In the 11 years since its discovery at the Large Hadron Collider (LHC), the Higgs boson has become a central avenue for shedding light on the fundamental structure of the universe. Precise measurements of the properties of this special particle are among the most powerful tools physicists have to test the Standard Model, currently the theory that best describes the world of particles and their interactions. At the Lepton PH๏τon Conference this week, the ATLAS collaboration reported how it has measured the mᴀss of the Higgs boson more precisely than ever before.

The mᴀss of the Higgs boson is not predicted by the Standard Model and must therefore be determined by experimental measurement. Its value governs the strengths of the interactions of the Higgs boson with the other elementary particles as well as with itself. A precise knowledge of this fundamental parameter is key to accurate theoretical calculations which, in turn, allow physicists to confront their measurements of the Higgs boson’s properties with predictions from the Standard Model. Deviations from these predictions would signal the presence of new or unaccounted-for phenomena. The Higgs boson’s mᴀss is also a crucial parameter driving the evolution and the stability of the universe’s vacuum.

The ATLAS and CMS collaborations have been making ever more precise measurements of the Higgs boson’s mᴀss since the particle’s discovery. The new ATLAS measurement combines two results: a new Higgs boson mᴀss measurement based on an analysis of the particle’s decay into two high-energy pH๏τons (the “dipH๏τon channel”) and an earlier mᴀss measurement based on a study of its decay into four leptons (the “four-lepton channel”).

The new measurement in the dipH๏τon channel, which combines analyses of the full ATLAS data sets from Runs 1 and 2 of the LHC, resulted in a mᴀss of 125.22 billion electronvolts (GeV) with an uncertainty of only 0.14 GeV. With a precision of 0.11%, this dipH๏τon-channel result is the most precise measurement to date of the Higgs boson’s mᴀss from a single decay channel.

Compared to the previous ATLAS measurement in this channel, the new result benefits both from the full ATLAS Run 2 data set, which reduced the statistical uncertainty by a factor of two, and from dramatic improvements to the calibration of pH๏τon energy measurements, which decreased the systematic uncertainty by almost a factor of four to 0.09 GeV.

“The advanced and rigorous calibration techniques used in this analysis were critical for pushing the precision to such an unprecedented level,” says Stefano Manzoni, convener of the ATLAS electron-pH๏τon calibration subgroup. “Their development took several years and required a deep understanding of the ATLAS detector. They will also greatly benefit future analyses.”

When the ATLAS researchers combined this new mᴀss measurement in the dipH๏τon channel with the earlier mᴀss measurement in the four-lepton channel, they obtained a Higgs boson mᴀss of 125.11 GeV with an uncertainty of 0.11 GeV. With a precision of 0.09%, this is the most precise measurement yet of this fundamental parameter.

“This very precise measurement is the result of the relentless investment of the ATLAS collaboration in improving the understanding of our data,” says ATLAS spokesperson Andreas Hoecker. “Powerful reconstruction algorithms paired with precise calibrations are the determining ingredients of precision measurements. The new measurement of the Higgs boson’s mᴀss adds to the increasingly detailed mapping of this critical new sector of particle physics.”

Source: CERN