Reviewed by Danielle Ellis, B.Sc.Sep 18 2024
Following an unforeseen measurement by the Collider Detector at Fermilab (CDF) experiment in 2022, physicists at the Large Hadron Collider’s Compact Muon Solenoid experiment revealed a new mass measurement of the W boson, one of nature’s force-carrying particles.
The Compact Muon Solenoid detector is located 100 meters underground on the Franco-Swiss borderer at CERN and collects data from the Large Hadron Collider. The detector has been operational since 2010 and is used by one of largest international scientific collaborations in history to study the fundamental laws of nature. Image Credit: Brice, Maximilien: CERN
This new measurement, the first for the CMS experiment, employs a novel technique, making it the most thorough investigation of the W boson's mass to date. After nearly a decade of analysis, CMS discovered that the W boson's mass is consistent with predictions, finally putting a multi-year-long mystery to rest.
The final analysis included 4 billion simulated events and 300 million events gathered from the 2016 run of the LHC. The scientists calculated the mass of over 100 million W bosons by reconstructing them using this dataset. Their discovery yielded a mass for the W boson of 80 360.2 ± 9.9 MeV, in agreement with the predictions of 80 357 ± 6 MeV from the Standard Model. To verify the theoretical hypotheses, they also conducted a different analysis.
The new CMS result is unique because of its precision and the way we determined the uncertainties. We have learned a lot from CDF and the other experiments who have worked on the W boson mass question. We are standing on their shoulders, and this is one of the reasons why we are able to take this study a big step forward.
Patty McBride, Distinguished Scientist, Fermi National Research Laboratory, US Department of Energy
Scientists have measured the mass of the W boson ten times since it was discovered in 1983.
One of the pillars of the Standard Model, the theory that explains nature at its most basic level, is the W boson. By precisely knowing the mass of the W boson, scientists can map the interaction of particles and forces, such as the strength of the Higgs field and the merger of electromagnetism with the weak force, which is responsible for radioactive decay.
The entire universe is a delicate balancing act. If the W mass is different from what we expect, there could be new particles or forces at play.
Anadi Canepa, Senior Scientist, Fermi National Research Laboratory, US Department of Energy
The precision of the new CMS measurement is 0.01%. This degree of accuracy is equivalent to measuring a 4-inch pencil to a distance of between 3.9996 and 4.0004 inches. The W boson, however, is a fundamental particle that has less mass than a single silver atom and no physical volume, unlike pencils.
Canepa added, “This measurement is extremely difficult to make. We need multiple measurements from multiple experiments to cross-check the value.”
The CMS experiment is distinct from previous experiments that have achieved this measurement due to its small size, specialized muon sensors, and a powerful solenoid magnet that bends the trajectories of charged particles as they pass through the detector.
McBride added, “CMS’s design makes it particularly well-suited for precision mass measurements. It is a next generation experiment.”
Due to the extremely short lifespans of most fundamental particles, scientists calculate their masses by summing the masses and momenta of all the entities they decay into. For particles such as the Z boson, a cousin of the W boson, which decays into two muons, this method is effective. However, the W boson presents a significant problem because a tiny fundamental particle known as a neutrino is one of its decay products.
Neutrinos are notoriously difficult to measure. In collider experiments, the neutrino goes undetected, so we can only work with half the picture.
Josh Bendavid, Scientist, Massachusetts Institute of Technology
The physicists have to be resourceful because they are working with only half the picture. The scientists first simulated billions of collisions at the LHC before applying their analysis to actual experimental data.
“In some cases, we even had to model small deformations in the detector. The precision is high enough that we care about small twists and bends, even if they’re as small as the width of a human hair,” Bendavid stated.
Physicists also require many theoretical inputs, such as what happens inside the protons when they collide, how the W boson is created, and how it moves before decaying.
McBride noted, “It is a real art to figure out the impact of theory inputs.”
Physicists have previously used the Z boson instead of the W boson to calibrate their theoretical models. While this method has many benefits, it also introduces an element of uncertainty into the process.
Elisabetta Manca, a researcher at the University of California Los Angeles and one of the analyzers, added, “Z and W bosons are siblings, but not twins. Physicists need to make a few assumptions when extrapolating from the Z to the W, and these assumptions are still under discussion.”
To reduce this uncertainty, CMS researchers devised a novel analysis technique that constrains theoretical inputs only with real W boson data.
“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. This has allowed us to free ourselves from the Z boson as our reference point,” Bendavid further added.
They also analyzed 100 million tracks from the decays of well-known particles as part of this analysis, recalibrating a large portion of the CMS detector until it was an order of magnitude more accurate.
Manca noted, “This new level of precision will allow us to tackle critical measurements, such as those involving the W, Z and Higgs bosons, with enhanced accuracy.”
The most difficult aspect of the analysis was its time consumption, as it called for the development of an entirely novel analysis method and an exceptionally thorough comprehension of the CMS detector.
“I started this research as a summer student, and now I’m in my third year as a postdoc. It is a marathon, not a sprint,” Manca added.
The National Science Foundation and the Department of Energy's Office of Science provide partial funding for the Compact Muon Solenoid (CMS) experiment. It is one of two large general-purpose experiments running at CERN's Large Hadron Collider (LHC).