Why Waiting Two Years for Publication of His Dissertation Was Worth It for This Physics Ph.D. Student

Thursday, May 14, 2020
Haiyan Gao (left) with newly-minted physics Ph.D. Yang Zhang in 2018. (Photo courtesy of Min Huang, Ph.D. ’16)

Most of Physics Professor Haiyan Gao’s students see their doctoral dissertations posted on her lab’s web site very soon after they have been awarded their Ph.Ds.

But Yang Zhang, Ph.D. 2018, had to wait two years, because his thesis work had a very good chance of being accepted by a major journal. And this week, it has been published in the journal Science.

What Zhang did was to create the world’s most precise value for a subatomic nuclear particle called a neutral pion. It’s a quark and an antiquark comprising a meson. The neutral pion (also known as p0) is the lightest of the mesons, but a player in the strong attractive force that holds the atom’s nucleus together.

And that, in turn, makes it a part of the puzzle Gao and her students have been trying to solve for many years. The prevailing theory about the strong force is called quantum chromodynamics (QCD), and it’s been probed for years by high-energy physics. But Gao, Zhang and their collaborators are trying to study QCD under more normal energy states, a notoriously difficult problem.

Yang Zhang spent six years analyzing and writing up the data from a Primakoff  (PrimEx-II) experiment in Hall B at Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, VA. His work was done on equipment supported by both the National Science Foundation and the Department of Energy.  

Diagram of a quark: a blue cicle labeled "u" is separated from a yellow circle labeled "d with a line over it" by a squiggly line, and all are within a gray circle
This is the quark structure of the positive pion – an up quark and an anti-down quark. The strong force is from gluons, represented as the wavy lines (Arpad Horvath via Wikimedia Commons)

In a Primakoff experiment, a photon beam is directed on a nuclear target, producing neutral pions. In both the PrimEx-I and PrimEx-II experiments at Jefferson Lab, the two photons from the decay of a neutral pionwere subsequently detected in an electromagnetic calorimeter. From that, Zhang extracted the pion’s ‘radiative decay width.’ That decay width is a handy thing to have, because it is directly related to the pion’s life expectancy, and QCD has a direct prediction for it.

Zhang’s hard-won answer: The neutral pion has a radiative decay width of 7.8 electron-volts, give or take. That makes it an important piece of the dauntingly huge puzzle about QCD. Gao and her colleagues will continue to ask the fundamental questions about nature, at the finest but perhaps most profound scale imaginable.

The PrimEx-I and PrimEx-II collaborations were led by Prof. Ashot Gasparian from North Carolina A&T State University. Gao and Zhang joined the collaboration in 2011.

“Precision Measurement of the Neutral Pion Lifetime,” appears in Science May 1. Dr. Yang Zhang is now a quantitative researcher at JPMorgan Chase & Co.