Higgs certainty boosted by more complete analysis

Its discovery last month sparked a media storm but the Higgs boson is now even more of a sure thing. ATLAS, one of the two experiments behind the original discovery, has carried out a more complete analysis that boosts the statistical significance of the Higgs signal even further.

The analysis also shores up the classification of the particle as a Higgs boson ? though whether this is exactly the Higgs boson predicted by the standard model of particle physics, or one with slightly different properties has yet to be nailed.

On 4 July, the twin detectors at the Large Hadron Collider near Geneva, Switzerland ? CMS and ATLAS - announced that they had finally found the long-sought Higgs boson, the last missing piece of the standard model and the particle credited with giving other particles mass.

The Higgs wasn't glimpsed directly: once it is created inside the detector it decays so quickly that it can only see the other particles the boson decays into. ATLAS and CMS only see it by looking for five different sets of particles, known as decay channels. For a Higgs of a certain mass, the standard model predicts the rates of decay for each channel.

Strange Higgs

The CMS detector experiment presented rates for all five: how frequently the new particle decays into pairs of bottom quarks, tau quarks, photons, W bosons and Z bosons. ATLAS's analysis was less complete. The experiment was only confident enough to present evidence for the Higgs in two of the channels: the photons and the Z bosons.

That was still enough for each experiment to separately claim a statistical significance of 5 sigma, meaning there's a 5-in-10-million chance that the signal is due to background processes, rather than a new particle.

But ATLAS has now taken another step towards completing their analysis. On 31 July, they posted a paper to the physics pre-print server arXiv that included the rates for the W boson channel. This raised the statistical significance of their Higgs signal to nearly 6 sigma, which means the chance of the signal they observe being due to background processes has now sunk to 2 in a billion.

As well as improving certainty in the original result, the new results will help physicists determine whether the particle is the same Higgs boson predicted by the standard model, or something stranger. "It could well be that it's not the standard model Higgs boson," says ATLAS spokesperson Fabiola Gianotti.

Exotic object

The WW channel, as it is called, is not as important for detecting the Higgs boson as the other two channels ATLAS cited in July. But it's one of the most important probes of the standard model, which makes specific predictions about the likelihood of Higgs boson decaying into W and Z bosons.

"It would essentially be poking a huge hole in the interpretation of this particle as a type of Higgs if the ratio of the rates of Higgs decay to W versus Higgs decay to Zs weren't as predicted," says Matt Strassler, a blogger and theoretical physicist based at Rutgers University in Piscataway, New York. "That's the importance of the measurement: it's testing the Higgs mechanism at its most fundamental."

The next step for ATLAS will be to check the other two channels for deviations from the standard model. CMS saw fewer tau particles than the standard model predicts. Although the significance of the deviation could disappear, it is fuelling speculation that the Higgs boson may deviate from the predictions of the standard model ? something that would open the door to more exotic, comprehensive theories of the universe.

"We're very interested to see what ATLAS has to say about that," says Strassler. "When the uncertainties become even smaller, when we have even more data and more studies, we'll be able to understand better the properties of this particle, if it's a Higgs boson or a more exotic object," says Gianotti.

CMS posted a paper to the arXiv the same day as ATLAS, but its analysis was the same as the one presented on 4 July.

References: ATLAS paper: arXiv:1207.7214v1; CMS paper: arXiv:1207.7235v1

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