If I have understood this correctly, a 10[sup]20[/sup] eV proton interacts with a background radiation photon to invariably produce a pi meson. Do 10[sup]19[/sup] eV (or less) protons interact at all with such photons? If yes, doesn’t that screw up the study of cosmic rays (making their energy determination problematic)? If no, why not (why shouldn’t cosmic ray particles with less than 10[sup]20[/sup] eV interact with background photons to produce particles, albeit less massive particles than pi mesons?)
IIRC, 1 Electron Volt (eV) is equal to 1.6x10^-19 Joules… that is, 10^20 eV is somewhat less than 10 Joules… not exactly enormous energy… It might heat a litre of water a thousandth of a degree or something, but not a whole lot else…
The key aspect here seems intrinsically technical to me, so apologies to anybody hoping for a non-technical answer.
It’s called the Griessen cut-off and it’s entirely down to kinematics. While 100 billion GeV is a lot by subatomic standards, a 2.7 K photon isn’t carrying a lot of energy; the mismatch between the two particles in the proton-photon collision is about 24 orders of magnitude. As a result, the centre-of-mass energy in the collision is comparitively small, only 0.17 GeV. That’s however just enough for the collision to produce a pion in addition to the proton. Hence the proton-photon cross section jumps up at this energy, producing the effect. At various higher energies, you’ll be able to produce hadrons other than pions and there will be corresponding jumps in the collision cross section.
Since pions are the lightest stable hadrons, this is the first energy the effect can kick-in at. There are lighter stable leptons (notably the electron), but they would have to be produced in an electromagetic process rather than a strong force one. The EM force is much weaker than the strong force and so the cross sections for such processes are much smaller.
Indeed protons with less energy do interact with such photons via the Compton effect, which is electromagnetic, but the cross section is significantly smaller and the energy losses less. It doesn’t make much difference even over inter-Galactic distances.
Yes, the effect does screw up what we can know about cosmic rays: very energetic ones that are produced far away don’t look energetic by the time they get here.
It’s the GZK limit (I think that the G might be Griessen, but there’s a couple of other guys, too), and it’s a mostly absolute cutoff. Below the GZK energy (10[sup]18.5[/sup]eV or so), the process will not occur at all. Above that energy, there’s still no guarantee it’ll occur over any particular distance, but give it a long enough distance and it’ll happen eventually.
The real problem in cosmic ray physics is not that we don’t see any such high energy cosmic rays, but that we do. Not very many, admittedly, but it’s more than anyone would have expected, and we’re not sure why. The simplest explanation is that ultra high-energy cosmic rays (UHECRs) just come from someplace close enough that a few of them can survive the GZK process, but there don’t seem to be any potential sources close enough in the right direction. M87, a particularly active galaxy, might fit the bill, but there don’t seem to be any more UHECRs coming from the vicinity of M87 than from anywhere else.