Solutions to a scalar discontinuous conservation law in a limit case of phase transitions

For a particular discontinuous flux function that can be associated to the limit case of a phase transition, we introduce an appropriate notion of entropy weak solution to the Cauchy problem for the corresponding conservation law. Then, for a class of initial data, that includes the Riemann data, we prove, by the vanishing viscosity method and with a suitable regularisation of the flux function, the existence of an entropy weak solution. This result can be easily extended to more general flux functions.