The mass of the so-called W boson is greater than previously thought, a team of physicists has concluded after years of analysis. And that could indicate new particles.

In itself, physicists have a well-functioning theory for particles and the forces that act between them: the Standard Model. The problem is that this theory cannot explain everything. For example, gravity is not part of it. Also, dark matter—the invisible stuff that makes up about 84 percent of the universe—isn’t made up of particles that are in this Standard Model.

That is why it is always very interesting if a measurement does not seem to match the standard model. Because that can be a first indication of particles, forces or other phenomena that still fall outside the standard model – and with which a number of open questions may be answered. The latest example: the mass of the W boson, which, according to the team behind the American particle experiment CDF II, is slightly larger than the standard model prescribes.

messenger particles

First of all, what kind of particle are we talking about here? The Standard Model contains three forces, or three ways in which particles can interact with each other. These are the electromagnetic force, the strong (nuclear) force and the weak (nuclear) force.

For each of these forces there are one or more ‘messenger particles’. For example, particles that attract or repel each other as a result of the electromagnetic force exchange photons. The strong force is transmitted by so-called gluons. And the messenger particles of the weak force, which play a role in radioactive decay, among other things, are the so-called W and Z bosons.

These are the W bosons. It is known that it must weigh about eighty times as much as protons and neutrons – the particles that make up atomic nuclei. Particle physicists have been trying to determine more and more precisely what that mass is for decades.

They do this with particle accelerators such as the Large Hadron Collider (LHC) near Geneva. In it, particles collide with each other at enormous speed, resulting in other particles being formed. One particle that can appear in such a collision is the W boson. That then decays within a fraction of a second to other, lighter particles, but from those other particles you can deduce – with a lot of pain and effort – what that short-lived W boson must have weighed.

Surprising result

The physicists behind the Collider Detector at Fermilab II (CDF II) have now done the latter with twice the precision of previous measurements. They used fairly ‘aged’ data: the CDF experiment recorded collisions in the US particle accelerator Tevatron, which was already retired in 2011. But the data that this machine has produced is still being analysed.

And in this case, such an analysis has produced quite a surprising result. Based on over 4 million particle collisions that produced W bosons, the CDF scientists arrive at a mass of 80.433 megaelectronvolts (MeV). (The electron volt is the unit of mass that particle physicists like to use. A proton or neutron weighs about 1 MeV.)

Why is that crazy? Because you can also calculate the mass of the W boson, using the Standard Model. You plug in the masses of all kinds of other particles, such as those of the Higgs boson discovered in 2012, and you get… 80,357 MeV.

dark photons

What could cause the difference between the calculated and measured mass of the W boson? For example, due to particles that have so far gone unnoticed in experiments, because they are too heavy or because they show too little of themselves. “Such particles can change the predicted mass of the W boson,” says CERN particle physicist Maarten Boonekamp, ​​not involved in the CDF study.

What kind of particles should those be? In their article, incidentally the cover story of the scientific magazine science, the CDF scientists list a number of options. Perhaps the Higgs boson is secretly made up of smaller particles. Or who knows, there may be so-called ‘dark photons’ that hardly interact with ordinary matter.

But actually, physicists at this point have no idea. “Dozens of theoretical papers are likely to appear in the coming weeks and months studying the implications of this result,” Boonekamp predicts.

With caution

At the same time, we shouldn’t be too hard on ourselves. “The result of CDF is quite different from previous measurements – including those of CDF itself,” says Boonekamp. “So caution is advised. And because it is such an exceptional result, which is quite different from the standard model, we have to be extra careful.”

What could be behind the result if it were not new particles or phenomena? According to Boonekamp, ​​the experimental analysis of the CDF team is exceptionally good – but the team makes use of outdated theoretical calculations. “The team does not mention the many developments in that area over the past 20 to 25 years. Whether this is important for their results remains to be seen. But at the moment it is an important point of attention.”

Far-reaching consequences

An important next step is to see if other teams can confirm the result. The eyes are mainly focused on the various experiments that study collisions in the particle accelerator LHC. So far, they are still a long way behind the CDF team in terms of precision, “but they are working on reaching the same level,” says Boonekamp.

Furthermore, the data collected with another Tevatron experiment, D0, could be reconsidered, write particle physicists Claudio Campagnari and Martijn Mulders in a commentary on the CDF article. And future particle accelerators such as the 100 kilometer long Future Circular Collider will also be able to say more about the mass of the W boson – but that is for the longer term.

For now, at least one team has come up with a surprising measurement. A measurement that could potentially have far-reaching consequences – but which could also be brushed aside if other physicists pounce on it.