Search for a Fourth Generation Charge -1/3 Quark via Flavor Changing Neutral Current Decay

Nov. 26, 1996

The theory of elementary particles referred to as the "Standard Model" is able to explain all physical phenomena related to all known elementary particles. The Standard Model organizes the known elementary particles into one of the following four classifications: quarks, leptons, gauge bosons, and higgs bosons. The following table summarizes the currently known particles.

Elementary particles of the Standard Model
Quarks u c t
d s b
Leptons nue numu nutau
e mu tau
Gauge bosons photon, g, Z0, W+, W-
Higgs boson H0

The known quarks and leptons fall into three "generations," corresponding to the three columns in the quark and lepton rows of the above table. However, nothing in the standard model itself fixes the number of quarks and leptons that can exist. Since the first three generations are full, any new quarks and leptons would be members of a "fourth generation."

This paper reports on a dedicated search for a fourth generation quark, called the b'. In this paper, we look for the b' quark by looking for its decay into either a b quark and a photon or a b quark and a g boson (gluon). Such decays are called "flavor changing neutral currents," because they involve an electrically neutral gauge boson (photon or gluon).

It is unusual for quarks to decay by emitting neutral gauge bosons. Most of the time, quarks decay into one of the electrically charged gauge bosons, W+ or W-, and another quark. Such decays are called "charged currents," because they involve a charged gauge boson. Several experiments have searched for fourth generation quarks by looking for their charged current decays, and not found any.

In recent years, it was pointed out by particle theorists that a fourth generation b' quark might decay by emitting neutral gauge bosons instead of charged gauge bosons. In that case, a fourth generation quark could exist that had been missed by previous experiments.

This experiment did not find any evidence for neutral current decays of b' quarks. The conclusion of this paper is that b' quarks with masses less than the mass of the Z0 boson are ruled out if they decay by neutral currents.

A copy of the paper, in preprint form, is available from hep-ex-9611021 .

For further information contact Dr. Herbert Greenlee, Fermilab, email:

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Figures from "Search for a Fourth Generation Charge -1/3 Quark via Flavor Changing Neutral Current Decay",
The DØ Collaboration.
Figure 1 (postscript) Figure 2 (postscript)

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