An international collaboration of physicists conducting experiments at the Thomas Jefferson National Accelerator Facility has determined for the first time the weak charge of the proton. The findings also include the determinations of the weak charge of the neutron, and of the up quark and down quark.

The weak force is one of the four fundamental forces in our Universe, along with gravity, electromagnetism and the strong force.

Although the weak force acts only on the sub-atomic level, we can see its effects in our everyday world. It plays a key role in the nuclear reaction processes that take place in stars and is responsible for much of the natural radiation present in our Universe.

Lead author Prof Darko Androic from the University of Zagreb, Croatia, and his colleagues proposed the first direct measurement of the proton’s weak charge, denoted by the symbol Qp w – this represents the strength of the weak force’s pull on the proton, a measure of how strongly a proton interacts via the weak force.

Since the weak charge of the proton is precisely predicted by the Standard Model, which is a well-tested theoretical framework that describes the elementary particles and the details of how they interact, it is an ideal parameter to measure experimentally as a test of the Standard Model.

To perform the experiment, the team directed a very intense beam of electrons into a container of liquid hydrogen. The electrons were longitudinally polarized – spinning along or opposite their direction of motion. Electrons that made only glancing collisions with the protons – elastic scattering, where the proton remained intact – emerged at small angles and were deflected by powerful electromagnets onto eight symmetrically placed detectors.

The weak force is far weaker than the electromagnetic force. In classical terms, one might think of this as for every one million electrons that interact with the protons via the electromagnetic force, only one will interact via the weak force.

The physicists measured those few weak interactions by exploiting an important difference between the two forces – the weak force violates a symmetry known as parity, which reverses all spatial directions and turns our right-handed world into a left-handed one. In an opposite-parity world, the electrons spinning with their axes along their direction of motion would interact with protons via the electromagnetic force with the same strength. Where the weak force is concerned, electrons with right-handed spin interact differently than left-handed ones.

By keeping all other parameters of the experiment the same, and only reversing the polarization direction of the electron beam, the scientists can use the difference or asymmetry of the measurements between two polarization directions to isolate the effect of the weak interaction.

The goal is to measure this difference, only 200 parts per billion, as precisely as possible. This precision is equivalent to measuring the thickness of a sheet of paper laid atop the Eiffel Tower.

The initial analysis of the experimental data yielded a value for Qp w = 0.064 ± 0.012 that is in good agreement with the Standard Model prediction, Qp w (SM) = 0.0710 ± 0.0007.

“Readers should view this result primarily as a first determination of the weak charge of the proton. Our final publication will be focused on implications with respect to potential new physics,” said Dr Roger Carlini, a Jefferson Lab staff scientist, who is a co-author of a paper accepted for publication in the journal Physical Review Letters (arXiv.org).

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Bibliographic information: D. Androic et al. First determination of the weak charge of the proton. Physical Review Letters, accepted for publication August 30, 2013; arXiv: 1307.5275