  The study of interaction between electromagnetism and elementary particles is
a long-standing topic in physics. Likewise, the connection between particle
physics and emergent phenomena in condensed matter physics is a recurring theme
and condensed matter physics has often provided a platform for investigating
the interplay between particles and fields in cases that have not been observed
in high-energy physics, so far. Here, using angle-resolved photoemission
spectroscopy, we provide a new example of this by visualizing the electronic
structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in
both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs
and bulk Weyl fermion dispersion as well as the regulation of Weyl fermions by
ferrimagnetism. Our results establish NdAlSi as a magnetic Weyl semimetal and
provide the first experimental observation of ferrimagnetic regulation of Weyl
fermions in condensed matter.
