(uncountable) proton and proton (used attributively to describe a collision or other interaction of these two particles)
Comparisons with ppbar and pp data reveal improvement in the description of single diffractive events. 2015, Christine O. Rasmussen, Torbjörn Sjöstrand, “Hard Diffraction with Dynamic Gap Survival”, in arXiv:
(alt-of, countable, pronunciation-spelling, uncountable) Eye dialect spelling of pee-pee.