The LHC homes in on the Higgs

EPS-HEP 2011

In this issue, news from the LHC experiments focuses on a few highlights at the first big summer conference.

The outstanding performance of the LHC enabled the ATLAS and CMS collaborations to report remarkable progress in the hunt for the Higgs boson at EPS-HEP 2011. With an integrated luminosity of more than 1 fb^–1 each – the original luminosity goal for all of 2011 – the experiments have been able to extend significantly the exclusion region for the Standard Model Higgs boson and to achieve impressive advances in extending sensitivity in other mass ranges.

In the Standard Model, the Higgs boson endows other particles and itself with mass. At the same time, the dominant decay mode of the Higgs depends on the value of its mass. Consequently, a comprehensive search for the Higgs must look in numerous decay modes.

At the conference, each collaboration reported results on several possible Higgs decay modes. These results were based on the full sample of data recorded by the end of June; the ability to search for so many decay modes so promptly reflected the efficiency of the experiments and the dedication of the collaborations. The most generally promising decay modes, such as H→γγ, H→W⁺W^–, and H→Z⁰Z⁰, were well covered by both experiments, while early results on H→τ⁺τ^– from CMS and on H→bb from ATLAS were also produced. In each experiment the results of these searches can be combined to optimize sensitivity across the range of possible Higgs boson masses.

The CMS and ATLAS Higgs limits presented at the conference are summarized by the solid curves in the two figures. These plots show the result of combining the limits from all of the analysed decay modes in each experiment in terms of the range of possible Standard Model Higgs mass that can be excluded with 95% confidence.

The two experiments presented similar exclusion ranges. They have now excluded mass ranges for the Higgs boson from 150 to 200 GeV and 300 to 450 GeV; they have also established expected limits within 50% of the Standard Model prediction for the region in between. Moreover, they are homing in on both the low mass region (around 115–150 GeV), which is preferred by electroweak measurements, and the high mass region above about 450 GeV. Throughout these regions, the experiments have already achieved sensitivities, reflected by the dashed curves, within a factor of 2–3 of the Standard Model cross-section.

While it is still early in the hunt for the Higgs, the ATLAS and CMS data also show some excesses that participants at the conference found tantalizing. For instance, both experiments currently see a small excess of candidate events at a mass of roughly 140 GeV. However, given the large range of masses and modes investigated by the two experiments and the as yet limited statistics, the limits observed do sometimes fluctuate from the limits that are expected. In addition, although the two detectors are independent, the results can be somewhat correlated because their background estimates make use of the same theoretical predictions.

Even as the LHC provides the experiments with more data, the painstaking process of combining the limits of the two experiments is currently underway. A combination with the experiments at Fermilab’s Tevatron, whose searches are particularly complementary in the low mass region, will also eventually be done.

Will the Higgs boson be discovered soon, or will the Standard Model Higgs boson be excluded as more data are accumulated? The answer at present is “watch this space”.