Given the task definition, example input & output, solve the new input case.
You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.
Example: Category: HOMOPHONIC PAIRS 
Clue: It's a notation on a percussion store to clash
Output: cymbal symbol
"Cymbal" and "symbol" both have the same pronunciations but different meanings, hence they are homophonic pairs. A symbol is the notation and a cymbal is a percussion instrument that clashes and rings.

New input case for you: Category: PHYSICS 
Clue: Discovered in the early 20th century, these 2 particles, 1 with a positive charge, the other a packet of energy, differ by a letter
Output:
a proton & a photon