Consider the Z production at a proton–antiproton collider and its decay channel Z→ e⁺e^-. The energies of the two electrons as measured by the electromagnetic calorimeters bare E₁ = 60 GeV and E₂ = 40 GeV. The energy resolution is given by σ(E) / E = 0:15=√E. The measured angle between the tracks is θ = 140° ± 1°. Find the error on m_Z.