Multiple Parton Scattering In Nuclear Medium

In theoretical physics, Quantum Chromodynamics (QCD) is a quantum field theory of the strong interaction based on non-abelian gauge field theory. There are two peculiar properties in QCD, one is confinement, and the other is asymptotic freedom. Color confinement means that force between quarks does not diminish as they are separated. Because of this, quarks are forever bound into hadrons, which leads to the consistent failure of free quark search. Asymptotic freedom means that in high energies reactions, quarks and gluons interact weakly, which leads to the success of QCD predictions. In this thesis, I will first give a historical introduction of high energy physics and discuss the expected features of Quark Gluon Plasma (QGP) formed in heavy-ion collisions. In Chapter2, I will try to give a brief review of the history and some basic concepts and properties of QCD.According to asymptotic freedom, one can use perturbation theory to calculate the relevant interactions. However, perturbative QCD can not be applied to describe the long distance be-havior. Unfortunately, most of the processes contains both the short distance and long distance interactions. Fortunately, QCD factorization tell us that one can separate the cross section in-to two parts:the process dependent perturbative QCD calculable short-distance hard parton cross section, and the universal long distance soft functions. The universal long distance soft functions include the parton distribution functions, fragmentation functions, multiple-parton high-twist correlation functions and so on. All these functions can be obtained by experimen-tal measurements, and once they are measured with particular regularization scheme, one can make predictions on some other reactions. In chapter3, we discuss the applications of QCD factorization in processes of electron-positron annihilation to hadrons, single hadron production in lepton-hadron deeply inelastic scattering and Drell-Yan dilepton production in proton-proton collisions, respectively. We obtained the process dependent short distance finite terms due to hard parton interactions, and we also derived the regularization scheme dependent, but pro-cess independent long distance functions, i.e., parton distribution function and fragmentation function, both of these functions satisfy particular QCD evolution equation.Within the framework of generalized collinear factorization in perturbative QCD (pQCD), in chapter4, we study the effect of initial gluon-quark double scattering and induced parton energy loss in Drell-Yan (DY) process in proton-nucleus collisions. We express the contribution from multiple parton scattering and induced gluon radiation to the DY dilepton spectra in terms of nuclear modified effective beam quark distribution functions. The modification depends on the quark transport parameter in nuclear medium. This is similar to the final-state multiple parton scattering in deeply inelastic scattering (DIS) off large nuclei and leads to the suppression of the Drell-Yan cross section in p+A relative to p+p collisions. With the value of quark transport parameter determined from the nuclear modification of single-inclusive DIS hadron spectra as measured by the HERMES experiment, we calculate DY spectra in p+A collisions and find the nuclear suppression due to beam parton energy loss negligible at the Fermilab energy Elad=800GeV in the kinematic region as covered by the E866experiment. Most of the observed nuclear suppression of DY spectra in E866experiment can be described well by parton shadowing in target nuclei as given by the EPS08parameterization. The effect of beam parton energy loss, however, becomes significant for DY lepton pairs with large beam parton momentum fraction x'or small target parton momentum fraction x. We also predict the DY cross section in p+A collisions at lower beam proton energy Elab=120GeV and show significant suppression due to initial state parton energy loss at moderately large x'where the effect of parton shadowing is very small.In chapter5, we first review the applicability of generalized QCD factorization theorem to multiple scattering Drell-Yan processe in p+A collisions, we then derive the transverse momen-tum broadening of the single jet in DIS and dilepton in Drell-Yan processes by considering initial-and final-state multiple scattering, respectively. Furthermore, we calculate in perturbative QCD the transverse momentum imbalance of dijet and dihadron production in high energy p+A (d+A) collisions. We evaluate the effect of both initial-and final-state multiple scattering at the mean time, which determines the strength of this transverse momentum imbalance. We show that, at leading order, single jet broadening in DIS, dilepton transverse momentum broadening in Drell-Yan and dihadron transverse momentum imbalance in p+A collisions share the same four-parton correlation functions, which means that any measurement of these transverse momentum broad-ening can provide direct information of the multiple-parton correlation functions inside a nucleus. Combining the dihadron transverse momentum imbalance result with the suppression of the cross section in d+Au collisions, which arises from cold nuclear matter energy loss and coherent power corrections, we are able to describe the dihadron correlations measured by both PEHNIX and STAR collaborations at RHIC, including mid-mid, mid-forward, and forward-forward rapidity hadron pairs.Within the generalized collinear factorization approach, we also evaluated the nuclear broad-ening of transverse momentum imbalance in perturbative QCD for photon-jet and photon-hadron pair in p+A collisions, as well as that for dijet, dihadron and heavy-meson pair in DIS in Chapter 6. We factorize the contribution to transverse momentum imbalance into the calculable short-distance partonic hard part multiplied by universal parton-parton correlation functions (twist-4matrix elements). We explicitly show that the hard partonic rescattering can be related to that of corresponding single scattering, except for the additional color factors which come from the initial-state and final-state reseatterings. We verify that additional color factor CF and CA are caused by the effective quark and gluon rescattering. For the photon-jet transverse momentum imbalance in p+A collisions, we evaluated it by including both the initial-state and final-state partonic rescattering which should be larger than that of purely initial-state or final-state mul-tiple scattering. However, since the final photon do not have strong interactions with the soft partons in the nuclear medium, the transverse momentum imbalance of photon-jet pair caused by multiple scattering is smaller that of dijet. We also demonstrated the role of final-state rescat-tering by calculating the transverse momentum imbalance of dijet and heavy-meson pair in DIS, it is shown that the imbalance of heavy-meson pair is lager that of dijet because of different color factors, which arc caused by different partonic snbproccsscs in these two cases. For heavy-meson pair production, we only considered the charm quark fragmentation into D-meson, thus only gluon-channel (the initial parton from target is gluon) presence in the partonic subprocess, which gives rise to a color factor CA, lager than that of dijet production, combination of Cp and CA.At last,I will give some discussions and outlook on our research in chapter7.