Direct observation of jet quenching
Editor's note: This story first appeared in the CERN Bulletin on December 13, 2010, under the headline 2010 ion run: completed! For more information, see posts about heavy-ion collisions and what scientists can learn from them. You can also read more in symmetry breaking about first measurements from the heavy-ion run and new insight into the primordial universe.
After a very fast switchover from protons to lead ions, the LHC has achieved performances that allowed the machine to exceed both peak and integrated luminosity by a factor of three. Thanks to this, experiments have been able to produce high-profile results on ion physics almost immediately, confirming that the LHC was able to keep its promises for ions as well as for protons.
A seminar on 2 December was the opportunity for the ALICE, ATLAS and CMS collaborations to present their first results on ion physics in front of a packed auditorium. These results are important and are already having a major impact on the understanding of the physics processes that involve the basic constituents of matter at high energies.
In the ion-ion collisions, the temperature is so high that partons (quarks and gluons), which are usually constrained inside the nucleons, are deconfined to form a highly dense and hot soup known as quark-gluon plasma (QGP). This type of matter existed about 1 millionth of a second after the Big Bang. By studying it, scientists hope to understand the processes that led to the formation of nucleons, which in turn became the nuclei of atoms.
At the recent seminar, the LHC’s dedicated heavy-ion experiment, ALICE, confirmed that QGP behaves like an ideal liquid, a phenomenon earlier observed at the US Brookhaven Laboratory’s RHIC facility. This question was indeed one of the main points of this first phase of data analysis, which also included the analysis of secondary particles produced in the lead-lead collisions. ALICE's results already rule out many of the existing theoretical models describing the physics of heavy-ions.
Jet quenching in the CMS detector
ATLAS presented the first direct observation of jet quenching, a phenomenon indirectly seen at RHIC a few years ago. The experiment has shown an imbalance in the energy distribution of two back-to-back jets (see ATLAS picture) in so-called central collisions. Centrality is a parameter indicating how big is the overlap of the two ions is when they collide; it is minimal when they hit only in the corner and it’s maximum when they overlap completely. ATLAS’s result is the first direct demonstration that when one of the two jets of particles goes through denser regions of QGP, its total energy is distributed in the medium and the jet appears to be almost totally absorbed. The observation of this imbalance and the study of the distribution of the energy are powerful means to study the properties of QGP. Confirmation of the direct observation of jet quenching came from the CMS experiment, which also reported the first observation of the production of Z bosons in heavy ion collisions.
In about one month of running with ions, the LHC experiments also collected evidence of the production of particles such as J/Psi and Upsilon, which again provide excellent tools to study the properties of deconfined matter. In future, they will be important in understanding the detailed behaviour of QGP.
Studies of heavy-ion physics have just started at the LHC and a lot of new results are expected from the data analysis that will be done in the coming weeks and months. So far, all the detectors have performed remarkably well, with data taking efficiency as high as 95%. This has translated into several publications that are just the beginning of the LHC’s heavy-ion adventure.
