A search for leptoquark bosons and lepton flavor violation in e⁺p collisions at HERA

A search for new bosons possessing couplings to lepton-quark pairs is performed in the H1 experiment at HERA using 1994 to 1997 data corresponding to an integrated luminosity of 37 pb^-1. First generation leptoquarks (LQs) are searched in very high Q₂ neutral (NC) and charged (CC) current data samples. The measurements are compared to Standard Model (SM) expectations from deep-inelastic scattering (DIS). A deviation in the Q² spectrum previously observed in the 1994 to 1996 dataset at Q²≳15000GeV² remains, though with less significance. This deviation corresponded to a clustering in the invariant mass spectrum at M ≃ 200 GeV which is not observed with the 1997 dataset alone. The NC DIS data is used to constrain the Yukawa couplings λ of first generation scalar and vector LQs in the Buchmüller-Rückl-Wyler effective model. Scalar LQs are excluded for masses up to 275 GeV for a coupling of electromagnetic strength, λ = 0.3. A sensitivity to coupling values ≲ 1 is established for masses up to 400 GeV for any LQ type. The NC and CC DIS data are combined to constrain λ for arbitrary branching ratios of the LQ into eq in a generic model. For a decay branching ratio into e⁺u pairs as small as 10%, LQ masses up to 260 GeV are ruled out for λ = 0.3. LQs possessing couplings to mixed fermion generations, which could lead to signals of lepton flavor violation (LFV), are searched in events with a high transverse momentum μ or τ. No μ, + X or τ + X event candidate is found that is compatible with LQ kinematics. Constraints are set on the Yukawa coupling involving the μ and τ lepton in a yet unexplored mass range.