Studies of exotic hadrons such as the <math><mrow><msub><mi>χ</mi><mrow><mi>c</mi><mn>1</mn></mrow></msub><mrow><mo>(</mo><mn>3872</mn><mo>)</mo></mrow></mrow></math> state provide crucial insights into the fundamental force governing the strong interaction dynamics, with an emerging frontier to investigate their production in high energy collisions where a partonic medium is present. The latest experimental measurements from the Large Hadron Collider show an intriguing evolution pattern of the <math><mrow><msub><mi>χ</mi><mrow><mi>c</mi><mn>1</mn></mrow></msub><mrow><mo>(</mo><mn>3872</mn><mo>)</mo></mrow></mrow></math>-to-<math><mrow><mi>ψ</mi><mo>(</mo><mn>2</mn><mi>S</mi><mo>)</mo></mrow></math> yield ratio from proton-proton collisions with increasing multiplicities toward proton-lead and lead-lead collisions. Here we propose a mechanism of medium-assisted enhancement for the <math><mrow><msub><mi>χ</mi><mrow><mi>c</mi><mn>1</mn></mrow></msub><mrow><mo>(</mo><mn>3872</mn><mo>)</mo></mrow></mrow></math> production, which competes with the more conventional absorption-induced suppression and results in a nonmonotonic trend from small to large colliding systems. Realistic simulations from this model offer a quantitative description of all available data. Predictions are made for the centrality dependence of this observable in PbPb collisions as well as for its system-size dependence from OO and ArAr to XeXe and PbPb collisions. In both cases, a nonmonotonic behavior emerges as the imprint of the competition between enhancement and suppression and can be readily tested by future data.

China

Yu Guo

Xingyu Guo

United States

Jinfeng Liao

Enke Wang

Hongxi Xing

Medium-assisted enhancement of <math><mrow><msub><mi>χ</mi><mrow><mi>c</mi><mn>1</mn></mrow></msub><mrow><mo>(</mo><mn>3872</mn><mo>)</mo></mrow></mrow></math> production from small to large colliding systems

The magnetic fields in heavy-ion collisions are important ingredients for many interesting phenomena, such as the chiral magnetic effect, the chiral magnetic wave, the directed flow <math><msub><mi>v</mi><mn>1</mn></msub></math> of <math><msup><mi>D</mi><mn>0</mn></msup></math> mesons, and the splitting of the spin polarization of the <math><mi>Λ</mi><mo>/</mo><mover><mi>Λ</mi><mo>¯</mo></mover></math>. Quantitative studies of these phenomena, however, suffer from limited understanding of the dynamical evolution of these fields in the medium created by the collisions, which remains a critical and challenging problem. The initial magnetic fields from the colliding nuclei decay very fast in the vacuum but their lifetime could be extended through medium response due to electrically conducting quarks and antiquarks. Here we perform a detailed analysis of such medium effect on the dynamical magnetic fields by numerically solving Maxwell's equations concurrently with the expanding medium described by viscous hydrodynamics, under the assumption of negligible back reaction of the fields on the fluid evolution. Our results suggest a considerable enhancement of late time magnetic fields, the magnitude of which depends sensitively on the fireball expansion as well as the medium electric conductivity both before and during hydrodynamic stage.

Anping Huang

Duan She

Shuzhe Shi

Mei Huang

Dynamical magnetic fields in heavy-ion collisions

Gauge fields provide the fundamental interactions in the Standard Model of particle physics. Gauge field configurations with nontrivial topological windings are known to play crucial roles in many important phenomena, from matter-antimatter asymmetry of today’s universe to spontaneous chiral symmetry breaking in strong interaction. Their presence is, however, elusive for direct detection in experiments. Here we show that measurements of the chiral magnetic effect (CME) in heavy ion collisions can be used for accessing the topological fluctuations of the non-Abelian gauge fields in the quantum chromodynamics (QCD). To achieve this, we implemented a key ingredient, the stochastic dynamics of gauge field topological fluctuations, into a state-of-the-art framework for simulating the CME in these collisions. This new framework provides the necessary tool to quantify initial topological fluctuations from any definitive CME signal to be extracted from experimental data. It also reveals a universal scaling relation between initial topological fluctuations and particle multiplicity produced in the corresponding collision events.

