I suppose experimental can be interpreted in many ways and of course different people will have different takes on what it means...
I suppose for example you could say if a band known for rock music starts making pop music then that is it experimenting and therefore that music is experimental.
For me personally i think of experimental in terms of music as an actual genre - as per this description:
"Experimental music is a general label for any music that pushes existing boundaries and genre definitions.
Experimental compositional practice is defined broadly by exploratory sensibilities radically opposed to, and questioning of, institutionalized compositional, performing, and aesthetic conventions in music.
Elements of experimental music include indeterminate music, in which the composer introduces the elements of chance or unpredictability with regard to either the composition or its performance. Artists may also approach a hybrid of disparate styles or incorporate unorthodox and unique elements"
So in that respect i personally say u2 have no songs that fall into that genre.
Passengers of course would be the exception - but then that isn't badged as a u2 album is it...