Scientific Method / Science & Exploration

Physicists find hints of a light Higgs boson in LHC data

At a presentation in France, teams from the LHC put stricter limits on where …

During Ars' trip to Fermilab earlier this spring, the staff was excitedly talking about their expectations for the summer. That's when the high-energy physics community has many of their meetings, and the expectation was that all of the major players—DZero and CDF at Fermi, and ATLAS and CMS at the LHC—would process as much data as they could and update the community on the search for things like supersymmetry and the Higgs boson, a particle that helps give all others mass. Right now, the Europhysics Conference on High Energy Physics is happening in Grenoble, France, and the folks from Fermi will not be disappointed. The first results from the LHC have greatly expanded the mass range in which the Higgs won't be found, and left open the possibility that it might eventually turn up in the area of 140GeV.

Results have been presented by people from both ATLAS and CMS. Each of these has looked for evidence of the Higgs in different "channels," with each channel representing a different process for producing a Higgs, which will then decay into a spray of distinct particles and photons. (Symmetry Breaking has a decent explanation of some of this.) Each one of these channels is sensitive to a different range of energies, both because of the process that triggers the event, and because the background of similar-looking events also depends on the energy. As a result, you get a complex set of graphs, each generated in a different channel.

Each of the dotted curves shows the expected number of events based on the background of similar-looking events produced by something other than a Higgs decay. The solid curves are the rates of events registered by (in this case) the ATLAS detector. Merging all of these individual channels together gets you a single curve that spans the whole energy range, shown below.

For the most part, the observed data (again, a solid line) is very close to that predicted by a Standard Model background that doesn't include the Higgs (within the green and yellow bands). This indicates the detector is not seeing anything unusual in these regions. In some areas, like the one around 375GeV, it's seeing so little of the Higgs-like events that the observed data is significantly below that expected from the background. This has allowed them to exclude the possibility that the Higgs is hiding between 294 and 450 GeV. In other areas, like the one centered around 175GeV, the observations were close to what was expected, but we've seen enough of these collisions to know the Higgs can't be there, or the detectors would have seen many more events in the area.

The net result is that we can exclude a Higgs with a mass between 155 and 190 GeV, and from 295 up to 450 GeV. The CMS detector is reporting similar results: its team is saying the Higgs is excluded from 149 to 206 GeV and 300 to 440 GeV, as well as several narrower intervals. (The CMS team hasn't made its slides available, so no images from them.)

Parts of that range had already been excluded by work at the Tevatron, so we're getting a very good idea of where the Higgs is not. Even better, the LHC, thanks in part to its higher energy collisions, has produced all that data with about a single inverse femtobarn of data, far less than the Tevatron has to work with. It's already produced more than an inverse femtobarn this year, and the intensity of collisions has continued to go up, meaning that the detector teams will have a lot more to work with before the year's out, and should be able to significantly broaden these exclusion regions.

Excluding the whole area would tell us something interesting—our extensions to the Standard Model that were done to give its particles mass through a Higgs field would have to be pitched. But I think it's safe to say that a lot of physicists are expecting the Higgs to show up, and the data presented so far does give them some hope. There is a small excess of events in the 130 to 150 GeV range in both detectors. "It should be noted that a modest excess of events is observed for Higgs boson masses below 145 GeV," the CMS team has stated, noting that they should have a better handle on whether this excess is real within the next several months.

ATLAS sees something similar in the area as well. The signal in both of these directions are just barely outside the two sigma measure of significance shown in yellow on the graph above, so they're difficult to discern in these graphs. But they're there, and you can expect the analysis in the channels that cover this region to be a top priority over the next few months.

What will additional data accomplish? A number of things. The simplest thing will be that it will raise the dashed horizontal line that indicates where the Higgs has been excluded. As the bar goes up, more and more of the dashed curve (which represents expected background signals) will fall below it. Eventually, only data that differs significantly from the expected values will lie above the exclusion line.

The other thing is that, with more collisions, the expected statistical deviations will shrink, narrowing the area in green and yellow. That data will either contain more events than predicted, in which case the deviation between predictions and observations will become larger, or the dotted and solid curves will end up in tighter alignment. All of this should enable any signals that exist to be easier to spot than they are now.

How long will we have to wait? Various things that I've seen suggest that we'll have enough data to have a clearer picture before the year's out. The coverage at Nature News, for example, quotes a CMS spokesman as saying "in a matter of months we'll know the answer."

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