“Particle Theory” by Edward Bryant was first published in Analog Science Fiction/Science Fact, February 1977. I read it in the collection The Legend Book of Science Fiction, edited by Gardner Dozois and published in 1991.
The story of the end of the universe, I think. And a man undergoing an experimental procedure to zap his prostate cancer with a pion beam.
It is a story that jumps about in time a bit, and I’m not sure I followed it very well or fully understood the ending. Perhaps a grounding in science would help? Anyway, I enjoyed it and there was much about the way it is written that will inform my writing. I enjoyed the conversational tone, but the physics was beyond me.