New results from CMS experiment put an end to mystery about W boson mass

Following an unexpected measurement by the Fermilab Collision Detector (CDF) experiment in 2022, physicists at the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) today announced a new measurement of the mass of the W boson, one of nature’s force-carrying particles.

This new measurement, the first of its kind from the CMS experiment, uses a new technique that makes it the most detailed investigation of the W boson mass to date. After nearly a decade of analysis, CMS has been found The mass of the W boson matches expectations, finally putting an end to a mystery that has persisted for many years.

The final analysis used 300 million events collected from the LHC run in 2016, and 4 billion simulation events. From this dataset, the team reconstructed and then measured the mass of more than 100 million W bosons.

They found that the W boson has a mass of 80,360.2 ± 9.9 megaelectronvolts (MeV), which is consistent with the Standard Model predictions of 80,357 ± 6 MeV. They also performed a separate analysis to verify the theoretical assumptions.

“The new CMS result is unique because of its precision and the way we quantified the uncertainties,” said Patty McBride, a senior scientist at the U.S. Department of Energy’s Fermi National Research Laboratory and a former CMS spokeswoman.

“We have learned a lot from the CDF experiment and other experiments that have worked on the W boson mass problem. We stand on their shoulders, which is one of the reasons why we are able to take this study a big step forward.”

Since the W boson was discovered in 1983, physicists have measured its mass in 10 different experiments.

The W boson is a cornerstone of the Standard Model, the theoretical framework that describes nature at its most fundamental level. A precise understanding of the mass of the W boson allows scientists to map out the interactions between particles and forces, including the strength of the Higgs field and the fusion of electromagnetism with the weak force, which is responsible for radioactive decay.

“The entire universe is a delicate balancing act,” said Anady Canepa, deputy spokesperson for the CMS experiment and chief scientist at Fermilab. “If the mass of W is different from what we expect, there may be new particles or forces at play.”

The new CMS measurement has an accuracy of 0.01%. That level of accuracy is equivalent to measuring a 4-inch pencil to between 3.9996 and 4.0004 inches. But unlike pencils, the W boson is a fundamental particle with no physical size and a mass less than one atom of silver.

“It is very difficult to make this measurement,” Canepa added. “We need multiple measurements from multiple experiments to verify the value.”

The CMS experiment is unique from other experiments that have made this measurement because of its compact design, specialized sensors for fundamental particles called muons, and an extremely powerful solenoid magnet that bends the paths of the charged particles as they move through the detector.

“The design of the CMS makes it particularly well-suited to accurately measure mass,” McBride said. “It’s a next-generation experiment.”

Because most fundamental particles are incredibly short-lived, scientists measure their masses by adding up the masses and momentums of everything they decay into. This method works well for particles like the Z boson, a close relative of the W boson, which decays into two muons. But the W boson poses a challenge because one of its decay products is a tiny fundamental particle called a neutrino.

“It’s notoriously difficult to measure neutrinos,” says Josh Bendavid, an MIT scientist who worked on the analysis. “In collision experiments, neutrinos are not detected, so we can only work with half the picture.”

Working with only half the picture meant physicists had to get creative. Before running the analysis on real experimental data, scientists first simulated billions of LHC collisions.

“In some cases, we even had to model small distortions in the detector,” Bendavid said. “The resolution is high enough that we care about small twists and bends; even if they’re as small as the width of a human hair.”

Physicists also need a lot of theoretical input, such as what happens inside protons when they collide, how the W boson is produced, and how it moves before it decays.

“Knowing the impact of theoretical inputs is a real art,” McBride said.

In the past, physicists have used the Z boson as a proxy for the W boson when calibrating their theoretical models. While this approach has many advantages, it also adds a layer of uncertainty to the process.

“The Z and W bosons are sisters, but they are not twins,” said Elisabetta Manca, a researcher at the University of California, Los Angeles, and one of the analysts. “Physicists need to make some assumptions when extrapolating the Z boson to the W boson, and these assumptions are still under discussion.”

To reduce this ambiguity, the CMS researchers developed a new analysis technique that uses only real W boson data to constrain the theoretical input.

“We were able to do this effectively by combining a larger dataset, the experience we gained from previous studies of the W boson, and the latest theoretical developments,” Bendavid said. “This allowed us to free ourselves from the Z boson as our reference point.”

As part of this analysis, they also examined 100 million tracks of known particle decays to recalibrate a huge section of the CMS detector so that it was more accurate by an order of magnitude.

“This new level of precision will allow us to handle critical measurements, such as those involving the W, Z and Higgs bosons, with increasing accuracy,” Manca said.

The most challenging part of the analysis was its time-consuming nature, as it required creating a new analysis technique and developing an incredibly deep understanding of the CMS detector.

“I started this research as a summer student, and now I’m in my third year as a postdoc,” Manca said. “It’s a marathon, not a sprint.”