Correction to: Nature Physics https://doi.org/10.1038/s41567-020-0863-3, published online 6 April 2020.

In the version of this Perspective originally published, in the sentence beginning “The Japan Proton Accelerator Research Complex (J-Parc) with a 50 GeV synchroton…”, ‘50 GeV’ should have been ‘30 GeV’. Also, in ref. 39, the first author name was incorrectly given as ‘Delahye’; it should have been ‘Delahaye’. The online versions of the Perspective have been amended.