The Theory Initiative decided not to include BMW’s value in its official estimate for several reasons. The data-driven approach has a slightly smaller error bar, and three different research groups independently calculated the same. In contrast, BMW’s trellis calculation was not published last summer. And while the result did match well with earlier, less accurate lattice calculations which were also found to be high, it was not reproduced independently by another group with the same precision.
The Theory Initiative’s decision meant that the official theoretical muon magnetic moment value was 3.7 sigma different from the experimental Brookhaven measurement. It set the stage for what has become the most anticipated revelation in particle physics since the Higgs boson in 2012.
A month ago, the Fermilab Muon g-2 team announced that they would present their first results on April 7. The particle physicists were thrilled. Laura Baudis, a physicist at the University of Zurich, said she was “counting the days” after anticipating the outcome for 20 years. “If Brookhaven’s results are confirmed by the new experience at Fermilab,” she said, “it would be a huge achievement.”
And if not – if the anomaly were to go away – some in the particle physics community feared nothing less than “the end of particle physics,” Stöckinger said. The Fermilab g-2 experiment is “our last hope for an experiment that truly proves the existence of physics beyond the Standard Model,” he said. If unsuccessful, many researchers might think that “we are giving up now and we need to do something else instead of doing physics research beyond the Standard Model”. He added: “Honestly, that could be my own reaction.”
The 200-person Fermilab team revealed the result just six weeks ago during an unveiling ceremony on Zoom. Tammy walton, a scientist on the team, rushed home to watch the show after working the night shift on the experiment, which is now in its fourth edition. (The new analysis covers the data from the first run, which is 6% of what the experience will eventually accumulate.) When the very large number appeared on the screen, plotted with the Theory Initiative prediction and the Brookhaven measure, Walton was delighted to see him land higher than the former and pretty much tapping on the latter. “People are going to be crazy excited,” she said.
Articles offering various ideas for new physics are expected to flood the Arxiv in the coming days. Yet beyond that, the future is unclear. What was once an illuminating gap between theory and experience has been clouded by a shock of far more hazy calculations.
It is possible that the calculation of the supercomputer turns out to be wrong – that BMW has overlooked a source of error. “We need to take a close look at the math,” El-Khadra said, stressing that it is too early to draw firm conclusions. “You have to push the methods to get that precision, and we have to understand if the way they pushed the methods has broken them.”
That would be great news for fans of the new physics.
Interestingly, however, even though the data-driven method is the approach with an unidentified problem under the hood, theorists find it difficult to understand what the problem could be other than unexplained new physics. “The need for new physics would only change elsewhere,” said Martin hoferichter from the University of Bern, one of the main members of Theory Initiative.
Researchers who have explored possible problems with the data-driven method over the past year say the data itself is probably not in error. It comes from decades of ultra-precise measurements of 35 hadronic processes. But “the data, or the way it is interpreted, may be misleading,” said Andreas Crivellin from CERN and other institutions, co-author (with Hoferichter) of a paper study this possibility.