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Charged Particle Production at High Rapidity in p+p collisions at RHIC

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Charged Particle Production at High Rapidity in p+p collisions at RHIC

Ramiro Debbe

CIPANP2006 High Energy Hadron 1-June-2006

Outline of presentation

- Brief description of the measurement.
- These are the first measurements of identified charged particle production at this energy (200 GeV) at high rapidity (y~3)
- Comparison of spectra and NLO pQCD calculations.
- Will describe the measurement of the p/+ ratio at high rapidity.

CIPANP2006 High Energy Hadron 1-June-2006

CC

3.25<<5.25

The data at forward rapidities were collected with FS at 4º ( ~3) and 2.3º (~3.4)

CIPANP2006 High Energy Hadron 1-June-2006

- Covers ~70% of pp inelastic cross-section (41mb)
- 3.25 < || < 5.25 range
- Vertex resolution (z)~ 1.6cm
- ±40cm in z used in the analysis

CIPANP2006 High Energy Hadron 1-June-2006

Correction for trigger bias.

We use Pythia 6.326 to extract the trigger bias introduced on the yields and translate them into differential cross-sections normalized to total inelastic (41 mb )

Total: Msel=0 and exclude elastic events.

NSD: semi hard QCD 2->2, SDA, SDB, DD, low Pt and at least one hit in both CC

GEANT3 has CC FFS and BFS

CIPANP2006 High Energy Hadron 1-June-2006

Particle Identification is done withBRAHMSRICH

One angle setting, several (6) magnetic field settings

Radiator gas with high index of refraction.

Radius resolution: 1.2 %

Efficiency 97%

CIPANP2006 High Energy Hadron 1-June-2006

p+p identified spectra at high rapidity

Red : positive particles

Blue empty: negative particles

Built with data from 4 and 2.3 degrees and up to six magnetic field settings.

Geometrical acceptance corrections applied as well as absorption and decay in flight.

Trigger bias (~20%) is also corrected.Normalization to total inelastic cross-section (41 mb)

BRAHMS Preliminary

CIPANP2006 High Energy Hadron 1-June-2006

Comparison of measurement and NLO pQCD calculations

NLO pQCD can reproduce the data at RHIC energies. This is a strong indication that the correct description of this should be done with partonic degrees of freedom

The fragmentation functions differ by the amount ofg->The data points toward a dominance of gluon-gluon and gluon-quark below 10 GeV/c

CIPANP2006 High Energy Hadron 1-June-2006

NLO-pQCD can reproduce y~0 hadron production at ISR but fails at higher rapidities.

√s=23.3GeV

√s=52.8GeV

Data-pQCD difference at pT=1.5GeV

Neutral pion production at small angles at ISR

Lloyd et al.

PRL 45 89 (1980)

Ed3s/dp3[mb/GeV3]

Ed3s/dp3[mb/GeV3]

q=6o

q=10o

q=15o

q=53o

q=22o

xF

xF

Bourrely and Soffer Eur. Phys. J. C36 371-374 (2004)

CIPANP2006 High Energy Hadron 1-June-2006

NLO pQCD for proton+anti-proton compared to data

A recent update of the KKP fragmentation function is used for this comparison: AKK where g->p has increased relevance.

The AKK function does well at y=0 (STARp+p) where the ratio anti-p/p~1 can be seen as consistent with dominance of gg or gq processes,but in my opinion is not appropriate for high rapidities because of the small value of the p/p ratio.

CIPANP2006 High Energy Hadron 1-June-2006

STAR 0 at high rapidity

FPD: Lead-glass arrays 3.4<<4.0 on both sides of collision.

Spectra at 3.3 and 3.8 obtained with a smaller FPD

KKP frag. func. has higher g-> than Kretzer

arXiv:nucl-ex/0602011

CIPANP2006 High Energy Hadron 1-June-2006

NLO pQCD comparisons to data

BRAHMS Preliminary

KKP has only 0 frag. Modifications were needed to calculate charged pions

KKP FF does a better job compared to Kretzer,can we extend the conclusion about gg and gq dominance at these rapidities?

Calculations done by W. Vogelsang. Only one scale =pT and the same fragmentation functions as used for the PHENIX comparison.

CIPANP2006 High Energy Hadron 1-June-2006

Particle/anti-particle ratios at y=3.0 and NLO comparisons

The NLO calculation reproduces the data closely, the pion ratio does not have corrections from weak decays (we estimate that the correction will be small).

CIPANP2006 High Energy Hadron 1-June-2006

Ratios p/+ at y=3.0 and 3.3

The -/+ ratio is consistent with dominance of valence quarks (at high pT) at these rapidities.

Small p/p ratio eliminates possible strong gluon -> p or p fragmentation (p/p~1)

The difference between protons and anti-protons indicates another mechanism besides fragmentation (as AKK) that puts so many protons at high pT at this rapidities.

BRAHMS Preliminary

Red: proton/+Blue: p/ -

CIPANP2006 High Energy Hadron 1-June-2006

e+e- p+pbar/++ - ALEPH

Summary and outlook

- BRAHMS has measured, for the first time, identified charged particle production at high rapidity.
- NLO pQCD calculations describe well the pion and kaon production with a favored set of fragmentation functions known as KKP. These agreements imply a dominance of g q and g g processes at these high rapidities as was the case for the measurements of neutral pions at mid-rapidity.
- The behavior of protons around y=3 cannot be explained with NLO calculation and the abundance of protons (with respect to positive pions) at high pT is an open question.

CIPANP2006 High Energy Hadron 1-June-2006