Physicists charge ahead with proton discovery
A University of Adelaide physicist has assisted in a discovery which may catalyse the ability to solve some of science's biggest remaining mysteries.
Associate Professor Ross Young, in collaboration with scientists at Jefferson Lab in Virginia, has precisely measured the ‘weak charge’ of a proton for the first time.
The discovery opens doors to finding a world of potential new particles that otherwise might only be observable using extremely high-energy accelerators.
Known as the Qweak experiment, scientists measured the proton's minuscule weak force by creating a test that could detect it.
An intense beam of electrons was directed onto a target containing liquid hydrogen. These electrons have an innate property called spin, which means they can either align with or against the direction they are travelling. The electromagnetic force does not care about the spin alignment of the electron, giving rise to precisely the same scattering rate either way. However, the ‘weak force’ causes a tiny dependence on the spin orientation of the electron. Physicists calculated the protons' weak charge by measuring this dependence.
What is a weak charge?
Four fundamental forces control how any two objects will behave when they meet. How each force is felt is determined by the innate properties of each matter.
- Gravitational and electromagnetic interactions produce significant long-range forces of which the effects can be seen directly in everyday life.
- Strong and weak interactions are effective only over a very short range. These produce forces at minuscule, subatomic distances and govern nuclear interactions.
Associate Professor Young from the School of Physical Sciences said, “the measurement is an incredible experimental accomplishment, reporting a precision of better than 10 parts per billion”.
“Achieving this precision is equivalent to testing if a coin is fair by flipping it 10 million billion times, while at the same time ensuring the conditions of the flip are identical every time.”
The measurement of the weak charge of the proton has been reported to be in excellent agreement with the theoretically predicted value. If a discrepancy with theory had been observed, this could have signified a new yet-to-be-discovered force of nature. The observed agreement consequently places stringent bounds on the existence of such new forces.
Figure 1: Parity-violating electron scattering from the proton.
An incoming electron, e, with helicity +1 scatters away from the plane of the ‘parity-violating mirror’.
The image in the parity-violating mirror shows the incoming electron with the opposite helicity, −1; instead of scattering into the plane of the parity-violating mirror (as it would in a real mirror), it scatters out of the plane of the parity-violating mirror.
The dominant electromagnetic interaction, mediated by the photon (γ, blue wavy line), conserves parity. The weak interaction, mediated by the neutral Z0 boson (dashed red line), violates parity.
The weak interaction is studied experimentally by exploiting parity violation through reversals of the incident-beam helicity, which mimic the parity-violating mirror ‘reflection’.
Further reading
- Precision measurement of the weak charge of the proton, Nature - international journal of science