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 $\tan β$ to the range $1.3 \le \tan β\le 2.1$. Further crunch on the top quark mass may determine $\tan β$ even more accurately within the fixed point scenario. On the other hand an experimental value of $\tan β> 2.1$ will rule out the fixed point scenario bounding $h^2_t(M_X)/4 π$ to 0.022 from above.