Jet Quenching at LHC: ALICE weighs in

Yesterday, the ALICE Collaboration weighed in with a jet quenching result of their own. In their preprint

the ALICE physicists analyze a quantity much more familiar from the RHIC experiments: the suppression of large-pT hadrons in nuclear collisions relative to binary collision scaled p+p reactions. Three aspects are remarkable:

  1. The maximal suppression factor (1/RAA ≈ 7) at pT = 7 GeV is about 40% stronger than that seen at RHIC;
  2. RAA rises strongly with increasing pT, reaching 0.35 at 20 GeV;
  3. The shape of RAA(pT) in the intermediate range 1 GeV ≤ pT ≤ 7 GeV is almost identical to that observed at RHIC.
  4. The lower minimum of RAA in spite the much flatter slope of the hadron spectrum indicates a much larger energy loss of the leading parton. It will be interesting to see whether the energy loss coefficient scales like the charged particle multiplicity between RHIC and LHC.

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.

First LHC Heavy Ion Data

Today the ALICE Collaboration posted the first analyses of data from Pb+Pb collisions at the LHC:


First Pb+Pb Collisions at 2.76 TeV !

The long-awaited day has arrived, in spectacular fashion. The LHC has declared “Stable beams with ions”, and ALICE, ATLAS and CMS are all seeing events, and sharing their beautiful event displays with the world.

It is enormously satisfying to see the field of relativistic heavy ion physics take this tremendous (large) step on the energy frontier. Congratulations and kudos to all who have worked for decades to make this happen. Surely extraordinary discoveries await.

Update: There is now a press release from CERN with useful links to various event displays.

First LHC Data!

Noted with great pleasure- the first preprint on LHC data has been posted by the ALICE Collaboration.


This is a marvelous tribute to the hundreds of physicists, students, engineers and technicians, many of whom have invested nearly two decades towards this achievement.

The paper presents ALICE’s measurement of the charged particle pseudo-rapidity density and its distribution for |η|<1 in √s=900 GeV p+p collisions. The results, obtained from only 284 events,   are in good agreement with UA5 data for proton-antiproton collisions at this energy. But what’s important is this demonstration that ALICE can (very!) quickly extract physics data from a few collisions, suggesting that new results from new energy regimes will appear very quickly as the LHC increases the collision energy.