Mostly brownish grey object, something physicists study (3,8)

'something physicists study' is the definition.
I know nothing about this answer so I can't judge whether this works.

'mostly brownish grey object' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'brownish' becomes 'tan' (I've seen this in another clue).
'grey' becomes 'up' (I can't explain this - if you can you should give a lot more credence to this answer).
'object' becomes 'article'.
'tan' with its final letter taken away is 'ta'.
'ta'+'up'+'article'='TAU PARTICLE'

Can you help me to learn more?