Professor Katrina Johnson was head of the Large Hadron Collider project in Switzerland. In 2009, Jack Harkness warned Professor Johnson to shut down the Large Hadron Collider. She couldn't simply shut it down, but made preparations to put the facility in lockdown.
With Jack's help, she reversed the polarity of one of the LHC's proton beams to fire protons and antiprotons at one another, cancelling each other out. This prevented the portal to the neutron eaters' dimension from opening. It also created the Higgs particle as a side-effect. (AUDIO: Lost Souls)