When beams of particles crash into each other at high speeds, the collisions yield hundreds of new particles, most of which fly away from the collision point at close to the speed of light. However, the Compact Muon Solenoid (CMS) team at the LHC found that in a sample of 2 million lead-proton collisions, some pairs of particles flew away from each other with their respective directions correlated.
"Somehow they fly at the same direction even though it's not clear how they can communicate their direction with one another. That has surprised many people, including us," says MIT physics professor Gunther Roland, whose group led the analysis of the collision data along with Wei Li, a former MIT postdoc who is now an assistant professor at Rice University. A paper describing the unexpected findings will appear in an upcoming issue of the journal Physical Review B and is now available on arXiv. The MIT heavy-ion group, which includes Roland and MIT physics professors Bolek Wyslouch and Wit Busza, saw the same distinctive pattern in proton-proton collisions about two years ago.
The same flight pattern is also seen when ions of lead or other heavy metals, such as gold and copper, collide with each other. Those heavy-ion collisions produce a wave of quark gluon plasma, the hot soup of particles that existed for the first few millionths of a second after the Big Bang. In the collider, this wave sweeps some of the resulting particles in the same direction, accounting for the correlation in their flight paths