Canada

Shu Lin

Accessing topological fluctuations of gauge fields with the chiral magnetic effect

We present a calculation of the heavy quark momentum diffusion coefficients in a quark-gluon plasma under the presence of a strong external magnetic field, within the lowest Landau level (LLL) approximation. In particular, we apply the hard thermal loop (HTL) technique for the resummed effective gluon propagator, generalized for a hot and magnetized medium. Using the derived effective HTL gluon propagator and the LLL quark propagator we analytically derive the full results for the longitudinal and transverse momentum diffusion coefficients for charm and bottom quarks beyond the static limit. We also show numerical results for these coefficients in two special cases where the heavy quark is moving either parallel or perpendicular to the magnetic field.

Aritra Bandyopadhyay

Heavy quark dynamics in a strongly magnetized quark-gluon plasma

Hadron spectroscopy provides direct physical measurements that shed light on the nonperturbative behavior of quantum chromodynamics (QCD). In particular, various exotic hadrons such as the newly observed <math><msubsup><mi>T</mi><mrow><mi>c</mi><mi>c</mi></mrow><mo>+</mo></msubsup></math> by the LHCb collaboration, offer unique insights on the QCD dynamics in hadron structures. In this paper, we demonstrate how heavy ion collisions can serve as a powerful venue for hadron spectroscopy study of doubly charmed exotic hadrons by virtue of the extremely charm-rich environment created in such collisions. The yields of <math><msubsup><mi>T</mi><mrow><mi>c</mi><mi>c</mi></mrow><mo>+</mo></msubsup></math> as well as its potential isospin partners are computed within the molecular picture for Pb-Pb collisions at center-of-mass energy 2.76 TeV. We find about three-order-of-magnitude enhancement in the production of <math><msubsup><mi>T</mi><mrow><mi>c</mi><mi>c</mi></mrow><mo>+</mo></msubsup></math> in Pb-Pb collisions as compared with the yield in proton-proton collisions, with a moderately smaller enhancement in the yields of the isospin partners <math><mrow><msubsup><mrow><mi>T</mi></mrow><mrow><mi>c</mi><mi>c</mi></mrow><mrow><mo>′</mo><mn>0</mn></mrow></msubsup></mrow></math> and <math><msubsup><mi>T</mi><mrow><mi>c</mi><mi>c</mi></mrow><mrow><mo>′</mo><mo>+</mo><mo>+</mo></mrow></msubsup></math>. The <math><msubsup><mi>T</mi><mrow><mi>c</mi><mi>c</mi></mrow><mo>+</mo></msubsup></math> yield is comparable to that of the <math><mi>X</mi><mo>(</mo><mn>3872</mn><mo>)</mo></math> in the most central collisions while shows a considerably stronger decrease toward peripheral collisions, due to a “threshold” effect of the required double charm quarks for <math><msubsup><mi>T</mi><mrow><mi>c</mi><mi>c</mi></mrow><mo>+</mo></msubsup></math>. Final results for their rapidity and transverse momentum <math><msub><mi>p</mi><mi>T</mi></msub></math> dependence as well as the elliptic flow coefficient are reported and can be tested by future experimental measurements.

Yuanyuan Hu

Qian Wang

Hui Zhang

Production of doubly charmed exotic hadrons in heavy ion collisions

In this work, we study the relativistic quantum kinetic equations in <math><mn>2</mn><mo>+</mo><mn>1</mn></math> dimensions from Wigner function formalism by carrying out a systematic semiclassical expansion up to <math><mi>ℏ</mi></math> order. The derived equations allow us to explore interesting transport phenomena in <math><mn>2</mn><mo>+</mo><mn>1</mn></math> dimensions. Within this framework, the parity-odd transport current induced by the external electromagnetic field is self-consistently derived. We also examine the dynamical mass generation by implementing four-fermion interaction with mean-field approximation. In this case, a new kind of transport current is found to be induced by the gradient of the mean-field condensate. Finally, we also utilize this framework to study the dynamical mass generation in an external magnetic field for the (<math><mrow><mn>2</mn><mo>+</mo><mn>1</mn></mrow></math>)-dimensional system under equilibrium.

