Home
News
Members
Research
Group Meeting
Others
High Energy Nuclear Theory Group
≡
High Energy Nuclear Theory Group
Research
Topics:
Quantum kinetic theory for massless and massive fermions
Matter under strong electromagnetic fields
Relativsitic hydrodynamics and magneto-hydrodynamics
Numerical simulations for kinetic theory (RBG)
Bell inequality, quantum information and their applications in high energy physics
Invited Reviews
Quantum kinetic theory for massless and massive fermions Kinetic theory can describe the phenomenon far away the equilibrium.
The most well-known theory is the Boltzmann equations.
In order to understand the quantum transport phenomenon for massless fermions, we have derived the chiral kinetic theory from the first principle calculations ( from quantum electrodynamics).
We have implemented the Wigner function W(x,p), which the the two point Green functions under the Wigner transformation.
The chiral kinetic theory is the kinetic theory for the massless fermions with quantum corrections. One of the most imporatant correction is related to the famous chiral anomaly.
The Omega in the above equations is also found in the condensed matter physics, named the Berry curvature.
Applying the chiral kinetic theory, one can derive the macroscopic quantum transport phenomenon order by order, e.g. the chiral magnetic effect (disscussed in the topic "Matter under strong electromagnetic fields"), chiral vortical effect, chiral anomaly and other higher order electromagnetic responses.
Matter under strong electromagnetic fields
Very high magnetic fields and orbital angular momenta (OAMs) are generated in these collisions. The magnetic fields are on the order of 10^(17−18) Gauss and are the strongest magnetic fields observed in nature.
One of the most important quantum transport phenomenon is the Chiral Magentic Effect. The chiral magnetic effect (CME) and chiral separation effect (CSE) are two quantum effects on the magnetic field in a chiral fermion system. In the CME, a charge current is induced along the magnetic field:
where μ5 is the chiral chemical potential. The magnetic field can also generate a chiral current
where μ is the charge chemical potential. These two quantum effects are related to the chiral anomaly, which is absent from classical theories. The chiral chemical potential represents a chirality imbalance. The nonzero μ5 arises from topological fluctuations in quantum chromodynamics (QCD), which are related to the local violation of parity and charge parity. Therefore, the observation of the CME in heavy-ion collisions implies local parity and charge parity violation. In addition, the CME has been observed in condensed matter and can be applied in quantum computing.
The mass correction to the CME is related to another well-known phenomenon, Schwinger pair production. The operator equation for the CME with finite mass corrections is the axial Ward identity. We have computed by the famous non-perturbative method, named world-line formalism, to compute these operators in the In-In states.
Relativsitic hydrodynamics and magneto-hydrodynamics To study the magnetic-field-related effects in heavy-ion collisions, we need to know the magnetic field as a function of time. The electromagnetic field can be estimated using the Lienard–Wiechert potential. Although the peak value of the magnetic field can be as large as a few times mass of pions, it decays very rapidly in vacuum. Such a magnetic field in vacuum could not provide sufficient time to generate the CME and other chiral transport effects; thus, intermediate effects must be present.
Magnetohydrodynamics (MHD) is widely used in astrophysics to investigate the coupling between a charged medium and a magnetic field. Ordinary MHD consists of the hydrodynamic conservation equations coupled with Maxwell’s equations. Very recently, our MHD studies showed that the magnetic fields decay as 1/τ in the infinite electrical conductivity limit, where τ is the proper time. The analytic solution for the anomalous MHD with the CME and chiral anomaly in a Bjorken flow has been derived. The magnetic fields decay approximately as ∼ 1/τ or ∼ e^(−στ)/τ, where σ is the electrical conductivity.
Another very important results found in our group is the casuality condition for relativistic hydrodynamic. In the relativistic hydrodynamic, the signal cannot move faster than the speed of light. This will provide a condition for the transport coefficients, which we named the asymptotic casuality condition,
We have also found that the fluid satisfying the above condition is always stable.
Numerical simulations for kinetic theory (RBG)
Many studies for non-equilibrium systems, e.g., the pre-equilibration puzzle in heavy-ion collisions, require solving the relativistic Boltzmann equation (BE) with the full collisional kernel to high precision. It is challenging to solve relativistic BE due to its high dimensional phase-space integrals and limited computing resources.
We have developed a numerical framework for a full solution of the relativistic Boltzmann equations for the quark-gluon matter using the multiple Graphics Processing Units (GPUs) on distributed clusters. Including all the 2 to 2 scattering processes of 3-flavor quarks and gluons, we compute the time evolution of distribution functions in both coordinate and momentum spaces for the cases of pure gluons, quarks and the mixture of quarks and gluons. By introducing a symmetrical sampling method on GPUs which ensures the particle number conservation, our framework is able to perform the space-time evolution of quark-gluon system towards thermal equilibrium with high performance. We also observe that the gluons naturally accumulate in the soft region at the early time, which may indicate the gluon condensation.
Bell inequality, quantum information and their applications in high energy physics In theoretical physics, Bell inequality is a powerful mathematical inequality about whether there is a complete theory of local hidden variables. In 1935, Einstein, Podolsky and Rosen proposed the famous thought experiment "EPR paradox", criticizing the "completeness" of quantum theory. They believed that quantum theory is incomplete, and there must be a more complete theory in which there are some variables that we have not yet observed. The local hidden variable theory is a complete theory by adding one or more "hidden variables" to make "local reality" established. Under the dual assumptions of locality and reality, Bell inequality establishes a strict limit on the possible correlation of the results when two separate particles are measured at the same time, which provides an opportunity to make a judgment between quantum nonlocality and Einstein's localized reality using physical experiments. In order to determine whether quantum theory violates Bell inequality under any circumstances, physicists designed many rigorous experiments. Up to now, most of the experiments show the violation of Bell inequality, which means that the prediction of local realism advocated by Einstein does not conform to the theory of quantum mechanics. In other words, quantum nonlocality does exist, and disciplines such as quantum information and quantum computing have also been based on it.
But the important question is, what is nonlocality? Can the so-called quantum entanglement really exceed the speed of light? Unfortunately, we cannot transmit information through quantum entanglement, and of course, it is impossible to exceed the speed of light to obtain information about another particle that is entangled with the particle we are measuring. As for what nonlocality is, now no one can understand but regarded it as a basic property of quantum theory. At present, many physicists believe in the hypothesis of EPR=ER, which assumes that quantum entanglement is connected by "wormholes" formed between particles. But the verification of this hypothesis is still hopeless. First of all, its establishment is based on the framework of string theory, and current experiments have not seen any signs of string theory. Secondly, wanting to directly observe the "wormhole" is also an extremely sci-fi thing. Therefore, in the previous "125 Scientific Questions" published by Science, "Do Deeper Principles Underlie Quantum Uncertainty and Nonlocality?" is on the list lively. More puzzles are still waiting to be solved in this field.
Recent invited reviews
Patrick Copinger, Shi Pu,
Chirality Production with Mass Effects-Schwinger Pair Production and the Axial Ward Identity
, arXiv: 2008.03635.
Jian-Hua Gao, Guo-Liang Ma, Shi Pu, Qun Wang,
Recent developments in chiral and spin polarization effects in heavy-ion collisions
,
Nucl.Sci.Tech.
31 (2020) 9, 90.
Powered by
w3.css
.