Science – The W particle shakes up particle physics


The Standard Model of particle physics cannot explain certain idiosyncrasies of the W particle, a fundamental particle of matter.

An image of CERN, in Geneva.

An image of CERN, in Geneva.


One of the fundamental particles of matter, the W particle, would have a greater mass than the theory predicts, shaking the “house of cards” of the Standard Model of particle physics, according to a study published in the journal Thursday. “Science”

“There are hints that some pieces are missing from the Standard Model, and we’re bringing a new one, which is very interesting and very important,” the lead author, Professor Ashutosh Kowtal, a physicist at American Duke University, told AFP.

Basis of radioactivity

This theory explains all the measurements made in the field of elementary particle physics, that is, the world of the infinitesimal, whose elements make up the atoms and the forces that control them. The standard model, completed in the second half of the XXand century, makes it possible to “make fantastically accurate predictions” about the behavior of these particles, explains physicist Harry Cliff of the University of Cambridge.

Like that of the W particle, a particle that mainly transmits a so-called weak interaction between other matter particles. It is the basis of radioactivity and, beyond that, of nuclear fusion reactions, such as those that power the sun. All these particles and forces are connected to each other in a kind of equilibrium. For example, the mass of the W particle is limited by that of the Higgs boson.

“Extraordinary Affirmations”

“The Standard Model predicts an equilibrium, and the experimental result presented to us contradicts this prediction,” notes AFP, CNRS physicist and research director Jan Stark. This “house of cards” falters with the study’s announcement that the mass of the W particle is stronger than expected. The achievement of the CDF collaboration, a group of some 400 scientists led by the Pr Kowtal measured this mass, at 80,433 megaelectronvolts, with unprecedented precision (0.01%), twice as great as the best existing.

It is the culmination of ten years of analysis of a 4 million particle sample produced at the Tevatron, a particle accelerator at Fermilab in the United States, which is now closed. This accelerator, like the LHC at CERN in Europe (which made it possible to identify the Higgs boson), causes particles to coalesce at phenomenal speeds, revealing the elements that make up them by breaking them down.

It’s now up to another team, on a different instrument, to confirm the result of this study to prove it. Because, as Jan Stark reminds us, “extraordinary claims require extraordinary evidence”.

“Animated Discussions”

A major challenge, given the extreme precision of the measurement, which cannot be a matter of statistical chance. Therefore, “it is either a big discovery or a problem in the analysis of the data,” says Jan Stark, predicting “fairly lively discussions in the coming years.”

The announcement of the CDF collaboration is the latest of “cracks that have been appearing in the Standard Model for several years, with accurate measurements contradicting model predictions,” note the physicists and authors of another paper in “Science.” If confirmed, this discovery could reveal the existence of “new interactions or new particles”, which today’s experiments are not yet able to reveal.

“A great thing in the infinitely great”

So if physicists are looking for lice on the head of the Standard Model, it’s because the latter struggles mainly with explaining “a grand thing in the infinitely large,” dark matter, said Jan Stark, who leads the appropriate called Laboratory. des 2 infinis (L2IT), at Paul Sabatier University in Toulouse.

Several observations, such as the speed of galaxies in galactic clusters or the anomalous speed of rotation of certain stars, have forced astrophysicists to theorize about the existence of a hypothetical “dark matter” animating these phenomena. But nothing in the Standard Model explains what particle this “dark matter” would consist of. “We follow the path, without neglecting any trace. So in the end we will have understanding”, says the P . to believer Kotwal.