In 2015, the LHC resumed collisions at an unprecedented energy of 13 TeV—an increase of 60 percent over the run that ended in 2012. Physicists pored over the data resulting from those collisions, searching for any hints of physics beyond known Standard Model processes. One of the most straightforward approaches for searching for those hints is to look for an obvious sign: an excess, or “bump” in a well-populated and measured data distribution. In a recent CMS paper accepted by Physical Review Letters, the CMS collaboration presented one such search using the newest data set of LHC collisions.
In this new measurement, CMS scientists looked at events where collisions resulted in two energetic jets, so-called “dijet” events. If the decay of some particle produced these two jets (or rather the two quarks or gluons that resulted in jets), the sum of the energies of the jets would be proportional to the mass of the parent. This quantity, called the dijet mass, can be calculated for all collisions resulting in two jets. Particle jets comprising quarks and gluons, however, are copiously produced in the quark- and gluon-rich environment of LHC collisions. The vast majority of these two-jet combinations do not, in fact, result from a single parent particle decaying. The amount of energy in each collision sets the upper bound of the possible dijet mass. This limitation also means that the dijet mass distribution across all collisions falls smoothly as a function of increasing mass. A particle decaying into two jets would cause a bump in that falling distribution — the obvious signpost that scientists look for. The more massive the particle, the easier it is to see in against the decaying background.
Numerous theories describing new physics prescribe particles, often massive, that decay into two jets. There has been no sign of these new particles using dijet decays at the LHC or Tevatron in the past. However, the much higher energy of Run 2 of the LHC meant that, even with just a few months of collision data, the new CMS measurement could look for even higher-mass particles, allowing scientists to find evidence for new, massive particles or at least extend the exclusion range.
Although the CMS search did include dijet events with unprecedented masses, upwards of 6 TeV, there were no obvious signs of new physics. CMS scientists were able to place a systematic limit on the mass of any hypothetical new particle decaying into jets, providing an important test of several proposed theories that predict such new particles. In all cases, the limits set were significantly more stringent than any set in previous searches for similar decays.
While an obvious signpost for new physics at the start of the LHC run would have been exciting, the lack of such a sign does not imply the lack of new physics. Just as some roads are not clearly marked, some hints of potential new physics can only be teased out with much larger data sets, the kind that the resumption of LHC collisions this year are expected to bring.