News

Shortly after the August Quark Matter conference, the Large Hadron Collider geared up for a test run of proton on lead (pPb) collisions. Proton nucleus collisions are the intermediate stage between the short length scale proton proton physics and finite sized physics of lead lead collisions. These types of collisions are going to be the main focus of the January running period and this short test was meant to get us ready for what’s to come. One of the test fills, a few hour continuous running period from midnight to 6am on September 13, gave us about two million good proton lead collisions.

The 2011 PbPb run was the run for rare physics and pushing the boundaries of what can be measured and high precision. With an arsenal of dedicated triggers we showed a whole range of interesting new results at this years Quark Matter conference.

Our measurement of medium induced energy loss in QGP using photon-jet events was finally submitted to arXiv:1205.0206 and for publication in Physics Letters B (PLB). This paper has been long awaited since the conception of the heavy ion program at CMS more than 12 years ago (See early CMS Note, p122). It is the first time in the heavy ion field where the medium induced energy loss can be directly studied with a fully unbiased probe (photon).

Our paper on the measurement of high transverse momentum charged particle suppression, accepted by the European Physical Journal C (EPJC) in February 2012, was recently published and featured in the cover of the new EPJC issue (March 2012). The figure that was featured in the cover is the main result of the paper showing the suppression of the charged particles over a large transverse momentum range, where a number of theoretical predictions show large variation, in particular at high transverse momentum regime, exhibiting large theoretical uncertainty. Together with other jet quenching related quantities that we have measured, this measurement should help elucidate the mechanism of jet quenching and the properties of the medium produced in heavy-ion collisions at collider energies.

For the month of November the LHC switched to colliding 2.76 TeV lead nuclei. The heavy ion group was heavily involved in the data taking which resulted in a very rich, high statistics dataset. This year’s recorded luminosity is 20 times higher than last year’s data giving us access to more precise measurements of known phenomena as well as new measurements and probes available for the first time ever in heavy ion physics.

The new year brought a new type of collision at the LHC: the accelerator smashed protons and lead nuclei together. Although we already caught a glimpse of these asymmetric proton-lead (pPb) collisions during a pilot run last September, the data collected early this year was the first sustained pPb run at LHC energies.

After barely a year of pp and a month of PbPb collisions, several exciting new results emerged from analyses led by our group. From the first 7 TeV pp collisions two summers ago novel correlations were seen in collisions producing the highest number of particles. These were never before seen in pp collisions nor were they predicted by any of the existing Monte Carlo models. This was the first manifestation of unexpected physics at the LHC which has gathered significant interest in both the media and the pp and heavy ion community.