Transverse Single Spin Asymmetries And Cross-sections for Forward π0 And η Mesons at

Len K. Eun, for the STAR CollaborationLawrence Berkeley National Laboratory

PhD: Penn State UniversityThesis Advisor: Steve Heppelmann

1Thursday, June 14, 2012

Transverse Single Spin Asymmetry

Previous STAR Measurement shows that π0 asymmetry is large in the region where the

production cross-section is reasonably well described by NLO pQCD prediction, up to xF of ~0.5.

2Thursday, June 14, 2012

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STAR Time Projection Chamber

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East FPD in Run 2006

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STAR Forward Pion Detector (FPD)

STAR FPD is a Pb-glass calorimeter in the very forward region of the

STAR.

Each of the main modules, placed on either side of the

beam line, consists of 49 Pb-glass cells, forming a

7X7 array.

For this analysis, we employed the “Center Cut”, defined by (ηγγ-3.65)2 + Φγγ2 < 0.15.

We look at di-photon event sample dominated by π0 decays.

3Thursday, June 14, 2012

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10MC Data

π0 - γ Separation at high xF

The separation between merged π0 clusters and single photons is primarily achieved through studying the second moment of clusters vs. cluster energy.

Previously, the weighting function was simply the energy deposited in a cell. Using logarithm of energy instead improves the separation at high xF significantly.

OLDLinear E

NEWLog of E

4Thursday, June 14, 2012

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2 cluster events, Pythia + GSTAR

Mass shift in data not explained by systematic error in di-photon separation measurement: Shift in gain.

Charged particle energy loss and Cherenkov with no attenuation has no energy dependence of gain.

Energy Dependence of Invariant MassStudy of di-photon mass vs. E

indicates that the gain increases with photon energy.

Cherenkov effect + attenuation of optical photons reproduces the observed mass shift vs. E.

Incident angle (~5˚) effect alters the shower shape. Superposition of 4 shower functions corresponding to 4 “z-slices” approximates the deformation.

Distance from cell to photon (cm)-6 -5 -4 -3 -2 -1 0

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Shower Shape in Vertical Direction

Distance from cell center to photon (cm)-4 -3 -2 -1 0 1 2 3 4

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ϒϒ

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EM-Shower Modeling

5Thursday, June 14, 2012

Physics Results)2

GeV

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c (3

dp

3E

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210 = 200 GeV s + X at , 0 STAR p + p

>=3.68 <0

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>=3.68 NLO pQCD <0

97 (2006)PRL>=4.0 STAR 2003 <0

>=3.8 STAR 2002 <0

>=3.3 STAR 2002 <0

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101 (2008)PRL>=3.7 no center cut, <0

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>=3.68 center cut, <

Cross-sections for π0 and η are consistent with expectations.

Transverse spin asymmetry for η is 2.2 sigma larger than that of the π0 for xF > 0.55

arXiv:1205.6826, recently submitted to PRD RC

6Thursday, June 14, 2012

Summary

Transverse single spin asymmetry at RHIC, measured by STAR, PHENIX, and BRAHMS, provides a useful avenue to test the current understanding of the pQCD.

STAR has measured both the cross-section and the asymmetry of forward neutral pions up to xF of 0.5. Cross-section is an important complimentary measurement that enables us to understand the spin measurement within the framework of pQCD.

We have made a number of improvements in analysis techniques to extend the xF reach of both cross-section and asymmetry measurements up to xF of 0.75.

In addition to π0, we also measured η-meson cross-section and asymmetry.

We find that the cross-sections are consistent with expectations based on pQCD + global fragmentation analysis.

The transverse spin asymmetry for the η is 2.2 standard deviation larger than that for the π0.

7Thursday, June 14, 2012