Following an unexpected measurement by the CDF (Collider Detector at Fermilab) experiment in 2022, physicists at the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) today announced a new mass measurement of the W boson, one of nature’s force-carrying particles.
This new measurement, a first for the CMS experiment, uses a new technique that makes it the most elaborate study of the W boson mass to date. After nearly a decade of analysis, CMS has found that the W boson mass is consistent with predictions, finally putting an end to a years-old mystery.
The final analysis used 300 million events collected during the LHC run in 2016 and 4 billion simulated events. From this dataset, the team reconstructed and then measured the masses of more than 100 million W bosons.
They found that the mass of the W boson is 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 that tests the theoretical assumptions.
“The new CMS result is unique because of its precision and the way we determined the uncertainties,” said Patty McBride, a distinguished scientist at the U.S. Department of Energy’s Fermi National Research Laboratory and a former CMS spokesperson.
“We have learned a lot from the CDF and other experiments that have worked on the question of the mass of the W boson. We rely on them, and that is one of the reasons why we are able to advance this study significantly.”
Since the W boson’s discovery in 1983, physicists have measured its mass in ten different experiments.
The W boson is one of the cornerstones 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 the interaction of 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 Anadi Canepa, deputy spokesperson for the CMS experiment and a senior scientist at Fermilab. “If the mass of W is different than we expect, new particles or forces could be at play.”
The new CMS measurement has an accuracy of 0.01 percent. This level of precision is equivalent to measuring a 4-inch pencil between 3.9996 and 4.0004 inches. But unlike pencils, the W boson is a fundamental particle with no physical volume and a mass less than that of a single atom of silver.
“This measurement is extremely difficult to perform,” 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 charged particles as they move through the detector.
“The design of the CMS makes it particularly suited for precision mass measurements,” McBride said. “It’s a next-generation experience.”
Most fundamental particles have extremely short lifetimes, so scientists measure their mass by adding together the masses and momenta of everything that decays. This method works well for particles like the Z boson, a cousin of the W boson, which decays into two muons. But the W boson poses a big challenge because one of its decay products is a tiny fundamental particle called a neutrino.
“Neutrinos are notoriously difficult to measure,” said Josh Bendavid, a scientist at the Massachusetts Institute of Technology who worked on the analysis. “In collider experiments, the neutrino is not detected, so we can only work with half the picture.”
Working with only half the picture means physicists have to get creative. Before performing the analysis on real experimental data, the scientists first simulated billions of LHC collisions.
“In some cases, we even had to model small deformations in the detector,” Bendavid said. “The precision is high enough that we care about small twists and bends, even if they are as small as the width of a human hair.”
Physicists also need a lot of theoretical information, such as what happens inside the protons when they collide, how the W boson is produced, and how it moves before it decays.
“It’s a real art to determine the impact of theoretical inputs,” McBride said.
In the past, physicists have used the Z boson as a surrogate for the W boson to calibrate their theoretical models. While this method has many advantages, it also adds a layer of uncertainty to the process.
“The Z and W bosons are brothers, but not twins,” said Elisabetta Manca, a researcher at the University of California, Los Angeles, and one of the analysts. “Physicists have to make some assumptions when they extrapolate from the Z to the W, and those assumptions are still under debate.”
To reduce this uncertainty, CMS researchers developed a new analysis technique that uses only real W boson data to constrain theoretical inputs.
“We were able to do this efficiently by combining a larger data set, experience from a previous study of the W boson, and recent theoretical developments,” Bendavid said. “This allowed us to free ourselves from the Z boson as a reference point.”
As part of this analysis, they also examined 100 million tracks from the decay of well-known particles to recalibrate a massive section of the CMS detector until it was an order of magnitude more accurate.
“This new level of precision will allow us to approach critical measurements, such as those involving the W, Z and Higgs bosons, with increased precision,” Manca said.
The most difficult part of the analysis was its sheer 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 am now in my third year as a postdoc,” Manca said. “It’s a marathon, not a sprint.”
More information:
Measurement of the mass of the W boson in proton-proton collisions at √s = 13 TeV, cms-results.web.cern.ch/cms-re … MP-23-002/index.html
Provided by Fermi National Accelerator Laboratory
Quote:New results from CMS experiment resolve mystery of W boson mass (September 22, 2024) retrieved September 23, 2024 from
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