The CMS experiment at CERN’s Large Hadron Collider (LHC) has recorded its first lead-lead collisions at an energy of 2.76 TeV per nucleon pair, marking the start of its heavy ion research program. Physicists around the world expect a wealth of new results and phenomena from these collisions, which occur at energies 14 times higher than previously achieved by the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory.
At 11:20:56 on Monday, Nov. 8, the LHC Control Center declared stable colliding beams of heavy ions. CMS immediately detected the first collisions, each producing thousands of particles whose trajectories are reconstructed in the CMS silicon detectors and whose energies are measured in the calorimeters. Moments later, the data were analyzed and the first images of these events were produced.
“We are very excited to see the first heavy ion collisions in CMS. The data will allow us to study hot matter at much higher temperatures than ever before,” said CMS heavy-ion run operations manager Bolek Wyslouch, a professor of physics at MIT. “I am very happy for the students and the physicists who have worked so much to prepare CMS for heavy-ion collisions and will certainly benefit from the richness of the physics.”
At RHIC, a hot and dense state of matter interpreted as the ”quark gluon plasma” was produced. Heavy-ion collisions at LHC are expected to produce a similar type of matter under even more energetic conditions. CMS will study the numbers of particles produced (several thousand in the most energetic collisions) and their distributions in space. Angular asymmetries, such as those seen at RHIC, the so-called “elliptic flow” of particles, will be used to characterize the strength of the interactions in the plasma.
“It is just amazing to see how well the CMS detector is performing under the challenging conditions of heavy ion collisions in LHC,” said CMS spokesperson Guido Tonelli. “The quality of the data is so good that we will soon be able to publish our first measurements. Having data collected by the same detector in proton-proton and heavy-ion modes is a powerful tool to look for un-ambiguous signatures of new states of matter.”
The CMS (compact muon solenoid) experiment, one of the major LHC experiments studying these record-setting “heavy-ion” collisions, is led by MIT researchers. MIT has a team of about 20 people in the heavy ion group at CMS, led by Wyslouch and Associate Professor Gunther Roland, both of the Department of Physics. Two other LHC experiments — ALICE and ATLAS — will also record and analyze data from these high-energy collisions, which will each produce up to 10,000 particles.
The current plan is for the LHC to collide heavy-ions until Dec. 6, delivering enough data for CMS to exploit new probes such as jets of hadrons, Upsilon mesons and Z bosons. Every collision will be recorded, thanks to the large bandwidth of the CMS data acquisition system. The upcoming results will shed light on the nature of the strong interaction as well as on the state of the universe in the first microseconds after the Big Bang.
The pattern of many months of proton-proton collisions followed by about a month of heavy-ion collisions is one that will be followed in subsequent years.
MIT Professor Wit Busza is also a member of the CMS team, and faculty members Christoph Paus, Markus Klute and Steven Nahn are working on the proton-proton collision experiments at LHC.
Following the first 7 TeV collisions at the end of March, the intensity of the LHC beams has rapidly increased, allowing the “re-discovery” of all known Standard Model particles – from the humble neutral pion through to J/psi and Upsilon mesons and culminating in the detection of thousands of W and Z bosons, as well as many hundreds of Top quarks. Dozens of papers have already been produced by CMS, on the detector performance and physics measurements. In addition, searches for new physics phenomena such as leptoquarks, quark-compositeness (internal structure of quarks) and Supersymmetry have begun, with CMS setting new limits on these and more. In September, CMS also produced a paper describing an unexpected result — two-particle correlations in proton-proton collisions. This last is reminiscent of similar effects seen in heavy-ion collisions.