Machine Heart Compilation
Editors: Zhang Qian, Chen Ping
If this discovery is confirmed by other experiments, it would be a major breakthrough in subverting the Standard Model of particle physics.
Physicists have found that an elementary particle known as the W boson appears to be 0.1 percent heavier than predicted by the Standard Model, a small difference that could herald a major shift in fundamental physics, and the results are on the cover of a new issue of Science.
The measurement comes from an old particle collider at the Fermi National Accelerator Laboratory in the United States, the Tevatron, which smashed the last batch of protons a decade ago. After that, the CDF (Collider Detector at Fermilab) team of more than 400 members continued to analyze the W boson produced by the collider, tracking countless sources of error for unmatched accuracy.
If the "extra 0.1% weight" of the W boson can be independently confirmed, it means that there are still some particles or forces in nature that we have not yet discovered, which will lead to the first major rewriting of the laws of quantum physics in half a century.
"This will revolutionize the way we see the world," and its significance may even rival the discovery of the Higgs boson in 2012, says Sven Heinemeyer, a physicist at the Institute for Theoretical Physics in Madrid. "The Higgs particle fits very well with previously known scenarios. But this discovery represents a whole new field."
The discovery comes at a time when the physics community is eager to discover flaws in the Standard Model of particle physics. The Standard Model is a set of equations that have long dominated the physics community, covering all known particles and forces. The Standard Model is known to be incomplete, and many problems are difficult to explain with the help of the Standard Model, such as the properties of dark matter. The CDF team's proven track record makes their new results a credible threat to the Standard Model.
In particle physics, the Standard Model (SM) is a set of theories that describe the three fundamental forces of strong, weak, and electromagnetic forces, and the elementary particles that make up all matter. It falls under the category of quantum field theory and is compatible with quantum mechanics and special relativity. The results of almost all of the experiments on these three forces are in line with the predictions of this theory. But the Standard Model is not yet a theory of everything, mainly because it does not describe gravity. The Standard Model has 61 elementary particles, including fermions and bosons—fermions are particles with semi-odd spins and adhere to the Pauli exclusion principle ( which states that no identical fermions can hold the same quantum state ) — and bosons have integer spins and do not adhere to the Pauli exclusion principle.
Aida El-Khadra, a theoretical physicist at the University of Illinois at Urbana-Champaign, said, "They've made hundreds of beautiful measurements. They are known for their rigor."
But no one has yet opened champagne to celebrate. Although the W boson mass measured by the CDF is far from the Standard Model's predictions individually, the results of other groups are not so different from the Standard Model (although less precise). For example, in 2017, the Atlas experiment of the European Large Hadron Collider (LHC) also measured the mass of the W boson and found that it was only one hair heavier than the Standard Model predicted. The conflict between CDF and ATLAS suggests that at least one team overlooked some of the subtle oddities in their experiments.
Aerial view of the Large Hadron Collider. The circular tunnel is 27.36 km long.
"I want the CDF results to be confirmed, and I want to understand the difference between it and previous measurements," said Guillaume Schmidt, a physicist at CERN whose lab owns the Large Hadron Collider and is a member of the ATLAS experiment.
"It's a monumental work," says Frank Wilczek, a Nobel Prize physicist at MIT, "but it's hard to know how to take advantage of this result."
W(Weak) boson
In physics , the W and Z bosons are elementary particles responsible for transmitting weak forces. Weak force is one of the four fundamental forces of the universe (gravity, electromagnetic force, strong interaction force (strong force), weak interaction force (weak force)). The four fundamental force parameters constitute the cosmic environment known today in a delicate balance, and these parameters can be accurate even to countless digits after the decimal point, and the change of any one of these parameters will completely change the entire universe.
Unlike gravity, electromagnetism, and force, weak forces do not push or pull too much, but instead convert heavier particles into lighter particles. For example, a μ meson spontaneously decays into a W boson and a neutrino, which in turn becomes an electron and another neutrino. The associated subatomic deformation produces radioactivity and helps keep the sun shining continuously.
Computer images of particle collisions in Fermilab's CDF detectors show W bosons decaying into positrons (magenta blocks, bottom left) and invisible neutrinos (yellow arrows). Image source: Fermilab/Science Photo Library
Over the past 40 years, the masses of W and Z bosons have been measured through a variety of experiments. The mass of the W boson proved to be a particularly appealing goal. The masses of other particles can only be simply measured and accepted as natural facts, but the mass of the W boson can be predicted by incorporating some other measurable quantum properties in the Standard Model equations.
For decades, experimenters at Fermilab and elsewhere have used the network of connections around the W boson to try to detect more particles. Once the researchers have accurately measured the factors that have the greatest impact on the mass of the W particle—such as the strength of the electromagnetic force and the mass of the Z boson—they can begin to feel the smaller effects that pull on their mass.
This approach allowed physicists to predict the mass of a particle called a top quark in the 1990s. They repeated this feat in the early 2000s: predicting the mass of the Higgs boson before it was detected.
