The mass of the W boson is “more than expected”, and there may be undiscovered particles or forces in the universe – Programmer Sought

Beijing time on May 9, according to foreign media reports, a new analysis of the mass of the W boson shows that these particles are much heavier than the standard model of particle physics predicts.Physicists have found that the elementary particle, which transmits the weak nuclear force, appears to be 0.1% heavier than previously predicted, whileIt’s this tiny difference that could herald a major change in fundamental physics. The results of the study were published recently in the journal Science.

▲ The W boson is one of 17 known elementary particles whose peculiar mass measurements could point to unknown particles or forces.

Latest measurements of the W bosonAn old-fashioned particle collider from the Fermi National Accelerator Laboratory in Illinois, the Tevatron. It was a particle accelerator built in 1983 and closed on September 30, 2011. Since then, approximately 400 members of the Fermilab Collider Detector (CDF) collaboration have continued to analyze the W bosons produced by the Tevatron, tracking down countless possible sources of error to an unrivaled level of precision.

If the W boson is “0.1% overweight” relative to the predictions of standard theory, it would beIt means that there are some particles or forces in the universe that have not yet been discovered by us, the laws of quantum physics in half a century may be about to undergo the first major rewrite that will revolutionize the way we see the world. The Higgs boson fits the previously known picture, but this discovery will open up a whole new field.The new findings, if validated, could even rival the Higgs boson discovered in 2012.

Currently,The physics community is desperately looking for flaws in the Standard Model of particle physics, and the discovery came at just the right time. The Standard Model is a set of equations that describe the three fundamental forces, the strong, the weak, and the electromagnetism, and the fundamental particles that make up all matter. It has long dominated particle physics, covering almost all known particles and forces. However, the Standard Model is still considered incomplete, and many unsolved mysteries remain to be explained, such as the nature of dark matter. The proven track record of CDF collaborative projects makes their new measurements quite credible, thus challenging the Standard Model.

Still, no one popped the champagne to celebrate. While the new W boson mass measurements differed considerably from those predicted by the Standard Model in isolation, other measurement experiments yielded less dramatic (though less precise) results. For example, in 2017, the ATLAS experiment at Europe’s Large Hadron Collider (LHC) measured the mass of the W boson and found it to be just a tad heavier than the Standard Model predicted — only the weight of a hair. The inconsistent results between CDF and ATLAS suggest that at least one of the teams overlooked some subtle quirks in the experiments.

If the CDF result is confirmed, and the researchers also want to understand how it differs from previous measurements, the W bosons on both sides of the Atlantic must be the same, which is a landmark work, but also difficult for us know what to do with it.

W boson

▲ The CDF project is one of two experiments carried out at different locations on the 6.3-kilometer ring of the Tevatron particle accelerator, shown here in 2001 during its installation.

The W and Z bosons, the elementary particles responsible for transmitting the weak nuclear force, were discovered in 1983 and were considered a triumph for the Standard Model. The W boson is named after the “weak” of the weak nuclear force. The weak nuclear force, also known as the weak interaction or weak force, is one of the four fundamental forces of the universe. Unlike gravity, electromagnetism, and the strong nuclear force (the strong interaction), the weak nuclear force doesn’t push or pull much, but rather converts heavier particles into lighter ones. For example, a muon spontaneously decays into a W boson and a neutrino, and then the W boson decays into an electron and another neutrino. The associated subatomic deformation produces radioactivity, a process that keeps the sun’s rays shining.

Over the past 40 years, researchers have measured the masses of the W and Z bosons through a variety of experiments. The mass of the W boson turned out to be a particularly enticing research target. While the masses of other particles are simply measured and accepted as a fact of nature, the mass of the W boson can only be predicted by incorporating some other measurable quantum properties in the Standard Model equations.

For decades, experimental physicists at Fermilab and other research institutions have exploited the network of connections around the W boson in an attempt to detect other particles associated with it. Once the researchers have precisely measured the terms that have the most influence on the mass of the W boson—such as the strength of the electromagnetic force and the mass of the Z boson—they can begin to examine other factors that have less of an effect on its mass.

Using this method, physicists in the 1990s predicted the mass of a particle called the top quark. The top quark interacts with other elementary particles through the strong force and decays into the W boson and bottom quark through the weak force. In 1995, physicists detected and determined the mass of the top quark. In 2000, physicists repeated the feat: predicting the mass of the Higgs boson before it was discovered.

However, while theoretical physicists have every reason to expect the top quark and the Higgs boson, and link them to the W boson through the Standard Model equations, there is no obvious missing piece to today’s theory. Any difference in the mass of the W boson points to the unknown.

