LHCb实验利用电弱探针与光诱导过程研究核物质效应的进展

Advancements in the nuclear matter studies using electroweak probes and photon-induced processes at LHCb

本文介绍了LHCb实验中的两项重要研究,旨在以电弱相互作用玻色子为工具探索原子核中的部分子分布函数(nuclear parton distribution functions,n PDFs).第一项研究利用质子-铅核对撞数据测量了Z玻色子的产生,用Z玻色子作为探针研究量子色动力学(quantum chromodynamics,QCD)和nPDFs.通过测量Z玻色子的产生截面,研究人员可以更准确地确定和理解部分子在核物质环境中的分布,从而完善现有的QCD理论并提高nPDFs的精度.第二项研究在铅核-铅核超周边对撞中研究了J/ψ和ψ(2S)介子的光致产生.这种对撞中,两个铅核通过光子和核内胶子的相互作用产生粲夸克偶素.相干光致产生涉及光子与整个原子核的胶子相互作用,能量交换较小,原子核保持完整.该研究验证了现有的微扰QCD和胶子交换模型,并提供了关于核遮蔽效应和核内胶子分布的重要信息.LHCb实验首次测量了粲夸克偶素随横动量变化的相干光致产生截面,并观测到了衍射效应,为未来高能核物理研究提供了宝贵数据.

The LHCb experiment at CERN's Large Hadron Collider has made significant progress in studying nuclear parton distribution functions(nPDFs)through two key investigations:The production of Z bosons in proton-lead collisions and the photoproduction of J/ΨandΨ(2S)mesons in ultra-peripheral lead-lead collisions.The study of Z boson production in proton-lead collisions has provided crucial insights into the behavior of quarks and gluons within nucleons.Z bosons,which are neutral electroweak gauge bosons,serve as excellent probes for studying quantum chromodynamics(QCD)because their production and decay processes are dominated by weak interactions and do not involve strong interactions.By measuring the production cross-section of Z bosons,researchers can accurately determine the distribution of partons(quarks and gluons)within nucleons in a nuclear environment.This measurement helps refine the existing QCD theory and improve the precision of nPDFs.The experimental results showed good agreement with theoretical predictions,particularly in the forward direction,providing valuable constraints on nPDFs and enhancing our understanding of nuclear effects on parton distributions.In the second study,LHCb researchers investigated the coherent photoproduction of J/ΨandΨ(2S)mesons in ultraperipheral lead-lead collisions.These heavy quarkonia,composed of charm-anticharm quark pairs,are produced through photon-induced interactions without direct contact between the nuclei.Coherent photoproduction involves minimal momentum exchange,preserving the integrity of the nuclei and providing a unique opportunity to study gluon distributions within the nucleus.The transverse momentum(pT)of the produced mesons is determined by the size of the nucleus,following the uncertainty principle.The LHCb experiment s forward design allowed for effective capture and analysis of particles traveling in the forward direction,distinguishing signal events from background noise.The results of the photoproduction study revealed differential cross-sections of J/ΨandΨ(2S)mesons as functions of rapidity and transverse momentum,compared with various theoretical models,including vector meson dominance and perturbative QCD with nuclear shadowing effects.A notable observation was the diffraction pattern in the p_T spectrum,a first-time experimental observation in such processes.This observation provides new constraints on theoretical models and improves our understanding of gluon dynamics within nuclei.The significance of these results lies in their contribution to refining QCD and enhancing the precision of nPDFs.The precise measurement of Z boson production in proton-lead collisions offers crucial data for understanding parton distributions in nuclear environments,aiding in the development of more accurate theoretical models.Similarly,the investigation of J/ΨandΨ(2S)meson photoproduction in ultra-peripheral collisions provides new insights into gluon distributions and nuclear shadowing effects,which are essential for comprehending the strong force and the behavior of nuclear matter.In conclusion,these studies at LHCb have advanced our understanding of QCD,nPDFs,and photon-nuclear interactions.The experimental results not only validate existing theoretical models but also highlight their limitations,paving the way for future high-energy nuclear physics research.The findings contribute to a deeper understanding of the fundamental structure of matter,offering a clearer picture of the complex interactions within nuclei and the universe.