54797
Its positive energy unitary irreducible representations are indexed by mass ( nonnegative number ) and spin ( integer or half integer ) and are associated with particles in quantum mechanics ( see Wigner 's classification ) .
 Wigner
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Wigner