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

Motivation Several channels which are sensitive to p0 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.

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

Reconstruction of p0 --> gg Trigger and shower maximum detector (SMD) completely installed for 2005, providing full azimuthal coverage over 1.086 < 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

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

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

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

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

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

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...

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: 21.0 pp2006: 1.4 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.

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