Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........4g&link_type=abstract
Thesis (PH.D.)--MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 1995.Source: Dissertation Abstracts International, Volume: 56-02, Secti
Physics
Scientific paper
Using data recorded by the L3 detector at LEP during 1991, 1992 and 1993, corresponding to 67.9 pb ^{-1} of integrated luminosity at c.m.s. energies around the Z pole ({~ }1.9 million Z decays), a search for supersymmetric particles, in particular the two lightest neutralinos, chi and chi^ {'}, via the reactions e ^+e^- to Z to chi chi^{'} and e ^+e^- to Z to chi ^{'}chi^{' } has been performed. The assumption is made that the chi^{'} decays into a stable and only weakly-interacting (hence invisible) chi plus a fermion pair via chi^{'} to chi + Z*, Z* to f=f. The signature searched for is an electron, muon or hadronic jet pair or monojet recoiling against isolated missing energy and significant missing transverse momentum. No significant excess of signature events over predicted rates from Standard Model background processes has been found in the data sample. Model-independent limits at 95% confidence on the branching ratios of Z into neutralinos of a few 10^{-5} have been set, with maximum values at Br(Z to chichi^{'}) < 5.5 times 10^{ -5} and Br(Z to chi ^{'}chi^{' }) < 5.2 times 10 ^{-5}. These model-independent results have been interpreted in the context of the Minimal Supersymmetric Standard Model (MSSM), currently the most widely-accepted specific model of supersymmetry. The theory has been excluded for triplet points (M, mu, tan beta ) in the MSSM parameter space where either (a) The number of MSSM-predicted Z decays into neutralinos exceeds the experimentally-determined Poisson upper limit at 95% confidence or (b) the MSSM-predicted width from either all Z decays into neutralinos or those into only the invisible final states exceeds the current L3 upper limits on DeltaGammaZ(37.5 GeV) or DeltaGamma_{inv }(15 GeV), respectively. For tan beta >= 4 the MSSM has been totally excluded at LEP I energies. Lower limits at 95% confidence on the neutralino masses m_χ and m_{chi^ {'}} have been established for tan beta >= 2. These are, for tan beta = 2, 4, 8 respectively, (mchi > 20 GeV; m_{chi^{' }} > 44 GeV), (m_χ > 25 GeV; m_{ chi^{'}} > 53 GeV), and (mchi > 26 GeV; m_{chi^ {'}} > 54 GeV). (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.).
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