A Search for Leptoquark Bosons and Lepton Flavor Violation in Positron-Proton Collisions at HERA

TL;DR

This paper reports a search for leptoquark bosons and lepton flavor violation in positron-proton collisions at HERA, setting limits on their masses and couplings based on observed data and deviations from the Standard Model.

Contribution

It provides new constraints on leptoquark masses and couplings, especially for first-generation scalar and vector LQs, and explores lepton flavor violation involving mu and tau leptons.

Findings

Scalar LQ masses up to 275 GeV excluded for lambda=0.3

Sensitivity to couplings up to lambda ≤ 1 for masses up to 400 GeV

No evidence of lepton flavor violation observed in mu or tau channels

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 37pb^-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 \gsim 15000GeV^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 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 \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^+ 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.