Nature reports:

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 Science 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. [Continue reading…]