Search for Leptoquark Bosons in ep Collisions at HERA

Just as in the Standard Model (SM), where colored objects with fractional electrical charge such as quarks exist, one expects to find similar objects in theories beyond the SM. One such example is leptoquark, a hybrid state formed by an electron and a quark, predicted by Grand Unifying theories which unify all forces of nature into one single theory and treat electrons and quarks as two members of the same family of particles.

If a leptoquark exists, it can naturally be formed and detected at HERA, a unique machine in which an electron beam collides with quarks obtained from a beam of protons. Leptoquarks are expected to decay quasi-immediately into an electron or a neutrino and a quark giving similar final states as those produced in the SM processes of Neutral Current (NC) and Charged Current (CC) interactions. On an event by event base, a leptoquark decay can not thus be differentiated from a NC or CC event. On the other hand, some final state distributions are expected to be different from the SM processes in particular if the mass of leptoquarks is smaller than the HERA kinematic limit of about 300 GeV, one expects to see a resonance peak in the invariant mass distribution calculated from the decay final state.

Search for leptoquarks have been carried out previously by many experiments including H1. In the current search, the largest data sample collected by the H1 experiment in the first phase of the HERA operation has been analyzed together with all previous H1 data. The analysis is performed using a new statistical method maximizing the search sensitivity. No evidence has been found for leptoquark production of all 14 leptoquark types considered. For a coupling strength as large as the electromagnetic force, leptoqurks with masses below 275-325 GeV are excluded at 95% confidence level. These limits improve and supercede earlier H1 limits based on subsamples of the data used here. The limits can be compared with those from other experiments. One example is shown here for a scalar leptoquark comparing the excluded region from this analysis (shaded area) with that from D0, OPAL and an H1 contact interaction (H1 CI) analysis.

Last Update 22.12.2005