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Image: Diphoton event
A computer graphic shows the spray of particles created by a proton-on-proton collision in the Large Hadron Collider’s CMS detector. The two green lines at lower center and upper right indicate the emission of two photons. Physicists say that could be part of an intriguing pattern, or merely a coincidence. (Thomas McCauley / CERN / CMS)

The Higgs boson is the biggest find of the century in particle physics, but for the past few weeks, physicists at the Large Hadron Collider have been considering whether there’s a mystery that’s even bigger. Or at least more massive.

The potential mystery has to do with a pattern of particle decay that results in the emission of two photons. The readings collected so far by the teams using the ATLAS and CMS detectors point to a slight “bump” in the expected pattern.

That may hint at the existence of a previously undetected particle with a mass of about 750 billion electron volts – six times heavier than the Higgs, French physicist Adam Falkowski (a.k.a. Jester) writes in his Resonaances blog.

Could it be a second Higgs boson? Evidence for gravitons or extra dimensions? Ever since the findings were made public three weeks ago, theories have been flying around like speeding muons, and with good reason. “If the diphoton excess is really a new particle, we are basically guaranteed to find other phenomena beyond the Standard Model,” Falkowski says.

However, the two-photon excess may be merely a coincidence – the sort of pattern that pops up in an early stage of data collection, but fades away when more readings are factored into the findings. It’s happened before.

In an email, Don Lincoln, a physicist at Fermilab who’s part of the CMS team, advises caution:

“When all effects are taken into account, the CMS experiment expects to see an excess like this about 10 percent of the time, while the ATLAS measurement should happen by chance about 1 percent of the time.  While these probabilities seem small, you should remember that the two experiments make thousands of plots each of which might have tens or hundreds of points.  With that many measurements, statistical fluctuations like this are expected; indeed, you’d be surprised if you didn’t see a few tantalizing coincidences like this.

“Still, the fact that the two experiments see an excess at the same place does mean we need to take it seriously.  This is because a statistical fluctuation and the first hints of a discovery initially look very much the same.  To figure out which this is will require the data we expect to record in 2016.  If I were a betting man, I would expect that this signal will turn out to be a statistical will o’ the wisp; however if it turns out to be a discovery it will be very exciting indeed.”

The LHC is currently shut down for maintenance but should resume proton-on-proton collisions by April. Stay tuned … or as Falkowski says, “Cross your fingers and fasten your seat belts.”


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