The Muon Fails to Upend Physics as We Know It

Another nail whitethorn seal the coffin of the muon arsenic a venue for caller physics. A squad of physicists taking highly precise calculations of the muon’s properties successful simulations recovered the particle’s properties much successful enactment with the Standard Model than antecedently believed.

The squad is called the BMW Collaboration and its research is presently hosted connected the pre-print server arXiv, meaning it has not yet been published successful a peer-reviewed journal. The team’s erstwhile findings, published successful Nature successful 2021,  “weaken[ed] the long-standing discrepancy betwixt experimentation and theory.” In different words, their enactment has brought experimental physics person successful enactment with theoretical predictions erstwhile it comes to our knowing of the muon.

In its caller paper, the squad performed large-scale lattice quantum chromodynamic (QCD) simulations connected finer lattices than successful its erstwhile work, yielding a much precise calculation. In essence, the squad took QCD arsenic an input, enactment a grid connected spacetime, and simulated it. Their results predicted an anomalous magnetic infinitesimal of the muon that was conscionable 0.9 modular deviations disconnected the experimental mean for measurements of the property.

The muon and the Standard Model

The muon is an simple particle astir 207 times arsenic monolithic arsenic the electron. For astir 20 years, scientists person considered the muon a imaginable venue for the find of caller physics. The contented lies with measurements of the particle’s anomalous magnetic moment, oregon g-2, a spot which describes quantum mechanics’ publication to the particles’ wobble successful the beingness of a magnetic field. G-2 of the muon disagreed with predictions of the Standard Model of particle physics, the foundational acceptable of theories undergirding physics for the past 50 years.

Unlike ample experiments measuring g-2 done particle collisions, the team’s probe “doesn’t request immoderate experimental input. It conscionable needs the activation of the underlying theory, which is QCD,” said survey co-author Zoltan Fodor, a theoretical particle physicist astatine the University of California successful San Diego, successful a telephone with Gizmodo. “You extremity up with what you spot connected our figures today: that the effect agrees wholly with the experimental result.”

In different words, the team’s findings suggest the evident gulf betwixt the muon’s predicted anomalous magnetic infinitesimal and that predicted by the Standard Model is not arsenic ample arsenic erstwhile findings suggested.

The g-2 storage-ring magnet astatine Fermilab. Photo: Reidar Hahn / Wikimedia Commons

Major experimental results suggested caller physics

The anomalous magnetic infinitesimal of the muon was archetypal measured astatine CERN successful the 1960s, but the measurement was imprecise. In 2006, the E821 experimentation astatine Brookhaven National Laboratory released its final measurements of g-2 of the muon, which differed from Standard Model predictions by much than 2 modular deviations, swelling to a quality of more than 3 modular deviations aft consequent calculations.

“Explaining g-2 of the muon with caller physics is not that easy,” said Andreas Crivellin, a theoretical physicist astatine the University of Zurich and the Paul Scherrer Institute, successful a telephone with Gizmodo. “It’s not thing that comes retired naturally; you alternatively person to enactment to find a exemplary that gives you a sizable effect.”

The statistical milestone astatine which physicists judge a existent find has been made—indicating that the probability of the effect occurring by accidental nether the Standard Model is highly small—is 5 modular deviations, oregon “five sigma.”

In 2021, the Muon g-2 Collaboration announced a measurement of the muon’s magnetic infinitesimal that disagreed with the Standard Model by 4.2 modular deviations. The gulf betwixt the figures widened since the Brookhaven result. But past year, experimental results from CMD-3, an accelerator successful Russia, seemed to marque the discrepancy betwixt the figures shrink. Two steps forward, 1 measurement back, depending connected however you look astatine it.

“This archetypal rule calculation from the lattice and the CMD-3 measurement some hold and some don’t constituent towards caller physics,” Crivellin said. “I americium not precise hopeful that determination is truly a sizable caller physics effect successful g-2 of the muon.”

Where does this permission us?

There are different ways of exploring the muon’s properties. In 2022, Gizmodo asked respective physicists what the adjacent large breakthrough successful particle physics mightiness be, fixed the comparative quiescent since the reflection of the Higgs Boson successful 2012. One physicist suggested a muon collider—”if we person a occupation with muons, let’s usage muons to find out,” they said.

Just past week, a antithetic squad of researchers published its investigation of a muon beam experimentation that could pave the mode for muon colliders successful the future. But gathering a caller collider tin beryllium costly and time-intensive.

With existing experiments, much information is ever useful, and retesting erstwhile results successful much precise ways could bespeak whether the Standard Model continues to clasp up. Fermilab’s Muon g-2 experimentation is expected to merchandise its final result adjacent year. If erstwhile results are immoderate indication, adjacent year’s fig volition beryllium different information constituent successful the muon’s saga, not its last chapter.