Measure the mass of the W boson

▲ Fermilab’s Tevatron particle collider was once the most powerful accelerator in the world.

The CDF project’s latest measurement of the W boson mass is based on an analysis of about 4 million W bosons produced by Tevatron between 2002 and 2011. When Tevatron smashes antiprotons with protons, W bosons often appear in the ensuing chaos. The W boson then decays into a neutrino and a muon or electron, both of which can be directly detected. The faster the muon or electron, the heavier the W boson that produced it.

Ashutosh Kotwal, a physicist at Duke University who is behind these recent CDF collaborative analyses, has spent his career refining the framework. At the heart of the W boson experiment is a cylindrical chamber containing 30,000 high-voltage wires that react when muons or electrons pass through it, allowing CDF researchers to infer the particle’s path and speed. Knowing the exact location of each wire is the key to obtaining precise trajectories of particles. For the new analysis, Kotvall and his colleagues used muons falling from the sky in the form of cosmic rays. Like bullets, the particles traveled through the detector in near-perfect straight lines, allowing the researchers to detect any unstable wire and pin the wire’s position to within 1 micrometer.

The researchers also spent years doing exhaustive cross-checks between data releases, repeating measurements independently to ensure that every Tevatron property was fully understood. At the same time, measurements of the W boson accumulate faster and faster. The most recent CDF analysis was published in 2012 and covered the first five years of Tevatron data. Over the next four years, the data volume quadrupled again. “It’s rushing in like water from a fire hose, faster than you can drink it,” Kotwaal said.

Nearly a decade after the last analysis, the CDF Collaborative Project has finally released its results. During a Zoom meeting in November 2020, Kotwaal pressed a button to decipher the team’s results (they used encrypted data so the numbers wouldn’t affect their analysis). The physicists in the room fell silent, all seeming to ponder what the results might mean. They found that the W boson has a mass of 80.433 billion electron volts (MeV), with an error of around 9MeV. This makes it 76MeV heavier than the Standard Model predicts, an error of about 7 times the measurement or prediction error.

Scientists usually use several sigma to judge the importance of a measurement,When sigma exceeds 5, scientists can confidently declare that they have made a definitive discovery,and The CDF measurement results reached “7 sigma”, which can be said to be a very clear result. However, low measurements from ATLAS and other experiments gave the researchers pause.

Is the new measurement wrong, or does it mean a new breakthrough?

▲ Computer image of particle collisions from Fermilab’s CDF detector, showing a W boson decaying into a positron (lower left magenta block) and an invisible neutrino (yellow arrow).

With Tevatron shutting down, the onus on confirming or denying the CDF measurements falls on the Large Hadron Collider (LHC). In fact, the device has produced more W bosons than Tevatron, but its higher collision rate complicates the analysis of W boson masses. Still, by collecting more data — possibly at lower beam intensities — the LHC is expected to address these issues in the coming years.

At the same time, theoretical physicists had to start thinking about what “superheavy” W bosons might mean. A muon briefly releases a W boson when it decays into an electron, and this intermediate W boson can interact with other particles, even with as yet undiscovered particles. This interaction with the unknown particle could skew the mass measurement of the W boson.

Another possibility is that the heavier W boson could be caused by another Higgs boson, which is less active than the one we know. Alternatively, it could be due to a new massive boson mediating a variant of the weak force, or a “composite” Higgs boson composed of multiple particles, a new force that will They are combined.

Some theoretical physicists suspect that the anomaly in the mass of the W boson may stem from particles predicted by supersymmetry theory. Supersymmetry theory is a long-studied framework that links matter particles to force-carrying particles, positing for every known particle a yet-to-be-discovered opposite type of particle — or “partner” (Supersymmetric particles of fermions). However, scientists have been unable to find supersymmetric particles at the Large Hadron Collider, and supersymmetry theory has fallen out of fashion, but some theoretical physicists still believe the theory is correct.

Sven Heinmeier and his collaborators have recently calculated that certain supersymmetric particles can resolve another putative phenomenon that does not fit the Standard Model, the muon g-2 anomaly. In doing so, these particles may also give the W boson a slight mass increase, although not enough to match the CDF measurements. “It’s interesting that particles that help us study g-2, and possibly W boson masses, may also help us,” he said.

The hard work of experimental physicists on precise measurements has made researchers more optimistic that the long-awaited breakthrough is coming. By and large, physicists are approaching the moment of breakthrough and truly surpassing the Standard Model.