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

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

Global Detectors Front Forward Spectrometer Back Forward Spectrometer

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

Particle Identification TIME-OF-FLIGHT 0<  <1 (MRS) 1.5<  <4 (FS) p max (2  cut) TOFW (GeV/c) TOFW2 (GeV/c) TOF1 (GeV/c) TOF2 (GeV/c) K/  K/p RICH: Cherenkov light focused on spherical mirror  ring on image plane Ring radius vs momentum gives PID  / K separation 25 GeV/c Proton ID up to 35 GeV/c CHERENKOV (2 settings)

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

Experimental Coverage

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 Whereand Performed global fit using T = T 0 + ay, n = n 0 + by

200 GeV Pion Spectra T 0 = GeV, n 0 = 4.45T 0 = GeV, n 0 = 4.38

200 GeV Kaon Spectra T 0 = GeV, n 0 = 6.44T 0 = GeV, n 0 = 6.23

200 GeV Proton Spectra T 0 = GeV, n 0 = 8.36T 0 = GeV, n 0 = 14.58

62 GeV p+p spectra

dN/dy

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(  )+Net(Casade)+Net(n), where each part involves feed-down corrections. We have measured net proton dN/dy Simply dN/dy p – dNdy pbar shown previously

net proton dN/dy  y ~ 1.26 (momgaus)  y ~ 1.20 (Hijing/B; remember dN/dy!)

Limiting Fragmentation

Net proton dN/dy Limiting Fragmentation Nucl Phys. A661 (1999) 362.

Rapidity dependence of Mean pt

NLO pQCD comparisons to data at large rapidity BRAHMS Phys. Rev. Lett. 98, (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 fragmentating into pions –gg and gq processes dominate at mid rapidity (STAR PRL 91, (2003). –Processes continue to dominate at larger rapidity. –AKK (p+pbar)/2 (p~pbar) reproduces experimental p, but not pbar

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

Global fits to data including BRAHMS large rapidity data PRD 75, (2007) Charged separated fragmentation functions Fragmentation functions significantly constrained compared to previous “state of the art” when adding RHIC data into fits.

NLO pQCD comparisons of 62 GeV  +, K + data at large rapidity scale factor of μ=p T DSS 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?  - KKP  + KKP

K/ 

K/  comparison to Au + Au Larger K/  for Au+Au –Radial flow –Absence of cannonical K suppression

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

Strangeness enhancement p+p evolution with pbar/p –cannonical K suppression – larger for K- Larger values for Au+Au – strangeness effects turing on –More energy available.

What Can we say about LHC Physics Net proton dN/dy –Use lower energy limiting fragmentation data

LHC p+p stopping prediction Merge limiting fragmentation plots Add LHC beam rapidity to them Fit with momGaus  y ~ 2 CMS will measure to 2.2 Stopping with energy (subtract from incoming energy)

Summary Particle production –dN/dy –Net proton dN/dy -> Stopping Limiting Fragmentation –dN/dy –Net proton dN/dy High pt pQCD calculations Strangeness enhancement Prediction for LHC