1. 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

2. 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 –m t -m 0 spectra - dN/dy distributions –Particle ratios - rapidity and centrality dependence –Comparisons with SPS 17.2 GeV results Summary

3. 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 (25 th ) to 06:26am (26 th )

4. 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. 5 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 ZDC Trigger (147953) CTB Trigger (27513) Nch Counts

6. 6 Centrality Centrality determination is done offline by selecting on the charged particle multiplicity. 20-GeV N ch 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. 0-10%10-30%30-50% 50-70% 70-100%

7. 7 PID by dE/dx 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) h+, |  | < 0.5 * only 3 free parameters, in most cases

8. 8 Efficiency corrections Efficiency embedding: Eric Hjort Method: Olga Barannikova 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 (r curv ~ p/B) and don’t reach the CTB. p t distributions of embedded and reconstructed tracks The ratio is the efficiency.

9. 9 Background protons corrections Interactions between fast pions and detector material generate secondary protons. background protons have low momenta, are negligible for m t -m 0 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. 10 Transverse mass spectra ++ K+ p+ pbarK- -- thermal fits: Bose-Einstein statistics for pions and kaons Fermi-Dirac for protons blast wave fit parameters: T th = 103 +/- 4 MeV  r = 0.47 +/- 0.04 c (M. Kaneta, Jul 14, 2002)

11. 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. 12 Comparison to published spectra STAR 19.6 GeV Data NA49 (5% - 10%)  -, K’s PRC66, 054902 (2002) p,p-bar private comm. NA44 (top 10%) PLB388, 431 (1996) WA98:  - NPA698, 647 (2002)

13. 13 Average p t From the models, we can approximate midrapidity *: O. Barannikova AuAu 200 GeV Mean p t rises with centrality, particle’s mass, and beam energy. * calculated from sqrt( 2 -mo 2 ) AuAu 20 GeV

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

15. 15 dN/dy comparisons to published data STAR (top 10%) new result NA49 (top 5%)  -, K’s PRC66, 054902 (2002)  +,p,p-bar NPA661, 45 (1999) NA44 (~top 10%) nucl-ex/0202019 (2002) There is good agreement between the new results and the published yields

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

17. 17 K/  ratios K/  (s NN ) 1/2 K+/K+/ K-/K-/ E917 NA49 NA44 BRAHMS There is very different behavior between the positive and negative K/  ratios. K+/  + ratios in p+p follow the trend of the K-/  -. PHENIX 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

18. 18 Kaon and Proton Ratios Results are consistent with expectations STAR Preliminary results 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.     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. 19 Particle ratios: pions  +/  - ratios show Coulomb effect: with positively-charged source,  + get extra kick, whereas  - are pulled toward the source.

20. 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) R446WA98 QM2001 KaoS PLB420 (1998) 20NA44 1996

21. 21 dN/d  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 WA98 WA97/NA57 Phobos NA49 E917/866 STAR PRELIMINARY RESULT E877 PHENIX BRAHMS Phobos 19.6 GeV Result - QM2002

22. 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.