The observed symmetry in the spectrum of elementary particles between leptons and quarks motivates the existence of leptoquarks . Leptoquarks (LQ) are bosons carrying both quark and lepton quantum numbers and fractional electric charge. Although leptoquarks could in principle decay into into any combination of a lepton and a quark, indirect experiemental limits lead to the assumtion that there would be three different generations of leptoquarks, each coupling to only one quark and lepton family and therefore individually conserving the family lepton numbers .
At the TeVatron, leptoquarks would mainly be produced in pairs. In
this analysis, the selection of di-muon + di-jet events is presented,
and about 104 pb-1 of proton-antiproton collisions, collected
during Run IIa with the DØ detector between September 2002 and
June 2003, have been compared to Monte-Carlo Simulations of the expected
Standard Model background as well as simulated scalar leptoquark events.
The data have been shown to be compatible with the expected background,
arising predominantly from Drell-Yan Z/gamma
One event is remaining in the data after the last cut while 1.59 ± 0.47 are expected from the background. Therefore, no excess of the data over the expected background was found, and the 95 % confidence level (C.L.) upper limits on the cross-section of scalar leptoquarks have been calculated, assuming 100 % branching fraction into charged leptons, beta = BF(LQ2->µ j) = 1. These limits have been compared to theoretically calculated cross-sections, and a lower bound of the mass of scalar 2nd-generation leptoquarks of M(LQ2) > 186 GeV was extracted.
A detailed description is available as DØ
note 4190 (restricted access; non-DØ-speakers, please contact
Reconstructed mu+j mass for all di-muon + di-jet events with M(µµ) > 110 GeV. There are two
possibilities to combine the two highest-pT muons with the two highest-ET jets. Only the
combination with the smaller mass difference of the two leptoquark candidates of the event is chosen,
and the reconstructed "leptoquark mass" of this event is the mean of the masses of the two µ+j systems.
Calculated cross-sections for scalar 2nd-generation leptoquarks as compared to the 95 % C.L.
upper limit on the cross-section, assuming 100 % branching fraction into charged leptons. The
LO and NLO cross-sections are calculated using reference  (taking into accound the correct
center-of-mass energy). The error band has been estimated by varying the renormalization and
factorization scale between M(LQ)/2 and 2 M(LQ).
Event diplays of the highest-ST candidate, remaining in the data after
the last cut: M(µµ) = 112 GeV, ST = 372 GeV.
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