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The Nuclear Effect of Azimuthal Asmmetry in Drell-Yan

Author: ChenLong
Tutor: LiangZuoTang; GaoJianHua
School: Shandong University
Course: Theoretical Physics
Keywords: Drell-Yan k_T-broadening transverse gauge link Boer-Mulders func-tion k_T width azimuthal asymmetry of leptonic pair nuclear effect/modification
CLC: O572.2
Type: Master's thesis
Year: 2013
Downloads: 4
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The multiple parton scattering suffered by a fast-moving parton when propagat-ing across the intermediate matter such as a nucleus gives rise to two essential conse-quences. One is the loss of energy of the parent parton carried away by the induced gluon bremsstrahlung and the other is just the kT-broadening of jet. These remarkable topics have received extensive and detailed investigation[1-7,56-58]. According to the work of Zuo-tang Liang, Xin-nian Wang, Jian Zhou in2008, the nuclear effects of gange link absorbed into the definition of transverse momentum dependent distribution function (TMD) can lead to kT-broadening of TMD and an elegant formulae repre-senting 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 semi-inclusive deep inelastic scattering by Jian-hua Gao, Zuo-tang Liang, Xin-nian 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 Drell-Yan process.This thesis calculates at first the nuclear modification of the azimuthal asymme-try of leptonic pair in unpolarized pA Drell-Yan process. We find that for a large weakly bounded nucleus, under the "maximal2-gluon correlation approximation"[58], this azimuthal asymmetry of leptonic pair depends on Boer-Mulders function in the large weakly bonded nucleus which can indeed be expressed as a convolution of Boer-Mulders function in a single nucleon and a Gaussian broadening weight-function. In order to give a quantitative estimation as well as an explicit result, we assume a Gaus-sian parameterizations of TMD functions within a nucleon and thus get a definite ex-pression for Boer-Mulders function in the large weakly bonded nucleus. The explicit result shows the nuclear modified Boer-Mulders 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 spe-cific, 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 Boer-Mulders func-tion; 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 Boer-Mulders function, while the suppression factor decreases with increasing of transverse momentum of the leptonic pair if the width of the Boer-Mulders 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 Drell-Yan. Therefore, the nuclear modification of the azimuthal asymmetry and especially its various dependence of the transverse momentum of leptonic pair in Drell-Yan are sensitive experimental probes of the relative width of transverse momentum distribution between the Boer-Mulders 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 Drell-Yan 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 nu-clear effect of Boer-Mulders function. The dependence behavior of nuclear modifi-cation of these azimuthal asymmetries on the transverse momentum of the leptonic pair resemble that in the unpolarized Drell-Yan 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 Drell-Yan are very similar to their counterparts in SIDIS. To be more specific, the nuclear modification behavior of azimuthal asymmetry terms cos2Φ and sin2Φ in Drell-Yan share the same pattern as cos2Φ in SIDIS [33], while the nuclear modification behaviors of sin Φs and sin(2Φ-Φs) closely resemble that of cos Φ in SIDIS[32]. Among the four par-ticular asymmetry terms calculated, the nuclear effect of the azimuthal asymmetry sin(2Φ+Φs) manifests the strongest suppression.

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