Higgs boson faces the perils of predictability

Fulfilling expectations is normally a good thing. But the fact that the newly discovered Higgs boson is behaving exactly as expected is cutting its chances of lighting a path to new physics.

The world rejoiced in July when physicists at the Large Hadron Collider (LHC) at CERN announced a new particle with some, but not all, of the properties predicted for the Higgs. It was the final member of the standard model of particle physics left to be discovered.

Questions quickly followed the confetti. The particle was not observed directly but through the particles it should decay into, not all of which were seen. This raised hopes that the Higgs might have different properties to those prescribed by the standard model. Because the model is known to be lacking ? it offers no explanations for gravity or dark matter ? such deviations might provide clues to these mysterious entities.

Now results from Fermilab's Tevatron collider in Batavia, Illinois, which shut down last year, show the Higgs decaying into one of the missing particle sets, a pair of bottom quarks.

Wiggle room

This boosts the standard model status of the Higgs. "In my view, it's important evidence that the particle which has been seen is the standard model Higgs boson," says Dmitri Denisov of the Tevatron's D0 experiment. "It's not something else."

The result is also interesting because the LHC has so far only glimpsed the Higgs decaying into other bosons ? photons, Ws and Zs. This confirmed that the Higgs gives mass to these "force carrying" particles, but left open the possibility that the particles that make up matter, like quarks, could get their mass some other way. The latest Tevatron result suggests otherwise.

The Tevatron signal is relatively weak: it only has a statistical significance of 3 sigma, short of the 5 sigma that was required for the LHC to claim its discovery of the Higgs. What's more, the Higgs still has not been glimpsed decaying into tau particles, as prescribed by the standard model. Nonetheless, the wiggle room for an exotic Higgs is shrinking fast.

Journal reference: Physical Review Letters, doi.org/h62

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