ATLAS precision record: uncertainty Higgs mass now less than one promille

24 July 2023

The ATLAS experiment at CERN has determined the mass of the Higgs particle to less than one promille accuracy, a record.

This was announced at the Photon Lepton conference in Melbourne. The new measurement looked at the decay of the Higgs particle to two photons. The result was then also combined with previous mass measurements from other decays.

The Higgs boson was discovered at CERN in 2012. The particle plays a central role in the mechanism that gives elementary particles their mass, but the mass of the higgs itself is not predicted in the theory. It must be determined experimentally.

Both ATLAS, an experiment in which Nikhef plays a prominent role, and the competing CMS experiment have published increasingly precise estimates over the past 11 years. This is possible because they are collecting more measurement data and because the analyses are getting better and better. “This precision measurement is the result of relentless investment in better understanding our measurements,” said ATLAS spokesman Adreas Hoecker.

An exact value of the Higgs mass is important to make calculations with the Standard Model of particle processes as precise as possible. The precision is needed to find any discrepancies in the experiments, which may indicate unknown particles or forces.

For cosmologists, moreover, the Higgs mass is a crucial ingredient for theories about the evolution of the universe and even about the stability of empty space itself, in which the so-called Higgs field extends.

From the double-photon decay measurements comes a mass estimate of 125.22 GeV plus or minus 0.14 GeV. For this, we looked at measurement data up to fall 2018, run1 and run2 of the LHC accelerator. After combining with the earlier mass estimate from Higgs decay to Z particles, ATLAS arrives at 125.11 GeV plus or minus 0.11 GeV. That is an accuracy of 0.09 percent, less than a promille.

Nikhef post-doc Luca Franco (Radboud University) calls the precision achieved impressive. He was previously involved in the calibration for photon measurements as a PhD in France. The precision, he says, is about twice better than in the previous estimate due to more data. But the biggest gains come from a better understanding of the detector itself. “The systematic precision is four times better than achieved before,” he says.

When the Higgs boson was discovered in 2012, ATLAS estimated a mass of 126.0 GeV plus or minus 0.4 GeV and CMS estimated a value of 125.3 plus or minus 0.4 GeV. CMS last year, thanks to much more data, already published a value of 125.35 plus or minus 0.15 GeV, just over 1.1 promille accuracy.