Physics at 13 TeV: CMS - scanning the unknown

CMS is getting ready to use its accurate detector to scan the many ripples of the unknown physics that may lie beyond the Standard Model. Foremost in everyone’s mind is the search for signs of the production of dark matter at a man-made machine.


Interestingly, the observation of dark matter or new weakly interacting particles implies detecting that some energy has escaped, i.e. it’s missing from the upcoming proton collisions at the LHC. This is why measuring and understanding “missing energy” will be a very important step in that direction. “A large amount of missing energy is the signature for many processes of physics beyond the Standard Model. However, it’s not the only interesting signature that we will be able to exploit when the new collision energy is available,” explains Luca Malgeri, CMS Physics Coordinator.

Among the interesting things not associated with missing energy are high-mass resonances, which are revealed by peaks in the invariant mass distribution spectra of pairs of leptons or hadronic jets. “The high-mass resonance peaks are strongly linked to the available energy in the collisions,” says Malgeri. “The probability of creating high-mass objects in 13 TeV collisions is much higher than during run 1, when the collision energy was 7 or 8 TeV.”

Theoretical studies also confirm that the standard cross-sections – i.e. interaction probabilities – to produce Standard Model particles at 13 TeV will be between 1.5 and 2 times higher than those at 8 TeV. For experiments, this translates into a quicker acquisition rate. “We will use the initial weeks of running at low luminosity to familiarise ourselves once more with our detector and, in particular, with the improvements that we have made during the long shutdown,” explains Malgeri. “Subsequently, if everything goes well, a few weeks of data acquisition should be enough for us to be able to observe, if they exist, the first new heavy particles, such as the Z prime, a particle that many theories predict and that should give us clues as to how the forces relate to one another.”

While high-mass objects will be within reach of the multi-purpose experiments very soon after startup, other particles could take months or years of accurate analysis before appearing on the physicists’ screens. “Some theories predict the existence of particles having signatures – such as long decay chains whose final states are several low-energy leptons or hadronic jets– that are difficult to disentangle from Standard Model signals. This type of information will require a lot of statistics and long studies,” Malgeri confirms.

Since exploring the unknown means not knowing what exactly we are looking for, CMS plans to explore strategies that allow scientists to potentially look for “anything” having a non-standard signature. “Given the finite data-storing capacity, we cannot write all the raw data we acquire onto disks,” explains Malgeri. “If we did this, we would fill thousands of terabytes per day! So, what any experiment does is use the trigger to rapidly select the information we want to store. However, when exploring unknown territory, this selection procedure could mean losing interesting information. A potential addition to this strategy being considered by CMS is to run data-scouting algorithms at trigger level. Those algorithms were already being used in our previous run. They scan the events that would normally be rejected as dominated by non-interesting processes and store minimal information about them at a very high rate. If interesting signatures are found, we can then modify our main trigger to look specifically into that specific type of event.”

With very powerful analysis tools and an extremely flexible trigger, the only issue for CMS could be too high a pile-up. “Our detector and off-line analysis work very well with a pile-up of up to 40 or 50. In principle, this is fine for run 2 when the 25-ns spacing between the proton bunches should ensure a relatively low level of pile-up. However, should the pile-up increase, we would indeed suffer a bit and we will need to develop new strategies,” Malgeri confirms. But he concludes: “We are preparing for the tough experimental conditions of run 2. Actually, we are looking forward to exploiting them and discovering what nature has in store for us beyond the Higgs!”

Check out more of our Physics at 13 TeV series in "ALICE - scratching under the surface", "ATLAS - extracting the most from new LHC data", "LHCb - a new data-processing strategy " and "TOTEM - a new era of collaboration with CMS". For a theory perspective on the next run, read "Life is Good at 13 TeV".

by Antonella Del Rosso