The theories of particle physics suggest that there are four forces in the universe: gravity, electromagnetism, the weak force, and the strong force.
Like the W boson, Bosons mediate fundamental forces and are critical to the Standard Model, which describes nature at its most basic level. Understanding the W boson’s mass helps scientists explore the interactions of particles and forces, including the Higgs field and the weak force involved in radioactive decay. If the W boson’s mass changes, it suggests unknown physics might be at play.
A previous measurement of its mass significantly differed from theoretical predictions, sparking excitement in the physics community to investigate potential new physics. However, this may have been an overreaction.
This time, physicists, including core UCLA researchers, have accurately measured the mass of the W boson — more precisely than a previous attempt at measuring the mass — and found that it is consistent with the Standard Model’s predictions of 80 357 ± 6 MeV.
Another way of saying this is that the W boson‘s mass is 80 giga electron volts or 1.42e-25 kilograms.
Michalis Bachtis, a UCLA associate professor of physics whose research group played a key role in the experiment, said, “Everybody was hoping we would measure it away from the theory, igniting hopes for new physics. By confirming that the mass of the W boson is consistent with the theory, we have to search for new physics elsewhere, maybe by studying the Higgs boson with high precision as well.”
The new results came from the Compact Muon Solenoid (CMS) experiment at CERN’s Large Hadron Collider. This experiment features a compact design with specialized sensors for detecting muons and a powerful solenoid magnet that bends the paths of charged particles. Key components of the CMS detector have been built at UCLA since the 1990s.
They used a 14,000-ton scale to measure the weight of a particle with a mass of 1×10-25 kg, or about 80 times the mass of a proton.
To measure the masses of fundamental particles, scientists add up the masses of their decay products, similar to a Newton’s Cradle toy. This method works for particles like the Z boson, which decays into two easily measurable muons. However, measuring the W boson’s mass is challenging because one of its decay products is a neutrino, which is very difficult to detect.
Initially, it was thought impossible to make this measurement at the LHC due to the need for precise calibration of the muons’ energy within a 0.01% margin of error. Bachtis and postdoctoral researcher Elisabetta Manca have spent the last eight years working to achieve this level of precision.
Manca said, “This new level of precision will allow us to tackle critical measurements, such as those involving the W, Z, and Higgs boson, with enhanced accuracy.”
Bachtis said, “We found out that the magnetic field of the experiment changed significantly when the detector was lowered in the cavern 100 meters underground compared to the surface. This was negligible for most measurements but not for the W boson mass. These small variations matter. Our analysis also had to correct for the deformation of the detector by its own gravity.”
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