Flavor Tracker (HFT) Key Measurements of HFT include: (1) Rcp (2) Elliptic flow, v 2 (3) Charmed Baryon to Meson Enhancement The method developed here is a baseline for analysis involving the Heavy Flavor Tracker (HFT) 21

22. Thank you 22

23. Back-Up 23

24. Distance of Closest Approach resolution STAR preliminary  Including the silicon detectors in the tracking improves the pointing resolution.  with 4 silicon hits, the pointing resolution to the interaction point ~ 250 μm at P = 1GeV/c. run 7 Au+Au@200GeV (MinBias trigger). DCA resolution as a function of inverse momentum. Reflect the resolution and Multiple Coulomb Scattering. 24

25. Secondary Vertex fit – Simulation ✿ There is no systematic shift in reconstructed quantities. ✿ The standard deviation of the distribution is flat at ~ 250  m, which is of the order of the resolution of (SSD+SVT). Reco vs. MC [cm] Mean of the difference reconstructed -MC Rms of the difference reconstructed -MC Reco - MC [cm] 25

26. Strategy of Reconstruction Select Event – Apply Event Level Cuts Select Trigger Cuts on Z-Vertex Position and its error Loop over Tracks – Apply Track Level Cuts Number of Silicon Hits Transverse DCA (DCA XY ) Track Momentum etc. Pair Association - D 0 Candidate Level Cuts rapidity, Cosine of Kaon decay angle etc. Decay Vertex Fit – Decay fit Level Cuts probability of fit, decay length error of decay length etc. Particle Identification – Apply PID Cuts |nσ K |, |nσ π | Cuts are applied in the analysis code to reduce background and to increase the candidate pool Output Saved for offline Analysis 26

27. Measurement via Semi leptonic (indirect) channels Indirect measurement through Semi-leptonic decay channels: D 0  e + + X (BR : 6.9 %) D +/-  e +/- + X (BR : 17.2%) ✔ Large p T range. ✔ Use of specific triggers ✔ Relative contribution of electrons from B and D mesons are unknown. 27 Measurement using azimuthal correlation of D mesons with e - Azimuthal correlation of open charm mesons with non-photonic Electron can be utilized to disentangle the charm and bottom contributions

28. Measurement via hadronic (direct) channels Direct measurement using a combinatorial method –Measurement of hadronic decay modes via invariant mass analysis. D 0 (D 0 )  K -  + (K +  - ) BR : 3.8 % D +/-  K  BR : 9.2% Results using STAR Time-Of- Flight (TOF) Detector (TOF+TPC offers better PID) TPC Only (Low p T ) ✔ C and B contributions separated. ✔ Limited to low momentum range. ✔ No triggers, no decay vertex reconstruction ✔ Challenging for charm mesons due to small decay length 28

29. Cuts in 1st Production 2007 Production MinBias Cuts in 2nd Production EVENT level triggerId : 200001, 200003, 200013 Primary vertex position along the beam axis : |zvertex| < 10 cm Resolution of the primary vertex position along the beam axis: |  zvertex |< 200µm TRACKS level Number of hits in the vertex detectors : SiliconHits>2 (tracks with sufficient DCA resolution) Transverse Momentum of tracks: p T >.5GeV/c Momentum of tracks: p >.5GeV/c Number of fitted: TPC hits > 20 Pseudo-rapidity : |  |<1 (SSD acceptance) dEdxTrackLength>40 cm DCA to Primary vertex (transverse), DCA xy <.1 cm EVENT level triggerId : 200001, 200003, 200013 Primary vertex position along the beam axis : |zvertex| < 10 cm Resolution of the primary vertex position along the beam axis: |  zvertex |< 200µm TRACKS level Number of hits in the vertex detectors: SiliconHits>1 Transverse Momentum of tracks: p T >.5GeV/c Momentum of tracks p >.8GeV/c Ratio TPC hits Fitted/Possible > 0.51 Pseudo-rapidity : |  |<1.2 dEdxTrackLength>40 cm DCA to Primary vertex (transverse), DCA xy <.2 cm Radius of first hit on track : < 9 cm if number of silicon hits =2 < 13 cm else cut changed new cut 29

30. Cuts from Previous production Continued.. Cuts in New Production DECAY FIT level Probability of fit >0.1 && |sLength|<.1cm Particle ID : ndEdx : |n  K |<2, |n  π |<2 D 0 candidate |y(D 0 )|<1 |cos(  *)|<0.8 DECAY FIT level Probability of fit >0.01 && |sLength|<.1cm Particle ID : ndEdx : |n  K |<2.5, |n  π |<2.5 ndEdx : |n  K |<2, |n  π |<2 |cos(  *)|<0.6 DCA daughters < 300 µm In both productions we made a pico file for further analysis. Cuts Used for making a pico file Previous Production New Production |D0Eta|<1.85 |Cos(θ*)<0.6 30

31. Uncorrected pT Spectra 31

32. Physics @ RHIC Important observations (Light flavors) Partonic Collectivity Partonic Energy Loss Behaves like an ideal fluid Medium created at RHIC has very high opacity In central Au+Au collisions the light hadrons in away-side jets are suppressed. Different for p+p and d+Au In addition, a measurement of energy loss of high pT partons using R AB shows significant suppression partons lose energy via gluon radiation Substantial elliptic flow (v 2 ) signal observed for a variety of particle species. Rapid Thermalization v 2 scaled by the number of valance quarks shows an apparent scaling Development of anisotropy in the partonic stage of collision 32

33. ‘Same sign’ background subtraction 33

34. Heavy Quark Energy Loss Puzzle – NPE Method Surprising results - - challenge our understanding of the energy loss mechanism - force us to RE-think about the elastic-collisions energy loss - Requires direct measurements of c- and b-hadrons. 1) Non-photonic electrons (NPE) decayed from - charm and beauty hadrons 2) At p T ≥ 6 GeV/c, R AA (NPE) ~ R AA (h ± ) !!! Contradicts naïve pQCD predictions STAR: Phys. Rew. Lett, 98, 192301(2007) and nucl-ex/0607012v3 Still the main method at RHIC 34

35. Charm Cross-Section Comparison at 200 GeV NLO Ref: R. Vogt, arXiv:0709.2531v1 [hep-ph] STAR and PHENIX do not agree about total charm production x-section Need precise, exclusive measurements

36. Measurement via Semi leptonic (indirect) channels Indirect measurement through Semi-leptonic decay channels: D 0  e + + X (BR : 6.9 %) D +/-  e +/- + X (BR : 17.2%) ✔ Large p T range. ✔ Relative contribution of electrons from B and D mesons are unknown. ✔ Use of specific triggers Measurement using azimuthal correlation of D mesons with e - Azimuthal correlation of open charm mesons with non-photonic Electron can be utilized to disentangle the charm and bottom contributions[3] 36 ✔ Triggers on high p T electrons Any information from direct reconstruction of D and B-mesons would help