Zhenjun Xiao

Implications of the top quark mass measurement for the SM Higgs boson mass MH

Motivated by the recent determination of the top quark mass by the CDF Collaboration Mt=174+or-10-12+13 GeV, we study,the implications of the top quark mass measurement for the standard model Higgs boson mass MH based on the phenomenological analysis of the radiative correction Delta r at two-loop order. We find that: (a) the central value of MH is very sensitive to the variation of Mt; (b) the present data are in favour of the heavy Higgs boson, MH=340-270+770 GeV for Mt=174 GeV, but no reliable constraints on MH could be derived until the measurement of Mt reaches high precision.

Study on the rare radiative decay Bc-->Ds* gamma in the standard model and multiscale walking technicolor model.

Applying the perturbative QCD method, we study the decay {ital B}{sub {ital c}}{r_arrow}{ital D}{sub {ital s}}{sup {asterisk}}{gamma} in the standard model (SM) and multiscale walking technicolor model (MWTCM). In the SM, we find that the contribution of weak annihilation is more important than that of the electromagnetic penguin diagram. The presence of pseudo Goldstone bosons in the MWTCM leads to a large enhancement in the rate of {ital B}{sub {ital c}}{r_arrow}{ital D}{sub {ital s}}{sup {asterisk}}{gamma}, but this model is in conflict with the branching ratio of {ital Z}{r_arrow}{ital b{bar b}}({ital R}{sub {ital b}}) and the CLEO data on the branching ratio {ital B}({ital b}{r_arrow}{ital s}{gamma}). If top-color is further introduced, the calculated results in the top-color-assisted MWTCM can be suppressed and be in agreement with the CLEO data for a certain range of parameters. {copyright}{ital 1996 The American Physical Society.}

Does Rb in MSM deviate from the data

In this paper we compare the MSM predictions for Z-boson decay widths and the relevant ratios with new LEP data. We find that, among the parameters studied here, only the measured value of Rb exceeds that predicted in MSM by more than two standard deviations for mt=174+or-16 GeV.

Virtual corrections on the Zbb vertex from charged PGBs and the new electroweak data

In the one-generation technicolour model, the one-loop virtual corrections on the Zbb vertex from the P+or- and P8+or- can be rather large; the shift delta Rb=(Rb-RbSM)/RbSM in the range of 0-9.6% in the considered parameter space. By comparing the vertex corrections with the new electroweak data one can put some primary limits on the mass of the colour octet P8+or-: mp2>335 GeV(at 95% CL) for mt=150 GeV.

Negative Vertex Contributions to Parameters S and T From Charged Pseudo-Goldstone Boson*

We describe how to incorporate the one-loop vertex corrections on coming from the virtual charged pseudo-Goldstone bosons (PGB) and in one-generation technicolor model into the estimation for the value of Peskin–Takeuchi parameters S and T. We find that the vertex contributions SV and TV are all negative and can cancel out the positive oblique contributions Shad and Thad. Now the value of S can be negative and T is also much smaller than the traditional estimation.

The Effect of Virtual Vector Technimesons on the Electroweak Parameter S

We analyze the electroweak parameter S of a composite Higgs model by making use of an effective action approach. The effect of the virtual vector technimesons is calculated. We find that the large contributions to parameter S from vector technimesons can be suppressed substantially in the composite Higgs model.

The Higgs Boson Mass in MSM and the New Data

In this paper we study the implications of the top quark mass measurement at CDF for the Higgs boson mass . We find that the present data are most likely in favor of heavy Higgs boson, for , but no realistic constraints on could be derived until the measurement of reaches high precision.

Two-loop radiative corrections and effects on estimation of Mt

In the framework of the standard model we have calculated the effects on the estimation of the top-quark mass, Mt, caused by the inclusion of the two-loop radiative corrections. The two-loop contributions only made a small increase in Mt of 9-11 GeV, about 7% higher for both its central value and the upper bound than the one-loop estimation.

Upper Bounds on Top Quark Mass, Z Boson Decay Widths and Mass Ratio MW/MZ*

In this paper we made a comprehensive analysis of some latest experimental data—the Z boson decay widths, the mass ratio MW/MZ and effective vector coupling constant gv, within the framework of standard model. We considered the , order QCD corrections in the calculation of Γh and ΓZ. The analysis shows that the mass ratio (Mt/MZ)2 is rather sensitive to the change of the strong coupling constant αs(MZ), and the smaller value αs(MZ) ≤ 0.128 is preferred. The upper bounds on top quark mass Mt are obtained from different analyses. The final averaged result is Mt = 102 ± 37 GeV, or Mt < 186 GeV at 95% C.L. for Higgs boson mass MH in the range of 50 ~ 1000 GeV.