Measurement Of Charged Antiparticle To Particle Ratios by the PHOBOS Experiment at RHIC

1. Measurement Of Charged Antiparticle To Particle Ratios by the PHOBOS Experiment at RHIC Conor Henderson Massachusetts Institute of Technology For the PHOBOS collaboration APS April 2001

2. PHOBOS Collaboration ARGONNE NATIONAL LABORATORY Birger Back, Nigel George, Alan Wuosmaa BROOKHAVEN NATIONAL LABORATORY Mark Baker, Donald Barton, Alan Carroll, Stephen Gushue, George Heintzelman, Robert Pak, Louis Remsberg, Peter Steinberg, Andrei Sukhanov INSTITUTE OF NUCLEAR PHYSICS, KRAKOW Andrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki , Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak MASSACHUSETTS INSTITUTE OF TECHNOLOGY Wit Busza* , Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane , Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Gerrit van Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard Wadsworth, Bolek Wyslouch NATIONAL CENTRAL UNIVERSITY, TAIWAN Willis Lin, JawLuen Tang UNIVERSITY OF ROCHESTER Joshua Hamblen , Erik Johnson, Nazim Khan, Steven Manly, Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs UNIVERSITY OF ILLINOIS AT CHICAGO Russell Betts, Clive Halliwell, David Hofman, Burt Holzman, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter UNIVERSITY OF MARYLAND Richard Bindel, Edmundo Garcia-Solis, Alice Mignerey * Spokesperson APS April 2001

3. Why Measure Antiparticle/Particle Ratios? • Microscopic viewpoint: • Antiproton/proton ratio determined by: • Baryon stopping; • Pair production; • Absorption in nuclear medium • Thermodynamic viewpoint: • Particle ratios can be used to estimate hadro-chemical potentials APS April 2001

4. Paddle Trigger Counter TOF Spectrometer Octagon+Vertex Ring Counters The PHOBOS Detector Au+Au collisions 68,359 events 12% most central APS April 2001

5. The PHOBOS Spectrometer • One arm with 16 layers of Si sensors • Outer layers situated in 2T magnetic field • Coverage near mid-rapidity APS April 2001

6. By x 10 cm 1 2 Particle Tracking In Spectrometer z • Road-following algorithm finds straight tracks in field-free region • Curved tracks in B-field found by clusters in (1/p, ) space • Match pieces by , consistency in dE/dx and fit in yz-plane B-field inverted to obtain antiparticle/particle ratios: e.g. Antiparticles for B+/ Particles for B- APS April 2001

7. Particle Identification • dE/dx in Si sensors depends only on velocity • dE/dx vs. p therefore allows identification of particle • Ratios stable w.r.t. cut variation APS April 2001

8. Corrections to the Raw Numbers • Secondary particles (+2%) • Little material between interaction point and sensitive volume • Antiproton absorption in detector (+8%) • From GEANT simulations • Feed-down from weak decays (-2%) • Reduced by tracking within 10cm of vertex • Further limit by distance-of-closest-approach cut on tracks APS April 2001

9. The Results For pt < ~1 GeV, near mid-rapidity: (Submitted to PRL; preprint hep-ex/0104032) • No strong pt dependence observed APS April 2001

10. K-/K+ and pbar/p significantly higher than at AGS or SPS HIJING over-predicts our data, RQMD under-predicts it. Comparison To Lower Energies and Models APS April 2001

11. Statistical model of Redlich et al. (QM ’01) Assume freeze-out temperature 160–170 MeV Obtain B = 455 MeV SPS, B = 240-270 MeV Estimation Of Baryo-chemical Potential APS April 2001

12. Antiparticle/particle ratios measured near mid-rapidity by PHOBOS silicon spectrometer K-/K+ and pbar/p higher than at AGS or SPS B=45 MeV is lower Summary APS April 2001

13. Spectrometer Acceptance Averaged over vertex range and azimuthal angle APS April 2001

14. Net Baryon Number y HIJING & RQMD Comparison : Baryon Stopping APS April 2001