Nuclear Physics Seminar

There has been a great deal of discussion in the literature since 2005 of the "Perfect Liquid" description of the heavy ion collision system produced at RHIC. This purported state of matter equilibrates very rapidly, within the first fm/c, is completely opaque, and essentially non-viscous. Typically missing in scenarios of the RHIC collision environment is an acknowledgement that the collisions are peppered with minijets, i.e. regular jets with no lower transverse momentum cut-off. Minijets were observed over 20 years ago by UA1 and were included by Wang and Gyulassy in the HIJING Monte Carlo model for SPS and RHIC nucleus-nucleus collisions. The prevaling opinion in the RHIC community is that these objects are dissipated in the hot, dense medium. Yet I will show that they persist in most-central, 200 GeV Au-Au collisions.

Two-particle correlation studies from STAR will be presented for the 62 and 200 GeV Au-Au and Cu-Cu minimum bias data plus the 200 GeV p-p minimum-bias data. The evolution of the correlation structures generated by minijets will be studied from p-p to central Au-Au. Implications of these observations will be discussed.
For example, the collision system appears to be approximately transparent, at least to minijets, in centrality ranges where v2 (elliptic flow) is largest. Implications of these data for understanding the RHIC collision system will be discussed.