Precision Measurement of the Proton’s Weak Charge Narrows the Search for New Physics

May 10, 2018
Dr. Elie Korkmaz
Physics Professor Dr. Elie Korkmaz is part of an international team of researchers who have measured the proton's weak charge to a high degree of precision.

An international team of researchers, including University of Northern British Columbia Physics Professor Dr. Elie Korkmaz, has gained new insight into the most elusive of the four fundamental forces in nature, the weak force.

The Q-weak experiment has revealed the strength of the weak force’s grip on the proton, by measuring the proton’s weak charge to high precision. The research was carried out using the high quality polarized electron beam available at the Thomas Jefferson National Accelerator Facility (JLAB) in Newport News, Va. The result, published in the May 10 issue of Nature, Precision measurement of the weak charge of the proton, significantly narrows the search for new particles that could influence the behavior of matter at sub-nuclear distance scales.

The proton’s weak charge is analogous to its more familiar electric charge, a measure of the influence that the proton experiences from the electromagnetic force.

While the weak force is difficult to observe directly, its influence can be felt in our everyday world. For example, it initiates the chain of reactions that power the sun and it provides a mechanism for radioactive decays that partially heat the Earth’s core and that also enable doctors to detect disease inside the body without surgery. The weak and electromagnetic interactions are closely related in the Standard Model of particle physics, a highly successful theory that describes these two forces as different aspects of a single force.

“Despite its success, the Standard Model has a number of very significant shortcomings and is therefore widely believed to be only a low-energy approximation to a more complete fundamental theory of nature, yet to be discovered,” said Korkmaz, a member of the Canadian group involved in the Q-weak project. “Such a new theory, potentially involving new fundamental particles or even new forces, would be expected to provide explanation for, among other phenomena, the seemingly arbitrary properties of the currently observed particles and forces, dark matter and dark energy, and the dominance of matter over antimatter in the universe.”

To measure the proton’s weak charge, an intense beam of electrons was directed onto a target containing cold liquid hydrogen, and the electrons scattered from this target were detected in a precise, custom-built measuring apparatus.

“Since the proton’s weak charge is predicted very precisely in the Standard Model, a precise measurement of this quantity can then be used to look for hints of new physics, via small deviations from the Standard Model prediction,” Korkmaz said.

The key to the Q-weak experiment is that the electrons in the beam were highly polarized – prepared prior to acceleration to be mostly “spinning” in one direction, parallel or anti-parallel to the beam direction. With the direction of polarization rapidly reversed in a controlled manner, the experimenters were able to latch onto the weak interaction’s unique property of parity (spatial inversion) violation, in order to isolate its tiny effects to high precision: a different scattering rate by about two parts in 10 million was measured for the two beam polarization states. The proton’s weak charge was found to be QWp=0.0719±0.0045, in excellent agreement with the Standard Model prediction.

“This excellent agreement with the Standard Model, at the precision level achieved, can now be used to constrain predictions of, and help identifying any new heavy particles that may be produced at the Large Hadron Collider or at future high energy particle accelerators”, said Korkmaz. “For example, our result has set limits on the possible existence of leptoquarks, hypothetical particles that can turn quarks - the building blocks of nuclear matter- into leptons -electrons and their heavier counterparts- and vice versa.”

The Q-weak experiment, initiated in 2001, represents the sustained effort of a large, international team of about 100 scientists from 25 institutions over nearly two decades.

The Canadian group, involving scientists from the University of Manitoba, UNBC, University of Winnipeg and TRIUMF, was a founding member and represents approximately 15 per cent of the Q-weak collaboration; it was a leading contributor to the equipment design and construction, data production, and data analysis efforts.

More than $3 million of support has been provided through the Natural Sciences and Engineering Research Council of Canada subatomic physics Project Grant program to the Canadian group. More support was provided by the Canada Foundation for Innovation. These funds were used to build equipment and to support student and postdoctoral researchers’ salaries and travel to carry out the measurements at JLAB.

UNBC’s contributions to this project were primarily in setting up and testing various elements of the experimental apparatus at JLAB and in actual data acquisition spanning a period of about three years and completed in 2013.

The successful completion of the Q-Weak experiment is an important milestone in parity violating electroweak physics and sets the stage for a new measurement of the weak charge of the electron, at even higher precision – the MOLLER experiment – which is currently under development and in which the same Canadian group is also involved.