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The Nuclear Effect of Azimuthal Asmmetry in DrellYan
Author: ChenLong
Tutor: LiangZuoTang; GaoJianHua
School: Shandong University
Course: Theoretical Physics
Keywords: DrellYan k_Tbroadening transverse gauge link BoerMulders function k_T width azimuthal asymmetry of leptonic pair nuclear effect/modification
CLC: O572.2
Type: Master's thesis
Year: 2013
Downloads: 4
Quote: 0
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Abstract
The multiple parton scattering suffered by a fastmoving parton when propagating across the intermediate matter such as a nucleus gives rise to two essential consequences. One is the loss of energy of the parent parton carried away by the induced gluon bremsstrahlung and the other is just the kTbroadening of jet. These remarkable topics have received extensive and detailed investigation[17,5658]. According to the work of Zuotang Liang, Xinnian Wang, Jian Zhou in2008, the nuclear effects of gange link absorbed into the definition of transverse momentum dependent distribution function (TMD) can lead to kTbroadening of TMD and an elegant formulae representing the link between unpolarized parton distribution function f1(x,kT2) in nucleus and that in nucleon is given. Such kind of calculation has already been applied into the study of nuclear effect/modification of azimuthal asymmetries in semiinclusive deep inelastic scattering by Jianhua Gao, Zuotang Liang, Xinnian Wang[32,33]. The main purpose of this thesis is to extend this kind of method to the investigation of the nuclear modification of azimuthal asymmetries of leptonic pair in DrellYan process.This thesis calculates at first the nuclear modification of the azimuthal asymmetry of leptonic pair in unpolarized pA DrellYan process. We find that for a large weakly bounded nucleus, under the "maximal2gluon correlation approximation"[58], this azimuthal asymmetry of leptonic pair depends on BoerMulders function in the large weakly bonded nucleus which can indeed be expressed as a convolution of BoerMulders function in a single nucleon and a Gaussian broadening weightfunction. In order to give a quantitative estimation as well as an explicit result, we assume a Gaussian parameterizations of TMD functions within a nucleon and thus get a definite expression for BoerMulders function in the large weakly bonded nucleus. The explicit result shows the nuclear modified BoerMulders function is more dispersive compared to that of a nucleon. When the transverse momentum is integrated, the overall nuclear modification of the azimuthal asymmetry is always suppressed, while if the transverse momentum of the leptonic pair is kept unintegrated, an interesting dependence of the suppression factor of the azimuthal asymmetry of leptonic pair shows up. To be specific, we found that the suppression factor is independent of the transverse momentum of the leptonic pair if the width of f1(x,kT2) is equal to that of BoerMulders function; the suppression factor increases with increasing of transverse momentum of the leptonic pair if the width of f1(x,kT2) is smaller than that of BoerMulders function, while the suppression factor decreases with increasing of transverse momentum of the leptonic pair if the width of the BoerMulders function is smaller than that of the f1(x,kT2) and noticeably the azimuthal asymmetry could even be enhanced for large enough transverse momentum of the leptonic pair in DrellYan. Therefore, the nuclear modification of the azimuthal asymmetry and especially its various dependence of the transverse momentum of leptonic pair in DrellYan are sensitive experimental probes of the relative width of transverse momentum distribution between the BoerMulders function and the unpolarized parton distribution function f1(x,kT2).Apart from the aforementioned calculations, the nuclear effect/modification of different kinds of azimuthal asymmetry terms in DrellYan with polarized incoming nucleon have also been studied. These azimuthal asymmetries involve extra TMDs f1T⊥, h1T, h1T⊥,h1L⊥, while their nuclear modification still solely depend on the nuclear effect of BoerMulders function. The dependence behavior of nuclear modification of these azimuthal asymmetries on the transverse momentum of the leptonic pair resemble that in the unpolarized DrellYan process but receive different degree of suppression. We also notice that the shapes and features of nuclear modification of these azimuthal asymmetries associated with the polarization in DrellYan are very similar to their counterparts in SIDIS. To be more specific, the nuclear modification behavior of azimuthal asymmetry terms cos2Φ and sin2Φ in DrellYan share the same pattern as cos2Φ in SIDIS [33], while the nuclear modification behaviors of sin Φs and sin(2ΦΦs) closely resemble that of cos Φ in SIDIS[32]. Among the four particular asymmetry terms calculated, the nuclear effect of the azimuthal asymmetry sin(2Φ+Φs) manifests the strongest suppression.

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