LHCb实验上重夸克偶素产生机制研究进展

Progress on heavy quarkonium production mechanisms at LHCb

量子色动力学(quantum chromodynamics,QCD)非微扰机制是尚未完全理解的基本问题,重夸克偶素的产生过程为认识QCD微扰以及非微扰性质提供了理想的途径.理论家提出了色单态模型、色蒸发模型以及非相对论性量子色动力学等方法来计算重夸克偶素的产生截面等特征.自欧洲核子研究中心大型强子对撞机运行以来,LHCb实验对重夸克偶素产生机制进行了广泛的研究,测量了J/、(2S)和(nS)等介子的产生截面和极化,并测量了J/和J/介子、J/和(2S)介子,以及J/和(nS)介子的关联产生截面与有效截面.这些研究结果为深入理解重夸克偶素的产生机制、甄别不同的理论模型以及约束理论模型的参数空间提供了重要的实验信息,大大改进了我们对重夸克偶素产生过程中微扰和非微扰性质的认识.本文将介绍LHCb实验在重夸克偶素产生机制方面的相关研究和未来展望.

The strong interaction is one of the fundamental forces of nature.The 20th century witnessed the development of our knowledge of the strong interaction,from nuclear force to quantum chromodynamics(QCD),from the nucleon structure of nuclei to the partonic structure of various hadrons.The evolution of the coupling constant of QCD with energy scale makes the strong force behave very differently at high-and low-energies.At the large momentum transfer,corresponding to high energy domain,the strong interaction becomes so weak that physics processes can be calculated perturbatively in power expansions of the coupling constant.Oppositely,at low energies(<1 GeV),it falls into the nonperturbative regime,which hinders our understanding of the strong interaction through first-principle QCD calculations.Experimentally,quarks and gluons are confined inside color neutral hadrons,namely color confinement.The confinement mechanism of QCD is an unsolved fundamental problem in physics.The hadroproduction of heavy quarkonia involves both perturbative and nonperturbative processes of QCD,such that it is ideal to study QCD properties.The creation of heavy-quark pairs due to interactions of quarks and gluons can be calculated perturbatively,while the hadronization of heavy quark pairs into heavy quarkonia is nonperturbative.Various models have been developed to calculate heavy quarkonium production,differing mainly in the treatment of the hadronization process.In the color single model(CSM),the heavy quark pairs and the final-state quarkonium must have the same quantum numbers.The color evaporation model(CEM) uses parameters,which are independent of the interaction processes and heavy-quark quantum numbers,to describe the hadronization rates.In the nonrelativistic QCD(NRQCD) framework,intermediate heavy-quark pairs with all possible quantum numbers can hadronize into a certain quarkonium.Their transformation probabilities are expressed by long distance matrix elements(LDMEs),which are ranked by the powers of the heavy-quark velocity inside the heavy quarkonium.Both CSM and CEM models face difficulties to explain heavy quarkonium differential crosssections,while none of the models can give a coherent description of heavy quarkonium polarization results from experiments.Since the start of the Large Hadron Collider(LHC) at CERN,the LHCb experiment has conducted extensive research on the production mechanism of heavy quarkonia.The studies include the production cross-sections of various charmonia and bottomonia,the polarizations of J/Ψ,Ψ(2S) and Υ(nS) mesons,and the associated production of heavy quarkonium pairs,J/Ψ-J/Ψ,J/Ψ-Ψ(2S),and J/Ψ-Υ(nS),and their corresponding effective cross-sections.The cross-section ratios between different states,which cancel part of the uncertainties from data as well in theoretical calculations,are also determined.In particularly,the polarizations for vector S-wave heavy quarkonia are found to be small(less than 20% in absolute values) within the LHCb kinematic range.The effective cross-sections for various heavy quarkonium pairs are found to be in the range of 5-40 mb,showing hints of nonuniversality for different systems and for different kinematic ranges by comparison with measurements from other experiments.Compared with aforementioned heavy quarkonium production models,it is found that calculations with CSM and CEM are in poor agreement with the LHCb data,while NRQCD provides a good description of production cross-sections and transverse momentum distributions.The η_(c)(1S)cross-section is still puzzling,as the magnitudes of the LDMEs for η_(c)(1S) and those for J/Ψ contradict if the heavy quark spin symmetry is assumed.For polarization results,the small values are not expected by neither CSM nor NRQCD,while CEM does not provide any information on polarization.The rich experimental results on heavy quarkonia at LHCb provided important information on theoretical models that describe the production mechanism of heavy quarkonia,leading to a better understanding of the perturbative and nonperturbative properties of QCD.With more data that are being collected in Run 3 of the LHC,measurements of heavy quarkonia are expected to include cross-sections and polarizations at high transverse momentum,polarizations for P-wave states,and associated production of multiple heavy quarkonia.