who discovered protons?

Thomson discovered electrons in the late 1800s, and Rutherford discovered the nucleus in 1911. It was a slow process of realization and discovery involving many scientists that led to the realization the nucleus contained protons. Most sources list Rutherford as the most likely discoverer, but he was one of many scientists who contributed to this evolving understanding of matter.

It was Eugen Goldstein who discovered 'anode rays' in 1886 which led to the conclusion that subatomic particles having positive charge exist. Further important experiments such as the infamous 'Gold Foil' experiment by Lord Rutherford and important contributions by scientists like Prout and Moseley helped establish the nature of these positively charged particles. The name 'proton' was suggested by Rutherford.

Ernest Rutherford