The Higgs boson: “Two b” or not “two b”

The best-fit cross section times branching ratio (σWH + σZH) x Br(H → bb) as a function of Higgs mass. The dark-shaded region shows the 68 percent confidence level interval; the light-shaded region shows the 95 percent confidence level region; and the Standard Model prediction is shown as the smooth, falling curve with a narrow band indicating the theoretical uncertainty.

With the recent announcement of a new particle discovery by the LHC experiments, it seems very likely that the Higgs boson exists. So, we are finally done with the search for the Higgs boson, right? Not quite!

Now the question is: What kind of a Higgs boson is it? Normally, when physicists talk about the Higgs boson, they are talking about the Standard Model Higgs boson, the simplest mechanism that allows for the masses of the particles in the Standard Model, where “simple” means that the mechanism only requires the Higgs boson – no additional particles required.

For the Standard Model Higgs boson, theorists know how to predict the rate at which it would be produced in a collider experiment and the fraction of the number of times it would decay into each species of other particles. In order to understand whether the Higgs boson is a Standard Model Higgs boson or a Higgs boson related to a more exotic model, scientists must measure as many properties of the Higgs boson as possible.

The excess of Higgs-like events reported by the LHC experiments is primarily due to events in which the boson decays into either two photons or two Z bosons. However, the Higgs boson searches conducted by the Tevatron experiments are more sensitive to the case where the Higgs boson decays into two b quarks.

Recently, the CDF experiment submitted four papers to Physical Review Letters describing searches for the Higgs decay in the bb final state. Scientists search for the cases of the Higgs produced in association with a W boson, the Higgs produced in association with a Z boson, the Higgs produced in association with a W or Z and W/Z decays into invisible particles, and finally a combination of all three searches. The combined result is the most sensitive search to date for the Higgs decaying into two b quarks and, while not conclusive, the result shows slightly stronger hints for the Standard Model than expected. That is, the CDF search for the Higgs boson in the bb final state indicates that it might occur more often than predicted in the Standard Model. However the new result is also still consistent with the Standard Model within measurement uncertainties.

This is an exciting time in the search for the Higgs boson, and it will likely take all of the pieces from the Tevatron and the LHC experiments to complete this challenging puzzle.

The CDF collaboration celebrated the Tevatron on September 30, 2011. Photo: Cindy Arnold