The observed symmetry in the spectrum of elementary particles between leptons and quarks motivates the existence of leptoquarks [1]. 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 [2].

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
production.

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(LQ_{2}->µ
j) = 1. These limits have been compared to theoretically
calculated cross-sections, and a lower bound of the mass of scalar 2^{nd}-generation
leptoquarks of M(LQ_{2}) > 186 GeV was extracted.

A detailed description is available as DØ
note 4190 (restricted access; non-DØ-speakers, please contact
the DØ
NP Convenors).

Scalar sum of the transverse energies of the µj+µj system for all

di-muon + di-jet events with M(µµ)> 110 GeV.

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-p_{T}
muons with the two highest-E_{T} 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 2^{nd}-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 [3] (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-S_{T} candidate, remaining
in the data after

the last cut: M(µµ) =
112 GeV, S_{T} =
372 GeV.

[1] J.C Pati and A. Salam, Phys. Rev. D 10, 275 (1974);

E. Eichten et al., Phys. Rev. D 34, 1547 (1986);

W. Buchmüller and D. Wyler, Phys. Lett. B
177, 377 (1986);

E. Eichten et al., Phys. Rev. Lett. 50, 811 (1983);

H. Georgi and S. Glashow, Phys. Rev. Lett. 32, 438
(1974).

[2] M. Leurer, Phy. Rev. D 49, 333 (1994).

[3] M. Krämer, T. Plehn M. Spira and P.M. Zerwas, Phys. Rev.
Lett. 79, 341-344 (1997).