Polygraph examinations aren't admissible in that way. Otherwise crime would be as simple as pass/fail not-guilty/guilty.

This is why a (good) defense attorney advises you against taking a polygraph if one is offered during a trial. If you pass, the case is not magically dismissed: it's useless. If you fail, you aren't automatically convicted, but the jury has their view of you negatively influenced by your failure of the test. You have nothing to gain by taking and passing one in court, but you have an undefined jury sentiment to lose should you fail.