# Pi meson decay (relativistic momentum)

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1. Feb 1, 2017

### RyanP

1. The problem statement, all variables and given/known data
A charged π meson (rest mass = 273me) decays into a neutrino (zero rest mass) and a μ meson (rest mass = 207me). Find the kinetic energies of the neutrino and the mu meson.

2. Relevant equations
E = moγc2
K = mo(γ-1)c2
v = pc2/E
p = moγv

3. The attempt at a solution
In the rest frame of the pi meson (S frame),
Ei = 273mec2
Ef = 207meγc2 + K where γ is the gamma factor of the mu meson and K is the energy (kinetic) of the neutrino.

Since the neutrino has no rest mass, it travels at c, so pv = Ev/c = K/c.

I couldn't figure out how to carry on in this frame, so I switched to an S' frame where the mu meson is at rest.

In this frame,
p'i = -273meγv where v is the speed of the pi meson initially (equal to the speed of the mu meson in the pi rest frame).
p'f = -E'nu/c since the neutrino is the only thing moving after the collision in this frame.

E'i = 273meγc2
E'f = 207me + E'nu

How do I proceed from here?

2. Feb 1, 2017

### phyzguy

Go back to the rest frame of the pion. Since the pion is initially at rest, the initial momentum in this frame is zero. Since momentum is conserved, the final momentum is zero also. So what do you know about the momenta of the neutrino and the muon?

3. Feb 1, 2017

### RyanP

The momentum of the neutrino is equal and opposite to the momentum of the muon - should I solve for the speed of the muon?

4. Feb 1, 2017

### phyzguy

You should be able to write four equations for the four unknowns, Enu, Emu, pnu, pmu.

5. Feb 1, 2017

### RyanP

Ended up with a quadratic equation for v (speed of the muon), in terms of both masses and c. Brute-forced it with the quadratic formula and got the right answer.