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

Abstract.

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 $Q^2$ 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 $Q^2$ spectrum previously observed in the 1994 to 1996 dataset at $Q^2 \gtrsim 15000$ GeV$^2$ remains, though with less significance. This deviation corresponded to a clustering in the invariant mass spectrum at $M \simeq 200$ 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 Buchmüller–Rückl–Wyler effective model. Scalar LQs are excluded for masses up to 275 GeV for a coupling of electromagnetic strength, $\lambda = 0.3$. A sensitivity to coupling values $\lesssim 1$ is established for masses up to 400 GeV 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 $e^+ u$ 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.