But while theorists have a variety of reasons to expect the top quark and the Higgs boson and relate it to the W boson through the Standard Model equations, today's theories have no obvious missing parts. Any difference in W boson mass points to the unknown.
Measure the mass of the W boson
CDF's latest measurements of W boson quality are based on an analysis of about 4 million W bosons produced by Tevatron between 2002 and 2011. When Tevatron collides protons into antiprotons, the W boson usually emerges in the chaos that follows. W can then decay into a neutrino and a μ/electron, both of which can be detected directly. The faster the μ or electron, the heavier the W boson that produces it.
Ashutosh Kotwal, a physicist at Duke University and the driving force behind the CDF, worked to refine the scheme. At the heart of the W boson experiment is a cylindrical chamber of 30,000 high-voltage wires that react when μ or electrons fly over them. This allows CDF researchers to infer the particle's path and velocity. In this process, knowing the exact location of each line is key to obtaining an accurate trajectory of the particles. In the new analysis, Kotwal and his colleagues used μ that fall from the sky in the form of cosmic rays, bullet-like particles that constantly pass through the detector in nearly perfect straight lines, allowing researchers to detect any unstable lines and fix the position of the lines to within 1 micron.
Ashutosh Kotwal
The Kotwal team also conducted exhaustive cross-checking over the years between data releases and repeated measurements in an independent manner to ensure they understood every trait of Tevatron. At the same time, W boson measurements are piling up faster and faster. The CDF published a paper in 2012, "Precise measurement of the W-boson mass with the CDF II detector," which covered Tevatron's first five years of data, which quadrupled over the next four years.
Kotwal humorously said, "It rushes over like water in a fire hose, faster than you can drink."
At a Zoom conference in November 2020, Kotwal declassified the team's latest results. The physicists in the room fell silent as they digested the answers. They found that the weight of the W boson is 80.433 billion electron volts (MeV) with an error of about 9 MeV. This makes it heavier than the Standard Model predicts 76 MeV, a difference of about seven times the margin of error for measurements or predictions.
In the scientific community, scientists use several sigma to determine the importance of a measurement. If sigma is less than 3, scientists will think it's not interesting. If sigma is between 3 and 5, scientists will begin to become interested and refer to the condition as "evidence of a certain discovery." If sigma is greater than 5, scientists are confident that the difference is real and meaningful. For sigma above 5, scientists usually name their papers "Right... observations". 5 sigma is a major discovery. The CDF's measurement was 7 sigma and should theoretically be published as a major discovery. But the low measurements from ATLAS and other experiments brought them to a halt.
"I would say it's not a discovery, it's a provocation," said Chris Quigg, a theoretical physicist at Fermilab who was not involved in the study. "This gives us a reason to accept this outlier."
The CDF's new measurements are wrong, or the long-awaited breakthrough is coming
With the Tevatron dust buildup, the responsibility for confirming or refuting CDF measurements will fall on the Large Hadron Collider. The device has produced more W bosons than Tevatron, but its higher collision rate complicates the analysis of W masses. However, by collecting additional data — perhaps at lower beam intensities — the LHC could address these issues in the coming years.
At the same time, theorists can't help but wonder what the oversized W boson might mean.
μ briefly releases W bosons as they decay into electrons, which can interact with other particles, even undiscovered particles. This unknown may result in errors in the mass measurement of W.
In addition, the researchers gave a number of other possible reasons, such as the extra weight of the W boson may be caused by a second Higgs boson, which is less active than the Higgs boson we know; or that a new massive boson transmits a variant of a weak force; or that multiple particles make up the composite Higgs boson (a new force that binds them together).
Some theorists are beginning to question the particles predicted by the theory of supersymmetry. Supersymmetry is a symmetry between fermions and bosons that has not yet been observed in nature. The framework associates particles of matter with force-carrying particles, setting each known particle an undiscovered opposite type of particle (which can be called a "partner"). But this "super buddy" failed to materialize on the Large Hadron Collider, and supersymmetry was no longer popular, but some theorists still think this is true.
Supersymmetry in particle physics. The concept map shows the Standard Model particles introduced by the principle of supersymmetry (SUSY) and their heavier supersymmetric partner particles.
Heinemeyer and his collaborators recently calculated that certain supersymmetric particles can solve another hypothetical phenomenon that does not conform to the Standard Model, the muon g-2 anomaly. In this way, these particles also cause the mass of the W boson to rise slightly, although not enough to match the CDF measurements. "It's fascinating that the particles that help us solve the g-2 problem may also help us solve the W boson mass problem," Heinemeyer said.
Researchers believe the long-awaited breakthrough is coming.
"For me, it feels like we're approaching a tipping point where something is going to explode, we're approaching a real model that goes beyond the Standard Model." El-Khadra said.
https://www.quantamagazine.org/fermilab-says-particle-is-heavy-enough-to-break-the-standard-model-20220407/