Of Particular Significance

Exciting Day Ahead at LHC

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 12/15/2015

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.

ATLAS’s two-photon data in Dec. 2011.  The data are the black dots; the red line is there to guide the eye and show you that the data is smooth… except for a curious little bump at around 125 GeV.

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.

CMS’s two-photon data in July 2012.  The data are the black dots; the red dotted line is there to guide the eye to a smooth curve, while the red solid line shows that the hypothesis of a new particle of about 125 GeV fits the data very well.

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.

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4 Responses

  1. /The mass that a parent particle would have had, if the two photons had come from a parent particle./

    By spontaneous symmetry breaking, the Higgs itself breaks the standard model ?
    The symmetey is not globally broken.
    Even if the condensed matter physics saves the standard model, the lowest energy level of “rare earth material” which produce blue LED light – resembles the invariant mass of diphoton (black hole like) ?

      1. Hawking’s low frequency radiation are analogous to Goldstone modes, High frequency horizon signal is like a Higgs field signal.
        Bump is between them ?

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