I Can an atom absorb a photon, yet its total kinetic energy is decreased?

[PeterDonis]

some exotic particle in high-energy physics can absorb a photon to become a higher-mass particle without the KE or PE changing?

If the particle is free, it is impossible for it to absorb a single photon without its momentum (and hence its KE) changing.

 

[Dale]

If the particle is free, it is impossible for it to absorb a single photon without its momentum (and hence its KE) changing.

No. You can change momentum without changing KE. It would be pretty fine tuned though. The momentum would have to start out along e.g. the negative x axis and change to the positive x axis at whatever velocity corresponds to the same KE. There should be one frame where this is true for any absorption by a free particle.

 

[PeterDonis]

You can change momentum without changing KE.

Hm, yes, for a very fine-tuned case this is possible.

 

[Dale]

Hm, yes, for a very fine-tuned case this is possible.

Yes, not only is it fine tuned, but also I cannot think of any reason why you would use that frame naturally. You would only use it specifically to make this point.

 

[Vanadium 50]

Since the decay \Sigma^0 \rightarrow \Lambda + \gamma occurs, the inverse process \gamma + \Lambda \rightarrow \Sigma^0 must also occur.

However, both baryons cannot be at rest. There exist a frame where the baryon kinetic energies are the same and a (different) frame where the momentum magnitudes are the same. This example also relies on the baryons being composite particles.