I appreciate the feedback. Even the negative feedback. You guys really think I'm some kind of asshole 🤣. I typed this from my phone in bed, so now that I'm at a keyboard, let me explain fully.
When I said I gave the exam over spring break, I didn't mean it began at the beginning of spring break and ended at the end of it. That time was available for them to work on it. When I give exams, I give them a little over a week. From Tuesday to Thursday the following week. In this case, it began the Tuesday before spring break and ended the Thursday after. The reason I did this is because, like many of you, I remember papers being due immediately before or right at the end of breaks. By saying I gave it over spring break, I meant I gave them plenty of time.
I am very clear what is and is not permitted for an exam in my syllabus. They get an equation sheet, the allotted time, and they can work with a partner. Nothing else. Except for AI in which case they must screenshot everything. This is mostly for my curiosity. It still doesn't work for physics.
When I say normal at my institution, I mean to give a take home exam. I wasn't deviating from the norm by doing this, and it is the way I typically do it. As we have all experienced, you may have a day when you have 3 exams. Maybe that happens to only a few students. It disproportionately effects them. Giving this time, they can work it into their schedules.
So what did I see that constitutes cheating? It's very clear to me that the students used solutions from Chegg and/or other sites. If you've done this sort of thing with code, you know that folks will change the names of the variables, but not the structure or logic. It reads exactly the same. That was the case here. A few students were so (hilariously) guilty of cheating, they actually rewrote the solution to a similar, but different problem. Those problems had a different number of parts!
This is not my first time doing this. I've done this at several other universities. In those cases, I didn't have the issue of cheating, so I don't have a very explicit cheating policy in my syllabus. I'm taking the advice that some have given and giving them credit for what they've done. I will however be telling them on Tuesday (a conversation I am NOT looking forward to having) that I know many of them cheated, that I have evidence of it, and that I will refer them to the honor council should it happen again.
The part that sucks the most is I trust students. Having done this before, I've found that if you trust students, respect them, they in turn respect your expectations. Given how blatant this cheating is, it feels like a betrayal. Thanks again to everyone who replied, it has given me plenty to think on.