лp碰撞Drell-Yan过程的色偶极模型

【作者】 赵洪明；

【导师】 段春贵；

【作者基本信息】 河北师范大学 ， 理论物理， 2002， 硕士

【摘要】 在靶静止系中，用色偶极模型研究了π-p碰撞的Drell-Yan（DY）过程。在靶静止系，轻子对产生的Drell-Yan过程应看作是韧致辐射，而不是部分子湮灭。入射强子中的一个夸克（或反夸克）在靶的作用下辐射出一个虚光子，虚光子再衰变成正反轻子对。该模型的优点在于它是在靶静止系下研究强子碰撞过程，不仅便于实验比较，而且对于物理机制的理解也更为直观。 色偶极模型的两个关键在于光锥波函数和偶极截面。光锥波函数已由微扰论得到了很好的解决。γ^*q光锥（LC）波函数在π-p碰撞过程中起了权重的作用：无论是横向极化光子还是纵向极化光子，由于LC波函数的作用，在γ^*q横向距离p很小时对截面贡献是主要的。但是，垂直极化部分中大ρ的贡献也不可忽略。另外，轻子对不变质量M越大，对应权函数极大值的ρ越小。并且，纵向极化部分是高扭度的，比横向部分约小一个量级。 而偶极截面目前还没有完全确定，只能按软的和硬的能量行为进行参数化。偶极截面只有在小距离ρ时才能表示成胶子密度的函数，在整个范围内求解偶极截面，需要解非常复杂的演化方程，目前是不能做到的。文献中已给出了几种参数化的形式，虽然没有详细讨论胶子的QCD演化，但是能在整个动力学范围内描述σ_q（?）（x,ρ）。本文采用的是Golec-Biernat和Wüsthoff提出的饱和模型，它只需三个参数来确定偶极截面。 本文在RHIC(s^1/2=200GeV)和LHC(s^1/2=5.5TeV)能量下计算了π-p碰撞Drell-Yan过程的微分截面。并且在同样能量下，给出了DY轻子对的横动量分布。更多还原

【Abstract】 We study Drell-Yan(DY) dilepton production in -proton collisions within the light-cone(LC) color dipole picture, which is described in the target rest frame. It was pointed out that in the target rest frame, DY dilepton production looks like bremsstrahlung, rather than parton annihilation. A quark (or an antiquark) from the projectile hadron radiates a virtual photon on impact on the target, the photon subsequently decays into a lepton pair. The virtual of this model is that the high energy collision is studied in the target rest frame, so it is not only convenient to compare with the experiment data, but also easy to understand the physic mechanic.The basic blocks in the color dipole method are the LC wavefunction and the dipole cross section. The LC wavefunction is calculated from perturbation theory. We have shown that the LC wavefunction for the y’q configuration plays the role of a weight for the different transverse separation p contribution to the process. Namely, small transverse separations are selected by both the transverse and longitudinal pieces. However, the transverse one can select non-negligible large sizes p. In addition, larger invariant mass M scans smaller y’q separations. Moreover, the longitudinal piece is higher twist (suppressed by a factorI/A/2).The dipole cross section is largely unknown, only at small distance pit can be expressed in terms of gluon density. However, several parameterizations exist in the literature, describing the whole function r(x,p), without explicitly taking into account the QCD evolution ofthe density. The saturation model provided by Golec-Biernat and Wiisthoff is used here with very economical parameterization.Estimates for n-p Drell-Yan differential cross section atRHIC(= 200Ge )and LHC( = 5.57e )are presented. We also study the transverse momentum distribution of DY pairs for n - p collision atthe energies of RHIC and LHC.更多还原