ram@zedat.fu-berlin.de (Stefan Ram) wrote or quoted:
Mass is /not/ changed in this process when the mass of the system
"γ + γ" is being compared with the mass of the system "e⁺ + e⁻".
Incidentally, I was flipping through a physics book yesterday
(not searching for information about the topic quoted above)
and noticed that, for once, it actually lays things out right
(even if it calls it "invariant" mass like that's not real
mass, just "invariant mass"). Anyway, here's the quote:
(If you don't feel like reading the whole thing: The part about
mass comes up at the end: "In a particle decay . . ."!)
|From (2.11) it can be seen that a single particle at rest has
|four-momentum p^u = (m, 0, 0, 0) and therefore p^up_u = m^2.
|Since p^up_u is Lorentz invariant, the relation
|
|E^2 − p^2 = m^2
|
|holds in all inertial frames. This is, of course, just the
|Einstein energy-momentum relationship. For a system of n
|particles, the total energy and momentum
|
| n
|p^u = S p_i^u
| i=1
|
|is also a four-vector. Therefore for a system of particles the
|quantity
|
| / n \ 2 / n \ 2
|p^up_u = | S E_i | -| S p_i |
| \ i=1 / \ i=1 /
|
|is a Lorentz-invariant quantity, which gives the squared
|/invariant mass/ of the system.
|
|In a particle decay a -> 1+2, the invariant mass of the decay
|products is equal to the mass of the decaying particle,
|
|(p_1 + p_2)^u (p_1 + p_2)_u = p_a^u p_a^u = m_a^2.
|
"Modern Particle Physics" (2013) - Mark Thomson
--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)