Dirac mass induced by optical gain and loss

Mass is commonly regarded as an intrinsic property of matter, but modern physics shows that particle masses have complex origins . Elementary particles acquire mass from couplings to other fields: most fermions and bosons receive mass from the Higgs field, as well as other interactions (e.g., quarks gain mass from interactions with gluons). In low-energy physics, quasiparticles behaving like fundamental particles can arise in crystalline lattices, such as relativistic Dirac quasiparticles in graphene. Mass can be imparted to these quasiparticles by various lattice perturbations. By tailoring lattice properties, we can explore otherwise-inacessible phenomena, such as how particles behave when Hermiticity, the symmetry responsible for energy conservation, is violated. Non-Hermiticity has long seemed incompatible with mass generation; when Dirac points are subjected to energy-nonconserving perturbations, they typically become exceptional points instead of opening a mass gap. Here, we show experimentally that Dirac masses can be generated via non-Hermiticity. We implement a photonic synthetic lattice with gain and loss engineered to produce Dirac quasiparticles with real mass. By tuning this mass, we demonstrate a crossover from conical to non-conical diffraction , topological boundary states between domains of opposite Dirac mass, and anomalous tunneling into potential barriers.