“It’s quite a surprise, even for them,” said Freya Blekman, a particle physicist at the DESY Institutes in Hamburg and CERN in Geneva and not directly involved in the research of her colleagues at Fermilab near Chicago in the US. They published this Thursday in the trade magazine science the results of 26 years of measurements with the CDF detector, one of the two measuring instruments of the Tevatron particle accelerator. The detectors track the debris splashed off during head-on collisions between protons and antiprotons, particles that have been chased by the particle accelerator to nearly the speed of light.
In 4.2 million of those collisions, a W particle formed very briefly, an elementary particle discovered in 1983 at CERN. Such a W particle quickly disintegrates into other particles, but by following the tracks of the ejecting particles, it is possible to determine what the mass of the W particle must have been.
Final conclusion: the W particle has a mass of 80,433 megaelectronvolts, about the mass of a bromine atom, with a measurement uncertainty of 9 megaelectronvolts. That’s hardly different from the mass previously published by CDF physicists shortly after the detector was shut down in 2011.
A web of connections
“But this is a new, much more precise measurement,” says Blekman. “Every screw, every wire in the detector has been included down to the micrometer, and more precise calculations have also been made on the basis of the known theory to increase the measurement accuracy,” says Blekman.
As a result, the measurement uncertainty has been reduced from 79 to 9 mega-electronvolts, and that makes all the difference: now that we know the W mass so precisely, it no longer seems to match the rest of particle physics.
“The measurement took many years, and the reading was hidden all the while until all procedures were fully checked,” said CDF researcher Chris Hays of Oxford University in a Fermilab press release. “When we revealed the value, it was a surprise.”
“We have the Standard Model,” says Blekman, “a theory about elementary particles that can predict exactly what will come from all kinds of measurements. That works great. But if you fill in this measurement, it’s not consistent.”
Physicists believe that the formulas should be beautiful and simple
Freya Blekman particle physicist
The mathematics of the Standard Model predicts relationships between the mass of the W particle and that of other particles such as the top quark and the Higgs boson. According to those relations, the W-mass must be 80,357 plus minus 6 megaelectronvolts: 76 less than the new measurement. Blekman: “It’s a web of connections, and now one of the particles suddenly no longer fits on the lines of that web.”
That can mean two things: either a mistake has been made, or the theory needs to be expanded. Physicists hope for the latter, because there is something to criticize about the Standard Model despite all the performance.
“There are many reasons why the Standard Model cannot be the last word,” says Blekman. A famous example is the dark matter that astronomers see but that is not in the Standard Model. “Gravity isn’t in it either, and at high energies the theory just stops working. In addition, the Standard Model is ugly and complicated. Many physicists believe that the final formulas should be beautiful and simple.”
All possible feathers
With every deviating measurement, they hope that it can serve as a crowbar in search of an extension of the Standard Model: new physics. Blekman: “There are thousands of different proposals for this, and most of them come down to adding extra particles that we have not yet seen.”
That sounds illogical: why would a deviation of 1 per mille from the expected mass of one particle be an indication of the existence of a completely different particle? But in quantum mechanics that one particle is always influenced by a cloud of short-lived particles of all possible plumes – also by undiscovered particles, which can perhaps add just that one per mille.
The Large Hadron Collider, the particle accelerator at CERN, has also published a measurement of the W mass, and another is in the works. “But here we do things a little differently, with different detectors and slightly different methods,” says Blekman. “I know how difficult that measurement is and how much work it is. But this does take some courage.”