LHC Detects Evidence of New Physics

After nearly 6 months of smashing particles, the Large Hadron Collider has seen signs of something entirely new. Pairs of charged particles produced when two beams of protons collide seem to be associated with each other even after they fly apart.

“It is a small effect, but it is very interesting in itself,” said physicist Guido Tonelli, spokesperson for the LHC’s CMS experiment. Tonelli and colleagues announced the results in a seminar at CERN September 21 and in a paper submitted to the Journal of High Energy Physics.

The LHC finally got up and running in March after more than a year of false starts. Beams of protons were smashed together in the 17-mile-long ring at energies of 7 teraelectronvolts (TeV) — three times the energy that had been achieved before.When two protons collide, they produce a flurry of smaller, short-lived charged particles that fly away from each other at certain angles and speeds. The CMS (Compact Muon Solenoid) experiment at the LHC detects the path each of these particles takes. Physicists can then use those tracks to reconstruct what happened at the heart of the collision, like reassembling shards of glass from a broken window.

In the new experiment, the CMS team took data on the charged particles produced in hundreds of thousands of collisions. The team observed the angles the particles’ paths took with respect to each other, and calculated something called a “correlation function” to determine how intimately the particles are linked after they separate. The plot of the data ends up looking like a topographical map of a mountain surrounded by lowlands and a long ridge behind it.

In the most basic case (below, left), the data looked exactly like the physicists expected it to. But in cases where at least 110 charged particles were produced, the team saw a funny ridge-like structure extending away from the mountain peak (below, right).

That ridge essentially means that particles in some pairs are flying away from each other at close to the speed of light along one axis, but are oriented along the same angle in the other axis.

It’s as if two particles somehow talked to each other when they were produced, the physicists said. This phenomenon has never been seen before in proton-proton collisions, though it resembles something seen at RHIC (the Relativistic Heavy Ion Collider) at Brookhaven National Laboratory in New York. That effect was interpreted to be from the creation of hot dense matter shortly after the collisions.

The CMS team collected the data in mid-July, and spent the rest of the summer trying to blame it on an error or artifact of the data.

“We are here today because we didn’t succeed to kill it,” Tonelli said. As far as the team can tell, the effect is real.

But where it comes from, nobody knows. There are a lot of possible explanations, and the team is not ready to choose one yet.

“This is a subtle effect, and careful work is required to establish its physical origin,” said MIT physicist Gunther Roland at the seminar at CERN. “So fire away.”



Images: 1) Image of a 7 TeV proton-proton collision in CMS producing more than 100 charged particles. 2) The correlation functions for “minimum bias” collisions (left) and for collisions that produced at least 110 charged particles (right); the new ridge is indicated with an arrow. Credit: CERN/CMS Collaboration