Using data from over a billion proton collision events gathered at CERN’s Large Hadron Collider (LHC), physicists have achieved record precision in measuring the mass of the W particle. This measurement aligns with the predictions of the Standard Model, which reinforces researchers’ confidence that no unexpected forces are hidden within their data.
CMS collision event candidates show W particles decaying into muons (red lines) and neutrinos evading detection (pink arrows). Image credit: CMS/CERN.
Discovered in 1983, the W boson is one of two fundamental particles that embody the weak force, one of the four fundamental forces in nature.
The weak force enables the transformation of particles, such as protons changing into neutrons and vice versa. This process not only powers nuclear fusion in the Sun but also drives radioactive decay.
Detecting W particles is extremely challenging as they decay almost instantaneously into two types of particles; one of which, the neutrino, remains undetectable.
To measure the W boson’s total mass, physicists analyze the other particle produced during decay, known as a muon.
In a groundbreaking study, researchers employed the Compact Muon Solenoid (CMS) experiment, the LHC’s particle detector that meticulously tracks muons and other particles generated from proton collisions.
They identified 100 million instances where W particles decayed into muons and neutrinos from trillions of proton-proton collisions.
By conducting a detailed analysis for each of these events, they achieved precise mass measurements.
Ultimately, the mass of the W boson was measured at 80360.2 ± 9.9 megaelectronvolts (MeV).
This measurement is in agreement with the predictions outlined in the Standard Model, physicists’ most reliable framework for understanding fundamental particles and their interactions.
The new accuracy matches previous measurements made in 2022 by Fermilab’s Collider Detector (CDF).
This earlier measurement had surprised many in the field, revealing a mass significantly heavier than predicted by the Standard Model and suggesting the potential existence of new particles and forces.
The latest CMS measurements are as precise as the CDF results and are consistent with the Standard Model, providing physicists with a solid foundation for understanding the W boson.
“Honestly, this is a significant relief,” said Dr. Kenneth Long, a physicist at the Massachusetts Institute of Technology.
“This new measurement strongly supports the reliability of the Standard Model.”
The research team’s work was published in this month’s edition of Nature.
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CMS Collaboration. 2026. High-precision measurement of W boson mass by the CMS experiment. Nature 652, 321-327; doi: 10.1038/s41586-026-10168-5
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Source: www.sci.news
