Higgs boson

The latest results from the Atlas experiment indicate that there may be two different Higgs bosons—one that weighs 123.5 GeV (in blue) and another that's 126.6 GeV (in red).

A month ago scientists at the Large Hadron Collider released the latest Higgs boson results. And although the data held few obvious surprises, most intriguing were the results that scientists didn’t share.

The original Higgs data from back in July had shown that the Higgs seemed to be decaying into two photons more often than it should—an enticing though faint hint of something new, some sort of physics beyond our understanding. In November, scientists at the Atlas and LHC CMS experiments updated just about everything except the two-photon data.* This week we learned why.

Yesterday researchers at the Atlas experiment finally updated the two-photon results. What they seem to have found is bizarre—so bizarre, in fact, that physicists assume something must be wrong with it. Instead of one clean peak in the data, they have found two an additional peak.* There seems to be a Higgs boson with a mass of 123.5 GeV (gigaelectron volts, the measuring unit that particle physicists most often use for mass), and another Higgs boson at 126.6 GeV—a statistically significant difference of nearly 3 GeV. Apparently, the Atlas scientists have spent the past month trying to figure out if they could be making a mistake in the data analysis, to little avail. Might there be two Higgs bosons?

Although certain extensions of the Standard Model of particle physics postulate the existence of multiple Higgs bosons, none of them would predict that two Higgs particles would have such similar masses. They also don’t predict why one should preferentially decay into two Z particles (the 123.5 GeV bump comes from decays of the Higgs into Zs), while the other would decay into photons.

The particle physicist Adam Falkowski (under the nom de plume Jester) writes that the results “most likely signal a systematic problem rather than some interesting physics.” (By “systematic problem” he means something like a poorly-calibrated detector.) The physicist Tommaso Dorigo bets that it’s a statistical fluke that will go away with more data. Indeed, he’s willing to bet $100 on it with up to five people, in case you’re the kind of person who likes to wager on the results of particle physics experiments with particle physicists. The Atlas physicists are well aware of both of these possibilities, of course, and have spent the past month trying to shake the data out to see if they can fix it. Still, the anomaly remains.

But let’s not let this intriguing blip distract us from the original scent of new physics. Back when the preliminary data seemed to show that the Higgs was decaying into two photons more often than it should, I wrote that it could be “a statistical blip that would wash away in the coming flood of data.” But more data has now arrived, and the blip hasn’t gone anywhere. The Higgs boson continues to appear to be decaying into two photons nearly twice as often as it should.

All the more reason to stay tuned for the next big data release, currently scheduled for March.

*Update 12/17/12: In November, scientists at the Atlas and CMS experiments (not the "LHC" experiment—apologies for the dumb typo) updated everything except for the two-photon data and, in the case of Atlas, the data regarding the decay of the Higgs into four leptons. I have added "just about" to indicate that the two-photon data wasn't the only thing missing. I apologize for the imprecise language.

*Update 12/17/12: The sentence as originally written inadvertently implied that there are two peaks in the two-photon data. In fact the two photon data has one peak, but at a different mass than the peak found in other data sets. The Higgs to two-photon data shows a peak at 126.6 GeV, while the Higgs to four-lepton data (newly updated) shows a peak at 123.5 GeV. Apologies for the confusion.

Higgs boson at CMS

Image of the new Higgs data courtesy of Atlas/CERN. Image of Higgs to two-photon event courtesy of CMS/CERN.