Generating a ‘Clean’ pi0 Spectrum in STAR

The STAR detector at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider uses polarized proton collisions to investigate the origin of the proton spin, using measurements such as neutral pion (π0) asymmetries. STAR’s Endcap Electromagnetic Calorimeter (EEMC) is especially useful for detecting photons from π0 decays at forward-angle scattering from ≈15 to ≈40 degrees above the beam direction. We identify π0’s by constructing invariant mass spectra from these photons. Large background contributions are present in these spectra and distort the true value of the π0 invariant mass. By applying constraints (cuts) on parameters such as the opening angle of the photons in the reconstructed pair, decay vertex position, photon energy, and energy asymmetry, I have been able to produce a ‘clean’ π0 spectrum with little background using the data from 2006. In the future, a ‘clean’ π0 invariant mass spectrum can provide an avenue to calibrate the EEMC against the known pion mass. This method combined with our current calibration methodologies could improve understanding of the EEMC response, critical to our ability to investigate π0 spin asymmetries. The results, along with a comparison to simulated Monte Carlo data, will be presented

An Event-by-Event Comparison of Clustering Algorithms for Photon Detection in the STAR Endcap Calorimeter

The STAR detector at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory uses polarized proton collisions to determine the origin of the proton spin, using measurements such as neutral pion asymmetries. The Endcap Electromagnetic Calorimeter (EEMC) in the STAR detector is especially useful for detecting photons from π° decays at forward angles. This latter measurement is obtained from the Shower Maximum Detector (SMD) in the EEMC where narrow crossed scintillator strips measure the energy deposited in them and can be used to identify the location of the photon shower. The electromagnetic shower most often deposits energy in a small number of adjacent strips that collectively form a “cluster.” This work has focused on a qualitative and quantitative comparison of two different clustering algorithms that were developed to reliably identify π° events and to effectively discriminate against background cluster selection that produces false π° signals. This comparative analysis will be presented and the strengths and weaknesses of the algorithms will be discussed

Neutral Pion Cross Section and Spin Asymmetries at Intermediate Pseudorapidity in Polarized Proton Collisions at Root s=200 GeV

The differential cross section and spin asymmetries for neutral pions produced within the intermediate pseudorapidity range 0.8 \u3c eta \u3c 2.0 in polarized proton-proton collisions at root s = 200 GeV are presented. Neutral pions were detected using the end cap electromagnetic calorimeter in the STAR detector at RHIC. The cross section was measured over a transverse momentum range of 5 \u3c p(T) \u3c 16 GeV/c and is found to agree with a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry A(LL) is measured in the same pseudorapidity range and spans a range of Bjorken-x down to x approximate to 0.01. The measured A(LL) is consistent with model predictions for varying degrees of gluon polarization. The parity-violating asymmetry A(L) is also measured and found to be consistent with zero. The transverse single-spin asymmetry A(N) is measured over a previously unexplored kinematic range in Feynman-x and p(T). Such measurements may aid our understanding of the onset and kinematic dependence of the large asymmetries observed at more forward pseudorapidity (eta approximate to 3) and their underlying mechanisms. The A(N) results presented are consistent with a twist-3 model prediction of a small asymmetry over the present kinematic range