$R_b$ in supersymmetric models

Details $R_b$ in supersymmetric models $R_b$ in supersymmetric models

1995-06-01

arxiv.org/abs/hep-ph/9506217v1

Phys.Rev. D52 (1995) 4116-4124

Physics

High Energy Physics

High Energy Physics - Phenomenology

20 pages (LaTeX), 10 figures (included)

Scientific paper

10.1103/PhysRevD.52.4116

We compute the supersymmetric contribution to $R_{b}\equiv \Gamma (Z\to b{\bar b})/\Gamma (Z\to {\rm hadrons})$ in a variety of supersymmetric models. In the context of supergravity models with universal soft-supersymmetry-breaking and radiative electroweak breaking we find $R^{\rm susy}_b\lsim0.0004$, which does not shift significantly the Standard Model prediction ($R^{\rm SM}_b=0.2157$ for $m_t=175\GeV$; $R^{\rm exp}_b=0.2204\pm0.0020$). We also compute $R_b$ in the minimal supersymmetric standard model (MSSM), and delineate the region of parameter space which yields interestingly large values of $R_b$. This region entails light charginos and top-squarks, but is {\em strongly} restricted by the {\em combined} constraints from $B(b\to s\gamma)$ and a not-too-large invisible top-quark branching ratio: only a few percent of the points with $R^{\rm susy}_b>0.0020\,(1\sigma)$ are allowed.

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