LHC restart provides tantalizing hints of a possible new particle

The recently souped-up Large Hadron Collider isn’t revealing the universe’s secrets yet.

Two analyses of proton collisions in the retooled LHC, which restarted at record energy in June after a two-year hiatus, have failed to yield any discoveries. The results do contain at least one intriguing hint of a new particle, researchers announced December 15 at a meeting at CERN in Geneva, but it will require more collisions to evaluate that possibility. After a successful first wave of collisions from 2010 to 2013 that uncovered the Higgs boson, physicists are hoping the revamped machine exposes new particles that would expand the standard model, the catalog of nature’s fundamental components.

LHC physicists collect and analyze the subatomic shrapnel produced when protons slam into each other at nearly the speed of light. This year, the LHC’s protons collided with 13 trillion electron volts of energy; the machine was running at 8 trillion electron volts when it exposed overwhelming evidence for the Higgs boson in 2012. The increased energy and other upgrades mean protons collide more often and can produce more massive particles.

Despite the increased potential for discovery, researchers with the LHC’s two main detectors, CMS and ATLAS, announced that, for the most part, the fresh data matched the predictions of the standard model. One exception is a small bump, an excess of activity at a particular energy, that could indicate the existence of a particle with a mass of roughly 750 billion electron volts, or 750 GeV. That’s about six times as massive as the Higgs. CMS and ATLAS spotted the possible signature of such a particle decaying into pairs of photons.

But the tantalizing detection could be a statistical fluke. While the chances of seeing this bump at 750 GeV are fairly low, says theoretical physicist Matt Strassler, finding such a bump anywhere within the huge range of masses explored by the LHC is not nearly as unexpected. (He compares it to playing the lottery: There’s a very small chance that you win but a very good chance somebody wins.) The bump “is not dramatically better than other hints we’ve seen in the past,” says Strassler. In fact, researchers announced that a similar bump that turned up in the LHC’s initial run has all but disappeared in the new data.

The uncertainty won’t stop theorists from exploring the implications of the existence of a 750 GeV particle. And plenty of physicists remember that the Higgs boson also began as an inconclusive bump before subsequent data sealed its discovery. Yet once again, physicists have to play the waiting game in the search for unexpected phenomena. The LHC is done smashing protons for the year but is scheduled to resume in the spring.