Can a security guard stop and search you?

Ok so I'm walking out of a library and their damn magnetic sensor thingies go off. I naturally keep walking b/c no one ever comes when that happens at Walmart (and it does often). Either that or they take 5 minutes to show up. So i pretty much get 200 feet and 2 minutes later from the door and I get yelled at to stop by a security guard! I have to walk all the way back (pretty much talked to like I am a book thief), and walk through the scanner again. Of course it didn't go off again.

So my question is, does a security guard who isn't the police have any right to stop/search someone if they have no visual proof whatsoever that a crime took place. A defective machine doesn't seem like it should give them the right.

Oh, it's a 2 part question. 1) Can they stop you and 2)can they search you

A security guard has no more and no less legal authority than any other private citizen. If you are witnessed committing an offense they may detain you and, yes, physically restrain you if you resist. It's called a citizens arrest. They may only detain you long enough for police to arrive, however.
They may NOT search you without your consent.

Laws vary, but in California, a Security Guard can stop you in the situation you described. He may not do a physical search. He can ask you to open your coat or something like that, but he is not allowed to touch. If he knows you committed a crime he can detain you physically.

A security guard has no more right to stop you than any private person or store employee.

They can stop you and search you if they are conducting a citizen's arrest. Otherwise, and short of that, they have no authority to forcefully detail and/or search you. Of course nothing prevents them from asking you to stop or asking to search you or other questions. But you should be free to ignore them.

They cannot physically touch you unless you have been witnessed commiting a crime, and even then they cannot prevent you from leaving or you can sue them for assault or harrassment.

Yea Security guards are definity aloud to search you.

You seem to be asking if there is an absolute right for them to search you and I can't answer that.

However, if you have nothing to hide, why not just cooperate and tell them you have nothing to hide? I could see your point if they are overbearing and demanding...but consider if a friend saw you refuse to cooperate with reasonable requests. They might conclude you had something to hide. As long as the request is reasonably and politiely asked, why not address their concern? Everyone pays for theft, so it seem to me that reasonable measures to prevent or discourage theft help all innocent people.

On the other hand, if they are being jerks about it, I can see a point in refusing to surcumb to an intrusive search. If you refuse though, I think they can deny you future access to the facility.