How is an experimental particle physicist evaluated?
I happened to be a member of my college's executive committee when an experimental particle physicist was being considered for promotion to tenure. She was, indeed, a member of a huge collaboration, so a typical paper of hers had perhaps 100 co-authors. The papers provided no information about what, specifically, she had contributed (and we know better than to pay much attention to citation counts and impact factors). The information we needed, about the quality and quantity of her work, was provided by numerous letters from external reviewers. ("External" means not in our university.) Ordinarily, the college requires that most external letters come from senior researchers who are not co-authors of the candidate's papers, but an exception is allowed in areas, like experimental particle physics, where only co-authors really know about the candidate's work.  My recollection (about 15 years old and therefore somewhat vague) is that the letters gave us very detailed information, not only about what the candidate had contributed but also about how efficiently she had done her part of the work.