  2-group symmetries are generalized symmetries that arise when 1-form and
0-form symmetries mix with each other. We uncover the existence of a class of
2-group symmetries in general 4d N=2 theories of Class S that can be
constructed by compactifying 6d N=(2,0) SCFTs on Riemann surfaces carrying
arbitrary regular punctures and outer-automorphism twist lines. The 2-group
structure can be captured in terms of equivalence classes of line defects plus
flavor Wilson lines, which can be thought of as accounting for screening of
line defects while keeping track of flavor charges. We describe a method for
computing these equivalence classes for a general Class S theory using the data
on the Riemman surface used for compactifying its parent 6d N=(2,0) theory.

High Energy Physics - Theory