Identified Charged Hadron Production in p+p Collisions at √s = 62.4 and 200 GeV Masahiro Konno for the PHENIX Collaboration (University of Tsukuba) JPS @ Yamagata, 9/21/2008

Introduction Measure transverse momentum spectra for identified particles as reference to heavy ion data. - Experiment: RHIC-PHENIX - Data: p+p at √s = 62.4 and 200 GeV Study the properties of hadron production in p+p collisions - Inverse slope parameter in mT spectra - Particle ratios - xT scaling, its exponent neff Compare them with data taken at different collision energies Compare them between p+p and heavy ion data (Au+Au, Cu+Cu)

PID Spectra in p+p collisions Phenix Preliminary Phenix Preliminary Measured pT spectra for π, K, p with √s = 62.4 and 200 GeV p+p data PHENIX TOF counter used for particle identification Proton and antiproton spectra measured up to pT = 4 GeV/c

mT Spectra for identified particles Hard Soft mT spectra fitted with an exponential function to extract the inverse slope parameter (Fit range: mT-m ~0.3-1.0 GeV). mT scaling roughly applicable: Two components (soft, hard) to be taken into account * Feed-down correction not applied. If applied, Tinv increased by ~ 10-15 %

Energy dependence of Tinv Open: p+p/p+pbar Closed: Pb+Pb, Au+Au Compiled Tinv in p+p (p+pbar) and A+A data Tinv(p+p) < Tinv(A+A) for π/K/p Clear energy dependences seen in (anti-)proton for both p+p and A+A

Baryon-meson difference in mT spectra Mesons Baryons mT scaling in yield is roughly applicable within an order of magnitude. By scaling arbitrarily at mT~ 1-1.5 GeV, the spectra are split into meson and baryon groups. The harder meson spectra indicate that meson production requires only a quark pair in fragmentation, while baryon production requires a diquark pair.

Baryon/Meson Ratios ratios ratios Open: p+p/p+pbar Closed: Au+Au ratios ratios Baryon/meson ratios increase with collision energy even in p+p collisions. Λ/K ratios at high energies are surprisingly high as in heavy ion collisions. This could be due to NLO contribution, bulk effect such as coalescence, and so on. First measure particle ratios in p+p at LHC energy (√s = 14 TeV).

High pT Spectra - pQCD description PRD76(2007)051106 p+p 200 GeV p+p 62.4 GeV - π0 spectra are described over a wide pT range with pQCD calculation within experimental and theoretical uncertainties.

xT Scaling in p+p Collisions Invariant cross section for single-particle inclusive reaction is given by the general scaling formula: where Inclusion of QCD into the above equation leads to: - n(xT,√s) equals 4 in lowest order calculations as in QED. Measured values of n(xT,√s) in p+p collisions are in the range from 5 to 8 due to higher order effects. The data points deviate from the xT scaling for pT 2 GeV/c, which is interpreted as a transition from hard to soft processes in particle production. Ref: PL 42B 461(1972), PRD11(1975)1199

xT Scaling for PID Spectra Pions Antiprotons xT scaling works for both of pions and (anti-)protons at √s = 62.4 and 200 GeV Next compare the xT-scaling power neff between pions and (anti-)protons

Estimation of neff p xT-scaling power n is a function of xT and √s. Effective value of n(xT,√s), neff, is obtained by the following two methods: (1) Taking the ratio of yields between different energies (e.g. √s = 62.4, 200 GeV) (2) Fit xT distributions with a common function Data points: method (1) Lines: method (2) π0 p pbar neff 6.45 ± 0.23 6.84 ± 0.47 6.46 ± 0.18 (Fit range: xT = 0.07 – 0.20)

pT Spectra in A+A Collisions In central Au+Au collisions, hadron yields are suppressed at high pT compared to those in p+p collisions. - The suppression is thought to be a final state effect (parton energy loss). Au+Au and Cu+Cu RAA show a similar dependence on Npart. where <Ncoll> is the number of binary NN collisions

xT Scaling in A+A Collisions neff vs. Npart The assumptions is that structure and fragmentation functions scale with xT for A+A. The π0’s show xT scaling with the same number of n as in p+p in any systems, while (anti-)protons show xT scaling with similar value for peripheral only. In central, they show a larger value of n – due to baryon enhancement at 2-3 GeV/c in 62.4 GeV A+A. - The effective energy loss should scale ΔpT/pT = const., leading to constant RAA.

Summary Identified charged hadron pT spectra measured in p+p collisions at √s = 62.4 and 200 GeV (Phenix preliminary) for reference of heavy ion data mT scaling tested with p+p and A+A data Inverse slope parameter: Tinv(p+p) < Tinv(A+A) for π, K, p Tinv increases with collision energy √sNN. (Anti-)protons increase quickly than pions and kaons for both p+p and A+A - Baryon-meson difference of the yields seen in mT spectra xT scaling tested with p+p and A+A data xT scaling works for pions and (anti-)protons in p+p collisions (62.4, 200 GeV) - In heavy ion data, π0’s show xT scaling with the same number of neff as in p+p for both central and peripheral collisions (Au+Au, Cu+Cu). - (Anti-)protons show xT scaling with similar value for peripheral. In central, they show a larger value of neff. This is consistent with the observed baryon enhancement at intermediate pT region (2-4 GeV/c). - Need to measure (anti)proton spectra higher pT to know whether a similar value of neff (as pions, or as in p+p) is obtained or not.