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Physicists Move One Step Closer to a Theoretical Showdown

The deviance of a tiny particle called the muon might prove that one of the most well-tested theories in physics is incomplete.

The Muon g-2 ring at the Fermilab particle accelerator complex in Batavia, Ill.Credit...Reidar Hahn/Fermilab, via US Department of Energy

By Katrina Miller

Katrina Miller, a science reporter, recently earned a Ph.D. in particle physics from the University of Chicago.

Aug. 10, 2023

On July 24, a large team of researchers convened in Liverpool to unveil a single number related to the behavior of the muon, a subatomic particle that might open a portal to a new physics of our universe.

All eyes were on a computer screen as someone typed in a secret code to release the results. The first number that popped out was met with exasperation: a lot of concerning gasps, oh-my-God’s and what-did-we-do-wrong’s. But after a final calculation, “there was a collective exhale across multiple continents,” said Kevin Pitts, a physicist at Virginia Tech who was five hours away, attending the meeting virtually. The new measurement matched exactly what the physicists had computed two years prior — now with twice the precision.

So comes the latest result from the Muon g-2 Collaboration, which runs an experiment at Fermi National Accelerator Laboratory, or Fermilab, in Batavia, Ill., to study the deviant motion of the muon. The measurement, announced to the public and submitted to the journal Physical Review Letters on Thursday morning, brings physicists one step closer to figuring out if there are more types of matter and energy composing the universe than have been accounted for.

“It really all comes down to that single number,” said Hannah Binney, a physicist at the Massachusetts Institute of Technology’s Lincoln Laboratory who worked on the muon measurement as a graduate student.

Scientists are putting to the test the Standard Model, a grand theory that encompasses all of nature’s known particles and forces. Although the Standard Model has successfully predicted the outcome of countless experiments, physicists have long had a hunch that its framework is incomplete. The theory fails to account for gravity, and it also can’t explain dark matter (the glue holding our universe together), or dark energy (the force pulling it apart).

One of many ways that researchers are looking for physics beyond the Standard Model is by studying muons. As heavier cousins of the electron, muons are unstable, surviving just two-millionths of a second before decaying into lighter particles. They also act like tiny bar magnets: Place a muon in a magnetic field, and it will wobble around like a top. The speed of that motion depends on a property of the muon called the magnetic moment, which physicists abbreviate as g.

In theory, g should exactly equal 2. But physicists know that this value gets ruffled by the “quantum foam” of virtual particles that blip in and out of existence and prevent empty space from being truly empty. These transient particles change the rate of the muon’s wobble. By taking stock of all the forces and particles in the Standard Model, physicists can predict how much g will be offset. They call this deviation g-2.

But if there are unknown particles at play, experimental measurements of g will not match this prediction. “And that’s what makes the muon so exciting to study,” Dr. Binney said. “It’s sensitive to all of the particles that exist, even the ones that we don’t know about yet.” Any difference between theory and experiment, she added, means new physics is on the horizon.

To measure g-2, researchers at Fermilab generated a beam of muons and steered it into a 50-foot-diameter, doughnut-shaped magnet, the inside brimming with virtual particles that were popping into reality. As the muons raced around the ring, detectors along its edge recorded how fast they were wobbling.

Using 40 billion muons — five times as much data as the researchers had in 2021 — the team measured g-2 to be 0.00233184110, a one-tenth of 1 percent deviation from 2. The result has a precision of 0.2 parts per million. That’s like measuring the distance between New York City and Chicago with an uncertainty of only 10 inches, Dr. Pitts said.

“It’s an amazing achievement,” said Alex Keshavarzi, a physicist at the University of Manchester and a member of the Muon g-2 Collaboration. “This is the world’s most precise measurement ever made at a particle accelerator.” The results, when revealed to the public at a scientific seminar on Thursday morning, were met with applause.

“The kind of precision that these people have managed to attain is just staggering,” said Dan Hooper, a theoretical cosmologist at the University of Chicago who was not involved in the work. “There was a lot of skepticism they would get here, but here they are.”

But whether the measured g-2 matches the Standard Model’s prediction has yet to be determined. That’s because theoretical physicists have two methods of computing g-2, based on different ways of accounting for the strong force, which binds together protons and neutrons inside a nucleus.

The traditional calculation relies on 40 years of strong-force measurements taken by experiments around the world. But with this approach, the g-2 prediction is only as good as the data that are used, said Aida El-Khadra, a theoretical physicist at the University of Illinois Urbana-Champaign and a chair of the Muon g-2 Theory Initiative. Experimental limitations in that data, she said, can make this prediction less precise.

A newer technique called a lattice calculation, which uses supercomputers to model the universe as a four-dimensional grid of space-time points, has also emerged. This method does not make use of data at all, Dr. El-Khadra said. There’s just one problem: It generates a g-2 prediction that differs from the traditional approach.

“No one knows why these two are different,” Dr. Keshavarzi said. “They should be exactly the same.”

Compared with the traditional prediction, the latest g-2 measurement has a discrepancy of over 5-sigma, which corresponds to a one in 3.5 million chance that the result is a fluke, Dr. Keshavarzi said, adding that this degree of certainty was beyond the level needed to claim a discovery. (That’s an improvement from their 4.2-sigma result in 2021, and a 3.7-sigma measurement done at Brookhaven National Laboratory near the turn of the century.)

But when they compared it with the lattice prediction, Dr. Keshavarzi said, there was no discrepancy at all.

Rarely in physics does an experiment surpass the theory, but this is one of those times, Dr. Pitts said. “The attention is on the theoretical community,” he added. “The limelight is now on them.”

