Longitudinal Spin Asymmetry and Cross Section of Inclusive 0 Production in Polarized p+p

Collisions at 200 GeV

Outline

Introduction Experimental Overview Inclusive Cross Section Neutral Pions in Jets Longitudinal Spin Asymmetry

Frank Simon, MIT, for the STAR Collaboration

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 210/03/2006

Introduction Polarized p+p collisions provide sensitivity to gluon polarization in the nucleon

With current statistics the focus is on inclusive measurements (Jets, Pions) large contribution of gg and qg processes to overall cross section => good tool to study gluon polarization, but no constraint of event kinematics

Neutral pions probe the same processes as jets, but with different experimental systematics and effects from fragmentation

relative contributions of subprocesses to inclusive 0 production

W. Vogelsang

Unpolarized measurements of inclusive 0 are of considerable interest: study fragmentation functions via NLO pQCD vs measured cross section comparisons: currently large uncertainty in gluon FF

study fragmentation directly: electromagnetic trigger selects jets with leading 0

gg qg qq

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 310/03/2006

The STAR ExperimentMagnet 0.5 T SolenoidTriggering & Luminosity Monitor

Beam-Beam Counters 3.4 < || < 5.0

Central Tracking Large-volume TPC

|| < 1.5 Calorimetry Barrel EMC (Pb/Scintilator)

|| < 1.0 Shower-Maximum Detector

…and many other systems not used in the 0 analysis

Used Triggers: Minimum Bias (MB): BBC Coincidence, highly prescaled High Tower 1 & 2 (HT1 / HT2): MB + one BEMC cell (x = 0.05 x 0.05) above threshold (lower threshold trigger prescaled)

2005 run

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 410/03/2006

Neutral Pion Reconstruction 2005 p+p dataset (after rigorous quality cuts): ~ 1.7 pb-1

Trigger & shower maximum detector in 2005 operational only for 0 < < 1, full acceptance -1 < < 1 available for 2006 data

tower size ( x )0.05 x 0.05

0 invariant mass:

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minv2 = 2E1E2(1− cosθ)

SMD becomes crucial for 0 reconstruction for pt ~ 5 GeV/c (photon separation equal to tower size)

veto calorimeter hits that have a charged track leading to them

0 candidates accepted for 0.1 < y < 0.9 0 has to be able to fire the trigger (but does not have to be the triggering particle) rejection of beam background found with pattern recognition code

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 510/03/2006

Neutral Pion Reconstruction

invariant mass spectrum near 0 mass described by: MC 0 line shape low invariant mass background (caused by cluster splitting in the SMD)

combinatoric background & residual fit

Correction factor for cross section determination obtained from PYTHIA & HERWIG simulations

different cuts used for MB and HT triggers

HT1 HT2

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 610/03/2006

Inclusive Cross Section

Lsampled: 0.4 pb-1 (HT triggers), 44 b-1 (MB)

Point-to-point systematics from yield extraction

Total systematics dominated by 5% uncertainty in BEMC energy scale, significant contribution from correction factor uncertainty

Compared to NLO pQCD (CTEQ6M pdfs) using KKP and Kretzer fragmentation func. better agreement with KKP large scale uncertainties in pQCD calculations, indicated by choosing different scales for KKP calculations

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E d3σdp3

= 12πpTΔpTΔy

c N π

Lsampled

Invariant cross section:

consistent with previous results from PHENIX

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 710/03/2006

Neutral Pions in Jets STAR is capable of full Jet reconstruction reconstructed are associated with Jets (HT triggered) if the 0 lies within the Jet cone (0.4 in , )

0 direction is strongly correlated with the Jet axis: leading 0 typically within 5° of the Jet axis

<z> defined as the mean ratio of 0 pt to Jet pt

Affected by trigger, depends on pt evolution of cross section and z dependence of the fragmentation function

RMS in , ~ 0.055

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 810/03/2006

Double Longitudinal Asymmetry: Overview

Ingredients: Polarization: measured by RHIC polarimeters Relative Luminosity R measured with the STAR BBC & scaler system (relative luminosities for each bunch crossing available)

