charged particle production at high rapidity in p+p collisions at rhic

CIPANP2006 High Energy Hadron 1-June-2006

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

Ramiro Debbe


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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.


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CC

3.25<<5.25

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


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•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


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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

Correction for trigger bias.

GEANT3 has CC FFS and BFS


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Particle Identification is done with BRAHMS RICH

One angle setting, several (6) magnetic field settings

Radiator gas with high index of refraction.

Radius resolution: 1.2 %

Efficiency 97%


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p+p identified spectra at high rapidity

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

Red : positive particles

Blue empty: negative particles


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Comparison of measurement and NLO pQCD calculations

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

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


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√s=23.3GeV √s=52.8GeV

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

Neutral pion production at small angles

at ISR

Lloyd et al.

PRL 45 89 (1980)

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

xF xF

Ed3

dp3 [

b/G

e V3 ]

Ed3

dp3 [

b/G

e V3 ]

Data-pQCD difference at pT=1.5GeV


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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 (STAR p+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.


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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

arXiv:nucl-ex/0602011KKP frag. func. has higher g-> than Kretzer


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NLO pQCD comparisons to data

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

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

BRAHMS Preliminary

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


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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).


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Ratios p/+ at y=3.0 and 3.3 The -/+ ratio is consistent with dominance of valence quarks (at high pT) at these rapidities.

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.

Red: proton/+ Blue: p/ -

BRAHMS Preliminary

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

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


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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.

charged particle production at high rapidity in p+p collisions at rhic

Documents

high rapidities

high rapidityfpd

high rapiditybuilt

high rapidity y

hadron production

star p p

high index of refraction

p p collisions