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Mid-rapidity charged hadron spectra in the Au+Au collisions at sqrt(s NN ) = 19.6 GeV at STAR

Mid-rapidity charged hadron spectra in the Au+Au collisions at sqrt(s NN ) = 19.6 GeV at STAR. University of California, Davis for the STAR Collaboration. Daniel Cebra Roppon Picha, Juan Romero Trieu Mai, Mark Allen, Nathan Farr, David Cherney. Overview. 19.6-GeV Introduction Methods:

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Mid-rapidity charged hadron spectra in the Au+Au collisions at sqrt(s NN ) = 19.6 GeV at STAR

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  1. Mid-rapidity charged hadron spectra in the Au+Au collisions at sqrt(sNN) = 19.6 GeV at STAR University of California, Davis for the STAR Collaboration Daniel Cebra Roppon Picha, Juan Romero Trieu Mai, Mark Allen, Nathan Farr, David Cherney

  2. Overview • 19.6-GeV Introduction • Methods: • Event selection - centrality determination - Track select. • PID - Bethe-Bloch, dE/dx vs ,Gaussian fits • Efficiencies - Proton background - p-bar absorbtion • Results • mt-m0 spectra - dN/dy distributions • Particle ratios - rapidity and centrality dependence • Comparisons with SPS 17.2 GeV results • Summary

  3. STAR Time Projection Chamber The 19.6-GeV was run in Nov 2001*: • Magnetic field = 0.25 T (half field) • Gas: P10 (90% Ar + 10% CH4) at ~ 1 atm • Detectors: SVT, TPC, FTPCs • Trigger: • Minbias (CTB > 15, ZDC e-w coinc.) • Central (CTB > 600) • Total Event Pool: • 175466 events * from 09:15am (25th) to 06:26am (26th)

  4. Event selection • Event cuts: • vertex X, vertex Y: r < 1 cm (over 95% pass) • |vertex Z| < 30 cm (over 25% pass) • 43,131 events pass vertex cuts • Track cuts: • global DCA < 3 cm • fit points > 24 • 0.51 <= fit points/max points <= 1.05 • chi-squared of fit <= 2.5

  5. ZDC Trigger (147953) Counts CTB Trigger (27513) Nch Trigger • ZDC vs. CTB observations: • not a “boomerang” shape as in 130 and 200 GeV (20 GeV spectators are more dispersed, not many neutrons make it to the ZDCs.) • the discontinuity around CTB=1300 comes from the trigger criteria

  6. Centrality • Centrality determination is done offline by selecting on the charged particle multiplicity. • 20-GeV Nch distribution shape is scaled to those from other RHIC beam energies. • 130 and 200 GeV have the same shape. 20-GeV shows trigger inefficiencies in the 10-30% and 70-100% centrality bins. 50-70% 70-100% 30-50% 10-30% 0-10%

  7. PID by dE/dx h+, || < 0.5 • dE/dx is described by the Bethe-Bloch function • We use a 7-parameter* Bethe-Bloch function to calibrate the dE/dx centroids as a function of  (= p/m) • Distributions of ln(dE/dx) are fitted by 4 Gaussians for , K, pr, and e. (with fixed centroids and widths) * only 3 free parameters, in most cases

  8. Efficiency corrections Reconstruction efficiency is plotted as a function of the transverse momentum. The efficiency drops sharply below 0.15 GeV/c because the low-momentum particles stay inside the TPC (rcurv ~ p/B) and don’t reach the CTB. pt distributions of embedded and reconstructed tracks The ratio is the efficiency. Efficiency embedding: Eric Hjort Method: Olga Barannikova

  9. Background protons corrections • Interactions between fast pions and detector material generate secondary protons. • background protons have low momenta, are negligible for mt-m0 above 0.3 GeV/c2. • We find the ratio of background/total proton inside 0-3 cm DCA. And use this to calculate real proton yields. Method of Schweda PRL 86, 4778 (2003)

  10. Transverse mass spectra p+ + K+ thermal fits: • Bose-Einstein statistics for pions and kaons • Fermi-Dirac for protons - K- pbar blast wave fit parameters: Tth = 103 +/- 4 MeV r = 0.47 +/- 0.04 c (M. Kaneta, Jul 14, 2002)

  11. Transverse mass spectra • comparison between different particles • Low antiprotons produced at this energy • Protons yields are comparable to kaons • Similar inverse slopes for all particles

