Fresh calculation of obscure particle's magnetism could dim hopes for new physics - Science Magazine

On 7 April, a collaboration of more than 200 experimenters announced to great fanfare that a particle called the muon is slightly more magnetic than predicted by physicists’ standard model, a discrepancy that could signal new particles waiting to be discovered.

“The standard model is just fine, according to our calculation,” says Zoltan Fodor, a theorist at Pennsylvania State University, University Park, and the leader of the Budapest-Marseille-Wuppertal (BMW) collaboration, which produced the new theoretical result.

A heavier, unstable cousin of the electron, the muon acts like a tiny bar magnet, and its magnetism provides a means to dowse for hints of new particles.

Quantum mechanics and relativity demand that the muon have a certain basic magnetism.

Thanks to quantum uncertainty, particles and antiparticles also constantly flit in and out of existence around the muon.

These “virtual” particles cannot be observed directly, but they can affect the muon’s properties, including magnetism.

Standard model particles should increase its magnetism by about 0.1%, and as-yet-unknown particles would add their own boost.

That’s why physicists were so excited when the Muon g-2 experiment at Fermi National Accelerator Laboratory confirmed a 20-year-old hint that the muon is about 2.5 parts per billion more magnetic than the standard model predicts, according to the consensus value, hammered out last year by the 132-member Muon g-2 Theory Initiative.

To make that prediction, the theorists had to account for the thousands of ways standard model particles can flit about the muon and affect its behavior.

In it, the muon emits and reabsorbs particles known as hadrons, which consist of other particles called quarks.

Theorists can attempt brute-force QCD calculations on supercomputers, if they model the continuum of space and time as a lattice of discrete points occupied by quarks and particles called gluons, which convey the strong force.

Several groups have also applied the lattice to the muon’s magnetism, albeit with sizable uncertainties.

Now, using hundreds of millions of processor hours at the Jülich Research Center in Germany, Fodor’s group has produced a lattice calculation of the hadronic vacuum polarization and a value for the muon’s magnetism that rivals the consensus standard model value in precision.

Aida El-Khadra, a lattice theorist at the University of Illinois, Urbana-Champaign, and a leader of the Muon g-2 Theory Initiative, notes that the uncertainties in the consensus value reflect mainly the limited precision of the input data.

Their hybrid estimate of the muon’s magnetism agrees well with the consensus prediction, Lehner says.

“The BMW result needs to be confirmed by other independent lattice calculations,” says Alexey Petrov, a theorist at Wayne State University.

“The standard model calculation is solid,” she insists.