Within the particles that constitute atomic nuclei, protons and neutrons, there are the quarks, the elementary particles with fractional charges, linked to each other thanks to the gluons, bosons that carry the nuclear interaction. Thanks to the gluons it is impossible to observe, at present, free quarks, but it is expected that in the very first stage of the universe, matter was in a state called quark-gluon plasma. Thanks to the observation of so-called quark jets we know, indirectly, that the interior of the accelerators RHIC and LHC, in particular in heavy ion collisions, this kind of plasmas were created and, according to the theory, in the presence of axial anomalies, dued by the presence of strong electromagnetic fields, we can create two special effects: the Chiral Magnetic Effect (CME) and the Chiral Separation Effect (CSE).
The CME is the phenomenon of electric charge separation along the axis of the applied magnetic field in the presence of fluctuating topological charge.These two effects are generated by the topology of the system: indeed, within the theory(1, 2, 3) is contemplated the existence of some numbers (called topological invariants, or winding number(4)) that, while not associated with an observable, still generate effects physically relevant because of their link with the fundamental symmetries of the system.
The Chiral Separation Effect (CSE) refers to the separation of chiral charge along the axis of external magnetic field at finite density of vector charge (e.g. at finite baryon number density)(5)
In particular the two excitations CME and CSE are generated, and when they happen at the same time, they interact generating a new excited state inside the quark-gluon plasma. This new state of matter is called Chiral Magnetic Wave (CMW).
This excitation stems from the coupling between the density waves of electric and chiral charge. Let us illustrate this statement by a qualitative argument (...): consider a local fluctuation of electric charge density; (...) it will induce a local fluctuation of axial current. This fluctuation of axial current would in turn induce a local fluctuation of the axial chemical potential, and thus (...) a fluctuation of electric current. The resulting fluctuation of electric charge density completes the cycle leading to the excitation that combines the density waves of electric and chiral charges(5)It is clear, therefore, that the CMW, although in an indirect manner, is linked to the quark-gluon plasma. The exciting news is that this particular excitement was observed within the RHIC(6) in the collision of gold ions, a step that brings us very closer to the big bang.
(1) Belavina A.A., Polyakova A.M., Schwartza A.S. & Tyupkina Y.S. (1975). Pseudoparticle solutions of the Yang-Mills equations, Physics Letters B, 59 (1) 85-87. DOI: 10.1016/0370-2693(75)90163-X
(2) Hooft G.'t. (1976). Symmetry Breaking through Bell-Jackiw Anomalies, Physical Review Letters, 37 (8) DOI: 10.1103/PhysRevLett.37.8
(3) Hooft G.'t. (1976). Computation of the quantum effects due to a four-dimensional pseudoparticle, Physical Review D, 14 DOI: 10.1103/PhysRevD.14.3432
(4) Fukushima K., Kharzeev D.E. & Warringa H.J. (2008). The Chiral Magnetic Effect, Physics Review D, 78 (7) DOI: 10.1103/PhysRevD.78.074033 (arXiv)
(5) Kharzeev D.E. & Yee H.U. (2011). Chiral Magnetic Wave, Physical Review D, 83 DOI: 10.1103/PhysRevD.83.085007 (arXiv)
(6) STAR Collaboration (2015). Observation of charge asymmetry dependence of pion elliptic flow and the possible chiral magnetic wave in heavy-ion collisions, arXiv: 1504.02175v2