  12. Comparison to published spectra STAR 19.6 GeV Data NA49 (5% - 10%) p- , K’sPRC66, 054902 (2002) p,p-bar private comm. NA44 (top 10%) PLB388, 431 (1996) WA98: p- NPA698, 647 (2002)

  13. Average pt • From the models, we can approximate midrapidity <pt>*: AuAu 20 GeV O. Barannikova AuAu 200 GeV Mean pt rises with centrality, particle’s mass, and beam energy. * calculated from sqrt(<mt>2-mo2)

  14. Midrapidity dN/dy p + K+ The extracted rapidity densities are consistent with the values from the SPS. K- - pbar Note: NA49 results are the averages over |y| < 0.6

  15. dN/dy comparisons to published data There is good agreement between the new results and the published yields STAR (top 10%) new result NA49 (top 5%) p- , K’sPRC66, 054902 (2002) p+,p,p-bar NPA661, 45 (1999) NA44 (~top 10%) nucl-ex/0202019 (2002)

  16. Particle ratios Particle ratios tell us about the quark contents. top 10% central top 10% central K-/K+ and pbar/p flat over mt-mo pbar/p drops as collisions become more central, suggesting pbar absorption within nuclear material. no strong centrality dependence in K-/K+

  17. K/p ratios There is very different behavior between the positive and negative K/p ratios. K+/p+ ratios in p+p follow the trend of the K-/p-. NA44 K+/p+ PHENIX E917 BRAHMS NA49 K/p STAR 130 nucl-ex 02008 (2003) NA49 PRC66, 054902 (2002) NA44 nucl-ex 0202019 (2002) E917 PLB476, 1 (2000) PHENIX PRL88, 242301 (2001) BRAHMS QM2002 K-/p- (sNN)1/2

  18. Kaon and Proton Ratios Results are consistent with expectations Proton and kaon ratios for central events both increase with collisions energy. This illustrates the reduction in the net baryon content and/or the baryon chemical potential across this collision energy range from the AGS to the top energies at RHIC. At 19.6 GeV the collisions produced Baryon-rich matter. K-/K+ STAR Preliminary results E866/917 nucl-ex 0008010 WA97 JPhys G25 (1999) 171 NA44 JPhys G23 (1997) 1865 NA49 NP A661 (1999) 45c STAR PRL 86, 4778 (2003) STAR

  19. Particle ratios: pions +/- ratios show Coulomb effect: with positively-charged source, + get extra kick, whereas - are pulled toward the source.

  20. Pion Ratios - Imaging the Source A comparison of the pi+/pi- ratios at different energies. From this comparison, we can find the Coulomb potential assuming a spherically symmetric geometry. As can be seen, the potential decreases as the collision energy increases. This is evidence of the depletion of the initial baryons in the central region. E866 PRC57 (1998) R446 WA98 QM2001 KaoS PLB420 (1998) 20 NA441996

  21. STAR PRELIMINARY RESULT PHENIX BRAHMS WA98 Phobos WA97/NA57 NA49 E917/866 E877 dN/dh Collision Energy Systematic The charge particle yield at mid-rapidity for central events (top 5 or 6%) is consistent with the trends established by AGS, SPS, and higher energy RHIC data. Phobos PRL 85 (2000) 3100 Phobos nucl-ex 018009 (2002) PHENIX PRL 86 (2001) 3500 STAR(130) nucl-ex 106004 (2001) BRAHMS QM2001 NA49 WA98 nucl-ex 0008004 (2000) WA97/NA57 CERN-EP-2000-002 E866/917 PRC59 (1999) 2173 E877 PRC51 (1995) 3309 Phobos 19.6 GeV Result - QM2002

  22. Summary • Thermal models fit spectra of pions and kaons well; protons show visible peaks near 0.1 GeV/c2 -- a blast wave effect. • Flat rapidity distributions indicate incomplete stopping within |y| < 0.5. • Au+Au at 19.6 AGeV - not baryon free. • K-/K+ ~ 0.6, no strong dependence on centrality or rapidity • pbar/p ratio ~ 0.1 • decreases in more forward rapidity bins • decreases with centrality • Both kaon and pbar/p ratios increase with beam energy; our results consistent with the trend. • Future: more comparisons to SPS results; higher rapidity results for pions • Overall, the results confirm the consistency between RHIC and SPS apparatus.

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