Suggestions:
1.) Take some initiative. Grab a pen and paper and get ready to answer your own questions.
2.) Figure out the names/locations of some of the particle accelerators in the United States.
3.) Go to the websites of the labs that maintain the particle accelerators.
4.) Look for job openings at those websites.
5.) Read the job descriptions attached to those job openings. Make notes about what those job descriptions involve and what they are looking for in employees.
6.) Sit in satisfaction that you have discovered the answers through your own ingenuity.Bonus hint: I would start with the lab named for the physicist who came up with a rough estimate for the yield of the first atomic bomb by droping pieces of paper as the shock wave passed. I just looked and they have two mechanical engineering positions currently open.