摘要
相对论重离子碰撞实验的首要物理目标是产生极端高温高密的夸克胶子等离子体新物态并研究其性质.重味强子是碰撞中产生的热密核物质性质和强相互作用非微扰特性的重要实验探针.与之相关的多个实验观测量,比如核修正因子和粒子产额比,表现出对热密核物质效应的敏感性.然而,基于它们来定量提取热密核物质性质还需要对碰撞中的冷核物质效应进行定量限定.LHCb实验参与了LHC上重离子碰撞数据获取,在其独特的前向快度区系统地测量了质子-铅核等碰撞中多种粲、底强子和重夸克偶素的核修正因子以及多种重味强子的产额比,对核物质效应和重味夸克强子化机制进行了深入研究.本文将介绍LHCb实验在这些研究中取得的新进展和未来研究方向.
One of the key tasks of high-energy physics is to explore the fundamental composition of matter and its interactions.Bound quarks and gluons were discovered in the 20th century;however,free quarks and gluons have never been observed.This phenomenon is known as“quark confinement”which is one of the fundamental problems in modern physics.Quantum chromodynamics(QCD)is the fundamental theory describing the strong interactions between quarks and gluons,and it predicts that at sufficiently high temperatures,quarks and gluons will be deconfined and form a new state of matter known as the quark-gluon plasma(QGP).This hot and dense state of matter may have existed in the early universe or inside dense celestial bodies.The only way to produce and study the QGP state in the laboratory is through relativistic heavy-ion collisions.Heavy quarks(charm and bottom quarks)have larger masses and can only be produced in the early stages of collision through hard parton scattering processes.They then undergo the entire evolution of the hot and dense QGP state,experiencing frequent scattering with light partons(light quarks or gluons),and eventually become heavy-flavor hadrons either by escaping the medium or by hadronizing along with the QGP medium.The momentum distribution,yield,or cross-section ratios of these heavy-flavor hadrons carry rich information about the QGP state and serve as good experimental probes for the quantitative study of the properties of the hot and dense QGP state.The LHCb experiment,with its unique forward rapidity coverage and excellent capability in measuring heavy-flavor particles,has made numerous significant contributions in the studies of nuclear matter effects with heavy-flavor probes.This article reviews some of the latest results obtained from proton-proton,proton-lead,and lead-lead collision data.Precise measurements of the nuclear modification factors for charm and bottom hadrons in proton-lead collisions have significantly constrained the gluon parton distribution functions at small x,rigorously testing the predictions based on nuclear parton distribution functions and the color glass condensate theory.An additional suppression of charm hadron production at high transverse momentum has been observed in the backward rapidity region,suggesting the presence of additional nuclear matter effects.A suppression of excited heavy quarkonium states relative to the ground state production has been found in proton-lead and high-multiplicity proton-proton collisions,indicating that comover scattering has a significant impact on heavy quarkonium production.The charm baryon-tomeson ratio is found to be almost consistent in forward and backward proton-lead collisions and peripheral lead-lead collisions within the LHCb forward rapidity coverage,with no significant enhancement of charmed baryon production observed.However,a substantial enhancement of strange-charm meson production has been discovered in high-multiplicity proton-lead collisions,providing important insights for understanding the hadronization mechanism of charm quarks and the reaction dynamics in smallsystem collisions.The cross-section ratio of bottom baryon to meson in both proton-lead and proton-proton collisions is found to be essentially consistent,and significantly higher at low transverse momentum than results from electron-positron colliders,indicating that bottom quark hadronization is subject to changes in different collision environments.The LHCb experiment will explore further the heavy-flavor probes of the nuclear matter effects in small-system collisions with the current datasets.With the restart of the Run-III of the LHC,the LHCb detector,after its phase-I upgrade,is now capable of reconstructing semi-central lead-lead collisions and collect more heavy-ion collision data.Utilizing this data,the LHCb experiment will be able to perform more precise measurements of heavy-flavor hadrons,deepen the understanding of nuclear matter effects,and ultimately gain a better understanding of the strong interaction and the structure of matter.
作者
陈缮真
朱相雷
Shanzhen Chen;Xianglei Zhu(Institute of High Energy Physics,Chinese Academy of Sciences,Beijing 100049,China;Department of Engineering Physics,Tsinghua University,Beijing 100084,China)
出处
《科学通报》
北大核心
2025年第1期70-82,共13页
Chinese Science Bulletin
基金
国家自然科学基金(12061141007)资助。
