Latest experiment at Large Hadron Collider reports first results



By Massachusetts Institute of Technology

After a two-year hiatus, the Large Hadron Collider, the largest and most powerful particle accelerator in the world, began its second run of experiments in June, smashing together subatomic particles at 13 teraelectronvolts (TeV)—the highest energy ever achieved in a laboratory. Physicists hope that such high-energy collisions may produce completely new particles, and potentially simulate the conditions that were seen in the early universe.

In a paper to appear in the journal Physics Letters B, the Compact Muon Solenoid (CMS) collaboration at the European Organization for Nuclear Research (CERN) reports on the run’s very first particle collisions, and describes what an average collision between two protons looks like at 13 TeV. One of the study leaders is MIT assistant professor of physics Yen-Jie Lee, who leads MIT’s Relativistic Heavy Ion Group, together with physics professors Gunther Roland and Bolek Wyslouch.

In the experimental run, researchers sent two proton beams hurtling in opposite directions around the collider at close to the speed of light. Each beam contained 476 bunches of 100 billion protons, with collisions between protons occurring every 50 nanoseconds. The team analyzed 20 million “snapshots” of the interacting proton beams, and identified 150,000 events containing proton-proton collisions.

For each collision that the researchers identified, they determined the number and angle of particles scattered from the colliding protons. The average proton collision produced about 22 charged particles known as hadrons, which were mainly scattered along the transverse plane, immediately around the main collision point.

Compared with the collider’s first run, at an energy intensity of 7 TeV, the recent experiment at 13 TeV produced 30 percent more particles per collision.

Lee says the results support the theory that higher-energy collisions may increase the chance of finding new particles. The results also provide a precise picture of a typical proton collision—a picture that may help scientists sift through average events looking for atypical particles.

“At this high intensity, we will observe hundreds of millions of collisions each second,” Lee says. “But the problem is, almost all of these collisions are typical background events. You really need to understand the background well, so you can separate it from the signals for new physics effects. Now we’ve prepared ourselves for the potential discovery of new particles.”

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