Precise Measurement of Pions Confirms Understanding of Fundamental Symmetry

  • Feynman diagram of the Primakoff Effect to produce neutral pions in experiment

A result 20 years in the making: Most precise measurement yet of the neutral pion confirms predictions of a fundamental symmetry

Nuclear physicists have announced the most precise measurement yet of the ultra-short lifetime of the neutral pion. The result is an important validation of our understanding of the theory of quantum chromodynamics, which describes the makeup of ordinary matter. The research, carried out at the Department of Energy’s Thomas Jefferson National Accelerator Facility, was recently published in the journal Science

Pions are the unassuming workhorses of the subatomic world. They are the simplest particles built out of the same ingredients as protons and neutrons: quarks bound up by the strong force. It’s this strong force, described by the theory of quantum chromodynamics, or QCD, that is responsible for the generation of most of the mass inside protons and neutrons.

“One of the fundamental questions for our field is where the matter comes from and how it has evolved,” explained Liping Gan, an experiment spokesperson and professor of physics at the University of North Carolina Wilmington.

Measuring the ultra-short lifetime of the neutral pion provides a window into QCD, and thus the heart of matter, that is difficult to access otherwise. That’s because the particle’s lifetime – how long it exists once it has been produced – is tied to the breaking of a fundamental symmetry in the universe called chiral symmetry. Scientists refer to this symmetry breaking as the chiral anomaly. 

The chiral anomaly affects the neutral pion directly, causing it to disintegrate and shortening its lifetime from what it would be otherwise. 

Haiyan Gao is a professor at Duke University and a leading collaborator and the Ph.D. student supervisor for the experiment collaboration. She says that theorists have made precise predictions of what the neutral pion’s lifetime should be from calculations of the chiral anomaly from QCD.

“In the low-energy region that we measure at Jefferson Lab, we generally can not solve QCD analytically. The neutral pion lifetime is one of the very few exceptions where you can calculate exactly from QCD in the massless quark limit, a prediction of the chiral anomaly,” she said. 

To verify this prediction, nuclear physicists from more than 16 collaborating national and international groups formed the PrimEx collaboration to make the most precise measurement of the neutral pion’s lifetime. 

“When we first started 20 years ago, there were several experiments, but they didn’t have enough accuracy to check the anomaly prediction for the pion lifetime,” explained Ashot Gasparian, a spokesperson and main contact person for the experiment and a professor of physics at North Carolina A&T State University.

The PrimEx collaboration took its name from the principle that the nuclear physicists would use to allow them to produce neutral pions for study: the Primakoff Effect. In this effect, two photons collide -- one from the beam and one from the Coulomb field that surrounds the target nucleus -- producing a pion. When the pion disintegrates, it re-emits two photons. 

The researchers note that the experiment received unwavering support from Jefferson Lab leadership from very early on. This support, from both theorists and experimentalists, played a significant role in the experiments’ development and execution. In many cases, that support was critical for the successful completion of the experiments and results. 

The PrimEx collaboration performed the first experiment in 2004 and announced its initial results in 2007. At the time, it was the most precise measurement yet of the neutral pion’s lifetime.

The second run of the experiment in 2010, PrimEx-II, yielded five times greater precision than the other Primakoff-type experiments and a factor of two greater than PrimEx-I. This result, just announced in Science, yielded a measurement of the neutral pion’s mean lifetime of 8.337 ± 0.056(statistical error) ± 0.112(systematic error) x 10-17s, or roughly 83 attoseconds.

“We have found that this lifetime is much, much shorter than it should be. That means that the chiral anomaly prediction is correct,” Gasparian said. “I was always thinking back in my mind, what if our result comes 10% lower, and that could be an interesting result. But, happily, we ended up with exactly the same answer as the chiral anomaly in QCD predicts.”

This result not only confirms the theoretical calculations from QCD, it also definitively shuts the door on alternative theories of why the pion’s lifetime is so short. With its 1.50% total uncertainty, it also presents some challenges to existing theory corrections to the anomaly.

“It shows the importance of the anomaly in QCD. It shows we’re in good agreement. We are in good agreement with the leading order prediction of the chiral anomaly,” explained Rory Miskimen, a professor of physics at the University of Massachusetts Amherst and experiment spokesperson.

Further, the research provides important parameter information needed by other researchers who are looking for physics that exist beyond the Standard Model, the theory that describes the subatomic world. 

According to Gao, “We are approaching Jefferson Lab to become a precision frontier in the electromagnetic interaction and to have great potential to look for new physics beyond the Standard Model. That’s actually a very exciting new role.”

Further Reading: 
Jefferson Lab Experiment Pins Down Pion

Contact: Kandice Carter, Jefferson Lab Communications Office, 757-269-7263, kcarter@jlab.org

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Jefferson Science Associates, LLC, a joint venture of the Southeastern Universities Research Association, Inc. and PAE, manages and operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S. Department of Energy's Office of Science.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science.