Recent direct measurement of the Top quark mass and quasi-infrared fixed point

Abstract

We note that the recent direct measurement of the top quark mass at $173.3\pm 5.6(stat)\pm 6.2(syst)$ by D0 collaboration severely constrains the theoretically attractive infra-red fixed point scenario of the top quark Yukawa coupling in supersymmetric GUTs. For one-step unified models the above mentioned measurement bounds the arbitrary but experimentally determinable parameter $\tan \beta$ to the range $1.3\le \tan \beta \le 2.1$. Further crunch on the top quark mass may determine $\tan \beta$ even more accurately within the fixed point scenario. On the other hand an experimental value of $\tan \beta >2.1$ will rule out the fixed point scenario bounding $h_t^2(M_X)/4\pi$ to 0.022 from above.

Comments: 7 pages, Latex with epsf style, 1 figure, captions.sty