At CERN, the laboratory that hosts the Large Hadron Collider [LHC]. Four years ago, almost to the day. Fabiola Gianotti, spokesperson for the ATLAS experiment, delivered the first talk in a presentation on 2011 LHC data. Speaking to the assembled scientists and dignitaries, she presented the message that energized the physics community: a little bump had shown up on a plot. On the plot were the numbers of proton-proton collisions that had produced two energetic photons (particles of light), shown versus the “invariant mass” of the two photons (the mass that a parent particle would have had, if the two photons had come from a parent particle — although the vast majority of them do not.) Most two-photon production processes are pretty random, and produce data that forms a smooth curve on such a plot. But a new particle decaying to two photons would, if it existed, be observed as a little upward wiggle on that curve. And that’s what ATLAS’s data showed.
But those were early days. Statistical flukes happen more often than most people realize, and the bump wasn’t large enough to be convincing. Still, supporting the case, CMS (represented by Guido Tonelli) showed some hints too around the same mass, not just in two photon events but in some other plots. In retrospect, some of the CMS and ATLAS hints were real, though there was no way to be certain for the next six months.
Then on July 4th, 2012, Joe Incandela of CMS, followed by Gianotti, delivered the coup de grace. The data they showed confirmed a new type of particle, with behavior somewhat resembling a Higgs particle.
Within a few more months, it had become clear that the new particle was indeed a Higgs particle, resembling the one predicted in the Standard Model. (The Standard Model is the set of equations used to predict the behavior of the known particles, with the inclusion of the simplest possible type of Higgs particle. We are calling the new Higgs particle “Standard Model-like” because we have a lot of work left to do before we can be confident that it really is the one that is present in the Standard Model.)
Fast forward to today. Gianotti, as of January 1, will become Director General of CERN. It is time now for a new generation of leading scientists to give the presentations on LHC’s 2015 data. 2011 was the start of a thrilling, exciting, maddening time as we waited through half of 2012 before the existence of the new particle became certain. Will we be so lucky today? Rumors have been rife. Stay tuned.