Zeroing in on Higgs boson properties

As Run 2 at the LHC gains momentum, a combined analysis of data sets from Run 1 by the ATLAS and CMS collaborations has provided the sharpest picture yet on the Higgs boson properties (ATLAS 2015, CMS 2015).Three years after the announcement in July 2012 of the discovery of a new boson, the two collaborations are closing the books on measurements of Higgs properties by performing a combined Run 1 analysis, which includes data collected in 2011 and 2012 at centre-of-mass energies of 7 and 8 TeV, respectively. This analysis follows hot on the heels of the combined measurement of the Higgs boson mass, m_H = 125.09±0.24 GeV, published in May by ATLAS and CMS (ATLAS and CMS 2015).

The new results are the culmination of one and a half years of joint work by the ATLAS and CMS collaborators involved in the activities of the LHC Higgs Combination Group. For this combined analysis, some of the original measurements dating back to 2013 were updated to account for the latest predictions from the Standard Model. A comprehensive review of all of the experimental systematic and theoretical uncertainties was also conducted to account properly for correlations. The analysis presented technical challenges, because the fits involve more than 4200 parameters that represent systematic uncertainties. The improvements that were made to overcome these challenges will now make their way into data-analysis tools, such as ROOT, that are widely used by the high-energy particle-physics community.

The results of the combination present a picture that is consistent with the individual results. The combined signal yield relative to the Standard Model expectation is measured to be 1.09±0.11, and the combination of the two experiments leads to an observation of the H → τ⁺τ^– decay at the level of about 5.5σ – the first observation of the direct decay of the Higgs boson to fermions. Thanks to the combined power of the data sets from ATLAS and CMS, the analysis yields unprecedented measurements of the properties of the Higgs boson, with a precision that enables the search for physics beyond the Standard Model in possible deviations of the measurements from the model’s predictions. The figure shows clearly the increased precision obtained when combining the ATLAS and CMS analyses.

The combined analysis is performed for many benchmark models that the LHC Higgs Cross-Section Working Group proposed, so as to be able to explore the various different effects of physics models that go beyond the Standard Model. As Run 2 gains momentum, the two collaborations are looking forward to reaping the benefits of the increase in centre-of-mass energy to 13 TeV, which will make some of the most interesting processes, such as the production of Higgs bosons in association with top quarks, more accessible than ever. However, even with the first results from Run 2, this set of combined results from 7 and 8 TeV collisions in Run 1 will continue to provide the sharpest picture of the Higgs boson’s properties for some time to come.