Particle Production in p + p Reactions at GeV

1. Particle Production in p + p Reactions at GeV K. Hagel Cyclotron Institute Texas A & M University for the BRAHMS Collaboration

2. p + p collisions at high energy • Baseline measurement for Heavy Ion Reactions • Jet suppression and the sQGP • Information on baryon transport • Constrain pQCD models • Information on fragmentation functions

3. Outline • Description and characteristics of BRAHMS • Particle spectra • Fits and fit parameters • Rapidity densities • Nuclear Stopping • Limiting fragmentation • High pT pQCD comparisons to data • Strangeness • LHC “prediction” • Summary

4. Front Forward Spectrometer Global Detectors Back Forward Spectrometer

5. Mid-rapidity Spectrometer • TPC, TOF, Cherenkov • 30o – 90o= 0 - 1.5 • Forward Spectromter • TPC, DC, TOF, Cherenkov, RICH • 2.3o – 30o = 1.5 – 4 • Mid-rapidity Spectrometer • TPC, TOF, Cherenkov • 30o – 90o= 0 - 1.5

6. Particle Identification CHERENKOV RICH: Cherenkov light focused on spherical mirror ring on image plane Ring radius vs momentumgives PID  / K separation25 GeV/c Proton ID up to35 GeV/c (2 settings) TIME-OF-FLIGHT

7. The BRAHMS Acceptance Transverse momentum [GeV/c] Rotatable spectrometers give unique rapidity coverage : Broad RAnge Hadron Magnetic Spectrometers Rapidity

8. Experimental Coverage

9. Fitting particle spectra • One method to extrapolate to parts of the spectrum not measured. • Different functions might (or might not) be appropriate for different spectra. • It is still an extrapolation that adds to systematic error. • Fit used in this work is Levy Function • Has characteristics of an exponential at low pT and evolves toward a power law at high pT Where • Performed global fit using T = T0 + ay + a2y2, n = n0 + by + b2y2 • 6 parameter fit + dN/dy for each rapidity bin

10. 200 GeV Pion Spectra x10-10 x10-12

11. 200 GeV Kaon Spectra x10-5 x10-7

12. 200 GeV Proton Spectra x10-9 x10-11

13. 62 GeV p+p spectra

14. dN/dy

15. Stopping • Obtained from net baryon dN/dy • Gives information on initial distribution of baryonic matter at the first moment of the collision. • Net-Baryon = Net(p)+Net(L)+Net(Cascade)+Net(n), where each part involves feed-down corrections. • At GeV the correction from net proton to net baryon is expected to independent of rapidity • We have measured and will show net proton dN/dy • Simply dN/dyp – dN/dypbar shown previously

16. net proton dN/dy • y(200) ~ 1.45 (fit) • y (Hijing/BB) ~ 1.20 • y(62) ~ 1.00 (fit)

17. Limiting Fragmentation

18. Proper scaling for p + p collisions is dN/dxF If dN/dxF = const, dN/dy ~ coshy Longitudinal scaling Batisita and Covolan, PRD59 (1999) 054006

19. Net proton dN/dyLimiting Fragmentation Nucl Phys. A661 (1999) 362.

20. NLO pQCD comparisons to data at large rapidity BRAHMS Phys. Rev. Lett. 98, 252001 (2007) • Comparison of different fragmentation functions • Modified KKP (Kniehl-Kramer-Potter) does better job than Kretzer (flavored FFs) on -, K+ • Difference driven by higher contributions from gluons fragmenting into pions • gg and gq processes dominate at mid rapidity (STAR PRL 91, 241803 (2003). • Processes continue to dominate at larger rapidity. • AKK (p +p)/2 (where p ~p) reproduces experimental p, but notp

21. Rapidity dependence of NLO pQCD comparison to data • KKP describes data from mid-rapidity (PHENIX, 0) to large rapidity (BRAHMS, -; STAR 0)

22. Global fits to dataincluding BRAHMS large rapidity data DSS, PRD 75, 114010 (2007) K+ + K- - • Charged separated fragmentation functions • Fragmentation functions significantly constrained compared to previous “state of the art” when adding RHIC data into fits.

23. Updated AKK FFs with charge separated data from BRAHMS • These fits indicate that, at large rapidity, fragmentation from valence u and d quarks contributes strongly to p and pbar asymmetry. p -p p +p AKK2008 hep-ph:0803.2768

24. scale factor of μ=pT DSS (De Florian, Sassot, Stratmann) also shown (dashed lines) K- data suppressed order of magnitude compared to K+ (valence quark effect). NLO pQCD using the recent DSS fragmentation functions give approximately same K-,K- yield (?) Related to fragmentation or PDFs? Beam remnants that are not addressed in FFs NLO pQCD comparisons of 62 GeV +, K+ data at large rapidity - KKP + KKP

25. Decreasing K/ (+ and -) for larger rapidity. More pronounced for K-/- K/

26. K/ comparison to Au + Au • Larger K/ for Au+Au • Radial flow • Absence of canonical K suppression in Au + Au

27. Increasing K+/K- suppression with increasing rapidity K/ vs rapidity

28. p+p evolution with pbar/p canonical K suppression – larger for K- Larger values for Au+Au – strangeness effects turning on More energy available in heavy ion collision. Strangeness enhancement?

29. What Can we say about LHC Physics • Net proton dN/dy • Use lower energy limiting fragmentation data • Shift to LHC beam rapidity to have predicted distribution

30. LHC p+p stopping prediction • Merge limiting fragmentation plots • Add LHC beam rapidity to them • Fit with Function • y ~ 2 • Experiments at LHC will measure ,K,p to y=1

31. Summary • Particle production • dN/dy • Net proton dN/dy • Stopping • Variation in y with beam energy • Limiting Fragmentation • dN/dy • Net proton dN/dy • Constant dN/dxF • Comparison to pQCD calculations • Constraints on Fragmentation Functions • K/ ratios and canonical K suppression • Net proton dN/dy “Prediction” for LHC