Advances in the studies of nuclear matter effects using heavy-flavor particles as probes at LHCb

<sec><p indent="0mm">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. </sec><sec> 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. </sec><sec> 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 <italic>x</italic>, 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-to-meson 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 small-system 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. </sec><sec> 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. </sec>