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Narrowing In on the W Boson Mass

ATLAS experiment makes one of the most difficult measurements in particle physics

Note: This article is adapted from a Brookhaven National Laboratory article. View the original article.

–By Sarah Charley, U.S.-CERN/Fermilab

VIDEO: The ATLAS Collaboration reports the first high-precision measurement at the Large Hadron Collider (LHC) of the mass of the W boson, using data from proton-proton collisions at 7 TeV. (Credit: ATLAS Collaboration)


Scientists working on the ATLAS experiment at the Large Hadron Collider (LHC)—the world’s largest particle collider, hosted at CERN, the European particle physics laboratory—have precisely measured the mass of the W boson, a particle that plays a weighty role in a delicate balancing act of the quantum universe. This measurement is regarded as one of the most difficult in particle physics and the first of its kind for an LHC experiment. The new result is consistent with predictions and comparable to the precision of previous measurements made at the Tevatron collider at Fermilab.

“Achieving such a precise measurement despite the demanding conditions present in a hadron collider such as the LHC is a great challenge,” says the spokesperson of the ATLAS Collaboration, Karl Jakobs. “Reaching similar precision to that previously obtained at other colliders, with only one year of Run 1 data, is remarkable. It is a prominent example of our ability to improve our knowledge of the Standard Model and look for signs of new physics through highly accurate measurements at the LHC.”

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After completing her Ph.D. thesis in calculating the mass of the W boson – a heavier-than-iron elementary particle that mediates one of the universe’s fundamental forces – physics researcher Aleksandra Dimitrievska is now testing out components for a scheduled upgrade of the world’s largest particle detectors.

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The U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) is a part of the ATLAS Collaboration. Brookhaven National Laboratory serves as the U.S. host laboratory for the 45 U.S. institutions working on the ATLAS experiment.

The quantum universe is a delicate ecosystem in which particles and forces are intimately linked in a subatomic balancing act. Over the years, theorists have developed very precise predictions about each particle’s properties based on experimental measurements, and the mathematical relationships between them contained in the Standard Model—the world’s best field-guide to subatomic particles.

For instance, the W boson is a massive particle and the carrier of the weak force, a subatomic superpower that enables quarks (the point-like particles found inside protons and neutrons) to switch their identities.

This new measurement by the ATLAS experiment, published Feb. 6 in the European Physical Journal C, pegged the W boson mass as 80370±19 million electron volts (MeV), which is in the same ball park as the theoretical prediction, but not yet precise enough to see if theory lines up exactly with experiment.

Measuring the mass of the W boson as well as we can is especially important because it is one of the main ingredients in the scientific equation that predicts the mass of the Higgs boson, which was discovered at the LHC and is a crucial component of the Standard Model.

“Prior to the Higgs discovery in 2012, we restricted its possible masses to a small window based on our measurements of the W boson and Top quark,” says Bodhitha Jayatilaka, a researcher at Fermilab who worked on the W boson measurement at the Tevatron. “By further confining the mass of the W boson, we can see if there are other ingredients which might be tipping the scales and secretly influencing the mass of the Higgs boson.”

The LHC measurement is based on around 14 million W bosons recorded by ATLAS since 2011. It relies on a thorough calibration of the detector and of the theoretical modeling of W-boson production. Further analysis with the huge sample of now-available LHC data will allow even greater accuracy in the near future.

Image - ATLAS Experiment event display: a W boson decays into one muon and one neutrino (Credit: ATLAS Collaboration)

ATLAS Experiment event display: a W boson decays into one muon and one neutrino (Credit: ATLAS Collaboration)



Brookhaven National Laboratory serves as the host lab for managing the U.S. ATLAS Operations Program and the high luminosity upgrade projects. Brookhaven Lab’s contributions to ATLAS are funded by the DOE Office of Science.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel Prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit