Say we have a proton $p$ and an anti-proton $b$. If they're fired directly at each other, apparently they're annihilated and all that's left is energy. But protons/antiprotons have a significant mass, and radius of about 1 femtometer. What if, instead of being fired directly into each other, $p$ and $b$ instead "grazed" each other by some arbitrarily small amount. Would they always annihilate and produce energy? Would only a "portion" or them annihilate? Or would it be probabalistic, where they annihilate with some chance that depends upon how much they overlap?

  • $\begingroup$ As I mention here, proton + antiproton annihilation is messy. How would your proposed partial annihilation work? There aren't any stable baryons lighter than a proton. (And there aren't any heavier ones, either). $\endgroup$
    – PM 2Ring
    Commented Sep 17, 2022 at 6:16

1 Answer 1


Protons and antiprotons are quantum particles, this means they obey quantum mechanical kinematics, solutions of the quantum mechanical equation for "proton scattering on antiproton", and no concept as "grazing" can be quantified in the solutions.

The experiment was done in the '80s. Look how the detectors locate the multiplicity of tracks from the annihilation :

enter image description here

an event from UA1

The probability of proton antiproton hitting each other in the collider depends on the beam design and the quantum mechanical solution of proton antiproton scattering. Those solutions have also a probability for elastic scattering, where no annihilation takes place, the analogue of your "grazing".

  • $\begingroup$ so....is the answer that it's probabilistic? Either they hit each other (with some probability) and behave accordingly, or they don't hit each other (with some probability) and behave accordingly? And the probability depends upon how closely they're shot at each other? And there's nothing in between? (e.g. no partial "grazing" where "some of the proton" cancels out "some of the antiproton") $\endgroup$
    – chausies
    Commented Sep 17, 2022 at 10:46
  • 1
    $\begingroup$ @chausies the "hit" is a classical concept of interactions. quantum mechanically there is a probability there will be a strong interaction with many particles output, or an elastic one, no matter how far away the tracks are, the probability becomes infinitessimaly small for large distances, but quantum mechanically it can be calculated. $\endgroup$
    – anna v
    Commented Sep 17, 2022 at 18:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.