2018 Impact factor 4.843
Particles and Fields
Eur. Phys. J. C 11, 447-471
DOI 10.1007/s100529900195

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

The H1 Collaboration

C. Adloff33 - et al.

33 Fachbereich Physik, Bergische Universität Gesamthochschule Wuppertal, Wuppertal, Germany

Received: 2 July 1999 / Published online: 28 September 1999

Dedicated to the memory of our colleague and friend Marc David, deceased 23.01.99.

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 Q2neutral (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 $Q^2 \raisebox{-0.5mm}{$\stackrel{>}{\scriptstyle{\sim}}$ }15000$GeV2 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 275GeV for a coupling of electromagnetic strength, $\lambda = 0.3$. A sensitivity to coupling values $\raisebox{-1.5mm}{$\:\stackrel{\textstyle{<}}{\textstyle{\sim}}\:$ }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 260GeV 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.


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