CMS/CERN

Physicists at the Large Hadron Collider (LHC), the giant particle-physics experiment near Geneva, Switzerland, have searched for many possible subatomic particles and novel phenomena. They have tried to recreate dark matter, reveal extra dimensions of and collapse matter into microscopic black holes.

But the possibility of an electrically neutral particle that is four times heavier than the top quark — the current heaviest — and that could decay into pairs of photons has apparently never crossed anybody’s mind. No theorist has ever predicted that such a particle should exist. No experiment has ever been designed to look for one.

So when, on 15 December last year, two separate teams at the LHC independently reported hints of such a particle (see Nature http://doi.org/bc4t; 2015), the reaction of many experts was similar to that of US physicist Isidor Isaac Rabi when the muon, a heavier relative of the electron, was discovered in 1936: “Who ordered that?”

If the particle exists, the implications would be enormous. Precisely because it is so unexpected, it could be the most important discovery in particle physics since quarks — the elementary constituents of protons and neutrons — were confirmed to exist in the 1970s. Perhaps it would be the biggest deal since the muon itself.

The evidence so far is scant, however. It amounts to a few too many pairs of γ-ray photons produced with combined energies of 750 giga­electronvolts when the LHC smashes protons together. The fact that two separate detectors spotted it at almost exactly the same energies gives some hope, but anomalous signals such as this often show up in experiments only to later vanish back into the noisy background.

Still, people at CERN, the European particle-physics lab that hosts the LHC, have scarcely talked about anything else since. And theoretical physicists around the world have gone into overdrive: more than 200 papers have been posted online with theories that could explain the particle. One possibility is that it could be a heavier cousin of the Higgs boson; another, even more tantalizing one, is that it is a type of graviton, the particle hypothesized to carry the force of gravity. If so, it could point to the existence of extra dimensions of space beyond the familiar three.

Some have discounted the outburst of preprint articles as merely an attempt by authors to rake up citations. One physicist has even done a quantitative comparison of this spike in activity with other fads that have come and gone in the past (see M. Backović Preprint at http://arxiv.org/abs/1603.01204; 2016), charting theorists’ initially exploding, then fading, interest. But describing theorists’ interest as ‘ambulance chasing’ is a bit unfair. To paraphrase Albert Einstein, if people knew what they should be looking for, it wouldn’t be called research.

“The LHC is now providing the opportunity of a lifetime to break entirely new ground.”

And particle physicists’ excitement is understandable, if tempered by caution. For decades, their field has been finding evidence for the standard model of particle physics, a collection of theories that was put together in the 1970s and has been more successful than anyone expected. The current generation of young physicists was not even born when particle accelerators produced their last genuinely surprising results. Meanwhile, searches for physics beyond the standard model have so far come up empty — at accelerators such as the LHC but also in many tabletop experiments and at detectors built underground or sent into space to look for dark matter. The most notable exception to the standard model’s standard fare has been the discovery, beginning in 1998, that the elementary particles called neutrinos spontaneously oscillate between their three known types, or flavours — something that the original version of the standard model had not predicted. That breakthrough earned two physicists a well-deserved Nobel Prize last year.

The LHC is now providing the opportunity of a lifetime to break entirely new ground. In 2015, it restarted after a long shutdown that brought the energies of its collisions to a record 13 teraelectronvolts, from 8 TeV. This has put much more massive particles in reach — if any exist — but it will be the last substantial jump in collider energies in a generation. More-powerful machines, if they ever see the light of the day, will take decades to plan, develop and build.

The good news is that whether the new particle exists or the data bump is a statistical anomaly is not a question that will leave us hanging for long. The LHC experiments had time to observe only relatively few collisions in their first 13 TeV run last year, before the experiment shut down for its winter recess.

At a meeting in the Italian Alps that starts on 12 March, LHC researchers might present fresh analyses of those data that could provide more clues. And the machine will begin to collect vastly more data in April. If the bump seen last year was an anomaly, it should go away by the summer. If not, stay tuned for some interesting announcements at the next round of conferences.