Too heavy a particle disrupts physics

Too heavy a particle disrupts physics


Mathilde Fontez, editor-in-chief of the scientific magazine Epsiloontoday evokes a discovery announced a few days ago which is causing a lot of noise in the field of physics.

franceinfo: On measured the mass of a particle, and it is too high, which could mean that the theories of matter have to be rewritten?

Mathilde Fontez: Yes, the mass value that had been measured in the past, and more importantly, that had been predicted by theory was around 80,357 mega-electronvolts. And the mass just published by a collaboration of nearly 400 physicists is 80,443.5. So, we agree, the difference is not huge: only 0.09%. But these measurements are precise: this difference is enough to say that there is a problem, that there is something that we have not understood.

Is it a particle of matter that has been measured?

It is an elementary particle called the W boson : a particle of the same family as the photon, the particle of light. Except that the W boson does not transmit electromagnetism, it transmits a nuclear force: it participates in radioactivity. It was discovered in 1983, and studied a lot since, in particle colliders, these large tunnels in which particles are propelled at the speed of light, so that they explode against each other, and that they give rise to secondary particles that can be studied.

It is data of this type that speaks today: data from the Tevatron, an American collider which operated between 1984 and 2011. It took physicists 10 years to process them, extract the signal corresponding to millions of W bosons , and find its mass.

So the W boson is too heavy?

A bit too heavy yes. So we could say to ourselves that we were simply mistaken about this mass, that it doesn’t change much. Except that in the theory of matter – this theory is called the standard model – all the properties of the particles are described, and are related to each other. If we were wrong about the mass of the W boson, that means that the theory is wrong, that it has a problem. And that is great news!

Why good news?

Because physicists know that the Standard Model is incomplete. And they have been looking for decades for clues that could lead them to complete it. In particular, this theory fails to describe gravity – we do not know how to describe, on a microscopic scale, the force which makes apples fall from the trees, and which keeps our feet on the ground. It is therefore a great hope that opens up today, thanks to the W bosonto find a breach, to unravel this mystery.


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