The Standard Model of particle physics has an intriguing symmetry: All known "matter" particles fall into three generations, each consisting of a lepton, its neutrino and a pair of quarks. This symmetry between leptons and quarks motivates the possibility of a new kind of lepton-quark force, mediated through so-called leptoquarks. Leptoquark would have both lepton and quark properties, and decay into a lepton (a charged lepton or a lepton neutrino) and a quark.
At Dě Run II, we have searched for the first generation scalar leptoquark pair production for two cases: when both leptoquarks decay to an electron and a quark, and when one of the leptoquarks decays to an electron and a quark and the other to a neutrino and a quark. We used the data collected from April 2002 to March 2004, with the integrated luminosity of 252 ▒ 16 pb-1.
In the two electrons and two jets channel, the major Standard Model backgrounds are the Z/Drell-Yan production, the multijet events and the top quark pair production. We suppressed the majority of the Z boson production by requiring the invariant mass of the two electrons to be outside of the Z boson mass range. We further required a high value of ST to suppress the background while maintaining a significant part of the leptoquark signal, where ST is the scalar sum of the transverse energies of the two electrons and the two jets. After these requirements, we observed 1 event while 0.54 ▒ 0.11 events are expected from the background.
In the electron, neutrino and jets channel, the main Standard Model backgrounds are the W boson production, the multijet events and the top quark pair production. To suppress background from W boson production, events with a transverse mass of the electron and the missing energy compatible with the W boson mass are rejected. Similarly to the two electrons and two jets channel, a high value of ST is also required, where here ST is the scalar sum of the transverse energies of the electron, the two jets, and the missing transverse energy. After these requirements, 1 event was observed and 3.6 ▒ 1.2 events were expected from background.
The data are consistent with the expected Standard Model background and no evidence for the existence of leptoquarks is observed. Therefore, lower limits on the leptoquark masses were calculated as a function of ▀ (see plot), where ▀ is the branching fraction of a leptoquark decaying to a charged lepton and a quark.