Slimming down a heavy weight

CMS, the heaviest particle detector ever constructed, will lose some weight in preparation for the luminosity upgrade at the LHC, says Jordan Nash at Imperial College London.

As with the ATLAS upgrade work, the main challenge is to fit a more sensitive inner detector into the existing space. That means cramming more detecting power into the same volume – but that won’t come at the cost of extra weight, says Nash.

“Actually, we’re thinking of cramming less material in, but making it more functional,” he says, by taking advantage of modern technology not available when CMS was designed in the early 1990s. “The fewer interactions there are between the particles from the collisions and the structure of the detector the clearer the results we will get – and that makes the detector more functional.”

That will be especially important after the luminosity upgrade. At that point, there will be many more collisions occurring at the heart of CMS and the detector will have to be super-sensitive to identify the important collisions, which will be occurring much more frequently.

Speeding up the detector

When the LHC starts up again this year, there will be roughly one proton-proton collision every 40 millionths of a second. About one in every one hundred thousand of those collisions will be unusual enough to warrant recording for further investigation in the search for unusual subatomic particles.

The inner trackers at CMS are wired up to software that can pick out and store those unusual collisions. But, after the luminosity upgrade unusual events will be much more common. “We will need to process the data from the central tracking device at a much faster rate,” says Nash.

That means increasing the number of trackers in the centre of CMS but, importantly, also will involve a re-write of the software designed to analyse that data to improve the speed of analysis.

The inner detector will be entirely stripped out and replaced, but the outer regions of CMS will also be modified, says Nash. “In the next few years we’ll be finishing the detector, if you want to call it that,” he says. “There are areas of CMS that we could still cover with muon detectors.

“If you look at CMS at the moment it just looks red on the outside – there should be muon detectors there, and we want to add those to give the detector a full capability to detect muons.”

Planning takes time

That’s rather modest work, and it can be easily accomplished in the annual winter shutdown at the LHC without disrupting the scheduled running of CMS and the LHC as a whole. But the work to the inner detector is much more of an undertaking and will require at least a year of installation work.

“A big part of our work is learning how to organise and coordinate the change over,” says Nash. “We’ll have to move out a lot of old material and replace it with the new trackers, and then cable them up and test them to be sure we’re ready to start taking data again.”

That changeover period is likely to run from 2017-2018. “We’ve just finished doing that installation exercise for the first CMS but soon it will be done again,” Nash says.

The research and development that will ultimately lead to that changeover is well underway – although the first CMS will not start producing any data until later in the year. “It’s a little unusual to be having to work on upgrades before we’ve actually begun taking data,” admits Nash. “The problem is it takes so long to do the research and development planning that we have to begin early.”

CMS detector. It is red right now, but won't be after upgrade work.