Testing for top quark teams

DZero physicists search for signs of top quarks teaming up to form new particles that would indicate exciting new physics.

Top quarks live a short and lonely life in the Standard Model, where they are expected to survive such a brief amount of time that they never become part of something larger. While all other quarks team up in twos and threes to make mesons and baryons, respectively, the top quark keeps to itself. But some models of new physics envision pairs of top quarks teaming up to form new composite particles that may play an important role in the universe.

One theory predicts that a top-antitop bound state replaces the Higgs boson and plays the role of giving other particles mass. This theory also predicts a new particle called a topcolor Z-prime (Z’) boson. If such a Z’ boson exists, it would lead to enhanced top quark pair production at energies near the mass of the Z’. Alternatively, enhanced top quark production could be the first evidence for a Grand Unified Theory. In a Grand Unified Theory, all of the different forces that the Standard Model explains—the strong, weak and electromagnetic forces—would be understood as different low-energy behaviors of a single force that explains everything at very high energies.

A team of physicists at DZero studied a large sample of top quark pairs to search for evidence of enhanced production. For each event, they calculated the rest mass of the top quark pair. This would be the mass of a new particle that decays into top quarks, if such a particle exists. The Standard Model predicts a smooth distribution of this invariant mass. One of the new particles considered in this search would stand out as a bump in that spectrum. While this analysis improved the lower limit on a Z’ boson’s mass beyond the previous study at DZero, a slight excess of events was noted at invariant masses around 950 GeV. This excess is not significantly above the Standard Model expectation when the statistics are considered.

—Mike Cooke

These physicists made major contributions to this analysis.
The calorimeter calibration group provides precision calibration of the DZero calorimeter, which is critical for many measurements performed by DZero like the one featured above.