PARTICLE PHYSICS

DESY’s HERA collider, decommissioned in 2007, is still providing valuable results to scientists. A newly released paper shows that quarks, which were the main particles under investigation at the electron–proton collider, do not visibly interact with potential cosmic background fields. This means that they don’t violate a fundamental symmetry of nature, the rotation and Lorentz invariance. HERA was specifically well-suited for studying quarks, so these results set important limits for other experiments and searches.

Any experiment taking place on Earth has, at any given time, a well-defined orientation and velocity with respect to the centre of the solar system, the centre of the galaxy, and the rest frame of the universe. The rotation and Lorentz invariance states that these symmetries should not have any effect on particles in an experiment. While this can be verified with many particles in the Standard Model of particle physics, with quarks it is slightly trickier. Quarks make up hadrons – for example the proton and the neutron – and so belong to the fundamental building blocks of the atomic nucleus. As building blocks go, they are difficult to examine: they cannot exist in isolation, meaning that they always bind with other quarks. The HERA collider was a one-of-a-kind facility for probing hadrons, as it collided much smaller electrons into protons to investigate structures within the proton. HERA was used extensively to learn essential details about quark behaviour and how the proton is actually constructed – knowledge that is essential to current colliders like the Large Hadron Collider at CERN.

Almost two decades on from its shutdown, those details are still coming. Recently, two theoretical physicists from Indiana University and the University of Sussex joined the ZEUS team at DESY to look at the rotation and Lorentz invariance and quark behaviour. Using the data from the HERA detector ZEUS that was taken between 2003 and 2007, they re-analysed collisions, looking for anomalies among the quarks. Such anomalies could have been the indication that the experiment, and thus the space around the Earth, was passing through a yet-unknown field that could not be otherwise seen. However, the results show that the rotation and Lorentz invariance holds for quarks – meaning that either such fields do not exist, or, if they do, quarks do not significantly interact with them.

“These are important details for finding physics beyond the Standard Model,” says DESY scientist Achim Geiser, who was part of the analysis team. “This establishes some of the world's best limits on the violation of this invariance, since we can see that there is no significant effect on light quarks within the hadron.”

DOI:10.1103/PhysRevD.107.092008