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Nuclear Physicists Successfully Measure the Weak Charge of Proton

As part of an experiment performed at the Department of Energy’s Thomas Jefferson National Accelerator Facility, nuclear physicists have been successful in measuring the proton’s weak charge by shooting electrons at a cold liquid hydrogen target.

Kurtis Bartlett won the 2018 JSA Thesis Prize for making measurements that helped determine the weak charge of the proton. (Image credit: DOE’s Jefferson Lab)

The precision experiment, dubbed Q-weak, presented several technical issues for the physicists to solve for its successful ending.

The cold liquid hydrogen target itself was one potentially confusing variable. The target system was tailored for Q-weak, meticulously constructing a system that could maintain the hydrogen cold even while it was being hit by a hard yet precise beam of spinning electrons.

The physicists even had to take into account the influence the aluminum container that contained the hydrogen would have on their outcome. Kurtis Bartlett was awarded the 2018 Jefferson Science Associates Thesis Prize for his contribution toward solving this technical challenge and for the thesis he wrote about these efforts.

The weak charge of the proton explains the amount of weak force—one of the four basic forces of the universe—which acts upon the proton.

Probing the proton with an electron via the weak force, it allows you to actually measure the weak charge.

Kurtis Bartlett, Thomas Jefferson National Accelerator Facility

However, just like its name suggests, the weak force is weak. Electrons most probably interact with protons through the electromagnetic force, another fundamental force.

Luckily, the weak force has a distinctive marker: it infringes a universal symmetry known as parity. A process that preserves parity symmetry takes place with the same probability as its mirror image. The weak force shows asymmetry for parity transformations.

Measuring this asymmetry gives access to the weak force,” stated Bartlett. “However, it’s very difficult to actually do in the laboratory—it’s a mathematical type of operation.”

Alternatively, Q-weak employed a stand-in for parity transformation. The electrons were polarized before they were accelerated, so that they were all rotating either in the same direction as the beam, or in the opposite direction as the beam.

Since the electromagnetic force preserves the parity symmetry, it interacts with electrons rotating in either direction in the same manner. However, the weak force interacts more with electrons rotating in a single direction as it breaks parity symmetry. Physicists can leverage this difference to obtain a measurement of the proton’s weak charge.

However, achieving that measurement was not that easy. In the experiment, the small portion of electrons that the physicists measure never really bombarded the hydrogen target. Instead, a few electrons scattered off the aluminum container that contained the hydrogen, which contaminated the weak force signal that the physicists were trying to measure.

At this point, Bartlett gave his entry. His mission was to reduce this signal contamination by determining the amount of measured signal that came from the aluminum target container.

I went through the process of understanding how to correct our measured values,” stated Bartlett.

To carry this out, Q-weak took away the hydrogen target for some runs, replacing it with a piece of aluminum similar to the container. After that, Q-weak again shot polarized electrons at the target, except that Bartlett measured parity asymmetry using an aluminum nucleus instead of measuring it using a proton of hydrogen.

It’s the first time that type of asymmetry has ever been measured, which is a pretty exciting thing,” he said.

Bartlett worked on his thesis, “First Measurements of the Parity-Violating and Beam-Normal Single-Spin Asymmetries in Elastic Electron-Aluminum Scattering,” at Jefferson Lab while doing his PhD in experimental nuclear physics at William & Mary. Wouter Deconinck, an assistant professor of physics at William & Mary, who also worked on the Q-weak experiment was his thesis advisor.

Bartlett submitted his thesis work to the Jefferson Lab Users Organization Board of Directors, who supervise the JSA Thesis Prize award process. The users are researchers from the United States and across the world who carry out basic nuclear physics experiments with Jefferson Lab’s research facilities and capabilities.

I was excited to hear the news that I’d won, and I am very honored to receive it,” stated Bartlett. “Though I received this award for my dissertation, it is very much a group effort, and I want to highlight that Q-weak as a whole involved many scientists, engineers, technicians and administrative staff to get it all done.”

The JSA Thesis Prize is awarded every year for the best PhD student thesis on research associated with Jefferson Lab science, and it comprises a $2,500 cash award and a commemorative plaque.

Nominations are decided on four standards: the quality of the written work, the student’s contribution to the work, the effect of the research on the field of physics, and service (how the work helps Jefferson Lab or other experiments).

The Southeastern Universities Research Association started the JSA Thesis Prize in 1999. It is presently one among the several projects supported by the JSA Initiatives Fund Program, which was set up by Jefferson Science Associates to support initiatives, programs, and activities that promote the scientific outreach, and advance the education, science, and technology tasks of Jefferson Lab through means that harmonize its basic and applied research focus.

Graduate students are the driving force of any research enterprise, so the Jefferson Lab User Organization is proud to give out the thesis price this year again. We thank JSA for providing support for this prize.

Julie Roche, Chair and Professor, 2018-2019 JLUO, Ohio University

Roche continued, “As usual, the theses submitted were of very high quality and made deciding on a winner quite a challenge. I want to thank the selection committee led by University of Virginia Professor Kent Paschke for its careful examination of the submissions. In the end, we are delighted to recognize Kurtis’s work.”

Bartlett is presently a postdoctoral research associate for the Space Science and Application Group at DOE’s Los Alamos National Laboratory, where he creates spacecraft detectors that quantify radiation to help determine the composition of planetary bodies.

Although I’m developing hardware now, I’m still using the skill set developed in my dissertation research,” stated Bartlett.


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