Hybrid beams in the LHC

The first proton-ion beams were successfully circulated in the LHC a couple of weeks ago. Everything went so smoothly that the LHC teams had planned the first p-Pb collisions for Wednesday, 16 November. Unfortunately, a last-minute problem with a component of the PS required for proton acceleration prevented the LHC teams from making these new collisions. However, the way is open for a possible physics run with proton-lead collisions in 2012.


Members of the LHC team photographed when the first hybrid beams got to full energy. The proton and lead beams are visible on the leftmost screen up on the wall (click to enlarge the photo).

The technical challenge of making different beams circulate in the LHC is by no means trivial. Even if the machine is the same, there are a number of differences when it is operated with beams of protons, beams of lead or beams of proton and lead.

Provided that the beams are equal, irrespective of whether they consist of protons or lead nuclei, they revolve at the same speed and the bunches always encounter each other in the same conditions in the same places. However, to provide proton-lead collisions, the machine has to be operated with two beams that are unequal in mass and charge. “Since both beams see the same bending field in the two-in-one LHC magnets, the lead nuclei will revolve just a little slower than the protons,” explains John Jowett from the LHC Accelerator Physics Group of the Beams Department. “At injection energy, the protons make an extra turn of the ring every 15 seconds. The beam-beam encounter points move slowly along the experimental sections, disappearing into the separate beam pipes and emerging several seconds later around another experiment. The basic periodicity is gone. Furthermore, the LHC bunch train is broken up with assorted gaps so the complex pattern of encounters is constantly shifting. Moreover, as the beams are accelerated, each becoming even more relativistic, this motion slows down abruptly until, at collision energy, the remaining small difference in speed can be absorbed by a small shift of the orbits, the motion freezes and periodicity is restored. But there is no escape from these effects during injection and energy ramping.”

Indeed, experience of analogous situations at previous accelerators led some experts to regard this as fatal for beam stability. “Arguments to the contrary were subtle and required experimental tests, and this is what has now been achieved,” says John Jowett. On 31 October, after months of careful preparation of the systems, proton and lead beams were persuaded to co-exist quite happily in the two rings and were ramped to full energy.

This was possible because, although the LHC’s magnetic fields must be the same for the two beams, the electric fields do not need to be. The independent radiofrequency systems of the two rings can run at different frequencies and are capable of handling two different beams independently. “In the energy ramp, the LHC is transformed into a perfect gigantic roulette wheel: as the motion of the encounter points slows down they can finally end up anywhere,” explains John Jowett.

Indeed, initially, the bunches that normally collide at ATLAS ended up meeting each other some 9 km away! However, further virtuoso tuning of the radiofrequency system solved the problem. “Imagine two necklaces with many beads on an elastic string wrapped around a cylinder. To line up the bead patterns, you can make many little tugs on one necklace, stretching it and allowing it to spring back with a small shift,” says John. This is closely analogous to what the radiofrequency systems did, except of course that both necklaces were moving around the cylinder in opposite directions 11,000 times a second...


For a scientific description of the proton-lead programme, click here.

The physics of the proton-lead collisions

Proton-ion collisions will allow physicists to study the properties of the so-called “cold nuclear matter”. “We will be able to investigate more precisely than ever before which nuclear objects participate in the collisions. Moreover, during the collisions, the hitting proton will probe the structure of the nucleus. In other words, it will still be a proton-proton collision but with one of the protons bound in the nucleus,” explains Yves Schutz, Deputy Spokesperson of the ALICE experiment.

Some theories predict that, at the high collision energy of the LHC, yet a new state of matter could be explored. This state, known as Colour Glass Condensate, is different from the quark-gluon plasma – the hot and dense state created in lead-lead collisions. “Theory tells us that the Colour Glass Condensate is a gluon-dominated state of matter. By studying proton-ion collisions, we should be able to demonstrate the existence of this state, which would then be the precursor state of the quark-gluon plasma formed in ion-ion collisions,” says Yves Schutz.

The main purpose of the future p-Pb runs is to observe the same type of phenomena but with different types of collisions. For example, physicists will be able to compare the behaviour of jets when just two protons collide (jet propagation in the vacuum), when protons and ions collide (jet propagation in cold nuclear matter), and finally when lead and lead-ions collide (jet propagation in hot nuclear matter or QGP). These observations will allow scientists to disentangle the effects due to cold nuclear matter from those due to the formation of the quark-gluon plasma and therefore represent a powerful test of the current theories of the structure of matter.


by CERN Bulletin