1. The problem statement, all variables and given/known data Mass of neutrino = 0 Mass of proton/neutron = 1 GeV Mass of tau = 2 GeV 2. Relevant equations Energy of neutrino in lab = e Energy of neutrino in CM = e0 3. The attempt at a solution In the centre of mass frame, we will take the momentum of the neutron to be e0/c, c = speed of light. CoE in CM: e0 + sqrt(m_n^2*c^4 + e0^2) = (m_p + m_tau)c^2 this gives e0 =1.33 GeV therefore the velocity of CM in lab frame = (e0/c)/(m_v + m_n) = e0/(c*m_n) = 1.33c > c. Cannot find where I'm doing the mistake!!