Search for the Standard Model Higgs Boson at CDF

The Higgs boson is an artifact of the theory which explains the mass of fundamental particles.

The standard model of particle physics describes the interactions between elementary particles. In the most basic form of the model, the fundamental force carriers are treated as massless particles. One important part of the theory is the Higgs mechanism, by which the symmetry of massless particles is broken and non-zero masses become possible. If this theory is true, an extra particle, called the Higgs boson, should exist. The Higgs boson itself is a disturbance in the Higgs field, as explained in this fascinating analogy offered by D.J. Miller . (At this point we really have to point out that most of the mass in the universe, including the mass of your kitchen table and you yourself, comes from the interactions of particles through quantum chromo-dynamics . But here we are talking about the masses of the particles themselves: the fact that the top quark mass is 175 GeV/c2, while the electron mass is 0.5 MeV/c2.) It's the interactions with the Higgs field that relate to the particles' masses.

A Higgs boson can be produced with a W boson in the CDF detector.

Collider Detector at Fermilab In the Tevatron accelerator at Fermilab, protons and antiprotons collide at high energies, and some of those collisions could result in the production of a Higgs boson in association with a W boson. We call this the "WH signature." If the Higgs boson mass is around 115 GeV/c2, it decays most often to a pair of b quarks which form a pair of particle jets in the CDF detector. The interesting thing about this pair of jets from Higgs decay is that their total mass is the mass of the Higgs boson! When the W boson decays to a high-energy lepton and a neutrino, the CDF detector triggers on the lepton, whether it is an electron or a muon.

How to search for the Higgs boson:

It is easiest to look for Higgs bosons decaying to b quarks. To do this, we look for the characteristic decay length of the b quark. If one or more of the jets is "b-tagged," then we add it to our sample of possible Higgs boson events. Unfortunately for us, nearly all of these events comes from sources other than WH production. The big challenge is figuring out, on a statistical basis, which events come from these background sources, and which ones come from true WH production! We have to be especially careful about sources of fake b-tags or W+b-bbar events. This latter class of events looks like Higgs bosons, but the mass of the b jets do not form a Higgs mass peak.

We have not found any evidence for Higgs boson production at CDF.

The number of events we find in our search is about what we would expect even if the Higgs boson were not present. And the mass of the jets does not show the kind of peak we expect from the Higgs boson events. (One challenge of this search is that the uncertainties in the non-Higgs background totals is larger than the Higgs rate itself.)

The contributions from the different types of background sources are also about what we expect. We conclude that the Higgs boson production cross section (times the decay fraction to b-bbar) must be no more than 3-10 pb, depending on the Higgs mass we assume. This means that no more than 1 out of every million million Tevatron collisions produces a Higgs boson with this decay mode!

Dijet mass spectrum in tagged W+2jet data

With more data, we expect to have a better chance of discovering the Higgs!

This analysis used data corresponding to a total integrated luminosity of 320 pb-1, but we expect to collect between 10 and 20 times that amount by the end of the Tevatron era. This should give us sensitivity comparable to the previous Higgs boson searches at the LEP collider . This is about as well as we expected to do when we turned on the Tevatron for Run II.

We are also searching for Higgs bosons decaying to two W bosons and for Higgs bosons produced in association with a Z boson. And the D0 detector, another experiment at the Tevatron, is performing the same searches. Stay tuned for more results soon!

More information about the most recent results

A copy of the paper "Search for H &rarr b-bbar Produced in Association with W Bosons in p-pbar Collisions at &radic s=1.96 TeV" is available from hep-ex/0512051.

For further information, please mail Yoshio Ishizawa, Jason Nielsen, and Weiming Yao at CDF.

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Updated Dec. 14, 2005