Name: Sungho J.
If an electron with enough energy collides with a proton, then what happens?
They just form a hydrogen atom?
Or they form a neutron?
If both are possible, then which factor controls the process?
I am asking because I heard that a neutron can decay to a proton and an
electron and the reverse is possible.
If an electron collides with a proton, many things can happen, depending on
the energy of the collision. At very low energies, they could form a
hydrogen atom as you suggest. A hydrogen atom has less energy than an
electron and a proton sitting next to each other because they form a bound
state and they are bound together with the binding energy of 13.6 eV.
Normally the electron will be captured in a very high Bohr orbit from which
it quickly cascades down to the ground state, emitting photons to get rid of
the binding energy.
At very high energies, electron-proton collisions can produce other
particles such as pions. Each collision must, of course, conserve electric
charge, energy, momentum, angular momentum, etc.
For example, a neutron CANNOT decay into a proton and an electron since
angular momentum cannot be conserved when a spin 1/2 particle decays into
two spin 1/2 particles. A neutron can decay into a proton, and electron,
and a neutrino -- three spin 1/2 particles. So an electron colliding with a
proton can produce a neutron and a neutrino.
This subject is too involved to discuss in detail here. I am not sure what
to recommend if you want to pursue this topic further. Perhaps go to your
closest Barnes and Noble bookstore and search the physics section for books
at your level on elementary particles. The book I can find which is closest
to your needs (though probably not very close) is "The Particle Play" by J.
C. Polkinghorne published by Freeman.
Dick Plano, Professor of Physics emeritus, Rutgers University.
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Update: June 2012