1. Femtoscopy BNL Workshop 6/21/2005George Stephans Very Low p T in x Past, Present, and Prospects

2. Femtoscopy BNL Workshop 6/21/2005George Stephans Collaboration (March 2005) Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane,Piotr Kulinich, Chia Ming Kuo, Wei Li, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Donald Willhelm, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Shaun Wyngaardt, Bolek Wysłouch ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

3. Femtoscopy BNL Workshop 6/21/2005George Stephans Kinematic Regions at RHIC Au+Au Centrality: 0-15% 200 GeV Mid-rapidity  (   +   )/2 Phys. Rev. C70, 051901(R) (2004)  (h  +h  )/2 PLB 578 (2004) 297 TAIL BULK HEAD Physics Topics: Bulk - Particle production, sets the stage Tail - Jet quenching, probes medium Head - Sensitive to global properties

4. Femtoscopy BNL Workshop 6/21/2005George Stephans Spectrometer The PHOBOS Detector (2001/2002)

5. Femtoscopy BNL Workshop 6/21/2005George Stephans The Low p T Part

6. Femtoscopy BNL Workshop 6/21/2005George Stephans The Technique  Simple in principle, Si gives dE/dx which sums to total energy  Combination of dE/dx and E Tot gives mass  Complicated in practice  Energy in last plane is unreliable (decays, interactions, etc.)  Need to iterate that energy until (dE/dx) i and E i consistent  Many corrections: decays, secondaries, ghosts, tracking efficiency, PID efficiency, mis-identified particles, etc.

7. Femtoscopy BNL Workshop 6/21/2005George Stephans X[cm ] A B C D E F Z[cm] Beam pipe 0 10 20 Z [cm]... Particle Measurements at Very Low p T For now: Only find particles stopped in the 5 th spectrometer plane Reconstruction is based on the analysis of the measured energy depositions in the first 5 layers. Fewer than 5 planes per track leads to more background. Beyond that, magnetic field becomes a serious issue. Single stopping particles from GEANT simulations are used to calibrate the method.

8. Femtoscopy BNL Workshop 6/21/2005George Stephans Comparison of dE/dx for low momentum pions (red) PR01 data (black) HIJING events

9. Femtoscopy BNL Workshop 6/21/2005George Stephans Particle Measurements at Very Low p T Summary of data reconstruction Au+Au  s NN =200 GeV (RHIC Run II) Centrality: 15% most central events Vertex: -7cm < Z vtx < 15cm Event statistics: 2,070,000 Total reconstructed tracks: (   +   ) 14,000 ( K  + K  ) 1,900 ( p+p ) 1,000  + +  – Dominant background: Feed-down, ghost, mis-identified K& p p T -dependent corrections from 0.69 to 0.78 Use HIJING simulations with yields tuned to our measurements

10. Femtoscopy BNL Workshop 6/21/2005George Stephans Published Result Au+Au 200 GeV 0-15% PHENIX - open symbols PHOBOS –closed symbols Solid curves: Fits to PHENIX only Dashed curves: Extrapolations Log scale! PRC 70 051901 (R) Used PHENIX because 0-15% data published

11. Femtoscopy BNL Workshop 6/21/2005George Stephans Published Result (Blown Up)      Au+Au 15% central Extrapolation of fit to PHENIX data for p T >0.25 GeV/c PRC 70 051901 (R)

12. Femtoscopy BNL Workshop 6/21/2005George Stephans Comparison to Cramer et al. PRL curve, data fit problem Star data, fixed fit Fit to modified data Phobos 0-15% data Estimated yield 0-5% Data is (      Au+Au 0-15% central Cramer, et al. PRL 94, 102302 & private comm. nucl-ex 0506008 {}

13. Femtoscopy BNL Workshop 6/21/2005George Stephans Procedure to Estimate 0-5% Yield  PHENIX pion data for p T =250-450 MeV/c  STAR pion data for p T =126-214 MeV/c (estimate 15% numbers using 5-10, 10-20% data)  PHOBOS charged hadron data for p T =250-450 MeV/c  PHOBOS summed charged hadron data These ratios range from 16.2 to 20.1% Our extrapolation band spans 15-20% Of course, the final answer is to have the data…

14. Femtoscopy BNL Workshop 6/21/2005George Stephans Near-Term Future d+Au 200 GEV — Study system size dependence Au+Au 62.4 & 200 GEV — Study energy & centrality dependence ✔ Important to determine where any interesting properties “turn off”

15. Femtoscopy BNL Workshop 6/21/2005George Stephans Far-Term Future Au+Au 200 GEV – high statistics data from RHIC Run 4 — Study low p T yields vs many event parameters  Could identify negatives using excess energy in last plane — Attempt to get BE correlation function at very low m T  Momentum and angular resolutions are encouraging R 0 (fm) K T (MeV/c) Cramer, et al. PRL

16. Femtoscopy BNL Workshop 6/21/2005George Stephans Far-Term Future Au+Au 200 GEV – high statistics data from RHIC Run 4 R 0 (fm) K T (MeV/c) We will try, but the reality is that this data set is unlikely to yield usable pair statistics using very low p T stopping particles. There are several other options to explore to see how low in K T Phobos can reach.

17. Femtoscopy BNL Workshop 6/21/2005George Stephans Conclusions  i low p T data can play an important role in evaluating model(s) of the exotic systems formed at RHIC  More data for yields will be available soon  Looking forward to detailed comparison to theory(ies)  Very low K T HBT would likely require a dedicated experiment