Making top quarks one at a time

Since its 1995 discovery in 2-TeV proton–antiproton collisions at Fermilab, the ultramassive top quark (t) has mostly been produced in top–antitop quark pairs via the strong interactions (diagram a, for example), which forbid the production of single top quarks. The standard model of particle theory also predicts single-top production via weak interactions like that in diagram b, with the weak boson W^± replacing the gluon g⁰ in the intermediate state and a bottom quark (b) emerging. But single-top production is much harder to detect than pair production amidst the overwhelming background of more pedestrian processes that can mimic either rare process. That’s because a pair gives the experimenter two chances to see the telltale signals of t decay. So why bother? Yielding a direct measure of the coupling at the tbW vertex, the cross section for single-top production provides a particularly sensitive test of some aspects of the standard model, such as the presumed absence of a fourth generation of quarks beyond the t and b. Furthermore, the pattern-recognition techniques developed and tested in the search for single-top production are crucial to the quest for the Higgs boson. Now the DZero and CDF detector teams at Fermilab have reported robust observations of single-top production with a cross section of about 3 picobarns (3 × 10⁻³⁶ cm²), consistent with the standard-model prediction. That’s almost half as big as the pair-production cross section, which is severely suppressed by the kinematic requirements for making two ultraheavy quarks. But that modest cross-section disparity also reflects the unifying tendency of the weak and strong interactions to approach each other with increasing energy. (V. M. Abazov et al. , http://arxiv.org/abs/0903.0850 ; T. Aaltonen et al. , http://arxiv.org/abs/0903.0885 ; both in press at Phys. Rev. Lett. )