Especially when PANIC is PANIC 2008, Particles and Nuclei in Collision, which was held in Eilat, Israel this year. I had the honor of giving a plenary talk on the role of “soft physics”: both for helping understand the physics at RHIC, and how the LHC will contribute to our evolving understanding.
I’ve written about “soft” physics before, both as “day 1” contributions to the LHC physics program, and something fascinating in its own right (but heck, I’m biased). The idea is that the low-energy particles, which are generally seen as “gunk” to be cut away by analyses looking for new high-mass particles, have very simple features if one compares different energies and different systems (e.g. collisions of protons or heavy ions). RHIC has been interpreting the heavy ion data in terms of a hot, dense thermalized medium, but treating the very similar p+p data as “reference”.
One interesting thing I learned this week was from a talk by my colleague Mike Lisa from OSU that one can systematize the differences between the transverse momentum distributions in p+p and Au+Au simply by accounting for the basic fact that momentum and energy have to be conserved in detail in every event. Doing this one finds that both systems have the same parent distribution, and the observed differences are merely from the imposition of conservation laws. This has two immediate reactions based on people I spoke to: 1) the “trivial” interpretation that all systems are “nothing but phase space”, and 2) the “deep” interpretation that heavy ions both show indications (nearly identical ones!) that the system is similarly hot and dense and “flows” like a fluid. Unfortunately for those who buy in to #1, #2 has much more experimental evidence supporting it, as Mike and his student point out in their papers. Interesting stuff and very much worth a look.
And unsurprisingly, I’ve posted photos.
