One other nail might seal the coffin of the muon as a venue for brand new physics. A staff of physicists taking extremely exact calculations of the muon’s properties in simulations discovered the particle’s properties extra consistent with the Customary Mannequin than beforehand believed.
The staff is known as the BMW Collaboration and its research is at present hosted on the pre-print server arXiv, which means it has not but been revealed in a peer-reviewed journal. The staff’s earlier findings, published in Nature in 2021, “weaken[ed] the long-standing discrepancy between experiment and concept.” In different phrases, their work has introduced experimental physics nearer consistent with theoretical predictions in relation to our understanding of the muon.
In its new paper, the staff carried out large-scale lattice quantum chromodynamic (QCD) simulations on finer lattices than in its earlier work, yielding a extra exact calculation. In essence, the staff took QCD as an enter, put a grid on spacetime, and simulated it. Their outcomes predicted an anomalous magnetic second of the muon that was simply 0.9 normal deviations off the experimental common for measurements of the property.
The muon and the Customary Mannequin
The muon is an elementary particle about 207 occasions as large because the electron. For about 20 years, scientists have thought of the muon a possible venue for the invention of latest physics. The difficulty lies with measurements of the particle’s anomalous magnetic second, or g-2, a property which describes quantum mechanics’ contribution to the particles’ wobble within the presence of a magnetic discipline. G-2 of the muon disagreed with predictions of the Standard Model of particle physics, the foundational set of theories undergirding physics for the final 50 years.
In contrast to giant experiments measuring g-2 via particle collisions, the staff’s analysis “doesn’t want any experimental enter. It simply wants the activation of the underlying concept, which is QCD,” stated examine co-author Zoltan Fodor, a theoretical particle physicist on the College of California in San Diego, in a name with Gizmodo. “You find yourself with what you see on our figures at the moment: that the outcome agrees fully with the experimental outcome.”
In different phrases, the staff’s findings counsel the obvious gulf between the muon’s predicted anomalous magnetic second and that predicted by the Customary Mannequin is just not as giant as earlier findings recommended.
Main experimental outcomes recommended new physics
The anomalous magnetic second of the muon was first measured at CERN within the Nineteen Sixties, however the measurement was imprecise. In 2006, the E821 experiment at Brookhaven Nationwide Laboratory launched its final measurements of g-2 of the muon, which differed from Customary Mannequin predictions by greater than two normal deviations, swelling to a distinction of more than three standard deviations after subsequent calculations.
“Explaining g-2 of the muon with new physics is just not that straightforward,” stated Andreas Crivellin, a theoretical physicist on the College of Zurich and the Paul Scherrer Institute, in a name with Gizmodo. “It’s not one thing that comes out naturally; you reasonably must work to discover a mannequin that provides you a large impact.”
The statistical milestone at which physicists imagine a real discovery has been made—indicating that the chance of the outcome occurring by likelihood below the Customary Mannequin is extraordinarily small—is 5 normal deviations, or “5 sigma.”
In 2021, the Muon g-2 Collaboration introduced a measurement of the muon’s magnetic second that disagreed with the Customary Mannequin by 4.2 normal deviations. The gulf between the figures widened for the reason that Brookhaven outcome. However final yr, experimental results from CMD-3, an accelerator in Russia, appeared to make the discrepancy between the figures shrink. Two steps ahead, one step again, relying on the way you have a look at it.
“This primary precept calculation from the lattice and the CMD-3 measurement each agree and each don’t level in the direction of new physics,” Crivellin stated. “I’m not very hopeful that there’s actually a large new physics impact in g-2 of the muon.”
The place does this depart us?
There are different methods of exploring the muon’s properties. In 2022, Gizmodo requested a number of physicists what the next big breakthrough in particle physics could be, given the relative quiet for the reason that commentary of the Higgs Boson in 2012. One physicist recommended a muon collider—”if we now have an issue with muons, let’s use muons to search out out,” they stated.
Simply final week, a distinct staff of researchers published its evaluation of a muon beam experiment that would pave the way in which for muon colliders sooner or later. However constructing a brand new collider might be costly and time-intensive.
With present experiments, extra information is all the time helpful, and retesting earlier leads to extra exact methods might point out whether or not the Customary Mannequin continues to carry up. Fermilab’s Muon g-2 experiment is anticipated to launch its final result subsequent yr. If earlier outcomes are any indication, subsequent yr’s determine will likely be one other information level within the muon’s saga, not its last chapter.
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