It´s not abandoned yet, but if it will the patent application will be prior art.
Holmlid published quite a number of scientific articles that also contain the principles of muon generation as claimed in the patent application (mostly published after filing date of course). These publications are already prior art for new patent application attempts.
The EPO is stating that it considers the patent to be withdrawn, due to lack of response. I really don’t know if there’s a way to reverse this decision, hope there is, but doesn’t seems to me that it could be.
A withdrawn patent I think It’s not the same as an abandoned one, at least in my country a withdrawn patent is considered as if it was never filed.