Higgs update

One of the most publicised goals for the Large Hadron Collider (LHC) in Switzerland was to search for the Higgs boson, the hypothetical particle that could explain how other subatomic particles get their mass.

Well, the LHC has been running for nearly 18 months now, so what has it found? Any sign of the Higgs?

The LHC’s two general purpose experiments, ATLAS and CMS, announced their preliminary findings at August’s Lepton-Photon conference, held in Mumbai, India. And so far it’s not looking good for the Higgs boson.

(There are actually six experiments running at the LHC. As the name suggests, the Large Hadron Collider is a very large particle accelerator that collides hadrons, i.e. protons, at very high energy. Examining the outcome of these collisions are a range of detectors, run by six different groups; these make up the LHC’s six experiments.)

Plots of results from the Higgs search by the ATLAS experiment, showing how most masses under 466 Gev are ruled out (click to embiggen)
Plot of ATLAS experiment's Higgs boson search results to date. The wobbly black dashed line shows the prediction from simulations, and the green and yellow bands are the uncertainty in the predictions (at one and two standard deviations respectively). The solid black line is the actual result from the collected data. The Higgs boson is ruled out with 95% confidence wherever the solid black line dips below the horizontal line at 1. (Image ATLAS)

Things were looking promising a couple of months ago, when there were a couple of anomalous signals that could have been the Higgs, but twice as much data has been collected since and the signals have faded.

At the moment, there’s still a slight chance the Higgs exists with a mass between 115 and 145 GeV, or maybe 232-256 GeV, or possibly 282-296 GeV, or potentially even above 466 GeV. All other mass ranges have been ruled out with 95% confidence (GeV, or giga-electron volt, is the unit used to measure the mass of subatomic particles; protons have a mass of 0.938 Gev).

There’s still more data to collect though, so it’s possible there’ll be a definite result either way by the end of the year.

But it’s not necessarily a bad thing if the Higgs isn’t discovered: it simply means that some other, unexpected mechanism must explain the origin of mass. Which is kind of good, because it means new physics, and the potential for new discoveries.

What those might be is unknown – and in fact so far the LHC has ruled out a few popular theoretical possibilities. But that just means the real truth is likely to be something unexpected, and that’s where the really interesting discoveries come from.

All of which makes it an exciting time to be a particle physicist – not that it ever isn’t.

Read more about the results from the ATLAS experiment


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