Particle's surprise mass threatens to upend the standard model

[muriel_volestrangler]

From its resting place outside Chicago, Illinois, a long-defunct experiment is threatening to throw the field of elementary particles off balance. Physicists have toiled for ten years to squeeze a crucial new measurement out of the experiment’s old data, and the results are now in. The team has found that the W boson — a fundamental particle that carries the weak nuclear force — is significantly heavier than theory predicts.

Although the difference between the theoretical prediction and experimental value is only 0.09%, it is significantly larger than the result’s error margins, which are less than 0.01%. The finding also disagrees with some other measurements of the mass. The collaboration that ran the latest experiment, called CDF at the Fermi National Accelerator Laboratory (Fermilab), reported the findings in Science1 on 7 April.

The measurement “is extremely exciting and a truly monumental result in our field”, says Florencia Canelli, an experimental particle physicist at the University of Zurich, Switzerland. If it is confirmed by other experiments, it could be the first major breach in the standard model of particle physics, a theory that has been spectacularly successful since it was introduced in the 1970s. The standard model is known to be incomplete, however, and any hint of its failing could point the way to its replacement, and to the existence of new elementary particles. “We believe there is a strong clue in this particular measurement about what nature might have in store for us,” says Ashutosh Kotwal, an experimental particle physicist at Duke University in Durham, North Carolina, who led the CDF study.

Some physicists strike a note of caution. Generating a W boson mass measurement from experimental data is famously complex. Although the work is impressive, “I would be cautious to interpret the significant difference to the standard model as a sign of new physics,” says Matthias Schott, a physicist at the Johannes Gutenberg University Mainz in Germany, who works on the ATLAS experiment at CERN, Europe’s particle-physics lab near Geneva, Switzerland. Physicists should prioritize working out why the value differs from the other recent results, he says.

https://www.nature.com/articles/d41586-022-01014-5

Over the past 30 years, there have been ever more precise measurements of the W boson mass, and the CDF Collaboration now adds to these reports. Based on 10 years of data recorded at the CDF, they report a W boson mass with an impressive precision of 117 parts per million (ppm)—twice as precise as the previous most accurate measurement. Their measured W boson mass is in direct contention with the SM because it is heavier than the SM prediction by seven standard deviations. This could be a signature for new interactions or new particles that are either too massive to be produced or too hard to detect at existing accelerators. Nonetheless, such yet-to-be-known particles and physical interactions could alter the relationships between the various observables through hidden interactions with the W boson and cause the observed deviation from SM predictions.

Effects on the W boson mass from previously undetected particles have been observed before. Notably, the observations of these effects were used to probe the masses of the top quark and the Higgs boson before their direct detection. After the observation and precise measurement of each discovered particle, the web of SM predictions was weaved with greater strength and accuracy. With more and more precise measurements of physical quantities—such as cross sections, decay rates, and masses of fundamental particles—fissures between SM predictions and reality may have begun to show. When not in agreement with the theoretical predictions, such measurements can provide a first glimpse of physics beyond the SM.

Because extraordinary claims require extraordinary evidence, the claim by the CDF Collaboration will require additional experiments to provide an independent confirmation. Scientists at the Large Hadron Collider (LHC) have already collected samples of W bosons that are larger than those available at Fermilab and, in principle, could achieve better precision. The Tevatron experiment at Fermilab—DZero—may also get back in the W boson mass-measuring race. The result from the CDF Collaboration provides an impetus to improve the measurements of other SM parameters that can help to test and constrain the theory, such as the top quark mass, the strong coupling constant, and the Weinberg angle, named after the late Steven Weinberg (6, 7), a founding father of the electroweak model that is currently being challenged.
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Among possible theories that could explain the discrepancy with the SM prediction is the theory of supersymmetry (SUSY), which is an old favorite of particle physicists because it provides a plausible explanation for some of the SM’s unexplained properties and forms a natural connection to deeper level descriptions of the universe such as string theory. However, none of the many exotic particles predicted by SUSY have been observed, despite extensive searches at particle detectors around the world. The surprisingly high value of the W boson mass reported by the CDF Collaboration directly challenges a fundamental element at the heart of the SM, where both experimental observables and theoretical predictions were thought to have been firmly established and well understood. The finding of the CDF Collaboration offers an exciting new perspective on the present understanding of the most basic structures of matter and forces in the universe.

https://www.science.org/doi/10.1126/science.abm0101

The paper: https://www.science.org/doi/10.1126/science.abk1781

[SWBTATTReg]

1. I wonder if this finding will mean that masses are questionable on other particles too, that the

entire array of particles and their masses in explaining the workings of 'everything' is up in the air still? Of course, a lot of good work has been done in this whole field, but this finding goes to show that we still have a way to go. And we (I think) finally detected the long sought-after Higgs Boson after so long a search. All of this stuff is neat. Hopefully, unlike the Atom Bomb and other similar projects, these discoveries won't lead to new mass weapons of destruction but instead, to new sources of renewable and endless power/energy supplies for our constantly growing civilization. The cheaper the power is, the more that becomes available for use for new technologies, etc. The possibilities are endless.