Probably just a weird fluctuation, but still something to watch. Event Display of a Candidate Electron-Positron Pair with an Invariant Mass of 2.9 TeV Edit: Follow-up after data-taking in 2015 ended: no further events with that high energy, now one event is reasonable. Was just a weird fluctuation to have this event so early on. CMS result with the full 2015 dataset ATLAS result with the full 2015 dataset The CMS detector found an event with a very high-energetic electron and a very high-energetic positron. If they are from the decay of a particle, this particle would have a mass of around 2.9 TeV (the so-called "invariant mass" of the electron/positron pair). The most likely candidate would be the hypothetical Z', a heavier version of the known Z boson. The event is significantly more high-energic than everything seen in run 1 (2010-2012). Sure, the proton energy increased, but the analyzed data from this year represents just 0.3% of the collisions from run 1. How surprising is it? There are well-known processes that can produce those pairs, but they rarely do that at very high energy. One could calculate the probability to find an event with an invariant mass of at least 2.9 TeV, but that is unfair - the choice of 2.9 comes from the one observed event. CMS calculated the expected number of events above 1.0, 2.0 and 2.5 TeV for the size of their analyzed dataset: Above 1.0 TeV: 0.21 Above 2.0 TeV: 0.007 Above 2.5 TeV: 0.002 Those numbers don't have proper systematic uncertainties yet, so they might change a bit in the future, but it is clear that an event with 2.9 TeV was not expected that early in run 2. What comes next? CMS studied just 1/3 of their dataset, and the full ATLAS dataset is even a bit larger. I'm quite sure both collaborations look at that now. If they find a second event, things get really interesting. If they find nothing, it is probably just a statistical fluctuation (with then ~0.04 expected events above 2 TeV it is not that surprising any more. Keep in mind that there are many possible particle combinations where one can look for events). If it is a Z', it should also decay to muons with the same probability. Looking for high-energetic events with a muon and an antimuon will be interesting as well.