Isolated Lepton Events at Hera: Susy R-parity Violation?

Events with an isolated high pT lepton, a hadron jet and missing energy as observed in the H1 experiment at HERA, are potentially associated with R-parity violation in supersymmetric theories. However, stringent kinematic constraints must be fulfilled if the production of supersymmetric particles in R-parity violating scenarios were the correct path for explaining these puzzling events. A reference point R/ is specified for which these constraints are illustrated and implications of the supersymmetric interpretation for new classes of multilepton events are indicated. 1. Several events with an isolated lepton in association with a hadron jet and missing transverse momentum, ep → e/μ + jet + p T (1) have been observed in the H1 experiment at HERA. For events with the jet transverse momentum larger than 25 GeV, the yield is larger, at a level of 3.4σ, than the number expected in the Standard Model [1], in which W production [2], with subsequent leptonic decays, gives rise to these final states. This type of events has been observed in running HERA with positrons but not with electrons. ZEUS, on the other hand, has not reported the observation of excess of such events [3]. A direct comparison of both experiments is difficult however since H1 and ZEUS cover somewhat different phase-space regions [4]. A potential interpretation of the events (1) is based on supersymmetric theories incorporating R-parity violating interactions [for a recent review of R-parity violating supersymmetry see Ref. [5]]. Most of the models assume the formation of fairly light stop particles in ed fusion, via the R-parity violating interaction λ′131L1Q3D c 1 in the superpotential, with stops decaying to a b-jet and a chargino. (i) In Refs. [1, 3] the chargino decays to a W -boson and a neutralino χ̃01, with the W -boson decaying leptonically. The final-state configuration in Eq. (1), could be achieved if the neutralino is assumed to be meta-stable, decaying outside the detector. However, such an interpretation is very unlikely since the lifetime for χ̃01 decays is bounded from above by the χ̃01 → bb̃ → bdν channel mediated by a virtual b̃. The size of the partial width, estimated as Γ(χ̃01) ∼ 3αW (λ ′2 131/4π)m 5 χ̃ 1 /32πm b̃ , is set by the size of the edt̃ coupling responsible for the production process, see Eq.(3), and the b̃ mass (αW denotes the electroweak SU(2)I fine structure constant g 2/4π). Similar consequences can be drawn when the chargino decays to a lepton and a meta-stable sneutrino ν̃τ assumed to be the lightest SUSY particle, even if direct R-parity violating ν̃τ couplings are absent, cf. Ref. [6]. The sneutrino may decay “back” through the channel ν̃τ → τχ̃ + 1 → τbt̃ → τbe d involving virtual intermediate χ̃+1 and t̃ states, with Γ(ν̃τ ) ∼ α 2 W (λ ′2 131/4π)m 7 ν̃τ/256π m χ̃ 1 m t̃ . The estimated decay widths of the two modes, Γ(χ̃01) ∼ 1 eV and Γ(ν̃τ ) ∼ 10 −3 eV, for λ′131 ∼ 5× 10 −2 and SUSY masses O(100) GeV, suggest lifetimes of order ∼ 10 sec and ∼ 10 sec, respectively, much too short for the particles to escape the detector before decay. (ii) A different class of models is based on Rp / violation in both lepton-quark and lepton-lepton Our discussion applies equally well to scenarios in which a charm squark is produced in ed fusion via the λ′121 coupling) and decays to a s-jet and a chargino. However, due to possible strong mixing in the stop sector, the lightest stop can be expected considerably lighter than the charm squark. The models differ in the structure of the subsequent decay chains of the chargino.