Employment Law Questions? Ask an Employment Lawyer.
You can sue the individuals that lied about you for defamation of character. That is a personal injury claim that you can bring yourself.
However, in employment law, unless you had an employment contract stating that you could only be demoted "for cause" the employer doesn't actually need a good reason for demotion. This is called 'at will' employment. The employer here can legally ignore the other evidence and decide to demote you simply based on the fact that the others decided to gang up on you. The manager may feel that it is better for office harmony to simply go along and while that is absolutely horrible, it is NOT illegal unless the only reason you were being harassed in the first place was your race, religion, gender, age, disability or FMLA use (and then you'd file an EEOC complaint).