Possible sneutrino - pair signatures with R - parity breaking

If sneutrinos are the lightest supersymmetry partners and R-parity is not conserved , the process e + e − → ˜ ¯ ν ˜ ν can have striking signatures due to the decay modes ˜ ν → ℓ + ℓ ′− or˜ν → q ¯ q ′. We present cross section formulas and discuss event rates and detection at the upgraded e + e − collider LEP. Four-lepton signals should be detectable up to sneutrino mass˜m ν = 80 GeV and maybe beyond; four-jet signals should be detectable up tõ m ν = 70 GeV, but would probably be obscured thereafter by W W background. Searches for supersymmetry (SUSY) particles depend considerably on the identity of the lightest SUSY partner (LSP), usually believed to be a neutralino[1]. Stable sneutrinos ˜ ν are strongly disfavored as LSP candidates, by a combination of constraints from Z decay [2] that nowadays give mass˜m ν > ∼ M Z /2 [3] and galactic dark-matter searches that together exclude the range 4 GeV < ˜ m ν < 1 TeV [4,5]. But in the presence of R-parity violation (RPV)[6,7,8,9,10] the LSP can be unstable and sneutrinos are credible candidates once more; indeed the literature contains examples of SUSY-GUT parameter sets that lead to sneutrinos as LSP, either without[11] or with[12] RPV effects in the evolution equations. In the present paper we discuss and calculate the SUSY signals that will arise at an e + e − collider from the SUSY threshold process e + e − → ˜ ¯ ν α ˜ ν α (α = e, µ, τ), (1) if approximately degenerate sneutrinos˜ν α are the LSP and decay by RPV processes. We ignore single-SUSY-particle production[7,10], that can in principle take place with a lower threshold via RPV interactions, because the latter are either known[7,8] or suspected to have much much smaller couplings than the gauge couplings which control Eq.(1). Sneutrino pair production proceeds via t-channel exchange of charginos χ