Improved ATLAS result weighs in on W boson

Improved ATLAS measurement of the W boson mass is in line with the Standard Model of particle physics

23 March 2023

The W boson is an elementary particle discovered at CERN in 1983 that is responsible for mediating the so-called weak interaction. The determination of its mass is of particular importance, for example as a precise test of the validity of the Standard Model of particle physics. After a first determination and publication of the mass in 2017, the ATLAS collaboration has now presented a new result for this mass. The preliminary result was presented by Prof. Matthias Schott, experimental physicist at the PRISMA+ Cluster of Excellence of Johannes Gutenberg University Mainz (JGU) at the "57th Rencontres de Moriond", one of the most important conferences for particle physics.

New W-boson mass is the result of a new analysis

As a result, the mass of the W boson is 80,360 million electronvolts (MeV) with an uncertainty of 16 MeV. It is based on a reanalysis of 14 million W boson candidates recorded back in 2011 in proton-proton collisions at CERN's Large Hadron Collider (LHC). It is consistent with the expectation of the standard model of particle physics, directly contradicting the recent measurement of the CDF experiment at the Tevatron, which caused a major stir in the spring of 2022.

The theoretical prediction of the mass of the W boson in the scope of the Standard Model is 80354 MeV, with an uncertainty of 7 MeV. It is closely related to the strength of the electroweak couplings and the masses of the heaviest elementary particles, including the Z boson, the top quark, and the Higgs boson. However, in theories that extend the Standard Model, the particle's mass is also linked to additional, as-yet-unknown, particles or interactions. The measurement of the W boson mass can therefore be used to test the Standard Model and any deviation between theory and experiment would be an indicator of new physics phenomena.

First mass of the W boson published in 2017

In 2017, ATLAS released its first measurement of the mass of the W boson, which was determined using data from 2011. The mass of the W boson came out to be 80,370 MeV, with an uncertainty of 19 MeV. Even then, the result was in good agreement with the prediction of the Standard Model and all previous experimental results.

Last year, the CDF collaboration at Fermilab announced an even more precise measurement based on an analysis of its full dataset collected at the Tevatron. The result, 80,434 MeV with an uncertainty of 9 MeV, differs significantly from the Standard Model prediction and from the other experimental results, calling for more measurements to try to identify the cause of the difference.

In its new study, ATLAS analysed again the sample of W bosons from 2011, improving the precision of its previous measurement. The new W boson mass, 80,360 MeV with an uncertainty of 16 MeV, is 10 MeV lower than the previous ATLAS result. This result is also in agreement with the Standard Model.

"The measurement of the W boson mass is among the most difficult and demanding measurements performed at hadron colliders. It requires extremely precise calibration of the particle energies and momenta measured with the ATLAS detector, and a careful assessment and excellent control of modelling uncertainties," says ATLAS spokesperson Andreas Hoecker. "This updated result provides a stringent test and confirms the consistency of our theoretical understanding of electroweak interactions."

To achieve it, the ATLAS collaboration used significantly improved analytical methods for instance advanced data-fitting techniques and also took into account new insights into the structure of the proton from recent years. Matthias Schott's group had been pursuing this new approach within the PRISMA+ Cluster of Excellence in Mainz for many years, and it has now been brought to a preliminary conclusion. "I am more than happy after so many years to be able to present this result," explains Matthias Schott. "But now we already have the analysis of special data sets coming up, which we recorded in 2018. These will help us to find out why the CDF measurement deviates from all other measurements."