La mesure de la masse des bosons W a pris 10 ans – et le résultat n’était pas celui auquel les physiciens s’attendaient.
“Vous pouvez le faire rapidement, vous pouvez le faire à moindre coût ou vous pouvez le faire correctement. Nous l’avons bien fait. » Telles étaient quelques-unes des remarques d’ouverture de David Toback lorsque le chef du Collider Detector du Fermilab a dévoilé les résultats d’une expérience d’une décennie visant à mesurer la masse d’une particule connue sous le nom de boson W.
Je suis un physicien des particules à haute énergie et je fais partie de l’équipe de centaines de scientifiques qui a construit et exploité le détecteur de collisionneur au Fermilab dans l’Illinois – connu sous le nom de CDF.
Après des milliards de collisions et des années de collecte de données et de calculs, l’équipe CDF a découvert que le boson W avait légèrement plus de masse que prévu. Bien que l’écart soit minime, les résultats, décrits dans un article publié dans la revue Science le 7 avril 2022, ont électrifié le monde de la physique des particules. Si la mesure est en effet correcte, c’est encore un autre signal fort qu’il manque des pièces au puzzle physique du fonctionnement de l’univers.
Le modèle standard de la physique des particules décrit les particules qui composent la masse et les forces de l’univers. MissMJ / WikimediaCommons Le modèle standard de la physique des particules décrit les particules qui composent la masse et les forces de l’univers. Crédit : MissMJ / WikimediaCommons
Une particule qui porte la force faible
Le modèle standard de la physique des particules est le meilleur cadre scientifique actuel pour les lois fondamentales de l’univers et décrit trois forces fondamentales : la force électromagnétique, la force faible et la force forte.
Les noyaux atomiques sont maintenus ensemble par la force forte. Cependant, certains noyaux sont instables et subissent une désintégration radioactive, libérant lentement de l’énergie par émission de particules. Ce processus est piloté par la force faible, et les scientifiques essaient de comprendre pourquoi et comment les atomes se désintègrent depuis le début des années 1900.
Selon le modèle standard, les forces sont transmises par des particules. Dans les années 1960, une série de percées théoriques et expérimentales ont proposé que la force faible soit transmise par des particules appelées bosons W et Z. Il a également postulé qu’une troisième particule, le boson de Higgs, est ce qui donne la masse à toutes les autres particules – y compris les bosons W et Z.
Depuis l’avènement du modèle standard dans les années 1960, les scientifiques ont parcouru la liste des particules prédites mais non découvertes et mesuré leurs propriétés. En 1983, deux expériences à[{” attribute=””>CERN in Geneva, Switzerland, captured the first evidence of the existence of the W boson. It appeared to have the mass of roughly a medium-sized atom such as bromine.
By the 2000s, there was just one piece missing to complete the Standard Model and tie everything together: the Higgs boson. I helped search for the Higgs boson on three successive experiments, and at last we discovered it in 2012 at the Large Hadron Collider at CERN.
The Standard Model was complete, and all the measurements we made hung together beautifully with the predictions.
The Collider Detector at Fermilab collected data from trillions of collisions that produced millions of W bosons. Credit: Bodhita/WikimediaCommons, CC BY-SA
Measuring W bosons
It’s a lot of fun to smash particles together at really high energies to test the Standard Model. These collisions produce heavier particles for a brief period of time before decaying back into lighter particles. To analyze the properties and interactions of the particles created in these collisions, physicists employ massive and extremely sensitive detectors at facilities such as Fermilab and CERN.
In CDF, W bosons are produced about one out of every 10 million times when a proton and an antiproton collide. Antiprotons are the antimatter version of protons, with exactly the same mass but opposite charge. Protons are made of smaller fundamental particles called quarks, and antiprotons are made of antiquarks. It is the collision between quarks and antiquarks that create W bosons. W bosons decay so fast that they are impossible to measure directly. So physicists track the energy produced from their decay to measure the mass of W bosons.
In the 40 years since scientists first detected evidence of the W boson, successive experiments have attained ever more precise measurements of its mass. But it is only since the measurement of the Higgs boson – since it gives mass to all other particles – that researchers could check the measured mass of W bosons against the mass predicted by the Standard Model. The prediction and the experiments always matched up – until now.
The new measurement of the W boson (red circle) is much farther from the mass predicted by the Standard Model (purple line) and also greater than the preliminary measurement from the experiment. Credit: CDF Collaboration via Science Magazine, CC BY
Unexpectedly heavy
Fermilab’s CDF detector is excellent at accurately measuring W bosons. Between 2001 and 2011, the accelerator smashed protons and antiprotons trillions of times, creating millions of W bosons and collecting as much data as possible from each collision.
In 2012, the Fermilab team reported preliminary results based on a subset of the data. We discovered that the mass was somewhat off, but close to the prediction. The researchers then laboriously analyzed the entire data set for a decade. Numerous internal cross-checks were performed, as well as years of computer simulations. Nobody could see any results until the entire calculation was completed to avoid bias sneaking into the analysis.
When the physics world finally saw the result on April 7, 2022, we were all surprised. Physicists measure elementary particle masses in units of millions of electron volts – shortened to MeV. The W boson’s mass came out to be 80,433 MeV – 70 MeV higher than what the Standard Model predicts it should be. This may seem like a tiny excess, but the measurement is accurate to within 9 MeV. This is a deviation of nearly eight times the margin of error. When my colleagues and I saw the result, our reaction was a resounding “wow!”
The fact that the measured mass of the W boson differs from the anticipated mass in the Standard Model could indicate one of three things. Either the math is incorrect, the measurement is incorrect, or something is missing from the Standard Model.
What this means for the Standard Model
The fact that the measured mass of the W boson doesn’t match the predicted mass within the Standard Model could mean three things. Either the math is wrong, the measurement is wrong or there is something missing from the Standard Model.
First, the math. In order to calculate the W boson’s mass, physicists use the mass of the Higgs boson. CERN experiments have allowed physicists to measure the Higgs boson mass to within a quarter-percent. Additionally, theoretical physicists have been working on the W boson mass calculations for decades. While the math is sophisticated, the prediction is solid and not likely to change.
The next possibility is a flaw in the experiment or analysis. Physicists all over the world are already reviewing the result to try to poke holes in it. Additionally, future experiments at CERN may eventually achieve a more precise result that will either confirm or refute the Fermilab mass. But in my opinion, the experiment is as good a measurement as is currently possible.
That leaves the last option: There are unexplained particles or forces causing the upward shift in the W boson’s mass. Even before this measurement, some theorists had proposed potential new particles or forces that would result in the observed deviation. In the coming months and years, I expect a raft of new papers seeking to explain the puzzling mass of W bosons.
As a particle physicist, I am confident in saying that there must be more physics waiting to be discovered beyond the Standard Model. If this new result holds up, it will be the latest in a series of findings showing that the Standard Model and real-world measurements often don’t quite match. It is these mysteries that give physicists new clues and new reasons to keep searching for a fuller understanding of matter, energy, space, and time.
Written by John Conway, Professor of Physics, University of California, Davis.
This article was first published in The Conversation.