After years of analysis, a team of physicists has concluded that the so-called W boson has a mass greater than previously thought. This could indicate new particles.
By itself, we have a well-functioning theory of particles and the forces that work between them: the Standard Model. The problem is that this theory cannot explain everything. For example, gravity is not a part of it. Also, dark matter — the invisible things that make up about 84 percent of the universe — is not made of the particles found in this Standard Model.
That’s why it’s always interesting if a measurement doesn’t match the Standard Model. Because this could be the first indication of particles, forces, or other phenomena that are still outside the Standard Model – from which a number of open questions can be answered. The most recent example: the mass of the W boson, which, according to the team behind the American CDF II particle experiment, is slightly larger than the Standard Model it describes.
First of all, what kind of particles are we talking about here? The Standard Model contains three forces, or three ways that 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 as a result of the exchange of electromagnetic force photons. The intense force is transmitted by the so-called gluons. and weak force messenger particles, which play a role in radioactive decay, among other things, the so-called W and Z bosons.
These are the W bosons. We know 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 precisely what that mass is for decades.
They do this using particle accelerators such as the Large Hadron Collider (LHC) near Geneva. In it, particles collide with each other at a tremendous speed, which leads to the formation of other particles. One particle that can appear in such a collision is the W boson. This then decays within a fraction of a second into other, lighter particles, but from those other particles you can deduce – with a lot of pain and effort – what the boson must weigh W is short-lived.
The physicists behind the collider detector at Fermilab II (CDF II) have now made the latter with twice the accuracy of previous measurements. They used somewhat ‘outdated’ data: the CDF experiment recorded collisions at the American particle accelerator Tevatron, which was already discontinued in 2011. But the data produced by this machine is still being analyzed.
And in this case, such an analysis yielded a completely surprising result. Based on more than 4 million particle collisions that produced W bosons, CDF scientists have arrived at a mass of 80.433 megaelectronvolts (MeV). (The electron volt is the unit of mass that particle physicists prefer to use. A proton or neutron weighs about 1 electron volt.)
Why is this crazy? Because you can also calculate the mass of the boson W using the standard form. You connect the masses of all other kinds of particles, like the one in the Higgs boson discovered in 2012, and you get… 80,357 electronvolts.
What can cause the difference between the calculated and measured mass of the W boson? For example, due to particles that have not yet been observed in experiments, because they are too heavy or because they show too little of themselves. “Such particles can change the expected mass of the W boson,” says CERN particle physicist Martin Boncamp, who was not involved in the CDF study.
What kind of particle should it be? In their article, by the way, the cover story of the scientific journal to knowCDF scholars list a number of options. The Higgs boson may be secretly made of even smaller particles. Or who knows, there may be so-called “dark photons” that barely interact with ordinary matter.
But we actually have no idea at this point. Boncamp predicts that “dozens of theoretical papers are likely to appear in the coming weeks and months to examine the implications of this finding.”
At the same time, we should not be so hard on ourselves. “The result of the CDF is very different from previous measurements – including the measurements of the CDF itself,” Boncamp says. “So caution is advised. Because it is an exceptional result, completely 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 CDF team’s empirical analysis is very good – but the team uses outdated theoretical calculations. The team did not mention the many developments in this field over the past 20 to 25 years. It remains to be seen if this is significant to their findings. But at the moment it is an important point of interest.”
An important next step is to see if other teams can confirm the result. The eyes mainly focus on the various experiments that study collisions at the LHC particle accelerator. So far, they’re still far from the CDF team in terms of accuracy, “but they’re working to get to the same level,” Boncamp says.
Moreover, the data collected with another Tevatron experiment, D0, can be revisited, particle physicists Claudio Campaniari and Martin Mulders wrote in a comment to the CDF article. Future particle accelerators, such as the 100-kilometre Future Circular Collider, will also be able to say more about the mass of the W boson – but that’s in the long run.
Right now we’re only talking about one team that came up with a surprising analogy. A measurement that could have far-reaching consequences – but could also be ignored if other physicists pounce on it.
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