The cross section and double helicity asymmetry in pp collisions

Frank EllinghausUniversity of Colorado

(for the PHENIX Collaboration)10/03/06

Spin’06, Kyoto, Japan

F. Ellinghaus, Spin’06, Kyoto, Japan

Introduction

• Status: ALL for unmeasured, fragmentation function unknown no predictions for ALL yet

• Additional channel (“global analysis”)• Naïve expectation (additional s+s in wave

function): increased relative contribution of strange quarks and glue should lead to a different asymmetry (when compared to )

• -> Observation of difference could help to disentangle the contributions from q (u,d,s) and g

22~ qaGqaGaA qqqgggLL

Access to polarized gluon distribution function via double helicity asymmetry:

F. Ellinghaus, Spin’06, Kyoto, Japan

PHENIX Detector

Photon identification:

• EPbSC > 0.2 GeV, EPbGl > 0.2 GeV

• Cut on Shower shape of cluster• (Charge veto cut: closest track in PC3 is considered, should be far

away or very close (pair production originated from photon))• (Time-of-flight cut)

Photon detection:

• Electromagnetic calorimeter: PbSc + PbGl, Acceptance: < |0.35|, = 2 x 90

• Photon trigger: Threshold ~ 1.4 GeV

Luminosity:

• Beam-Beam Counter (BBC), 3.1 < < 4.0

F. Ellinghaus, Spin’06, Kyoto, Japan

reconstruction via decay• Invariant two photon mass: 0.48 GeV < M < 0.62 GeV

• Energy asymmetry cut: E1-E2 / E1+E2 < 0.7

• pT > 2 GeV

• |zvertex| < 30 cm

• Fit:Gauss+Pol3

2<pT<3 GeV

F. Ellinghaus, Spin’06, Kyoto, Japan

Invariant cross section

yp

ypN

effefffBRLppd

dE

T

T

recTrigAccT

),(1111

2

13

3

L = integrated Luminosity, based on Vernier scan using the BBC

BR= Branching ratio: 2 photons = 0.3943 ± 0.0026

fAcc = acceptance function from MC (includes smearing)

effTrig (Minimum Bias data) = Trigger efficiency of MB trigger

effTrig (high pT triggered data) = (Trigger effi. MB) x (Trigger effi. high pT

trigger)

effrec = Correct for loss due to photon conversion (~6% in PBSC, ~8% in PbGl) x loss due to cut on shower shape (~4%)

N = number of reconstructed

F. Ellinghaus, Spin’06, Kyoto, Japan

facc from Monte CarloAcceptance and smearing correction from MC(also accounts for dead regions in EmCal, minimum cuts on photon energies,…)

Up to 10% acceptance in PbSc for at high pT

dataMC

pT

F. Ellinghaus, Spin’06, Kyoto, Japan

Photon/MB Trigger Efficiency

Photon trigger efficiency roughly stable from 4 GeV on (high pT photon trigger threshold is set to 1.4 GeV)

Minimum Bias (MB) trigger efficiency about 80 %

F. Ellinghaus, Spin’06, Kyoto, Japan

cross section

The relative contributions of quarks and gluons are maybe not too different…?

Need more statistics Run 5 cross section analysis in progress

Enables extraction of fragmentation functions from (e+e- and) pp ( gluon FFs)

F. Ellinghaus, Spin’06, Kyoto, Japan

Inclusive Asymmetries

NRN

NRN

PPA

YBLL

1

L

LR Relative Luminosity R using

beam-beam counters

Asymmetries in bands around peak are consistent with each other

F. Ellinghaus, Spin’06, Kyoto, Japan

Background corrected Asymmetry

r

ArAA

BGLL

BGLL

LL

1

BG

BG

NN

Nr

F. Ellinghaus, Spin’06, Kyoto, Japan

Single Spin Asymmetries

NRN

NRN

PAL

1

L

LR

Parity violating asymmetries zero as expected

F. Ellinghaus, Spin’06, Kyoto, Japan

Comparison to and Outlook

Including Run 6 data will result in statistical power comparable to present Run 5 preliminary data.

F. Ellinghaus, Spin’06, Kyoto, Japan

Summary• Inclusive double helicity asymmetry has been extracted• Run 3 cross section for available, Run 5 result soon

enables extraction of fragmentation functions enables theoretical calculations in order to compare

extracted asymmetry to theory