Energy transfer ($\nu =E-E^{\prime }$)

The energy component of a transferred four-momentum in a specified frame. In fixed-target deep inelastic scattering the target rest frame gives $\nu =E-E^{\prime }$, where $E,E^{\prime }$ are incoming and scattered electron energies. It is frame dependent even though $Q^2=-q^2$ is Lorentz invariant.