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STARSTAR

Double Longitudinal Spin Asymmetries of Inclusive Charged Pion Production in Polarized

p+p Collisions at 200 GeV

Adam Kocoloski, MITFor the STAR Collaboration

Kyoto UniversityOctober 3 2006

• Motivation

• Analysis Method

• Asymmetries

• Systematic Uncertainties

• Summary

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Why Charged Pions?

• Two complementary measurements with different contributions from polarized quark PDFs

• For qg processes, ALL(+) - ALL(-) tracks sign of G

• STAR is capable of efficient reconstruction and identification of charged pions over a large range of transverse momenta

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Measuring ALL at STAR

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ALL =σ ++ −σ +−σ ++ +σ +−

= 1P1P2

× N++ − RN+−N++ + RN+−

3 Separate Measurements

•Beam Polarizations

•Measured at RHIC polarimeters

•Relative Luminosities

•Beam-Beam Counter (BBC) scintillator tiles

•Spin-sorted Yields

•BBC and Barrel EMC trigger on events

•Time Projection Chamber (TPC) reconstructs and identifies pions

STAR TPC

•|| < 1.6 and 0.5 < r < 2.0 m inside 0.5T solenoid

•Efficient reconstruction of charged tracks to 20 GeV; particle ID via dE/dx up to 10 GeV

J. Adams, et al. Phys. Lett. B637, 161-169 (2006).

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Jet patch pTMinimum bias Jet patch

Analysis Method

Triggering•All triggers require BBC coincidence

•Statistics dominated by jet patch triggers: EM energy deposition in x = 1.0 x 1.0 patch

Cuts•Select high-quality sample of TPC tracks at || < 1.

•Particle ID accomplished using TPC dE/dx

•Use BBC timing window for vertex selection

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

2005 Inclusive +/- ALL

GRSV polarized PDFs:M. Glück, E. Reya, M. Stratmann, W. Vogelsang, PRD63, 094005 (2001).B. Jäger, M. Stratmann and W. Vogelsang, PRD70, 034010 (2004).

Fragmentation functions modified from KKP:B. A. Kniehl, G. Kramer and B. Pötter, Nucl. Phys. B582, 514 (2000).

KKP modification:Charge-separated versions of KKP pion fragmentation functions obtained by multiplying favored partons by (1+z) and unfavored by (1-z).

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Systematic Summary

Trigger Bias{3.0 - 7.3}, pT dependent

Non-longitudinal polarization 3.0

Relative Luminosity 2.2

Particle Identification 1.7

Beam Background 0.3

All uncertainties are 10-3 on ALL:

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Trigger Bias

• Majority of pions are sub-leading particles in trigger jet– Significant statistics from “away-

side”, untriggered jet as well• PYTHIA afterburner used to

construct “polarized” event generator

• Calculate ALL in simulation with and without trigger requirement

• Bias estimated using average of GRSV-min and GRSV-std scenarios

• 3.0 - 7.3 x 10-3 as a function of pT and charge sign

Other Cross-Checks• Charge-summed asymmetry

consistent with neutral pions• “Near-side” and “away-side”

asymmetries consistent with each other

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Particle ID Background

• Plot track dE/dx in terms of standard deviations from mean of pion dE/dx band

• Estimate >90% purity in acceptance window from multi-Gaussian fit

• Shift dE/dx window to select a background sample and calculate its double-spin asymmetry

• B.g. fraction x (asymmetry + 1 sigma) 1.7 x 10-3

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Looking Ahead

• 2006 saw improvements in:– polarization– – trigger acceptance and

thresholds

• Begin to look at non-inclusive measurements…

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L • dt∫

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Looking Ahead: Away-side ALL

• Calculate for each pion relative to trigger jet

• Originally motivated by trigger bias studies

– “away-side” sample free from fragmentation bias

• Reasonable statistical precision

• Needs theoretical guidance

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R = Δη( )2 + Δφ( )

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Summary• First spin result for inclusive charged pion production at STAR.

• EMC-based triggering setup limits statistics at high pT and introduces a bias in the measured ALL. This bias is estimated in simulation to be less than the size of the statistical uncertainties.

• Promising Outlook in 2006:– Significant increase in FOM– Away-side ALL reduces bias from jet trigger

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STARSTAR

Backup Slides

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Inclusive Charged Pion ALLAdam KocoloskiSTARSTAR

Comparison With Neutral Pions