Physics – High Energy Physics – High Energy Physics - Phenomenology
Scientific paper
1994-02-04
Phys.Lett.B327:279-286,1994
Physics
High Energy Physics
High Energy Physics - Phenomenology
latex, 10 pages, 4 figures (included), CERN-TH.7138/94, CTP-TAMU-05/94
Scientific paper
10.1016/0370-2693(94)90729-3
We show that the currently experimentally preferred values of the top-quark mass (\ie, $130\lsim m_t\lsim180\GeV$) are naturally understood in the context of string models, where the top-quark Yukawa coupling at the string scale is generically given by $\lambda_t={\cal O}(g)$, with $g$ the unified gauge coupling. A detailed study of the Yukawa sector of $SU(5)\times U(1)$ supergravity shows that the ratio of the bottom-quark to tau-lepton Yukawa couplings at the string scale is required to be in the range $0.7\lsim\lambda_b/\lambda_\tau\lsim1$, depending on the values of $m_t$ and $m_b$. This result is consistent with $SU(5)\times U(1)$ symmetry, which does {\em not} require the equality of these Yukawa couplings in the unbroken symmetry phase of the theory. As a means of possibly predicting the value of $m_t$, we propose a procedure whereby the size of the allowed parameter space is determined as a function of $m_t$, since all sparticle and Higgs-boson masses and couplings depend non-trivially on $m_t$. At present, no significant preference for particular values of $m_t$ in $SU(5)\times U(1)$ supergravity is observed, except that high-precision LEP data requires $m_t\lsim180\GeV$.
Lopez Javier
Nanopoulos Dimitri
Zichichi Antonino
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