Oct 6, 2008Amaresh Datta (UMass) 1 Double-Longitudinal Spin Asymmetry in Non-identified Charged...

Oct 6, 2008 Amaresh Datta (UMass)1

Double-Longitudinal Spin Asymmetry in Non-identified Charged Hadron Production at pp Collision at √s = 62.4 GeV at

Amaresh Datta

(UMass, Amherst)


Proton Spin and Search for Gluon Polarization

Orbital ang.momenta

Gluon spincontrib.

Quark spin 0.33

At PHENIX, polarized proton-proton scattering is studied to probe gluonpolarizationAt √s = 62.4 GeV, mid-rapidity hadron production is sensitive to polarized gluon distribution and probes interaction of partons with momentumfraction 0.06 < x < 0.4


Subprocess Contributions to Mid-rapidity Charged Hadron Production as a Function of COM Energy

17.3 GeV62.4 GeV130 GeV200 GeV

ggqgqq

Calculations by W. Vogelsang

In the low transverse momentum range of our interest (0.5 < pT < 4.5 GeV/c), ‘qg’ process is the dominant contributor

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Hard Scattering Processes in p+p:Factorization and Universality

Hard Scattering Process

2P2 2x P

g 2f x

q 1f x1P

1 1x P

s

zhqD

qqgˆ

The cross section of a hard scattering process is convolution of:– Parton distribution functions (need experimental input)

– pQCD hard scattering rates (calculable in pQCD)

– Fragmentation functions (need experimental input)


Motivation for Charged Hadron Asymmetry Measurement at √s = 62.4 GeV

Measurements of ALL will be additional contribution to global analysis for constraining ∆g

Will probe slightly higher x-range than higher (200 GeV) energy measurements

Will be corroboration of gluon polarization found in neutral pion ALL at PHENIX

Charged hadron asymmetries can be helpful in determining sign of ∆g A significant positive ∆g can be reflected in the asymmetries as

ALL(π+) > ALL(π0) > ALL(π-); (for ∆g(x) without any node) A negative ∆g will cause reverse order in asymmetries


Cross-sections

Cross-section of charged hadrons will be measured and compared with NLO and NLL calculations

Charged hadron cross-section from 2002 run at 200 GeV and neutral pion cross-section from 2006 run at 62.4 GeV have been measured and show good agreement with theoretical predictions

62.4 GeV data suggests NLL corrections may be relevant at this energy

Neutral Pion cross-section in Run-6at 62.4 GeV (submitted to PRD)Charged hadron cross-section from Run-2(PRL 95, 202001)


Data Set

Run-6 data (2006) 1 week of longitudinally polarized pp collision data at

√s = 62.4 GeV recorded at Relativistic Heavy Ion Collider (RHIC)

Analyzed data ~ 40 nb-1

Events analyzed were triggered (PHENIX minimum-bias trigger) by requiring coincidence of hits on Beam-Beam Counters (BBC) on both sides of the centre of nominal interaction region


Measurement at PHENIX

Charged hadron tracking: Drift Chamber (DC) Pad Chamber (PC)

Luminosity Detectors: Beam-Beam Counter (BBC) Zero Degree Calorimeter (ZDC)

Background sources Electrons from event vertex and

decays and conversions Hadrons from decay of long-lived

particles Hadrons from decay of short lived

particles

Tracking starts from DC


Tracking of Charged Particles

Charged particles hit DriftChamber

Almost no magnetic field afterDC

Tracks projected onto outer Detectors (e.g. Pad Chamber)

Hits on outer detectors withinacceptable deviation are associated with tracks


Treatment of Backgrounds

Electron backgrounds vetoed by Ring Imaging Cherenkov Detector (RICH) Electrons fire RICH at ~ 17 MeV/c RICH theshold for pions ~ 4.7 GeV/c

Contribution from decays of long-lived (cτ ~ 1-10 m) particles are estimated using broadened distribution of hits on Pad Chamber and ALL is corrected for background ALL

Short-lived (cτ ~ 1-10 cm) decay contribution estimated to be ~ 7±7 % and no correction to ALL for this background is performed


Reduction of Backgrounds

Spectra of charged tracks (from data) before and after specific cuts within pT range of interest

Higher pT region contains particles that do not originate from event vertex and get reconstructed with (false) high-pT

RICH veto and constraints on matching variables from PC hits reduce background from analyzedtracks


Asymmetry and Statistical Uncertainty

++ same helicity+- opposite helicity

R = Relative Luminosity = L++/L+-


Fill-by-Fill Combination of ALL

Count N and luminosity for same and opposite helicity configurations (summed over all crossings) and calculate ALL for each fill

Average ALL over fills Perform checks:

Parity-violating AL consistency with zero

Comparison of statistical uncertainty to width of distribution of ALLwith randomized spin configurations

Fill-by-fill consistency (stability of fit checked with χ2/NDF)


Theoretical Predictions for ALL

NLO pQCD calculations in the GRSV (Phys. Rev. D 63, 094005 (2001) ) model assuming different input values of ∆g


ALL of Charged Hadrons

Double longitudinal spin asymmetries of charged hadrons


Asymmetries of Charged Hadrons and Neutral Pions (Neutral Pion Asymmetry result from hep-ex: 08100694)

xT scaling of Pi0 ALL shows consistency of asymmetries with resultsat different center of mass energy


Conclusion

Longitudinal double spin asymmetry results found to be consistent with small gluon polarization in the x-range measured by other probes (e.g. neutral pions) at PHENIX

ALL results will contribute to global analysis to constrain ∆g

With more events in future runs, the asymmetry of charged hadrons can be helpful in determining the sign of ∆g


Thank You


Backup: Single Spin Asymmetry (Blue Beam)


Backup: Single Spin Asymmetry (Yellow Beam)

Oct 6, 2008Amaresh Datta (UMass) 1 Double-Longitudinal Spin Asymmetry in Non-identified Charged...

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