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Overview of 20 GeV Au+Au Analysis at RHIC

Detailed analysis of 20 GeV Au+Au collisions at RHIC, covering details, methods, efficiency correction, and results including yields, ratios, and more.

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Overview of 20 GeV Au+Au Analysis at RHIC

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  1. Status of 20 GeV Au+Au Analysis • Overview: • 20 GeV run’s details • 2 centrality bins analyzed • PID method • efficiency correction • results: yields, ratios, dN/dy Roppon Picha 23 Oct 2002 UCD NPG meeting

  2. RHIC Runs Overview Refs: K. Schweda, HS02 RHIC BUR 2002 • Year 2000: • Au + Au @ 130 GeV 2 weeks • p + p @ 130 GeV (3) weeks • Year 2001: (Silicon Vertex Tracker installed) • Au + Au @ 200 GeV 15 weeks • Au + Au @ 20 GeV 1 day • p + p @ 200 GeV 5 weeks • Year 2002: 1st of November • d + Au @ 200 GeV 16 weeks • p + p @ 200 GeV 8 weeks NPG Meeting

  3. 20 GeV Au+Au at STAR • Au+Au at E = 19.6 GeV per nucleon; 0.25 Tesla magnetic field • Trigger: ZDC e&w and CTB > 15 [RHIC BUR 2002] • Details: • Production: /auto/stardata/pDST/flow_pDST_production/reco/minBias/22GeVZDC/ReversedHalfField/P02ge/2001/ • 12 runs: • 2329: 88, 89, 91, 92, 93, 94, 100, 101 • 2330: 2, 3, 4, 5 • # minbias events = 278,452 • # events with (Nch = 0) = 106,250 • # good minbias events = 172,202 NPG Meeting

  4. Selections: Events Vertex locations vY vZ vX |vZ| < 30 (center of TPC, which has uniform acceptance) (vX+0.246)2+(vY-0.378)2 < 1 (eliminates interactions with beam pipe material) NPG Meeting

  5. Selections: Events Multiplicity: 2 centrality bins are studied more central collisions more central collisions 0-10% central: Nch >= 237 10-30% central: 117 <= Nch < 237 NPG Meeting

  6. Selections: Tracks Fit points The distance of closest approach between the track and the main event vertex has a built-in cut at 3 cm to eliminate background protons. The c2 of the helix fit to the found points was limited to 2.5. The total number of points used in the fit was required to be greater than 25 (out of a maximum of 45) to eliminate short tracks. And the fraction of points used in the fit was required to be greater than half the possible number to avoid split tracks. rapidity:|y| < 0.5, 5 bins,bin’s width = 0.2 DCA (cm) Tracking % Chi squared NPG Meeting

  7. Particle ID: finding dE/dx Bethe-Bloch parametrization: Particles to identify: +, -, K+, K-, p, and pbar p- masses : 139.570 MeV K: 493.677 MeV p: 938.272 MeV K- e- - NPG Meeting

  8. PID: Fitting dE/dx distribution • From the parameters, 4 Gaussians are constructed in each mt-m0 slices. • mt-m0 range: 0-1 GeV/c2, divided into 40 slices (bins), bin’s width = 0.025 GeV/c2. • The integrations of the fits then give the raw yields of the particles (d2N/(2pi mt dmt dy) as a function of mt-m0). - e- K- p- NPG Meeting

  9. Efficiency Method: O. Barannikova and J. Romero • Plot pt distribution of Monte-Carlo embedded tracks (mMcTracks.mPtMc) and the reconstructed tracks (mMatchedPairs.mPtMc) from /auto/pdsfdv41/starprod/QA/MiniDst/20GeV/RevHalfField/ • Plot the ratio of the two • Fit the ratio to: eff = m1*exp(-(m2/pt)^m3) Efficiency depends on particle type, rapidity, and impact parameter. The higher the track density, the lower the efficiency. The embedding was run for -, K-, and pbar. We apply the same corrections for their positive counterparts. NPG Meeting

  10. Corrected yields • corrected yield = raw yield/eff • Fit the yields to distribution functions: use Bose-Einstein for pions and kaons (bosons); Fermi-Dirac for protons and antiprotons (fermions) • … obtain dN/dy and temperatures from the fits NPG Meeting

  11. Pion Spectra, Au+Au 20 GeV |y| < 0.1 Au + Au --> + + X Au + Au --> - + X NPG Meeting

  12. Kaon Spectra, Au+Au 20 GeV |y| < 0.1 Au + Au --> K+ + X Au + Au --> K- + X NPG Meeting

  13. p, pbar Spectra, Au+Au 20 GeV |y| < 0.1 low statistics for pbar Au + Au --> p+ + X Au + Au --> p- + X NPG Meeting

  14. Spectra shape 0-10% central h+ h- 10-30% central h+ h- NPG Meeting

  15. dN/dy distributions + K+ p+ • dN/dy increases with centrality, nearly doubles in some cases • more uniform as a function of rapidity in peripheral collisions p- - K- NPG Meeting

  16. Particle ratios • From the yields --> particle ratios can be obtained. The ratios give us basic information about the collision dynamics and the nuclear matter produced. • K/ ratios (not studied here)--> strangeness production (70% of strange quarks go into producing kaons) • K-/K+ ratios --> kaon medium modification • pbar/p ratios --> baryon transport up’s and down’s Refs: STAR Coll., nucl-ex/0206008 F. Wang, nucl-ex/9911004 C. Roy, nucl-ex/0111017 K- u K+ - d - u s + d up’s and down’s u d u d + u s + quarks’ masses u: 1.5 - 4.5 MeV d: 5 - 8.5 MeV s: 80 - 155 MeV p- d K- + 1 Au ion = 79 p, 118 n - NPG Meeting

  17. Ratios: Pions As the lightest mesons, pions are the most abundant particles produced. Unlike K and p ratios, the +/-ratio is not constant over mt-m0. We found a slight excess of negative pions. This is due to the isospin dependence in the branching ratios. There are more neutrons (118+118) than protons (79+79) in a collision. The direct + are produced in pp, while direct - are produced in nn collisions. [J. Klay, Ph.D. thesis 2001] 0-10% The +/-ratio also allows us to calculate the Coulomb potential (q1q2/r). We find that the potential decreases as the energy increases. [D. Cebra, QM 2002] 10-30% * errors omitted NPG Meeting

  18. Ratios: Kaons and Protons K-/K+ pbar/p • both vary very little as a function of rapidity • not a big difference between the two centrality bins NPG Meeting

  19. Ratios: Kaons and Protons Both (us)/(us)and (uud)/(uud) increase with collisions energy, with the kaon’s ratio increasing faster (more s quarks). They approach unity at E = 200 GeV. K-/K+ Sources: E866/917 nucl-ex 0008010 WA97 JPhys G25 (1999) 171 NA44 JPhys G23 (1997) 1865 NA49 NP A661 (1999) 45c NPG Meeting

  20. Summary • yields increase with centrality; similar shape in 2 centrality bins; uniform over |y| < 0.5 • dN/dy increases with centrality; uniform over |y| < 0.5, although some have curious shapes • Both K-/K+ and pbar/p ratios uniform over |y| < 0.5; dramatically increases with energy • K ratio - no strong dependence on impact parameter • pbar/p ratio slightly higher in peripheral collisions * • To do - antiproton absorption in detector material; analysis of other centrality bins; better understanding of the physics involved NPG Meeting

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