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It s Official: We ve Found the Higgs Boson. But Which One?


Potential Higgs to photon decay event as seen by the CMS experiment at the LHC

Higgs to two-photon candidate event as seen by CMS in May 2012

When last we checked in on the hunt for the Higgs, physicists weren’t yet ready to call the deal done. They were only willing to say that they had discovered a new particle—some sort of boson—and that this new boson was “Higgs-like.” Their reticence hinged on the measurement of the new particle’s spin, a fundamental quality that, for bosons, must take an integer value such as 0, 1 or 2. Both in July, when the proto-Higgs was first announced, and in November, when scientists released additional data analysis, they didn’t have enough data to definitively say that the boson had a spin of zero, which a Higgs must have.

That uncertainty has now melted away. This week, physicists gathered in Moriond, Italy announced that additional data from the Large Hadron Collider’s 2012 data run now conclusively show that the new boson has a spin of zero, and is thus a Higgs boson.

The question now becomes: just what kind of Higgs boson is it? And might it have hidden twins?

The Standard Model of particle physics—the extremely successful set of theories that physicists use to understand the universe but which most scientists believe is incomplete—predicts that the Higgs boson should behave in a particular way. Once we know its mass (which we do), the Standard Model precisely determines with what frequency the Higgs should change into other subatomic particles (a Higgs lasts for only a fraction of a second before decaying into other particles; these particles are what scientists measure at the LHC). Because scientists want to deeply probe the Standard Model to see where it fails—thus pointing the way to a new and deeper understanding of physical law—they have been closely monitoring just how the Higgs decays.

Early results indicated that the Higgs-like particle was behaving as the Standard Model predicts—with one intriguing exception. The Higgs seemed to be decaying into two photons about twice as often as it should. Could the discrepancy open a much-anticipated crack in the Standard Model?

Alas, no. New results presented at the Moriond Conference show that the apparent excess in photons was likely a statistical fluke. The Higgs appears to be a boring old Standard Model Higgs boson after all. Physicists hoping to find problems with the Standard Model will have to wait until the data from the next LHC run—which, because of a planned two-year shutdown to upgrade the machine, won’t begin before 2015.

Those who pine for strange Higgs physics now hope for a blockbuster: that the Higgs we’ve discovered isn’t the Higgs boson, but rather only the first of many. According to the decoupling theorem (PDF; see especially slide 15), if the Standard Model is not the whole story of the universe and many different Higgs bosons exist, one of them should look very much like a Standard Model Higgs boson. Therefore, the fact that we’ve seen a Standard Model Higgs boson does not mean that there aren’t additional non-Standard Model Higgs bosons out there waiting to be found.

So chin up, Higgs aficionados. Come 2015, we could be doing the Higgs dance all over again.

Image courtesy CERN/CMS

The views expressed are those of the author and are not necessarily those of Scientific American.

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