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It’s December 15, 2015, and an auditorium in Geneva is packed with physicists. The air is filled with tension and excitement because everybody knows that something important is about to be announced. The CERN Large Hadron Collider (LHC) has recently restarted operations at the highest energies ever achieved in a laboratory experiment, and the first new results from two enormous, complex detectors known as ATLAS and CMS are being presented. This announcement has been organized hastily because both detectors have picked up something completely unexpected. Rumors have been circulating for days about what it might be, but nobody knows for sure what is really going on, and the speculations are wild.  

The CMS spokesperson takes the stage first, giving a presentation with no surprises until the very end, when two plots appear showing the energies—theoretical and actual—carried by a flood of particles emerging from head-on collisions between protons traveling at nearly the speed of light. If you squint, there appears to be bump in the experimental curve, suggesting too many events at one point than theory would predict. It could be evidence for a new, unexpected particle—but at a level that’s merely interesting, not definitive. We’ve seen things like this before, and they almost always go away when you look more closely. 

Then Marumi Kado from ATLAS steps up, with a strangely confident look in his eye—and when the results finally flash on the screen, the audience understands why. ATLAS has seen the bump too, at the same point as CMS did, but now it’s so prominent that you can’t miss it. This really does look like a new particle, and if it is, there is suddenly an enormous crack at the very heart of high-energy physics.

The signal is one of the simplest you can imagine: it represents two high energy photons emerging from the decay of a subatomic particle created in a proton-proton collision. It’s very similar to the signal that led to the discovery of the Higgs boson in 2012. But this particle is not the Higgs boson: it is six times more massive. Nobody had predicted anything like this. It is shocking to the physicists in the auditorium. People look around, astonished, trying to confirm that their own reactions are reflected in what they see in their colleagues’ faces. If the observations are confirmed, it will be revolutionary. This could mean nothing less than the fall of the Standard Model of particle physics (SM), which has passed every experimental test thrown at it since it was first put together over four decades ago.

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