Spin dependent yields N++, N+- : determined from integration of invariant mass in the signal region after background subtraction

Spin direction in the interaction region verified by the STAR BBCs

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ALL =σ ++ −σ +−σ ++ +σ +−

= 1P1P2

× N++ − RN+−N++ + RN+−

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P12P22 ⋅ Ldt∫, FOM ~

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R = L++L+−

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 910/03/2006

Double Longitudinal Asymmetry ALL

constant fit (assumes no pt dependence): ALL = -0.017 +- 0.021

compared to NLO calculations(ignoring systematic errors): GRSV Std: 3 GRSV Max: 10 GRSV Min: 3 GRSV Zero: 2 GRSV max scenario disfavored

overall scale uncertainty from beam polarization measurement not included

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 1010/03/2006

ALL Systematic Studies & Errors Parity-Violating Single Spin Asymmetries

Come in through the weak interaction, and are limited to less than 10-4, so they should be consistent with zero at the present level of statistics

Good tool to investigate possible spin dependent background effects or issues with relative luminosities

For 0 all observed single spin asymmetries are within 1 of 0, no systematic assigned

Random Pattern Analysis Asymmetries calculated with randomized bunch patterns no indication of non-statistical effects found

Systematic Errors assigned for remaining Background (from beam background, not removed invariant mass background) pt dependent from 5 x 10-3 to 11 x 10-3

yield extraction (normalization of background model) from 3 x 10-3 to 7 x 10-3

non-longitudinal spin components in beams 3 x 10-3

relative luminosities 2 x 10-3

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 1110/03/2006

Comparisons to Published Results

Comparison with published results: STAR 2003/2004 Jets, pt divided by 2 PHENIX 2003/2004 0

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 1210/03/2006

Projections for 2006 Data

Significant increase in sampled luminosity Polarization typically ~60% acceptance increase by a factor of 2

significant increase in figure of merit!

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 1310/03/2006

Summary First spin result for inclusive 0 production at mid-rapidity from STAR

Inclusive cross section consistent with NLO pQCD calculations KKP fragmentation functions favored over Kretzer set scale uncertainty of pQCD of comparable size as preliminary systematics

Energetic 0 carry a significant fraction of the total transverse momentum of their associated jet

Double longitudinal spin asymmetry disfavors large positive gluon polarization consistent with previous observations with jets and 0 result limited by statistics

Significant increase in figure of merit with the already recorded 2006 data set

Backup

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 1510/03/2006

Backup: Systematics: Cross Section Point-to-Point (yield extraction, background subtraction)

Energy scale (5% uncertainty on BSMD gain calibration)

Correction factor (variation of cuts, uncertainty in SMD gain (to a large extend built into MC, additional uncertainties included in systematics), statistical limitation of MC dataset)

Acceptance Stability (changes in electronics status, modeling in MC)

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 1610/03/2006

Backup: Systematic Errors on ALL

Bin

remaining Background

Yield Extration

relative Luminosity

non-long. Effects

Total Systematic

stat. Error

1 0.0052 0.0062 0.0022 0.003 0.0089 0.0443

2 0.0052 0.0032 0.0022 0.003 0.0072 0.0291

3 0.0082 0.0047 0.0022 0.003 0.0102 0.0439

4 0.0112 0.0069 0.0022 0.003 0.0137 0.0836

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 1710/03/2006

Backup: BEMC 0 Acceptance Photon Candidates (Calorimeter Hits) above 1 GeV requiring information from both SMD planes: Data vs MC

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Data (all triggers) PYTHIA (no trigger selection)

Data histogram integrates over a running time of 12 days, MC takes the detector status at one specific time within that period

Overall good agreement, detector acceptance reasonably modeled in MC

Frank Simon: Inclusive 0 and ALL in p+p at 200 GeV 1810/03/2006

Backup: BEMC 0 Data/MC Comparison

HT1 triggers4.5 GeV/c < 0 pt

< 4.6 GeV/cblack: datared: Pythia

HT2 triggers7.0 GeV/c < 0 pt

< 7.2 GeV/cblack: datared: Pythia

Preliminary! Limited MC Statistics! 0 properties well reproduced in MC for different pt and triggers