Since discovering the Higgs boson 10 years ago, the ATLAS and CMS collaborations have been carrying out numerous measurements of its properties and its interactions with other particles, which have been  consistent with predictions from the Standard Model. The Higgs boson’s mass, for instance, has been measured to be 125 billion electronvolts (GeV), with a precision of 0.1%. However, one property that remains inaccessible via direct measurements is the particle’s decay width, which is linked to its lifetime and, if found to deviate from its predicted value, could indicate the presence of new physics. At the recent Higgs 2022 conference and at a CERN seminar this week, the ATLAS  collaboration presented the results of its latest study of this property.

Width is a fundamental parameter of any unstable particle with a finite lifetime – the shorter the lifetime, the broader the width. The Higgs boson's width, which represents the range of possible masses around the particle’s nominal mass of 125 GeV, is predicted to be 4.1 MeV – too small to be directly measured. However, its value can be determined by comparing the rate of Higgs boson production at the particle’s nominal mass (“on-shell” production) with that at much larger masses (“off-shell” production). This relies on the fact that the on-shell Higgs boson production rate depends not only on the Higgs boson’s interactions with other particles, but also on its width. By contrast, the off-shell rate is, to a good approximation, independent of the width.

In its new study, the ATLAS collaboration looked for off-shell Higgs boson production using proton–proton collision data collected during Run 2 of the Large Hadron Collider (LHC) from 2015 to 2018. In particular, ATLAS physicists searched for collision events where the Higgs boson transforms, or “decays”, into two Z bosons, which in turn decay into four charged leptons or two charged leptons plus two neutrinos, as thesedecay channels provided the highest sensitivity to the off-shell signal.

After isolating these events from those of background processes that resemble them but do not involve the Higgs boson, the researchers combined the results from both channels to measure the ratio of the off-shell Higgs boson production rate to its Standard-Model prediction. The data were found to be consistent with Standard Model predictions, rejecting the background-only hypothesis, which assumes no off-shell Higgs boson production, with an observed (expected) statistical significance of 3.2 (2.4) standard deviations. This result provides experimental evidence of off-shell Higgs boson production.

By combining these results with their previous on-shell Higgs boson measurements, the ATLAS researchers obtained a Higgs boson width of 4.6 MeV, with upper and lower uncertainties of 2.6 and -2.5 MeV, respectively. This value is in agreement with the Standard Model expectation and corresponds to a particle lifetime of 180 yoctoseconds (1 yoctosecond is 10^-24 seconds).

The results are compatible with those from a recent study by the CMS collaboration, which also found evidence of off-shell Higgs boson production and measured the particle’s width. With the increased collision energy and greater accumulated data expected from Run 3 of the LHC, more precise measurements of both the production process and the particle’s width are anticipated.

Read more on the ATLAS website.