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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)
Oct 6, 2008 Amaresh Datta (UMass)2
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
Oct 6, 2008 Amaresh Datta (UMass)3
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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Oct 6, 2008 Amaresh Datta (UMass)4
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)
Oct 6, 2008 Amaresh Datta (UMass)5
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
Oct 6, 2008 Amaresh Datta (UMass)6
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)
Oct 6, 2008 Amaresh Datta (UMass)7
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
Oct 6, 2008 Amaresh Datta (UMass)8
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
Oct 6, 2008 Amaresh Datta (UMass)9
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
Oct 6, 2008 Amaresh Datta (UMass)10
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
Oct 6, 2008 Amaresh Datta (UMass)11
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
Oct 6, 2008 Amaresh Datta (UMass)12
Asymmetry and Statistical Uncertainty
++ same helicity+- opposite helicity
R = Relative Luminosity = L++/L+-
Oct 6, 2008 Amaresh Datta (UMass)13
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)
Oct 6, 2008 Amaresh Datta (UMass)14
Theoretical Predictions for ALL
NLO pQCD calculations in the GRSV (Phys. Rev. D 63, 094005 (2001) ) model assuming different input values of ∆g
Oct 6, 2008 Amaresh Datta (UMass)15
ALL of Charged Hadrons
Double longitudinal spin asymmetries of charged hadrons
Oct 6, 2008 Amaresh Datta (UMass)16
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
Oct 6, 2008 Amaresh Datta (UMass)17
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
Oct 6, 2008 Amaresh Datta (UMass)18
Thank You
Oct 6, 2008 Amaresh Datta (UMass)19
Backup: Single Spin Asymmetry (Blue Beam)
Oct 6, 2008 Amaresh Datta (UMass)20
Backup: Single Spin Asymmetry (Yellow Beam)