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

2. 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)

3. 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

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

5. 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

6. 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~ 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.

7. 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).

8. 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.

9. 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

10. 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

11. 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)

12. 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

13. 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.

14. 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.