Particle’s surprise mass threatens to upend the standard model

Por • 18 abr, 2022 • Sección: Ciencia y tecnología

Davide Castelvecchi & Elizabeth Gibney

07 April 2022

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.

Overweight particle

Since its discovery in 1983, experiments have calculated the W boson to weigh as much as 85 protons. But its exact mass has been challenging to quantify: the first experimental estimate had error margins of 5% or more. “The measurement of the W boson mass is arguably the single most challenging parameter to measure in our field,” says Mika Vesterinen, a particle physicist at the University of Warwick, UK, who works on this measurement at CERN’s LHCb experiment.

With its cousin, the Z boson, the W is involved in most types of nuclear reactions, including the fusion that powers the Sun. The W and Z bosons carry the weak nuclear force — one of the four fundamental forces of nature — similar to how every electromagnetic interaction involves the exchange of photons.

Colliders produce W bosons by smashing together particles at high energies. Experiments typically detect them through their decay into either a kind of electron or its heavier cousin, the muon, plus a neutrino. The neutrino escapes the detector without a trace, whereas the electron or muon leave conspicuous tracks.

In the decay, most of the W’s original mass transforms into the energy of the new particles. If physicists could measure that energy and the path of all the decay particles, they could immediately calculate the mass of the W that produced them. But without being able to track the neutrino, they can’t say for sure which portion of the electron or muon’s energy comes from the W’s mass and which comes from its momentum. This makes the measurement “notoriously difficult”, Vesterinen says. “You try to construct the mass when you only see half of the decay.”

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