What A Difference a Day Makes (*)

The large value of charged multiplicity  dNch/dη = 1584 observed by ALICE in the 5% most central collisions, and the large value of elliptic flow they report in minimum bias collisions both suggest that the demise of the strong coupling paradigm at the LHC have been greatly exaggerated. I attribute this in part to “the tyranny of asymptotic freedom”, that is, the beauty of the running coupling constant in QCD has led to some wishful thinking about how fast a logarithmic term can vary. There is also the enormous appeal and simplicity of the “classical QGP” described as a nearly free, massless Boltzmann gas. But the data from RHIC, and now these first ever so exciting results from LHC, have taught us that there is also a wealth of fundamental physics and new phenomena in the strongly-coupled regime that is so much more interesting than a non-interacting gas!Predictions for dN_ch/d\eta excluded by ALICE

At the same time there has indeed been a demise, in fact several, in this case of the various predictions of (mostly) lower multiplicity densities than the value of ~1600 reported by ALICE. I was one of those who would have guessed the lower values of 1200-1300, but since that was only a guess, the surprise for me was a pleasant one. Ironically, earlier on the day this value was announced in Andrea Dainese’s talk at the LHCC , I had read with great interest this paper Hadron production at the LHC: Any indication of new phenomena by Levin and Rezaeian , describing in a very accessible fashion their predictions for LHC multiplicities based on saturation physics. There is also a nice discussion of the extensions incorporated in their approach over the KLN model, so it is more than a little puzzling that the simpler KLN model (at least in one of its various instantiations) better describes the Pb+Pb data at the LHC.

(See also Berndt Mueller’s post for a more quantitative discussion of what we learn from the first two ALICE papers on Pb+Pb collisions at the LHC.)

(*) The title of this post is drawn from one of the loveliest songs in the American jazz idiom.

Theorists confront the “Ridge”

The “ridge”, an elongated (in rapidity) structure in the two-particle correlations associated with a hard trigger hadron, keeps attracting the attention of theorists. [Stay tuned for a forthcoming review of the experimental status of the “ridge”.] Two recent preprints explore strikingly different explanations. (more…)

Color Glass Condensate Primer

We asked Larry McLerran, the world’s leading expert on the Color Glass Condensate, to write this CGC primer for us: (more…)