CMS measures Higgs boson’s mass with unprecedented precision

Measurement of the Higgs boson mass: candidate two-photon and four-lepton events
A candidate from CMS of a Higgs boson transforming into two photons; the two large green towers show energy deposits from the photons
(Image: Thomas McCauley, CMS/CERN)

The Higgs boson is a special particle. It is the manifestation of a field that gives mass to elementary particles. But this field also gives mass to the Higgs boson itself. A precise measurement of the Higgs boson’s mass not only furthers our knowledge of physics but also sheds new light on searches planned at future colliders.

Since discovering this unique particle in 2012, the ATLAS and CMS collaborations at CERN’s Large Hadron Collider have been busy determining its properties. In the Standard Model of particle physics, the Higgs boson’s mass is closely related to the strength of the particle’s interaction with itself. Comparing precise measurements of these two properties is a crucial means of testing the predictions of the Standard Model and helps search for physics beyond the predictions of this theory. In addition to probing its “self-interaction” strength, the researchers have also paid careful attention to the exact mass of the Higgs boson.

When it was first discovered, the particle’s mass was measured to be around 125 gigaelectronvolts (GeV) but it wasn’t known with high precision. Analysis of much more data was needed before reducing the errors in such a measurement. Now, the CMS collaboration has announced the most precise measurement so far of this property: 125.35 GeV with a precision of 0.15 GeV, or about 0.1%.

Like most members of the zoo of known particles, the Higgs boson is unstable and transforms – or “decays” – nearly instantaneously into lighter particles. The mass measurement was based on two very different transformations of the Higgs boson, namely decays to four leptons via two intermediate Z bosons and decays to pairs of photons. To arrive at the mass value, the scientists combined CMS results of these two decays from two datasets: the first was recorded in 2011 and 2012 while the second came from 2016.

This measurement adds another piece to the puzzle of the exciting world of subatomic particles.

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