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April 24th 2019

The outcome of an experiment cannot depend on the speed or orientation of the laboratory in which the experiment takes place. All forces in nature must satisfy this fundamental symmetry, called Lorentz invariance. Forces are also invariant under spatial inversion (parity, P), substitution of matter for antimatter (charge conjugation, C) and time reversal (T). The exception is the weak force. This force is responsible for beta decay, which occurs when an atomic nucleus emits an electron or positron. Only the combination of the three symmetries together, CPT symmetry, is retained by the weak nuclear force (see film). If CPT is violated nonetheless, then this also means that the Lorentz symmetry must be broken. Researchers from this research programme are therefore studying the Lorentz symmetry of beta decay. At present little is known about this, so the researchers have developed both the theoretical and experimental aspects.

The researchers slow down a radioactive beam of the short-living isotope 20Na until it is stationary in neon gas. They then polarise the 20Na atoms using laser light as a result of which the spin (the rotational axis) of the atomic nuclei is set in a certain direction. Using this system they determine whether the decay rate of the isotope is dependent on the spin direction of the nuclei. If the decay rate differs with spin orientation, Lorentz symmetry has been violated.

The researchers measure how many nuclei are oriented using the positron emitted in beta decay. That process breaks the spatial inversion symmetry and reveals the positioning of the 20Na nuclei with respect to the emission direction of the beta particle, just as in the famous experiment of madam Wu.

The decay rate is measured using gamma radiation. Here the counting rates are independent of the spin direction. However, if the Lorentz symmetry were violated   the rates would depend on the direction. With this, the experiment places a limit on the degree of the Lorentz symmetry violation in the order of 10-3.

Researchers want to continue their work at the CERN experiment LHCb, where weak decay very close to the speed of light can be measured. The sensitivity for violation of the Lorentz invariance is then at its highest.