Top quark physics at colliders

1.通过测量过程e^e^-→tT^^-阈值附近的截面，可以精确地确定顶夸克质量。我们发现，用&lt；MS&gt；^^-质量代替极点质量作为顶夸克质量的参数，可以减小理论上的测量误差。我们用高阶微扰QCD计算研究了底原子谱，发现该理论很好地再现了数据，这与微扰理论不适用的看法相反。我们在质量区研究顶夸克的衰变，在质量区运动学上不允许顶夸克的直接衰变。通过求解重整化群方程，计算了超引力模型中通过气味变化的中性流对魅力夸克的衰变。我们证明了当参数tanβ较大时，这种模可以占主导地位。我们进一步研究了这种模式与带电电流相竞争的情况，并提出了一种在强子对撞机4上探测顶夸克对产生事件的方法。在微扰理论中，混合矩阵的定义在高阶是非平凡的。我们研究了混合矩阵的规范依赖关系，并在几个例子中显式地计算了修正。众所周知，Higgs玻色子衰变为夸克和夸克的QCD修正是相当大的。我们发现，巧妙地选择最低阶参数可以减小修正的大小，从而导致更可靠的理论预测。

1. Precise determination of the top quark mass may be made by measuring the cross section of the process e^+e^- → tt^^- near threshold. We find that theoretical error in the determination can be made small by using the <MS>^^^- mass instead of the pole mass as the parameter representing the top quark mass.2. We examine the bottomonium spectrum using higher order perturbative QCD calculation and find that the theory reproduces the data quite well, in contrary to the belief that perturbation theory is not applicable.3. We study the decay of the top squark in the mass region where the direct decay to the top quark is not kinematically allowed. We calculated the decay to a charm squark a neutralino via flavor-changing neutral current in the supergravity model by solving renormalization group equations. We show that this mode can be dominant when the parameter tan β is large. We further examine the case this mode competes with the charged current decay to a bottom squark and a chargino and propose a method to detect the top squark pair production events at hadron colliders4. Definition of the mixing matrix is nontrivial at higher orders in perturbation theory. We examine the gauge dependence of the mixing matrix and explicitly calculated the correction in several examples.5. It is known that the QCD correction for the decay of a Higgs boson to quarks and squarks is rather large. We find that a clever choice of the lowest-order parameter can reduce the size of the correction, leading to more reliable theoretical predictions.