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CERN | Geneva | Switzerland

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Where do we stand on the Higgs boson search?

One of the most sought after particles in our field, the Higgs boson, is playing hard to catch. It might be that it does not even exist. All we physicists know is that something new is required by the theory. It might be the Higgs boson: that’d be the simplest solution, or we need to exclude its existence and move on to explore the next set of possibilities.

We have a theory called the Standard Model of particle physics that has withstood decades of experimental scrutiny without showing any cracks. The Standard Model tells us how many events containing a Higgs boson we should see if it exists but it does not predict its exact mass, making it much harder to find.

Each event is just a snapshot of what happens when protons moving at near the speed of light collide in the Large Hadron Collider (LHC). The Standard Model also tells us how many other types of events collected in our detectors could mimic the signature of a Higgs boson decay. These events are called the background. We design our searches specifically to select the desired signal events while minimizing the background. In the end, we work out how many signal events from Higgs boson decays and how many events from other processes we should retain given a specific set of selection criteria. Then we compare these estimates with what is collected with our detectors to see if Higgs bosons were present or not.

Imagine that all selected events were like the contents of a small lake. If a hidden fish creates a disturbance underneath, we will see a wave on a calm water surface. But of course, if there is some wind, ripples would appear, making it harder to spot the wave caused by a fish. The presence of a Higgs boson would do just that: appear like a wave on top of the calm water. As with the wind, the background creates small ripples one could easily mistake for a signal. The background can also fluctuate following statistical laws, like a random wind. In our case, having more data is equivalent to having more fish in the same spot, making their presence easier to detect.

To see if the Higgs boson or any new particle exists, we need to collect as many events as possible. Until then, it is pure guesswork since statistical fluctuations can easily fool us.

This is what happened this summer, when the first Higgs results were presented in July. We only had about one inverse femtobarn of data available (those are just the units we use to measure the data sample size). Some tantalizing ripples appeared as if we were seeing something. A month later, the CMS and ATLAS experiments each had two inverse femtobarns of data analyzed. The initial hint had completely disappeared, making statistical fluctuations once more the bane of a particle physicist’s existence. In the calm lake analogy, the first ripples we had seen were not caused by a real source like a hidden fish but simply by small variations at the water surface.

Now after much effort, the first combination of these August results was made public last Friday.  This is equivalent to one experiment having four inverse femtobarn of data, four times more than in July. This time, a wide mass range is excluded, namely between 141 and 476 GeV at the 95% confidence level. This means there is less and less space where the Higgs might still be hiding. In fact, it is now limited to be between 114 and 141 GeV.

This low mass range is where it was most expected, based on various theoretical hints and experimental factors. But this is also the range where it is most difficult to see, meaning more data is needed to see a real wave above all the small ripples.

This year, the ATLAS and CMS experiments each collected five inverse femtobarns of data. People are now bending over backwards to analyze these data and present new results at the scheduled meeting of the CERN Council planned at CERN in mid-December . Let’s hope both teams will manage and that some interesting signal will emerge. Combining this data will take a few more months and is expected in March.

What’s for sure, if the LHC, and the CMS and ATLAS experiments continue to perform as they did this year, we will have a final answer on finding the Higgs boson or excluding it definitively by the end of next year.

Let’s keep our fingers crossed…

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.

The combined Higgs boson search result for the ATLAS and CMS experiments using 2 inverse femtobarns of data. The dotted line represents what one would expect with this much data based on theoretical predictions and statistical laws. The green and yellow bands indicate the error margin around this prediction, respectively with 68% and 95% chances of being correct, if all sources of experimental and theoretical errors have properly been accounted for. The black points are the experimental results. The horizontal axis gives the possible Higgs mass value on a logarithmic scale. Every time the data (black line) falls below the horizontal red line means we exclude a Higgs boson with this possible mass. The region above 476 GeV is still allowed but disfavored by theoretical considerations, meaning all eyes and efforts are now on the low mass region. In fact, below 141 GeV, what we observe experimentally is slightly more events than expected, that is, the black line goes above the yellow band. The bigger the deviation, the more likely we are to find the Higgs boson in that area.