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Physicists have made the most precise estimations yet of the 鈥渨idth鈥� of the Higgs boson.

In the standard model of particle physics, the Higgs boson imbues all other fundamental particles with mass. It was discovered in the aftermath of extremely fast protons crashing into each other at the Large Hadron Collider (LHC) in Switzerland in 2012. But the properties of the Higgs are hard to pin down because it sticks around for such a short amount of time before decaying into more particles, and it doesn鈥檛 always appear with the same mass.

The latter is a consequence of the Heisenberg uncertainty principle which says that any particle that lives for a finite time must have a spread of possible energies and masses instead of a single value, says , part of one of the big groups working at the LHC called the CMS collaboration.聽The range of masses is characterised by a number that particle physicists call 鈥渨idth鈥� 鈥� particles with very small widths show up with the same mass in almost all experiments, and particles with large widths are very inconsistent. Up until now, physicists only had imprecise estimates of the width of the Higgs boson.

The CMS researchers determined the Higgs鈥� width from data collected during the LHC鈥檚 second run between 2016 and 2018. Their strategy was to compare data about two processes where the Higgs decays into two other particles. In one process an unusually massive Higgs boson decayed into two particles called Z bosons. In the other, the Higgs boson started with a mass that theoretical models predict to be more common. By comparing them, the researchers calculated that the Higgs could have a width of 3.2 megaelectronvolts. This is consistent with the standard model and more precise than a past measurement that only indicated that the width must be smaller than 9.2 megaelectronvolts.

鈥淚f you had shown me this result in 2010, I would have been utterly stunned to see it come from the LHC,鈥� says at the University at Buffalo in New York. He says that the collider was designed to detect differences in mass at the scale of gigaelectronvolts, which are a thousand times larger than megaelectronvolts. The new analysis surpassed that through a clever choice of processes to study, he says.

at Royal Holloway, University of London says that measuring the width of the Higgs accurately could reveal discrepancies in theoretical predictions and therefore reveal new physics, like the Higgs interacting with some exotic dark matter particle.

For the new analysis, it was crucial that the second run of the LHC produced more data than before 鈥� about 15 times more than the experiment that first revealed the Higgs, says Landsberg. The team hopes to improve its calculations after data from the collider鈥檚 current run becomes available in 2026.

鈥淣ow, we鈥檙e looking at the Higgs and its properties like in an impressionistic painting. With more data we hope to turn it into something like a sharp photograph,鈥� says Landsberg.