UNIVERSITY PARK, Pa. \u2014 A tiny discrepancy in particle physics has loomed for decades as an exciting possible crack in one of science\u2019s most successful theories, hinting at unknown forces or quantum objects. Now, an international team led by a Penn State physicist has published the most precise study yet to reveal the discrepancy was a fluke in calculation, not nature.<\/p>\n
More than half a century of measurements of a fundamental property of the muon \u2014 the more massive, short-lived cousin of the electron \u2014 did not line up with theoretical predictions, raising hopes that new physics might be behind the unexplained inconsistency.<\/p>\n
In a paper published today (April 22) in the journal Nature<\/a>, a team led by a Penn State researcher describes one of the most precise calculations ever performed in particle physics, showing that the Standard Model \u2014 the theory describing the known building blocks of matter \u2014 still holds.<\/p>\n \u201cThere were many calculations in the last 60 years or so, and as they got more and more precise they all pointed toward a discrepancy and a new interaction that would upend known laws of physics,\u201d said Zoltan Fodor, distinguished professor of physics at Penn State and lead author of the study. \u201cWe applied a new method to calculate this discrepancy quantity, and we showed that it\u2019s not there. This new interaction we hoped for simply is not there. The old interactions can explain the value completely.\u201d<\/p>\n The team\u2019s findings, which took over 10 years to calculate, bring theory and experiment into agreement to within half a standard deviation, with a level of precision that would have been unthinkable just a decade ago, Fodor said. The result strengthens confidence in the Standard Model to 11 decimal places, dramatically narrowing the space where new physics could be hiding.<\/p>\n \u201cPeople ask me how it feels to make this discovery and, to be honest, I feel somewhat sad,\u201d Fodor said. \u201cWhen we started to calculate this quantity, we thought we were going to have a good and trustworthy calculation for a new fifth force. Instead, we found there is no fifth force. We did find a very precise proof of not just the Standard Model, but also of quantum field theory, which is the foundation on which the Standard Model was built.\u201d<\/p>\n The new calculations and prior ones hinged on what scientists call the \u201cmagnetic moment\u201d of the muon, a measurement for how strongly the particle behaves like a tiny bar magnet. According to quantum theory, that number should be exactly two, representing the ratio of how the particle \u201cwobbles\u201d to the strength of the magnetic field in which it exists. But in experiments, researchers detected deviations from what theory predicted. Other particles popped in and out of existence, tugging on the muon just enough to alter its magnetic strength by a small amount known as the \u201canomalous magnetic moment,\u201d or g\u22122.<\/p>\n Because muons are about 200 times heavier than electrons, they are especially sensitive to such tugs and that extreme sensitivity made the muon g\u22122 one of the most carefully scrutinized quantities in physics.<\/p>\n