The Weak Side of the Proton

A precision measurement of the proton’s weak charge narrows the search for new physics.

Researchers conducted the Q-weak experiment in Jefferson Lab’s Experimental Hall C, and their goal was to very precisely measure the proton's weak charge, a term that quantifies the influence that the weak force can exert on protons. Engineers installed the Q-weak apparatus, shown here, in the Hall for the experimental run, which concluded in 2012.

The Science

The weak force is one of the four fundamental forces in our universe, along with gravity, electromagnetism, and the strong force. Researchers have made the first experimental determination of the weak charge of the proton, a measure of the precise strength of the weak force’s influence on the proton. They found that the measurement agrees well with predictions using the Standard Model. The Standard Model is the current theory used to describe how fundamental particles interact with each other.

The Impact

The determination of the proton’s weak charge provides a precision test of the weak force.  As it is in excellent agreement with the theoretical prediction of this parameter derived from the Standard Model, this constrains some possible types of new particles and forces beyond our present knowledge.


The weak force’s effects can be observed in our everyday world: 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 force acts directly on subatomic particles, such as the protons, neutrons, and electrons that make up atoms. These particles carry a weak charge, a measure of the influence that the weak force can exert on them. To measure the weak charge of the proton, scientists directed an intense beam of electrons generated by Jefferson Lab’s Continuous Electron Beam Accelerator Facility onto a target containing cold liquid hydrogen. A precise, custom-built apparatus detected the electrons that scattered from the protons inside the target. The electrons in the beam were highly polarized — meaning that the electrons were mostly “spinning” in one direction, parallel or anti-parallel to the beam direction. By precisely and rapidly reversing the direction of polarization, the nuclear physicists measured the proton’s weak charge. The new result is in excellent agreement with the predictions of the Standard Model of particle physics, and so it serves as a test of the model and can impact its predictions. For example, this result has set limits on the possible existence of leptoquarks, which are hypothetical particles that can reverse the identities of two broad classes of very different fundamental particles — turning quarks (the building blocks of nuclear matter) into leptons (electrons and their heavier counterparts) and vice versa.

Numerous technical achievements made this experiment possible. These include the high-current, high-polarization, extremely stable electron beam provided by the Continuous Electron Beam Accelerator Facility; the world's highest-power cryogenic hydrogen target; extremely radiation-resistant Cerenkov detectors; ultra-low noise electronics for signal readout and beam current measurement; and a system that measures the beam polarization to better than 1%.


Roger Carlini
Jefferson Lab


This work was supported by the Department of Energy, Office of Science, Nuclear Physics; the National Science Foundation; the Natural Sciences and Engineering Research Council of Canada; and the Canadian Foundation for Innovation, with matching and in-kind contributions from a number of the collaborating institutions.


The Jefferson Lab Qweak Collaboration, “Precision measurement of the weak charge of the proton.” Nature 557, 207 (2018). [DOI: 10.1038/s41586-018-0096-0]

Related Links

Jefferson Lab press release: The Weak Side of the Proton

Jefferson Lab press release: Proton's Weak Charge Determined for First Time

Jefferson Lab backgrounder: Q-weak Backgrounder: Probing the Proton's Weak Side

Jefferson Lab press release: Getting a Better Measure of Spin with Diamond

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