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.


  1. Xin-Nian Wang
    November 22nd, 2010 | 6:34 pm

    It is interesting to observe how our physics intuition are influenced by others’ views or opinions. During the days leading up to the last predictions for LHC, many people are so convinced that the dN_ch/d\eta will be as low as Wit’s ruler-prediction that they looked for variations and improvements of their models to give that low number. During a one-sided discussion session on saturation at HP2010 in Eilat, I was also so convinced by my colleagues that I had to proclaim that I would abandon HIJING model if the LHC number comes to be that low. I simply cannot get that low number within HIJING once constrained by RHIC data. Now with ALICE data, many people have to abandon their improved and more complete models (like KLN2 and KNL3) and go back the original KNL1 and start justifying why it did worked after all.

Leave a reply