R-Parity Violation at LEP2 : Virtual Effects

We investigate the ability of LEP2 to detect possible R-parity violation, especially for the case where direct production cross-sections are too small for all superparticles. We demonstrate that for coupling strengths allowed by present experiments, sfermion-exchange diagrams can contribute significantly to the ee → f f̄ process. This would be a useful tool in further constraining the parameter space. Similar arguments hold for leptoquarks and dileptons as well. CERN-TH/95–250 October 1995 ∗[email protected] Supersymmetry is perhaps the most popular and promising theoretical concept going beyond the Standard Model (SM). The Minimal Supersymmetric Standard Model (MSSM) [1] is obtained from the SM by the naive supersymmetrization of both the particle content and the couplings in the latter. However, the most general Lagrangian consistent with supersymmetry as well as with the SU(3)c⊗SU(2)L⊗U(1)Y gauge symmetry contains terms that have no analogue within the SM. This comes about as one of the Higgs supermultiplets has the same quantum numbers as the doublet lepton superfields and thus may be replaced by the latter in the Lagrangian. A similar effect may also be spontaneously generated if any of the sneutrino fields develops a vacuum expectation value. Furthermore, trilinear terms involving the singlet quark superfields are also allowed. The additional pieces in the superpotential may thus be parametrized as [2] W6R = λijkLiLjE k + λ′ijkLiQjDc k + λ ijkU c i D jDc k , (1) where E i , U c i , D c i are the singlet superfields and Li and Qi are the SU(2)-doublet lepton and quark superfields. The coefficients λ ijk are antisymmetric under the interchange of the last two indices, while λijk are antisymmetric under the interchange of the first two. Terms as in eq.(1) obviously have striking phenomenological consequences. For example, the λijk and λ ′ ijk couplings violate lepton number, while the λ ′′ ijk violate baryon number. Simultaneous presence of both may therefore lead to catastrophically high rates for proton decay. In a similar vein, pairs of such couplings may induce tree-level flavour-changing neutral currents. Such considerations led to the introduction of a discrete symmetry known as “R-parity”. Expressible in terms of baryon number (B), lepton number (L) and the intrinsic spin (S), R ≡ (−1) has a value of +1 for all SM particles and −1 for all their superpartners. Apart from ruling out each of the terms in eq.(1), an exact R-parity has the additional consequence of rendering the lightest superpartner (LSP) stable. While this discrete symmetry may be phenomenologically desirable, there is no clear theoretical motivation for it to exist. Even phenomenologically, an exact R-parity is an overkill. The constraints from proton-decay may be evaded by assuming that all of the λ ijk