What does SU stand for?

Context:

Hints for more fulfilling searches

Boolean Operators
'AND', 'AND NOT', and 'OR'
Metacharacters
The bad news: In general, punctation and other non-alphanumeric characters are not indexed and cannot currently be searched for. Currently some interesting characters like (,), and = are not searchable. Regrettably, this means that one cannot search for SU(3) or c=1 at this time (but searching for 'c' AND '1' will give hits on 'c = 1').
The good news: The characters ^,_,{,},+, and - are indexed. For example, searching for K^+ or nu_e will work - be aware that authors are not always consistent in how super- and subscripts are presented.
Non-indexed characters are stripped out from the search query.
Stemming
Most fields stem words automatically (searching for superconductors will match superconducting)
Wild card truncation
Wild card (*) can only be used anywhere but at the beginning of a term (but see author examples below).
Grouping
Grouping can be done with parentheses
Binary Booleans are not associative - parentheses are mandatory if a field has multiple Booleans
First two fields are grouped together in form before third field is added in
Exact phrases
Use double quotes (") - Warning: can be slow (try AND instead)

More:

http://arxiv.org/multi?group=grp_q-bio&%2Ffind=Search