Most of the particles produced at the LHC are unstable, and almost instantaneously decay into lighter particles that we can observe in detectors such as CMS. For example, many particles decay into pairs of leptons – electrons or muons (heavier cousins of electrons). If we measure the masses of these pairs of leptons, we should be able to determine the mass of the particle that gave rise to them.

However, due to the number of collisions taking place in the LHC and the variety of particles produced, we might observe pairs of leptons at lower masses that don’t come from the decay of particles but are produced from random interactions. In order to determine whether the pairs really came from the decay of a single particle, we plot the mass of the pairs against the number of instances when that particular mass was observed. This plot or histogram shows clear peaks at those masses where the lepton pairs have been formed from the decay of particles, as in Figure 1.

[Figure 1 shows the di-muon mass peaks for a variety of particles including […]. The peaks are said to represent the “resonance” of the muon pairs, corresponding to the masses of the particles from which they decayed.]

At much higher masses, however, there is very little background and peaks can be observed with fewer data. CMS data points to such a high-mass peak, with [X] events observed in the muon channel on [insert dates here] and [Y] events observed in the electron channel on [insert dates here].