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W boson: Is the new measurement for the subatomic particle's mass the first chink in the armour of the Stand Model? © Getty Images

W boson: Is the new measurement for the subatomic particle's mass the first chink in the armour of the Standard Model?

Published: 22nd April, 2022 at 15:33
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Data from an old experiment hints at answers to some of the biggest questions in physics.

A new measurement from an old experiment may have just given us a huge clue to some big unanswered questions in physics. The Collider Detector at Fermilab (CDF), a particle accelerator experiment which operated until 2011, recently caused a stir by re-measuring the mass of a particle known as the ‘W boson’.


Each of the four fundamental forces (the strong force, the weak force, electromagnetism and gravity) has associated particles which ‘carry’ the force: for example, the photon – a particle of light – is a carrier of the electromagnetic force. The W boson is one of the carriers of the weak force.

It is unusual for an experiment which stopped taking data more than a decade ago to rouse such interest. The reasons are subtle, but compelling. To see why, let’s step back and see where our knowledge of the fundamental forces and constituents of matter – expressed in the so-called ‘Standard Model’ – stands at present. The Standard Model describes the strong, weak and electromagnetic forces, and all known elementary particles.

The theory explains the mass of the W (and all the other fundamental particles), and also predicted the existence of the Higgs boson, which was then discovered at CERN in 2012. This ‘completes’ the Standard Model, but leaves several questions unanswered. For example, how does gravity (a glaring omission from the model!) fit in?

Why, according to astrophysical observations, is there a lot of so-called ‘dark matter’ in the Universe, and what is it? Why is there so much more matter than antimatter? The Standard Model is clearly not the full story, and many extensions to it have been postulated.

The Standard Model is a subtle framework, though. In the subatomic world of quantum mechanics, particles influence each other even when there is not enough energy around for them to exist. They can traverse tiny loops, forming and annihilating before they are directly observed. We call them ‘virtual’ particles, but their influence is very real and measurable. One thing they do is influence particle masses, with the consequence that while the Standard Model does not predict the absolute values of particle masses, it does predict – very precisely – some relationships between them.

Back to the W mass then. It can be measured directly, which is what CDF has done (and more on that in a moment). But it can also be calculated using all the other measurements we have made, combined with the relationships between masses in the Standard Model.

The directly measured value should agree with the calculated value, otherwise something is wrong. Excitingly, there could be new, beyond-the-Standard-Model virtual particles participating in those loops. The interest has risen because the new CDF mass measurement does not agree with the calculated W mass.

If you made this CDF measurement a million million times, you’d only expect one discrepancy this big, if the Standard Model is correct. As always, there are some reasons for caution. CDF measured W bosons produced in high-energy collisions between protons and anti-protons.

The measurement has taken over a decade because it is very hard to be so precise. When a W is produced, it decays instantaneously, and one of the things it produces is a neutrino, which CDF cannot detect. Information about the neutrino (and hence about the W mass) is calculated from assuming it must balance everything else produced in the collision.

This means many different sources of uncertainty can have a significant influence, such as the distribution of particles inside the proton, extraneous background particles, and of course the precise geometry and accuracy of the detector itself.

Even so, mistakes can never be completely ruled out, and the new measurement is in fact somewhat out of line with other measurements, even those made earlier by CDF. Now the result is out there, it will receive a level of scrutiny few other measurements get, and other experiments, especially those at CERN, will be trying hard to match its precision and confirm or refute the discrepancy.

That said, this is a very strong hint that the answers to some of the big questions left open by the Standard Model may soon be within reach, just as the Large Hadron Collider starts its third running period and will itself be increasing the precision with which it can probe the energy frontier.

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Jon is a Professor of Physics at University College London. He works on the ATLAS experiment at the CERN Large Hadron Collider.


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