The NA64 experiment at CERN has looked for signs of a fifth force Brice, Maximilien © 2016-2019 CERN

A new clue to the existence of a fifth fundamental force of nature may have emerged from a Hungarian laboratory.

Attila Krasznahorkay at the Atomki Institute of Nuclear Research, Hungary, and his colleagues have spent years studying the radioactive decay of beryllium-8, an unstable isotope. In 2016, they published details of an odd finding, suggesting it was being caused by a previously unknown particle.

Now the same researchers have found another anomaly, this time in an energy transition involving an excited state …