The DØ Collaboration has been searching for leptoquarks (LQ) for some time. Final results from the Run Ia (1992-1993 ) data for both first and second generation LQ searches were published in Physical Review Letters. Preliminary results from larger Run I data sets were presented at DPF'96 in Minneapolis. At Moriond '97 in France, DØ's cross section limits on first generation leptoquarks were more than sensitive enough to make it clear that gauge vector leptoquarks (those with spin 1, like a W-boson), could not be the source of the HERA event excess. Vector leptoquarks would be produced by the Tevatron, and the DØ cross section limit required that a gauge vector LQ decaying solely to an electron and a quark would need to be more massive than 298 GeV. For the scalar (spin 0) case, the Moriond limit (for the decay, solely to electron and quark) was 175 GeV using the best theoretical model of the cross section available at the time.
On April 16, a more precise cross section calculation by theorists raised the scalar mass limit from the DØ data to about 190 GeV (in the HERA observation region). An effort was already underway to search in this region. This effort was built on the pre-Moriond analysis, but called in some very powerful tools developed at DØ . In particular, DØ used improved particle identification algorithms, programs to optimize the sensitivity of the search to a particular LQ mass range, and programs to perform a maximum resolution mass fit to candidate events. However, the only new candidates that turned up in the optimized search did not match the lepto quark profile; they had the requisite high mass for the electron-jet pairs, but not the total transverse energy expected of events with two massive objects decaying. They were far more consistent, both in energy profile and in number of events, with the expectations from background samples. So DØ reverted to setting a limit, now improved, from the complete Run I (1992-1996) data set. In the scenario most likely to consistently explain the HERA data, the pure electron-quark decay of scalar leptoquark, the DØ result excludes masses of leptoquarks up to 225 GeV. This analysis was presented at Hadrons-in-Collision in June, 1997. A paper based on this analysis has been submitted to PRL. We then turned these techniques to the e nu j j channel. In addition, we employed a neural net algorithm to help separate the signal from the background. This result was presented at the HEP'97 conference in August, 1997. Combining the results of this analysis with that in the e e j j channel, we find that if the leptoquark decays half the time to a neutrino and quark, the new DØ analyses exclude masses below 204 GeV.
Our searches for second and third generation leptoquarks were also presented at the Hadrons-in-Collision conference. We exclude a second generation leptoquark which decays 100% of the time to mu mu jj below a mass of 184 GeV, and one that decays 50 % of the time to mu mu jj and 50% of the time to mu nu jj up to a mass of 140 GeV. We exclude a third generation leptoquark which decays 100% of the time to b b nu nu below a mass of 98 GeV.Last updated: Aug. 23, 1997
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