Shot noise in coupled electron-boson systems

The nature of charge carriers in strange metals has become a topic of intense current investigation. Recent shot noise measurements in the quantum critical heavy fermion metal YbRh$_2$Si$_2$ revealed a suppression of the Fano factor that cannot be understood from electron-phonon scattering or strong electron correlations in a Fermi liquid, indicating loss of quasiparticles. The experiment motivates the consideration of shot noise in a variety of theoretical models in which quasiparticles may be lost. Here we study shot noise in systems with co-existing itinerant electrons and dispersive bosons, going beyond the regime where the bosons are on their own in thermal equilibrium. We construct the Boltzmann-Langevin equations for the coupled system, and show that adequate electron-boson couplings restore the Fano factor to its Fermi liquid value. Our findings point to the beyond-Landau form of quantum criticality as underlying the suppressed shot noise of strange metals in heavy fermion metals and beyond.