The strongest electromagnetic fields in nature are created in high energy nuclear collisions and expected to change the dynamic scattering processes in the early stage. The magnetic field effect on the Drell-Yan process is investigated in this work. The single photon decay into quark pairs and lepton pairs in an external magnetic field leads to a significant Drell-Yan suppression in low and intermediate invariant mass region. The calculation up to the Landau level <math altimg='si1.svg'><mi>n</mi><mo linebreak='goodbreak' linebreakstyle='after'>&#8722;</mo><mn>1</mn></math> is complete in the energy region <math altimg='si2.svg'><mi>s</mi><mo linebreak='goodbreak' linebreakstyle='after'>&lt;</mo><mn>2</mn><mo stretchy='false'>/</mo><mn>3</mn><mi>n</mi><mo stretchy='false'>(</mo><mi>e</mi><mi>B</mi><mo stretchy='false'>)</mo></math>, and the enlarged phase space at higher landau levels may enhance the dilepton spectrum in the high mass region.

China

Shile Chen

Germany

Jiaxing Zhao

Pengfei Zhuang

Suppressed Drell-Yan process by an external magnetic field

The rotation effect on the QCD properties is an open question. We study the dynamic gluon mass in a dense QCD matter, the rotation is introduced by taking a covariant transformation between the flat and curved spaces. The law of causality which restricts the rotation strength of the system is carefully considered in the calculation. We find that the rotation effect is not monotonous. Overall, it behaves like an anti-screening effect, reflecting in the decreasing gluon mass, but the strength changes with the rotation. For a QCD matter with low baryon density, the screening effect in the flat space can be completely canceled by the rotation, and gluons are confined in a strongly rotating matter. When the rotation is extremely high, the matter approaches to a weakly interacting gas.

Guojun Huang

Yin Jiang

Rotation induced color confinement

The magnetic field created in high energy nuclear collisions will affect the QCD dynamical processes, such as the heavy quark production which happens in the initial stage of the collisions. We calculate in a strong magnetic field the heavy quark production cross section of the process   <math> <mi>gg</mi> <mo>→</mo> <mi>Q</mi> <mover> <mi>Q</mi> <mo>¯</mo> </mover> </math>  $$ gg\to Q\overline{Q} $$  at leading order and the corresponding transverse momentum distribution in nucleus-nucleus collisions. In comparison to the QED process, the heavy quark production is dominated by the unique QCD channel with gluon self-interaction. Due to the dimension reduction of quark phase space in a strong magnetic field, the production is concentrated in a very narrow energy region above the threshold. Since the rotational invariance is broken in a magnetic field, the production becomes anisotropic.

France

Heavy flavor production under a strong magnetic field

We calculate the Debye screening mass in thermal, dense, and magnetized QCD matter in the frame of resummed perturbation theory. In the limit of zero temperature, when the Landau energy level and Fermi surface of quarks match each other <math><msubsup><mi>μ</mi><mi>q</mi><mn>2</mn></msubsup><mo>=</mo><mn>2</mn><mi>n</mi><mo>|</mo><mi>q</mi><mi>B</mi><mo>|</mo></math>, where <math><mi>q</mi></math>, <math><msub><mi>μ</mi><mi>q</mi></msub></math>, and <math><mi>B</mi></math> are respectively the quark electric charge, baryon chemical potential, and external magnetic field, the screening mass diverges and the system is in the state of weakly interacting parton gas, which is very different from the known result of strongly interacting quark-gluon plasma at high temperature. The divergence disappears in the thermal medium, but the screening mass oscillates with clear peaks at the matched magnetic field.

Resonant screening in dense and magnetized QCD matter

We study the relation between chiral and   $ U_A(1) $   symmetries in the quark-meson model. Although quarks and mesons are described in mean field approximation, the topological susceptibility characterizing the   $ U_A(1) $   breaking comprises two components: one controlled by the condensate and the other by the meson fluctuation. The   $ U_A(1) $   restoration is governed by the competition of these components. In a hot medium, the condensates melt. However, the fluctuation is enhanced. Therefore, the   $ U_A(1) $   symmetry cannot be solely restored via the temperature effect. Nevertheless, the baryon density reduces the condensates and fluctuation, and thereby, the   $ U_A(1) $   symmetry can only be restored in a dense or dense and hot medium. The strange condensate plays a weak role in the susceptibility, and the chiral and   $ U_A(1) $   symmetry restorations occur almost at the same critical point.

