A Search for Leptoquark Bosons and Lepton Flavor Violation in Positron-Proton 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 37pb{\textasciicircum}-1. First generation leptoquarks {(LQs)} are searched in very high Q{\textasciicircum}2 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 Q{\textasciicircum}2 spectrum previously observed in the 1994 to 1996 dataset at Q{\textasciicircum}2 {\textbackslash}gsim {15000GeV{\textasciicircum}2} remains, though with less significance. This deviation corresponded to a clustering in the invariant mass spectrum at M {\textbackslash}simeq 200 {GeV} which is not observed with the 1997 dataset alone. The {NC} {DIS} data is used to constrain the Yukawa couplings lambda of first generation scalar and vector {LQs} in the Buchmueller-Rueckl-Wyler effective model. Scalar {LQs} are excluded for masses up to {275GeV} for a coupling of electromagnetic strength, lambda=0.3. A sensitivity to coupling values {\textbackslash}lsim 1 is established for masses up to {400GeV} for any {LQ} type. The {NC} and {CC} {DIS} data are combined to constrain lambda for arbitrary branching ratios of the {LQ} into eq in a generic model. For a decay branching ratio into e{\textasciicircum}+ u pairs as small as 10\%, {LQ} masses up to 260 {GeV} are ruled out for lambda=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 mu or tau. No {mu+X} or {tau+X} event candidate is found that is compatible with {LQ} kinematics. Constraints are set on the Yukawa coupling involving the mu and tau lepton in a yet unexplored mass range.