In Picture:Â ATLAS particle detector at the Large Hadron Collider, near Geneva
Geneva, June 17, 2016:Â 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.
Jim Olsen, the CMS physics coordinator, 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.
The SM describes what the building blocks of the universe are and how they work, and from there, at least in principle, explains every other phenomenon in nature. Originally theorists thought that the SM would be an approximation of a more fundamental theory that would be quickly discovered. This is what has always happened in the past. Newton’s theory of gravity, for example, doesn’t apply to bodies that are extremely massive, or which are moving close to the speed of light. It is accurate enough that engineers could use it to send the New Horizons space probe toward Pluto and have it arrive in just the right place nine years later. Einstein’s theory of General Relativity, however, is more fundamental, and applies in those extreme where Newton’s theory breaks down.
Moreover, there are many reasons to believe that the SM is incomplete. In particular, the mechanism that generates the mass of the elementary particles suggests that the theory must be modified at higher energies. To discover this new physics was the number one motivation for the construction of the LHC and several other experiments before that.
To theorists’ surprise, however, the SM has performed much better than originally expected. This has been both a blessing and a curse for particle physics for many years. On one hand, the discovery of the Higgs boson was an enormous success, identifying the SM’s last, and arguably most important, building block. On the other, the fact that the Higgs has just the mass and all the properties everyone expected generated a widespread pessimism about new discoveries. The search for a more fundamental theory might drag on indefinitely.
But the bumps in the ATLAS and CMS data, which showed up at an energy of 750 billion electron-volts (GeV), would completely change this situation overnight, making it virtually certain that more discoveries will be coming during coming years. If the hint of a new particle is real, the successes of the SM suddenly will have come to an end.
The importance of this result is clear to everybody working in the field and it has immediately triggered a huge amount of work on the possible implications. None of the more fundamental models that currently exist as possible replacements for the SM can explain the bump. If the SM has fallen it is likely not for any reason we expected.
If the new particle is real, it is absolutely unclear what might be its role in the greater scheme of things. Maybe it is related indirectly to the Higgs boson somehow, or maybe it is connected with the puzzle of dark matter in the universe. Or maybe it is just there by chance. Certainly these are questions that scientist will have to answer in the future and more data will help to understand what lies ahead.
This is by far the most exciting thing that has happened in particle physics over the last three decades. If this hint of new physics is confirmed-something that could happen within just a few weeks, or possibly even within days-it is difficult to state the importance of such a discovery. It would be bigger than the detection of the Higgs boson, which was just confirmation of what was already known.
If the bump is real, we are about to start writing a whole new chapter in the history of fundamental physics. It is impossible to imagine where this could lead.
We could know the answer very, very soon.