Which Higgs?

How many electroweak symmetry-breaking doublets are in that hand? Photo: Leo Bellantoni

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With all the discussion about “the Higgs,” it is worth remembering that what Peter Higgs (and Robert Brout, François Englert, Gerald Guralnik, Carl Hagen and Tom Kibble) gave us was not at first a particle. Originally, it was a trick.

Specifically, it was a mathematical trick to solve a particular physics problem — the problem of how to retain a lovely property called gauge invariance and still allow massive particles in the theory. The trick is to add mathematical expressions to the theory that have what are called doublets.

Now, that trick can be played more than one way. If the trick is played in the simplest way, by adding one doublet, the existence of one and only one new particle is predicted; that is the particle that we usually call “the Higgs.” But there is no particular reason to believe that the simplest way is how nature is playing with us. The next simplest play has two of these doublets and predicts both three new neutral particles and a pair of charged particles. There are many other ways in which nature might be playing its cards.

One way to figure out what is in this hand of cards is to measure the spin and parity of the Higgs that has been found. The spin of a particle is its intrinsic angular momentum; the parity has to do with how the particle’s interactions will appear if they are viewed in a mirror.

If there is only one doublet and there is only one Higgs boson, then the spin must be zero and the parity must be even. If there are two doublets and if we have found one of those three neutral particles, then the particle we have found must have a spin of zero but it might not have even parity. It could have an odd parity or be a mixture of even and odd parity. Because the found Higgs decays into a pair of photons, it can not have a spin of one; but a spin of two and a positive parity is possible in some theories with extra dimensions.

Although the spin and parity are properties of the particle itself and do not depend on what the particle decays into, it is valuable to check that one obtains the same result regardless of what the particle decays into. For this reason, DZero has leveraged the comparative advantage of the Tevatron to set constraints on the spin and parity of the Higgs that has been found in the case where it decays into a pair of bottom quarks.

The DZero result suggests that nature is playing the simplest hand — the single-doublet scenario. Comparing odd to even parities for the spin zero case, the odd parity hypothesis is disfavored with 97.6 percent confidence. Comparing spin zero to spin two for the even parity case, the spin two hypothesis is disfavored with 99.0 percent confidence. However, this does not quite prove that there is only one doublet. It is possible to get the same result with two doublets. We shall have to see a few more cards before we know exactly what is in that hand!

Leo Bellantoni

These DZero members all made significant contributions to this result.
The DZero collaboration thanks these members of the collaboration, who have ensured the highest quality of scientific output from DZero by serving as physics group conveners until this summer: Ashish Kumar (State University of New York, Buffalo), Yvonne Peters (University of Manchester, England), Elizaveta Shabalina (Georg-August Universität, Göttingen, Germany), Mark Williams (Indiana University), Hang Yin (Fermilab).