May 2014

Q-weak Experiment Determines Proton's Weak Charge

Scientists make the first experimental determination of the weak charge of the proton and extract the weak charges of the neutron and up and down quarks.

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Image courtesy of Jefferson Lab

The Q-weak experiment at Jefferson Lab precisely measured the weak force's hold on the proton.

The Science

One of the universe's four fundamental forces, the weak force, may act only on subatomic particles, but its effects are felt across the universe, as it enables stars to shine and powers natural radiation. For the first time and using just 4 percent of their collected data, scientists have experimentally determined the strength of the weak force's pull on the proton, the proton's weak charge.

The Impact

The proton's weak charge is precisely predicted by the Standard Model, which describes the particles and forces that make up all matter. This preliminary result is in good agreement with the Standard Model, and the final result will offer a stringent experimental test of the Standard Model and will provide constraints on new physics comparable to searches conducted at the Large Hadron Collider at CERN.


Physicists’ Standard Model of the universe—the current prevailing description of how the universe works—postulates that there are only four fundamental forces: electromagnetism, gravity, the “strong” force, and the “weak” force. The latter two are responsible for holding matter together; the weak force acts on subatomic particles, such as protons and neutrons. These particles carry a weak charge, a measure of the influence that the weak force exerts on them. Using just 4% of data collected in the Q-weak experiment, Jefferson Lab researchers have announced the first experimental determination of the proton's weak charge. When combined with previous atomic parity-violation and other parity-violating electron scattering measurements, the neutron's weak charge was also determined, which also allowed significant constraints to be placed on the weak charges of the up and down quarks. The final result, using 25 times more data, may provide insight into the Standard Model, which describes the particles and forces that make up all matter, and may show evidence for new heavy particles, such as those that may be produced by the Large Hadron Collider at CERN in Europe. Numerous technical achievements in the last decade made this experiment possible, including the high-current, high-polarization, extremely stable electron beam provided by Jefferson Lab's Continuous Electron Beam Accelerator Facility; the world's highest-power cryogenic hydrogen target, designed with the aid of computational fluid dynamics to minimize density variations at maximum beam power; extremely radiation-hard Cherenkov detectors; ultra-low-noise electronics to read out the signals and precisely measure the beam current; and a system which measures the beam polarization to better than 1 percent using a back-scattered laser. The net effect of these achievements yielded an astonishingly small total uncertainty of 47 parts per billion for the data published so far.


Roger Carlini
Thomas Jefferson National Accelerator Facility


The Q-weak experiment was funded by the U.S. Department of Energy, the U.S. National Science Foundation and the Natural Sciences and Engineering Research Council of Canada. Matching university contributions were also provided by The College of William and Mary, Virginia Tech, George Washington University and Louisiana Tech University. Technical support was provided by TRIUMF, MIT/Bates and Jefferson Lab.


D. Androic et al., "First Determination of the Weak Charge of the Proton." Physical Review Letters 111 141803 (2013). doi: 10.1103/PhysRevLett.111.141803.

Related Links

Q-weak: A Precision Test of the Standard Model and Determination of the Weak Charges of the Quarks through Parity-Violating Electron ScatteringExternal link

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Last modified: 11/19/2015 9:12:58 AM