Searching for Invisible and Almost Invisible Particles at e^+e^- Colliders

Department of Physics, University of Wisconsin–Madison, Madison, Wisconsin, United States
Physical Review Letters (Impact Factor: 7.51). 12/1995; 76(12). DOI: 10.1103/PhysRevLett.76.2002
Source: arXiv


We explore the techniques, cross sections and expected signal significance
for detecting invisible and almost invisible particles at LEP2 and the NLC by
means of a hard photon tag. Examples from supersymmetry include the lightest
chargino and second lightest neutralino when their masses are nearly the same
as that of the lightest neutralino (the LSP), and invisibly decaying
sneutrinos. The importance of particular features of the detectors is
discussed, instrumentation for vetoing a fast e^+ or e^- in the beam hole being
especially crucial.

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    ABSTRACT: The observable consequences of gauge-mediated supersymmetry breaking (GMSB) are reviewed here. Implications of scenarios both with a neutralino and a slepton as the next lightest supersymmetric particle (NLSP) are surveyed in relation to hadronic ande + e - colliders. We also discuss the phenomenological consequences of the NLSP decaying slowly, giving rise to delayed events in the detectors.The importance of distinguishing signals of GMSB models from those of the commonly discussed supergravity-type theories is emphasized.
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    ABSTRACT: We re-examine the phenomenological aspects of a recently proposed string-derived SU(5)$\times$U(1) one-parameter supergravity model, and explore the sensitivity of the model predictions to variations in the strong coupling and the string unification scale. We also perform an analysis of the constraints on the parameter space of the model in light of the recent Tevatron trilepton data and the LEP~1.5 chargino and slepton data. We obtain $m_{\chi^\pm_1}\gsim70\GeV$, which excludes one-third of the parameter space of the model. The remainder of the parameter space should be probed by ongoing analyses of the Tevatron trilepton data and forthcoming LEP~2 runs. Comment: 12 pages, LaTeX, 4 figures (included) Slightly revised version to appear in Phys. Rev. D (May 1 issue); noted that trilepton searches cannot probe the full parameter space of the model because the leptons from neutralino decay become too soft to be detectable
    Physical review D: Particles and fields 01/1996; 53(9). DOI:10.1103/PhysRevD.53.5253 · 4.86 Impact Factor
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    ABSTRACT: Currently popular search strategies for supersymmetric particles may be significantly affected due to relatively light sneutrinos which decay dominantly into invisible channels. In certain cases the second lightest neutralino may also decay invisibly leading to two extra carriers of missing energy (in addition to the lightest supersymmetric particle (LSP) ) — the virtual LSPs (VLSPs). A tree lavel calculation shows that if the sneutrino mass happens to be in the small but experimentally allowed range (m ≈ν ≈ 45–55 GeV), these particles together with neutralino pairs may contribute significantly to the missing energy in the process e+e− → γ+ E at LEP-2 energies as an enhancement over the Standard Model or the conventional MSSM predictions. It is further shown that a much larger region of the parameter space can be scanned at a high luminosity e+e− collider at 500 GeV like the proposed NLC machine. Moreover, at both LEP-2 and NLC this process may play a complementary role to direct chargino searches, which may fail due to a near mass degeneracy of the chargino and the sneutrino. Formulae for the cross sections taking into account full mixings of the charginos and the neutralinos are derived. The signal remains observable even in the context of more restricted models based onN=1 SUGRA with common scalar and gaugino masses. A preliminary study of the QED radiative corrections due to soft multiple photon emission as well as hard collinear bremsstrahlung indicates that these corrections play a crucial role in estimating the background.
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