Evrim Yazgin
Cosmos science journalist
Physicists rocked the boat in 2022 when they measured the mass of the W boson particle and came up with a result that was “in tension” with Standard Model of particle physics predictions.
A new precise measurement of the W boson mass by the Compact Muon Solenoid (CMS) experiment at CERN’s Large Hadron Collider (LHC) puts the mystery of the fundamental particle’s mass to rest.
The new result is consistent with predictions made in the Standard Model.
Nearly a decade of analysis used 300 million events collected from the 2016 run of the LHC, as well as 4 billion simulated events. The team then reconstructed and measured the mass from more than 100 million W bosons.
The results are published on the CMS Collaboration’s website.
The measured mass of the W boson is 80,360.2 ± 9.9 megaelectronvolts (MeV). For comparison, this is a little less than a single silver atom. Standard Model predictions place the W boson mass at 80,357 ± 6 MeV. The new CMS measurement has a precision of 0.01%.
W bosons are among the fundamental particles in the Standard Model which describes all the basic building blocks of the universe.
W bosons are force carriers – along with Z bosons – which translate the nuclear weak force that governs radioactivity.
Understanding the W boson’s mass allows scientists to grasp the interaction of forces and particles. This includes the strength of the Higgs field (responsible for giving all particles their mass) and the merging of the weak force with electromagnetism.
“The entire universe is a delicate balancing act,” says Anadi Canepa, deputy spokesperson of the CMS experiment and a senior scientist at the US Department of Energy’s Fermi National Research Laboratory (Fermilab). “If the W boson mass is different from what we expect, there could be new particles or forces at play.”
If new, exotic physics is to be found, the CMS measurement suggests it won’t be in the W boson mass.
“CMS’s design makes it particularly well-suited for precision mass measurements,” says Patty McBride, a distinguished scientist at Fermilab and the former CMS spokesperson. “It’s a next generation experiment.”
Most fundamental particles are only around for a fraction of a second before decaying into other particles. Physicists work their masses out in collider experiments by measuring the combined mass of the particles they decay into.
This works well for particles like the W boson’s sibling, the Z boson. But W bosons pose a problem – one of their decay products is a neutrino.
“Neutrinos are notoriously difficult to measure,” says Josh Bendavid, a scientist at the Massachusetts Institute of Technology, who worked on the analysis. “In collider experiments, the neutrino goes undetected, so we can only work with half the picture.”
To get around this problem, physicists have to use simulations of LHC collisions to supplement the experimental data. In the past, physicists used Z bosons as stand ins for W bosons in their simulations. But this adds uncertainty into the theoretical models.
“Z and W bosons are siblings, but not twins,” says Elisabetta Manca, a researcher at the University of California Los Angeles.
To reduce uncertainty, the CMS Collaboration used only real W boson data to build their theoretical models.
“We were able to do this effectively thanks to a combination of a larger data set, the experience we gained from an earlier W boson study, and the latest theoretical developments,” Bendavid explains. “This has allowed us to free ourselves from the Z boson as our reference point.”