Dr. Binney said, “We are on the edge of our seats to see how this theory discussion pans out.” Physicists expect to better understand the g-2 prediction by 2025.

Gordan Krnjaic, a theoretical particle physicist at Fermilab, noted that if the experimental disagreement with theory persisted, it would be “the first smoking-gun laboratory evidence of new physics,” he said. “And it might well be the first time that we’ve broken the Standard Model.”

While the two camps of theory hash it out, experimentalists will hone their g-2 measurement further. They have more than double the amount of data left to sift through, and once that’s included, their precision will improve by another factor of two. “The future is very bright,” said Graziano Venanzoni, a physicist at the University of Liverpool and one leader of the Muon g-2 experiment, at a public news briefing about the results.

The latest result moves physicists one step closer to a Standard Model showdown. But even if new physics is confirmed to be out there, more work will be needed to figure out what that actually is. The discovery that the known laws of nature are incomplete would lay the foundation for a new generation of experiments, Dr. Keshavarzi said, because it would tell physicists where to look.

“Physicists get really excited when theory and experiment do not agree with each other,” said Elena Pinetti, a theoretical physicist at Fermilab who was not involved in the work. “That’s when we really can learn something new.”

For Dr. Pitts, who has spent nearly 30 years pushing the bounds of the Standard Model, proof of new physics would be both a celebratory milestone and a reminder of all that is left to do. “On one hand it’s going to be, Have a toast and celebrate a success, a real breakthrough,” he said. “But then it’s going to be back to work. What are the next ideas that we can get to work on?”

Katrina Miller is a science reporting fellow for The Times. She recently earned her Ph.D. in particle physics from the University of Chicago. More about Katrina Miller

https://www.nytimes.com/2023/08/10/science/physics-muons-g2-fermilab.html

NASA Seeks a Nuclear-Powered Rocket to Get to Mars in Half the Time

By Kenneth Chang

Published July 26, 2023Updated July 27, 2023

In less than four years, NASA could be testing a nuclear rocket in space.

The space agency and the Defense Advanced Research Projects Agency, or DARPA, announced on Wednesday that Lockheed Martin had been selected to design, build and test a propulsion system that could one day speed astronauts on a trip to Mars.

BWX Technologies, based in Lynchburg, Va., will build the nuclear fission reactor at the heart of the engine.

The $499 million program is named DRACO, short for the Demonstration Rocket for Agile Cislunar Operations.

An illustration of Lockheed Martin’s proposed nuclear-powered spacecraft.Credit...Lockheed Martin

What if a spacecraft could get to Mars in half the time it currently takes?

Every 26 months or so, Mars and Earth are close enough for a shorter journey between the worlds. But even then it is a pretty long trip, lasting seven to nine months. For most of the time, the spacecraft is just coasting through space.

But if the spacecraft could continue accelerating through the first half of the journey and then start slowing down again, the travel time could be slashed. Current rocket engines, which typically rely on the combustion of a fuel like hydrogen or methane with oxygen, are not efficient enough to accomplish that; there is not enough room in the spacecraft to carry that much propellant.

But nuclear reactions, generating energy from the splitting of uranium atoms, are much more efficient.

The DRACO engine would consist of a nuclear reactor that would heat hydrogen from a chilly minus 420 degrees Fahrenheit to a toasty 4,400 degrees, with the hot gas shooting from a nozzle to generate thrust. Greater fuel efficiency could speed up journeys to Mars, reducing the amount of time astronauts spend exposed to the treacherous environment of deep space.

Nuclear propulsion could also have uses closer to home, which is why DARPA is investing in the project. The technology may allow rapid maneuvers of military satellites in orbit around Earth.

Nuclear propulsion for space is not a new idea. In the 1950s and 1960s, Project Orion — financed by NASA, the Air Force and the Advanced Research Projects Agency — contemplated using the explosions of atomic bombs to accelerate spacecraft.

At the same time, NASA and other agencies also undertook Project Rover and Project NERVA, efforts that aimed to develop nuclear-thermal engines similar in concept to those now being pursued by the DRACO program. A series of 23 reactors were built and tested, but none were ever launched to space. Until the end of this program in 1973, NASA had contemplated using nuclear reactors to propel space probes to Jupiter, Saturn and beyond, as well as to provide power at a lunar base.

“The technical capabilities, including early safety protocols, remain viable today,” Tabitha Dodson, the DRACO project manager, said in a news briefing on Wednesday.

A key difference between NERVA and DRACO is that NERVA used weapons-grade uranium for its reactors, while DRACO will use a less-enriched form of uranium.

The reactor would not be turned on until it reached space, part of the precautions to minimize the possibility of a radioactive accident on Earth.

“DRACO has already done all of our preliminary analyses across the entire spectrum of possibilities for accidents and found that we’re all the way down in the low probability and all the way down in the teeny tiny amount of release,” Dr. Dodson said.

The DRACO development is to culminate with a flight test of the nuclear-thermal engine. The launch is currently scheduled for late 2026 or early 2027.

The demonstration spacecraft would most likely orbit at an altitude between 435 and 1,240 miles, Dr. Dodson said. That is high enough to ensure that it stays in orbit for more than 300 years, or long enough for radioactive elements in the reactor fuel to decay to safe levels, she said.

A correction was made on 

July 28, 2023

: 

Using information from a news conference, an earlier version of this article misstated when a flight test of a nuclear-thermal engine could occur. The launch is scheduled for late 2026 or early 2027, not late 2025 or early 2026.

Kenneth Chang has been at The Times since 2000, writing about physics, geology, chemistry, and the planets. Before becoming a science writer, he was a graduate student whose research involved the control of chaos. More about Kenneth Chang