Neutral Pion Production in Nucleus-nucleus Collisions at 158 and 200 GeV/nucleon

Neutral Pion Production in Nucleus-nucleus Collisions at 158 and 200 GeV/nucleon
Author:
Publisher:
Total Pages: 11
Release: 1998
Genre:
ISBN:

Download Neutral Pion Production in Nucleus-nucleus Collisions at 158 and 200 GeV/nucleon Book in PDF, Epub and Kindle

Two years ago, at the 12th Winter Workshop on Nuclear Dynamics held here in Snowbird, the authors presented WA80 limits on direct photon production in 200-GeV 32S + Au collisions. It was found that the results were consistent (within 1[sigma]) with the absence of an excess of photons over those that can be accounted for by the two-photon decay branches of [pi]° and [eta] mesons and by the small photon contributions from other radiative decays. They are in the process of finalizing the direct-photon production results from collisions of lead nuclei at 158 GeV/nucleon. The author briefly discusses the status of the analysis and gives some preliminary results at the end of this talk. However, most of this presentation is concerned with a very different aspect of the photon measurements: distributions of neutral pions. In contrast to direct photons which probe initial collision conditions, hadrons, such as neutral pions, interact strongly and decouple late in the reaction evolution and, thus, provide information concerning the system at freeze out. Transverse momentum spectra at low and intermediate p{sub T} relate to thermodynamic and hydrodynamic descriptions of the hot, dense systems. In addition, the high-p{sub T} region reflects the hard-scattering regime and may help one understand initial-state particle production by forming a bridge to proton-proton and proton-nucleus results. It follows that it is essential that the [pi]° measurements cover a large p{sub T} range.

Neutral Pion Double Helicity Asymmetry in Polarized Proton-Proton Collisions at [the Square Root of Sigma]

Neutral Pion Double Helicity Asymmetry in Polarized Proton-Proton Collisions at [the Square Root of Sigma]
Author: William Axel Leight
Publisher:
Total Pages: 112
Release: 2012
Genre:
ISBN:

Download Neutral Pion Double Helicity Asymmetry in Polarized Proton-Proton Collisions at [the Square Root of Sigma] Book in PDF, Epub and Kindle

One of the primary goals of the spin physics program at the STAR experiment is to constrain the polarized gluon distribution function, [delta]g(X,Q2), by measuring the double longitudinal spin asymmetry, ALL, of various final-state channels. Neutral pions provide a potentially powerful final state because they are copiously produced in p+p collisions and have few backgrounds. In 2009, STAR took 14 pb-1 of integrated luminosity of 200 GeV p+p collisions, with average beam longitudinal polarization of 59%. Neutral pions produced in these collisions can be identified using STAR's large-acceptance electromagnetic calorimeter, with help from tracking from the STAR Time Projection Chamber. This work presents a measurement of the inclusive neutral pion ALL from this data, based on a new [pi]0 reconstruction algorithm. A comparison to theoretical predictions and other experimental results suggests that the current best-fit value of AG, the gluon contribution to the proton spin, is too small and that [delta]G is actually comparable in magnitude to the quark contribution to the proton spin [delta][sigma].