UCL Mathematical & Physical Sciences


Particle mass measurement not in line with Standard Model

8 April 2022

An international team including researchers at UCL have made the most precise measurement of an elementary particle – which does not match predictions under the guiding theory of physics.

The Collider Detector at Fermilab

The Collider Detector at Fermilab (CDF) collaboration, which involves 400 scientists from around the world, have measured the mass of the W boson, one of nature’s force-carrying particles.

The measurement does not match the value expected based on the Standard Model of particle physics, suggesting there may need to be new, as yet undiscovered, pieces added to the long-standing theory.

Professor David Waters (UCL Physics & Astronomy), UCL High Energy Physics Group Leader and CDF Collaboration Member, said: “This new result from the CDF experiment, in which UCL was closely involved over many years, is potentially very exciting. The W boson is responsible for the weak nuclear force that mediates radioactive beta-decay and some of the nuclear processes taking place inside the Sun.

“By measuring very precisely how much the W boson weighs, we are testing Standard Model calculations that take into account quantum fluctuations involving heavy particles.

“This latest measurement is different to what is predicted, and adds to recent evidence from other experiments, such as those at Fermilab and CERN, that the Standard Model of particle physics may be missing some key elements - perhaps new particles or forces.

“The result needs to be scrutinised and confirmed. If this, and other, deviations from the Standard Model are indeed confirmed over the next few years, this would usher in a new era in particle physics.” 

Decade of careful analysis
The CDF collaboration at the Fermi National Accelerator Laboratory (Fermilab) in the US announced the result today after 10 years of painstaking analysis.

Using data collected by the collaboration, scientists have now determined the particle’s mass with a precision of 0.01%—twice as precise as the previous best measurement.

This new mass measurement is different to the result expected when scientists work out the mass using the theory outlined in the Standard Model.

The team used data collected from the experiment’s run which started in 2003, through to when it stopped taking data in 2011. The Science and Technology Facilities Council (STFC) funded CDF from 2000-2012.

Professor Grahame Blair, STFC Executive Director for Programmes, said: “Results like this demonstrate the hidden effort of science – the years of careful number-crunching to progress closer to a new understanding. Physics beyond the Standard Model can come in careful iterations, edging us towards exciting new answers. STFC proudly continues to support UK scientists in the hunt for physics beyond the Standard Model in experiments including the Large Hadron Collider and others around the world.”

Although the new value matches up with some previous W boson mass measurements, there are also significant disagreements compared to previous results, so more measurements are needed to confirm the finding.

Painstaking work
The W boson is a messenger particle of the weak nuclear force and it is responsible for the nuclear processes that make the sun shine and radioactive-particles decay.

Using high-energy particle collisions produced by the Tevatron collider at Fermilab, the CDF collaboration collected huge amounts of data containing W bosons from 1985 to 2011.

CDF researchers have worked on achieving increasingly more precise measurements of the W boson mass for more than 30 years. This result uses the entire dataset collected from the second run of the Tevatron collider at Fermilab. It is based on the observation of 4.2 million W boson candidates, about four times the number used in the analysis the collaboration published in 2012.

The CDF collaboration comprises 400 scientists at 54 institutions in 23 countries, including the UK. It involves researchers at UCL and the universities of Glasgow, Liverpool and Oxford.




  • The Collider Detector at Fermilab recorded high-energy particle collisions produced by the Tevatron collider from 1985 to 2011. About 400 scientists at 54 institutions in 23 countries are still working on the wealth of data collected by the experiment. Credit: Fermilab

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