Xianglei Zhu

Lianyi He

Pengfei Zhuang

Quantum kinetic equation and dynamical mass generation in <math><mrow><mn>2</mn><mo>+</mo><mn>1</mn></mrow></math> dimensions

Exploring the nature of exotic multiquark candidates such as the X(3872) plays a pivotal role in understanding quantum chromodynamics (QCD). Despite significant efforts, consensus on their internal structures is still lacking. As a prime example, it remains a pressing open question to decipher the X(3872) state between two popular exotic configurations: a loose hadronic molecule or a compact tetraquark. We demonstrate a novel approach to help address this problem by studying the X(3872) production in heavy ion collisions, where a hot fireball with ample light as well as charm (anti-)quarks is available for producing the exotics. Adopting a multiphase transport model (AMPT) for describing such collisions and implementing appropriate production mechanism of either molecule or tetraquark picture, we compute and compare a series of observables for X(3872) in Pb-Pb collisions at the Large Hadron Collider. We find the fireball volume plays a crucial role, leading to a 2-order-of-magnitude difference in the X(3872) yield and a markedly different centrality dependence between hadronic molecules and compact tetraquarks, thus offering a unique opportunity for distinguishing the two scenarios. We also make the first prediction of X(3872) elliptic flow coefficient to be tested by future experimental measurements.

Deciphering the Nature of X(3872) in Heavy Ion Collisions

The possibility that nuclear matter at a density relevant to the interior of massive neutron stars may be a quarkynoic matter has attracted considerable recent interest. In this work, we construct a phenomenological model to describe the quarkyonic matter, that would allow quantitative calculations of its various properties within a well-defined field theoretical framework. This is implemented by synthesizing the Walecka model together with the quark-meson model, where both quark and nucleon degrees of freedom are present based on the quarkyonic scenario. With this model we compute at mean-field level the thermodynamic properties of the symmetric nuclear matter and calibrate model parameters through well-known nuclear physics measurements. We find this model gives a very good description of the symmetric nuclear matter from moderate to high baryon density and demonstrates a continuous transition from nucleon-dominance to quark-dominance for the system.

Gaoqing Cao

A field theoretical model for quarkyonic matter

A strong magnetic field influences significantly the masses of the charged light mesons. For example, the mass of charged pion increases with the magnetic field increasing. We propose this mechanism as a possible way to extract the magnitude of magnetic field at freeze-out in heavy ion collisions and thus help constrain its lifetime which is currently a major open question to resolve. Specifically we show that the ratio between the yield of charged pions and that of charged rhos is very sensitive to the magnetic field value at freeze-out. By using a viscous-hydrodynamic framework (iEBE-VISHNU) to simulate heavy ion collisions and implementing magnetic-field-dependent meson masses, we compute their yields and predict the dependence of such ratio on the magnetic field. We suggest to use this ratio of charged rho yield over charged pion yield as an experimental observable to extract the possible magnetic field at freeze-out in heavy ion collisions.

Kun Xu

Defu Hou

Extracting the magnitude of magnetic field at freeze-out in heavy-ion collisions

Heavy quark production provides a unique probe of the quark-gluon plasma transport properties in heavy ion collisions. Experimental observables like the nuclear modification factor $$R_\mathrm{AA}$$ <math><msub><mi>R</mi><mi>AA</mi></msub></math>  and elliptic anisotropy $$v_\mathrm{2}$$ <math><msub><mi>v</mi><mn>2</mn></msub></math>  of heavy flavor mesons are sensitive to the heavy quark diffusion coefficient. There now exist an extensive set of such measurements, which allow a data-driven extraction of this coefficient. In this work, we make such an attempt within our recently developed heavy quark transport modeling framework (Langevin-transport with Gluon Radiation, LGR). A question of particular interest is the temperature dependence of the diffusion coefficient, for which we test a wide range of possibility and draw constraints by comparing relevant charm meson data with model results. We find that a relatively strong increase of diffusion coefficient from crossover temperature $$T_c$$ <math><msub><mi>T</mi><mi>c</mi></msub></math>  toward high temperature is preferred by data. We also make predictions for Bottom meson observables for further experimental tests.