The Higgs boson is sending mixed signals: its mass seems to vary depending on how it is measured. What’s more, oddities in the way it decays into other particles, first noticed when the team at the Large Hadron Collider announced the discovery of a new boson in July, do not seem to be going away.

Experiments at the LHC, part of the CERN particle physics laboratory near Geneva, Switzerland, do not detect the Higgs directly. Instead, they collect and analyse the slew of particles it is predicted to decay into. The latest results from the ATLAS detector at the LHC suggest that when we look at its decay into two photons, we find that the new boson’s mass is about 3 gigaelectronvolts greater than when calculated from its decay into particles called Z bosons.

Albert De Roeck, one of the key Higgs hunters at ATLAS’s sibling detector, CMS, finds this puzzling. “The results are barely consistent,” he says.

But he says the inconsistency almost certainly reflects a problem with the measurements rather than strange physics.


“There’s probably a large statistical fluctuation pulling the data around,” says Matt Strassler of Rutgers University in Piscataway, New Jersey. He says the problem may have implications for other results.

No fluke

The ATLAS team also announced new results from analysing the Higgs boson’s rate of decay into pairs of photons. The standard model of particle physics predicts exactly how often this should happen. Intriguingly, when early hints of a new particle were upgraded to a “discovery” in July, it was doing that more often than the standard model says it should. But there was not enough data to say whether this was a fluke.

If the Higgs really decays into photons at too fast a rate, it would offer some clues as to where to look for new physics that might explain mysteries like dark matter, gravity and the dearth of antimatter in the universe.

Today, physicists announced that the excess has not gone away – but it also does not look much more likely to be a genuine event, even though physicists have now seen twice the number of decays.

“It keeps us titillated,” says Raymond Volkas from the University of Melbourne in Australia. Strassler calls the new results “very interesting and tantalising”, but adds that they are still not quite enough to tell us anything.

“I guess the CMS results are now very highly anticipated,” says De Roeck. The CMS team has not released their data on the Higgs decay to photons yet, saying they need more time to do their analysis.

According to Strassler, the Higgs mass problem could be a sign that we should not trust the unusually high rate of photon decays measured. “This unfortunately tends to make me less confident that the excess seen in the photon signal will survive with more data.”

Next week, the LHC will move into a different phase before shutting down in early 2013 while the machine is upgraded. You’d think that might let physicists’ blood pressure recover, but there is no rest for the wicked. Marumi Kado, presenting the ATLAS results today, said they have only analysed a fraction of the data collected so far. “Surprises might be waiting for us in the present data,” he said.