1. Inclusive p0 Production in Polarized pp Collisions using the STAR Endcap Calorimeter Jason C. Webb, Valparaiso University, for the STAR Collaboration
Outline
1. Motivation -- ALL(p0)
2. Experimental Overview
3. Data Analysis
4. Longitudinal Spin Asymmetry
5. Outlook

2. Motivation Several channels which are sensitive top0
p+/-
Several channels which are sensitive to
the gluon polarization in the nucleon are
available at STAR
With current luminosities at RHIC, STAR
is focusing on inclusive observables: jets
and identified hadrons
With increased luminosity
in future runs, gamma-jet
coincidences will be used
to probe Dg(x). Measuring
ALL(p0) is a necessary first
step, since neutral pion decays
are the largest source of
background in the prompt-g
measurement.
h =0
h =3.3
Relative contributions of partonic subprocesses change significantly with rapidity.

3. Experimental Overview
Magnet
0.5 T Solenoid
Triggering & Luminosity Monitor
Beam-Beam Counters
3.4 < || < 5.0
Central Tracking
Large-volume TPC
|| < 1.5
Calorimetry
Endcap EMC (Pb/Scintilator)
1.086 < h < 2.0
Shower-Maximum Detector
And other systems...
2005
h = 1.09
h = 2.0
Triggers:
Minimum Bias (MB): BBC Coincidence, highly prescaled
High Tower Trigger: MB * Endcap calorimeter cell (tower) above threshold
Two thresholds: HT1 and HT2 with lower (HT1) prescaled
Dedicated runs with HT2 w/out BBC coincidence to study beam-related backgrounds
Presentation today based on HT2 only

4. Reconstruction of p0 --> gg
Trigger and shower maximum detector (SMD) completely installed for 2005, providing
full azimuthal coverage over < h < 2.0
pp2005 dataset after quality cuts: 497 production runs with 1.1 pb-1
SMD
p0 invariant mass: Mgg = E √(1 - zgg2) sin(f/2)
- total energy measured by towers
- opening angle measured by SMD
- energy sharing zgg = |E1-E2|/E measured by SMD
p0 candidates accepted over 1.09 < h < 2.0
p0 must satisfy software HT ET > 3.0 GeV
beam backgrounds difficult to reject on an
event-by-event basis w/out compromising
p0 yields.
Photon candidates identified
by forming clusters around
“seed” strips in the SMD

5. Double Longitudinal Spin Asymmetries
Spin-sorted mass spectra
like-sign helicity
unlike-sign helicity
The double longitudinal spin asymmetry is defined as
where “++” and “+-” refer to the like- and unlike-
sign helicity states respectively.
Polarizations (PY, PB) are measured by the RHIC
polarimeters.
Relative luminosities for the two helicity states
are measured by the BBCs.
Spin dependent yields are determined by counting the number of events in each helicty state

6. Beam Backgrounds
sector ~ f
Online QA plots showed a significant f- asymmetric background in 2005 in all
layers of the endcap
Asymmetry persists when looking at p0
candidates
Beam backgrounds get worse with increasing pT
towers
Beam background tracks parallel to beam-axis
reconstructed w/ TPC agree with the observed
hit pattern.
sector ~ f
Mgg [GeV]
p0 gains [GeV/ch]
7.0 < pT < 8.0 GeV
Ngg 0.1 < M < 0.18 GeV
Gains determined by MIPs
and confirmed by p0's. h

7. Beam Backgrounds III Spectrum reasonably described by four components
1. p0 + h MC
2. cluster splitting
3. beam background
4. combinatoric background

8. Beam Backgrounds II purity = signal/total
Dedicated beam-background runs were taken
with HT2 triggers without the BBC coincidence
requirement.
These dedicated runs allow us to:
1. Extract the purity of the p0 sample by fitting
the f-dependence of the production data
2. Estimate the spin-asymmetry of the beam
backgrounds
purity = signal/total
purity = “signal” / total
HT w/out BBC coincidence
HT with BBC coincidence
f-asymmetric
beam background
f-symmetric signal
7.0 < pT < 8.0 GeV
sector ~ f
sector ~ f

9. Beam Backgrounds IV Measured asymmetries for the beam background
To compute ALL(p0):
To compute systematic uncertainties:
1. Assume ALL(bg) = 0% +/- stat. error
2. Estimated uncertainty in purity = 5%
Measured asymmetries for the beam background
are consistent with zero with large statistical
uncertainties.
In each pT bin fiducial cuts were made
to maximize the purity of the p0 sample

10. Double Longitudinal Spin Asymmetry: Results
ALL(p0) is consistent with zero
Current statistical and systematic uncertainties
do not rule out any gluon polarization scenario.
Systematic uncertainties are dominated by
contributions from the beam background
The 2005 ALL(p0) results from the endcap
are limited by the systematic uncertainties
associated with the magnitude and spin-
dependence of the beam background. But
the future looks bright...

11. Outloook In 2006 RHIC had its first long polarized
pp run. This realized several improvements
over the 2005 run:
1. Increased luminosity from 1.1 to 3.5 pb-1
2. Increased polarization from 47% to 60%.
3. Improved gamma and p0 triggers.
4. Shielding in the tunnels supressed the
observed beam backgrounds by more than
an order of magnitude
Background triggers/real triggers:
pp2005:
pp2006:
The 2006 data will:
1. Increase our reach to higher pT
2. Begin to yield meaningful tests of the
various polarization scenarios
3. Provide an important baseline for the
future prompt-g measurements.

13. Calibration Current calibration based on MIPs
As a cross check, a tower-by-tower calibration using p0's was performed
1. p0 mass reconstructs to the correct value using
the MIP derived gains
2. The eta-p0 mass difference is reconstructed
properly
3. Tower-by-tower calibration using p0's agrees
to MIP-gains to w/in ~10%.
Mgg [GeV]
Mgg [GeV]
p0 gains [GeV/ch]
tile number
All runs which pass detector QA
first guess at background shape