Graduate assistantship assignment/research interests do not align

[AMB8706]

I was very excited to get into my 1st choice PhD Psychology program, and was under the assumption that I would be in a specific professor's research lab (I mentioned this many times throughout my interview, was personally interviewed by them, and my MA thesis/previous years of research experience was the exact same material as this lab). After confirming my acceptance, I was notified by this professor that this lab did not have funding for a new graduate student. 

I just received my assignment for my graduate assistantship, and the professors have no similarities with my research interests, as far as I can tell. Will my dissertation be advised by these professors? Or will I have a different dissertation supervisor? (The professor I was hoping to work with did offer to help me with my dissertation, but it is very unclear in what capacity). I am disappointed that I will not be involved in the research I was expecting (trying to be open minded to new experiences...). I will feel MUCH better if I am just working for these professors, and will be able to conduct the research that I am interested in. Can someone please explain to me how this might work? Or how it works at other schools?

Thank you in advance!

[TakeruK]

Short answer: You should ask people in your department and find out. The most direct people to ask would be the professors you mentioned of course, but if you want to ask around for some info before approaching them (not necessary but I'd understand how that might make someone more comfortable), then ask the other students in your new program.

Longer answer:

In some places and programs, your grad research assistantship is also your dissertation work. But at other places and programs, it's common to work with one professor for your dissertation while also doing assistantship work with another professor for the purposes of funding. I don't know what the norm is in your field, so I am not sure if your situation is unusual or par for the course.

In addition, it might be possible for you to get an assistantship with your thesis advisor in the future, if more money opens up later. Definitely talk to your desired advisor about this sooner rather than later. I can't speak for everyone, but most people I know prefer having the work they do for their funding and the work they do for their dissertation be the same when possible.

Try to get the full answer ASAP because it's still early enough for some changes to happen if necessary. That is, would you really be happy if you have to switch your research field for your PhD? Is this still your first choice school if either you cannot work with your first choice lab, or if you have to get funding by doing unrelated work in a second lab? If not, I would consider checking in with other schools that accepted you and see if you can still be admitted for this fall. I would even consider deferring to Fall 2018 start if they are willing to take you but don't have any more funding room this year. I would also not rule out reapplying entirely and making sure you are in the right fit. However, before you take any action, it's important for you to get a realistic and accurate view of what your current department expects from you and what are some actual possibilities for your dissertation. You're about to start a 5-6 year (or more) chapter of your life, so it's okay to ask questions and ensure you're on the right path for you.

Finally, this last part is too late to be useful to you, @AMB8706 (sorry!), but in case others are reading this, it's really important to confirm the process for research and teaching assistantship appointments prior to accepting an offer. It's not enough to assume that if you say you want to work in X or if you say you want to work with X that there will be a place for you. At many places, being accepted to the department and being accepted to a specific lab/group/professor are independent processes. Many places don't even assign you to a specific group/lab until the end of your first year, if you do rotations etc. prior to having a thesis advisor. At another department in my school, each year, about half of the incoming class wants to work with Prof. X and the majority of them will not be able to do so. Make sure you find out whatever the process is and that you're okay with that process (i.e. if it's a competition, do you have a backup plan in case you don't get to work with the person you wanted to?).

[Eigen]

@TakeruK has some excellent advice, but I just want to reiterate for lab based people: I always tell my students not to strongly consider a department if there aren't at least 3 people they'd be happy working with. 

Ideally, you get the person you want to work with- but things happen. It might be a tight year, and everyone wants to work with one person and it gets competitive. They might lose a grant, and not be able to afford to take on a new student (or a current student might not graduate, leaving no opening). On the much less predictable and more severe end, they might get in an accident/get sick/die (I've seen all of the above happen to friends). You also might love the research, and find out you can't work with them personally, or have deep personal issues with someone else in the lab and need to switch. 

If you only have one person you want to work with, realize you're taking a heavy risk- if anything doesn't work out with that person, you're out of luck.