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**1. A neutron decays into a proton, an electron and an electron antineutrino. In the frame of the neutron, the proton is in rest after decay. Calculate the ''invariant mass of the electron and electron antineutrino together''. The rest masses of the proton and neutrno are known**

**2. Conservation of energy, momentum and conservation of four momentum.**

**3. The problem is that I don't understand what is asked of me, are they asking for ##m_{e} + m_{v}##? If so, I start like this in natural units (h bar = 1 , c=1)**

##m_{n}=m_{p} + E_{e} + E_{v}##

Plugging in the relativistic formulas for energy, and knowing that since the proton has no momentum the momenta of the electron and antineutrino have to be equal in size:

##\sqrt{p^{2}+m_{e}^{2}}+\sqrt{p^{2}+m_{v}^{2}}=m_{n}-m_{p}##

Seems like not enough info, hence I'm doubting I'm correctly interpreting the question.

##m_{n}=m_{p} + E_{e} + E_{v}##

Plugging in the relativistic formulas for energy, and knowing that since the proton has no momentum the momenta of the electron and antineutrino have to be equal in size:

##\sqrt{p^{2}+m_{e}^{2}}+\sqrt{p^{2}+m_{v}^{2}}=m_{n}-m_{p}##

Seems like not enough info, hence I'm doubting I'm correctly interpreting the question.