Search for 2^nd-Generation Leptoquarks in the 

µj+µj Channel in ppbar Collisions at sqrt{s} = 1.96 TeV

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(LQ₂->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(LQ₂) > 186 GeV was extracted.

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

Di-muon mass M(µµ) of all highly energetic µj+µj events.

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.

Bibliography

[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).