Jianing Li

Jin Gui

 Symmetry Restoration at High Baryon Density  

We calculate the color screening mass in thermalized and magnetized QCD matter in the frame of loop resummation theory without restriction to the magnetic field strength. Our full calculation covers the often-used approximations for a weak magnetic field at high temperature, and for a strong magnetic field at low temperature. We find that while the magnetic field created in heavy ion collisions at RHIC and LHC energies is probably the strongest one in nature, its effect on the QCD matter is still weaker in comparison with the high temperature of the fireball, and therefore can safely be treated as a perturbation.

Quantum color screening in external magnetic field

The interaction potential between a pair of heavy quarks is calculated with the resummed perturbation method in Gribov-Zwanziger (GZ) approach around the deconfinement phase transition. The resummed loop correction makes the potential complex. While the real part is suppressed by color screening and becomes short-ranged at high temperatures, the imaginary part is enhanced through decay processes in a hot medium and becomes comparable with the real part around the phase transition. The strong imaginary potential comes from the magnetic scale in the GZ approach and shows that both the color screening and decay processes play important roles in quarkonium dissociation in high-energy nuclear collisions.

Wan Wu

Heavy-quark potential in the Gribov-Zwanziger approach around the deconfinement phase transition

Transverse momentum integrated and differential <math><mrow><mi>J</mi><mo>/</mo><mi>ψ</mi><mspace width="4pt"></mspace><msub><mi>R</mi><mrow><mi>A</mi><mi>A</mi></mrow></msub></mrow></math> are systematically studied in Au-Au collisions at energies selected for the beam energy scan at the BNL Relativistic Heavy Ion Collider using a transport approach, including cold and hot nuclear effects respectively in the initial conditions and the collision terms. With decreasing energy, while the temperature, lifetime, and size of the quark-gluon plasma (QGP) fireball decrease, the nuclear absorption of the initially produced charmonia is more and more strong, and the nuclear shadowing effect on charmonium regeneration goes to antishadowing first and then to shadowing again. As a competition between the cold and hot nuclear effects, the QGP phase is still important for charmonium production at <math><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>200</mn></mrow></math>, 62.4, 54,4 and 39 GeV but becomes negligible at <math><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>14.5</mn></mrow></math> GeV.

Effects of cold and hot nuclear matter on <math><mrow><mi>J</mi><mo>/</mo><mi>ψ</mi></mrow></math> production at energies selected for the beam energy scan at the BNL Relativistic Heavy Ion Collider

The electromagnetic response of hot QCD matter to decaying external magnetic fields is investigated. We examine the validity of Ohm's law and find that the induced electric current increases from zero and relaxes towards the value from Ohm's law. The relaxation time is larger than the lifetime of the external magnetic field for the QCD matter in relativistic heavy-ion collisions. The lower-than-expected electric current significantly suppresses the induced magnetic field and makes the electromagnetic response incomplete. We demonstrate the incomplete electromagnetic response of hot QCD matter by calculations employing the parton transport model combined with the solution of Maxwell's equations. Our results show a strong suppression by two orders of magnitude in the magnetic field relative to calculations assuming the validity of Ohm's law. This may undermine experimental efforts to measure magnetic-field-related effects in heavy-ion collisions.

Zeyan Wang

Carsten Greiner

Zhe Xu

Incomplete electromagnetic response of hot QCD matter

We calculate <math><mrow><mi>J</mi><mo>/</mo><mi>ψ</mi></mrow></math> triangular flow in high energy nuclear collisions. The charmonium motion in medium is controlled by a transport equation with loss and gain terms, and the evolution of the hot medium is governed by a single-shot hydrodynamic equation with a triangularly deformed initial condition. In comparison with the elliptic flow <math><msub><mi>v</mi><mn>2</mn></msub></math>, the triangular flow <math><msub><mi>v</mi><mn>3</mn></msub></math> comes almost completely from the charmonium regeneration in the quark gluon plasma and therefore is more sensitive to the medium properties.