Something slightly odd seems to be going on with the Higgs boson. Separate measurements of its properties are showing two slightly different masses, according to scientists who presented their latest data on Dec. 13.

Ever since physicists found a particle that looks very much like the Higgs boson in July, they have been probing its properties, essentially running their experimental hands all over it to check out its features. They do this by smashing protons together at insanely high speeds in the Large Hadron Collider and watching the resulting rain of particles that gets produced. Within this melee, several Higgs bosons appear and almost immediately decay into other particles.

The LHC can detect the Higgs decaying in two different ways. One channel produces two characteristic photons while another creates four particles known as leptons. The two decay paths each give scientists a distinct value for the mass of the Higgs. But there's a little problem.

"There turns out to be a slight tension between the two masses," said physicist Beate Heinemann of the University of California, Berkeley, who works on ATLAS, one of the LHC’s Higgs-searching experiments. "They are compatible, just not super compatible."

The two photon channel is saying that the Higgs mass is 126.6 gigaelectronvolts (GeV), or about 126 times the mass of a proton. The four lepton decay route suggests the mass is 123.5 GeV. A very tiny disagreement that is nonetheless very strange because the Higgs should have one identifiable mass. ATLAS scientists noticed the discrepancy in their data previously and thought it might simply be a problem with calibrating their machinery. Yet even after calibration and analyzing more data, the difference remained.

Several physics blogs have noted that one way to explain this would be to have two different Higgs bosons, each with a very similar mass. This would be a truly bizarre and unexpected result. But it's much more probable that scientists are seeing a statistical mirage.

Heinemann said the four lepton channel has only analyzed about 10 Higgs bosons and the two photon channel about 500 Higgs. Physicists need to see the same result over and over in thousands or even millions of particle events before they are sure it's not just a statistical coincidence. "The most likely explanation is that it’s one particle," said Heinemann.

The LHC's other Higgs-searching experiment, CMS, has been measuring the particle's mass as well and so far is showing a value of about 125 GeV, which agrees with ATLAS within the two experiments' resolution. Everything seems to be pointing to a fairly well-behaved Higgs boson that has no hints of physics beyond the Standard Model, the framework that explains the interactions of all known particles and forces. The slight discrepancy is all par for the course in the business of physics discovery. We'll have to wait to see how it plays out when the LHC scientists present their next round of data and analysis at the Moriond conference in March.

Image: Claudia Marcelloni/ATLAS/CERN