“You can do it fast, you can do it cheap, or you can do it. We’ve done it well.” decades to measure the mass of a particle called the W boson.
I am a high energy particle physicist and part of the team of hundreds of scientists who built and operated the collision detector at the Fermilab in Illinois, known as the CDF.
After billions of collisions and years of data collection and calculations, the CDF team has discovered that the W boson has a little more mass than expected. Although the difference is small, the results are described in an article published in the journal Science on April 7, 2022, having electrified the world of particle physics. If the measurement is correct, it is another strong signal that pieces are missing from the physical puzzle of how the universe works.
A particle that carries weak strength.
The standard model of particle physics is the best current scientific framework for the fundamental laws of the universe and describes three basic forces: electromagnetic force, weak force, and strong force.
Atomic nuclei are held together by strong force. However, some nuclei are unstable and disintegrate radioactively, slowly releasing energy by emitting particles. This process is driven by weak force, and scientists have been trying to understand why and how atoms have been separated since the early 20th century.
According to the standard model, forces are transmitted by particles. In the 1960s, a number of theoretical and experimental advances proposed that the weak force was carried by particles called W and Z bosons. He also postulated that a third particle, the Higgs boson, is the one that gives mass to all the other particles, including the W and Z bosons.
Since the advent of the Standard Model in the 1960s, scientists have been working on the list of predicted particles that have not yet been discovered and on measuring their properties. In 1983, two experiments of
Measurement of bosons W
It is a lot of fun to break very high energy particles to test the standard model. These collisions produce heavier particles for a short period of time before breaking into lighter particles. To analyze the properties and interactions of the particles created in these collisions, physicists use massive and extremely sensitive detectors at facilities such as Fermilab and CERN.
In CDF, W bosons occur approximately once in 10 million when a proton and an antiproton collide. Antiprotons are the antimatter version of protons, with exactly the same mass but with opposite charges. Protons are made up of smaller fundamental particles called quarks, and antiprotons are made up of antiquarks. It is the collision between quarks and antiquarks that creates W bosons. W bosons decay so rapidly that it is impossible to measure them directly. Therefore, physicists track the energy produced by their decay to measure the mass of W bosons.
In the 40 years since scientists first detected the presence of the W boson, successive experiments have made increasingly accurate measurements of its mass. But it was only from the measurement of the Higgs boson, as it gives mass to all other particles, that the researchers were able to compare the measured mass of the W bosons with the mass predicted by the standard model. Prediction and experiments have always coincided, so far.
Fermilab’s CDF detector is excellent for accurately measuring W bosons. Between 2001 and 2011, the accelerator collided with protons and antiprotons billions of times, creating millions of W bosons and collecting as much data as possible with each collision.
In 2012, the Fermilab team reported preliminary results based on a subset of data. We found that the mass was a little low, but close to the prediction. The researchers then thoroughly analyzed all the data for a decade. Many internal cross-checks have been performed, as well as years of computer simulations. No one was able to see the results until the entire calculation was completed to prevent the bias from leaking into the analysis.
When the world of physics finally saw the result on April 7, 2022, we were all amazed. Physicists measure the masses of elementary particles in units of millions of volts of electrons, abbreviated as MeV. The mass of the W boson turned out to be 80,433 MeV – 70 MeV above what the standard model predicts. This may seem like a small excess, but the measurement is accurate to 9 MeV. This is a discrepancy of almost eight times the margin of error. When my classmates and I saw the result, our reaction was a “Wow!
What does this mean for the standard model
The fact that the measured mass of the W boson does not match the mass predicted in the standard model could mean three things. Either the calculations are incorrect, the measurement is incorrect or something is missing from the standard model.
First, math. To calculate the mass of the W boson, physicists use the mass of the Higgs boson. CERN experiments have allowed physicists to measure the mass of the Higgs boson by a quarter of a percent. In addition, theoretical physicists have been working on W boson mass calculations for decades. Although the calculations are sophisticated, the prediction is solid and unlikely to change.
The next possibility is a failure of the experiment or analysis. Physicists around the world are already looking at the result to try to break it. In addition, future experiments at CERN may produce a more accurate result that confirms or disproves the mass of Fermilab. But in my opinion, experience is the best possible measure right now.
This leaves the last option: there are inexplicable particles or forces that cause the mass of the W boson to move upwards. Even before this measure, some theorists had proposals for new particles or potential forces that would lead to the observed deviation. In the coming months and years, I look forward to a series of new articles that seek to explain the baffling mass of the W bosons.
As a particle physicist, I am convinced that there must be more physics to discover beyond the standard model. If this new result is maintained, it will be the last of a series of findings showing that the standard model and real-world measurements often do not match at all. It is these mysteries that give physicists new clues and new reasons to continue seeking a more complete understanding of matter, energy, space, and time.
Written by John Conway, Professor of Physics at the University of California, Davis.
This article first appeared in The Conversation.