Trying to explain the unexpected

Distribution of events versus reconstructed top-jet (mtj) invariant mass for the observed data and expected backgrounds in the signal region. Three signal hypotheses are shown.

CDF and DZero have both observed an unexpected asymmetry in the production of pairs of top and anti-top quarks in proton-antiproton collisions. More top quarks are produced in the direction that protons travel than in the antiproton direction (and more anti-tops are produced in the antiproton direction than in the proton direction). Particle physicists find this strange because it is a deviation from the expected symmetry. Such behavior is also not predicted by the Standard Model, and thus could be an indication of new physics.

Theorists have tried to explain the anomaly. One possible explanation would be a new, heavy particle that would alter the Standard Model prediction. Unfortunately, these models in general cause known measured production rates (cross sections) to deviate from their measured values.

More recently, theorists have made this new postulated particle an object that is not its own antiparticle. This gets around the conflict with measured processes and also yields a decay process that can be cleanly measured at both the Tevatron and the Large Hadron Collider.

The above-proposed model predicts the new heavy particle, called M, would be produced in association with a top quark. The M particle will then decay to an anti-top quark and a light quark. CDF physicists looked for a top-anti-top quark pair plus an additional light quark. This process is just like Standard Model top-anti-top pair production but with an additional quark in the event. This postulated new physics process, if real, would produce a bump in the mass distribution of the top-anti-top + quark mass (see Figure).

Using CDF’s complete and final data analysis, the data are consistent with our understanding of the backgrounds (top-anti-top pair production, W+ jets, and other backgrounds) and no M particle is found. This allows us to set limits on searches for top quark plus jet resonances. We expect that further studies at the LHC will either extend these limits or perhaps find something new. In the meantime, the search for an explanation for the mysterious asymmetry continues.

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—edited by Andy Beretvas

These physicists were responsible for this analysis. First row from left: Jahred Adelman (Yale), Nate Goldschmidt (University of Florida). Second row: Daniel Whiteson and Kanishka Rao (University of California, Irvine).