defamation of character at work

[rogers38]

i was recently accused of breaking the code of conduct at work by a coworker. Prior to being accused I had a dispute with coworker about how a case we were working on was being handled (in front of our supervisor during our weekly supervision). This same coworker reported the allegations to our supervisor two days after the disagreement. The allegations are false and allegedly occurred twice. Once incident allegedly occurred 14 months ago and the second one allegedly occurred three months ago. My supervisor informed me that there was no way they could prove that the allegations were true however she reported that they have to take action and put me on probation for a period of up to six months during which time I will be watched closely. They want me to sign a letter stating their concerns and their plan of action. I pointed out to them that I feel I am being unfairly accused due to the argument that occurred with the same coworker prior to the alleged accusations. I am concerned about signing the letter for many reasons most importantly because the allegations are false and being made by a disgruntled employee. The letter will be placed in my file and I am concerned how it can or will impact future employment else where.