Hot and heavy at the LHC

Protons collide with lead nuclei, sending a shower of particles through the LHC’s detectors. (Courtesy: ALICE/CERN)

By Tushna Commissariat

In the early hours of this morning the Large Hadron Collider (LHC) successfully collided protons and lead ions for the first time, with the collisions being recorded by all of the detectors: ATLAS, CMS, ALICE and LHCb.

Late last year the LHC trialled a similar run, during which it accelerated separate beams of lead ions and protons. However, at that time the beams were not successfully collided, and the run was postponed. To learn more about the 2011 run, take a look at this news story.

The whole business of colliding different particles is a difficult one, as it presents physicists with a number of technical challenges. “Firstly, the collisions are asymmetric in energy, which is a challenge for the experiments,” explains accelerator physicist and lead-ion team leader John Jowett. “At the accelerator level we don’t really see the difference in particle size, but the difference in the beam size and the fact that the beam sizes change at different rates may affect how the particles behave in collisions.”

Also, the LHC normally accelerates two opposing proton beams from 0.45 to 4 TeV, before they collide at a total energy of 8 TeV. Radio-frequency (RF) cavities are used to give the beams the necessary energy boost, as well as to keep them in strict synchrony. But here is where another problem arises: the system ties the momentum of one beam to the momentum of the other, while it needs to account for the differences between the protons and the much heavier lead ions. A lead nucleus, containing 82 protons, is accelerated from 36.9 to 328 TeV, or from 0.18 to 1.58 TeV per proton or neutron, which means that the RF cavities need to be tuned to different frequencies for each beam. This allows both beams to achieve stable orbits within their own ring during injection and acceleration. In the past, other projects have experienced difficulties in getting this just right, as have researchers at the LHC.

“The RF systems of the two rings can be locked together only at top energy before collisions, when the small speed difference that still remains can be absorbed by shifts of the orbits that are acceptably small,” says Jowett. He further explains that the beams then have to be adjusted again by the RF system so that the collisions take place inside detectors, where experiments take physics data, so a lot of preparation has been needed to allow the LHC systems to carry out this new operational cycle.

Researchers are hopeful that this latest short run will deliver the first data for proton–nucleus collisions before a scheduled main run takes place from January to February 2013, just before the accelerator is shut down for maintenance.