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 Buchmuller–Ruckl–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.