Nuclear Physics Seminar

The PHOBOS multiplicity detector allows the study of 2-particle angular correlations over a broad range of pseudorapidity and azimuthal angle. Data have been extracted for both inclusive charged particles and for events containing a high-pT trigger particle in the
PHOBOS spectrometer.

Projections of inclusive 2D correlations in relative azimuth (delta-phi) and pseudorapidity (delta-eta) onto 1D delta-eta correlations have been interpreted in the context of a simple cluster model. Average cluster sizes and widths from Cu+Cu and Au+Au collisions are found to be similar for events with the same fractional cross-section. Results for semi-peripheral heavy ion collisions show an enhancement of the cluster size compared to p+p, an effect which displays a pronounced decrease towards central events. Finally, using corrections derived using dynamical and cluster models, the measured data can be extrapolated to full phase space, leading to much larger
e?ective cluster sizes and widths than previously noted in the literature.

For central Au+Au collisions, correlations using a pT > 2.5 GeV/c trigger particle exhibit a near-side `ridge' structure, extending to at least |delta-eta|>4. The yield associated with the ridge contribution diminishes in magnitude with decreasing Npart and eventually seems to disappear for collisions with less than ~80 participating nucleons. Comparison of these data with models should
be useful for understanding the origin of this phenomenon in heavy ion collisions.