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star focus: Observation of charge asymmetry dependence of pion elliptic flow and the possible chiral magnetic wave in heavy-ion collisions

The STAR collaboration has found supporting evidence for what’s called a “chiral magnetic wave” rippling through the soup of quark-gluon plasma created in RHIC’s energetic particle smashups, as presented in a recent publication, Phys. Rev. Lett. 114, 252302 (2015).

In the picture with a chiral magnetic wave, the strong magnetic field created by the passing-by of spectators, with the presence of finite density of electric charge, causes chiral charges to separate along the axis of the magnetic field – a process called Chiral Separation Effect (CSE). The chiral separation acts like a seed that, in turn, causes particles with different electric charges to separate – in a process described as Chiral Magnetic Effect (CME). The evidence supporting the existence of the latter has been previously presented by STAR and hotly debated. The CSE and CME trigger each other and the process goes on like a wave. In the end, these two effects together will push negative particles into the equator and positive particles to the poles.

To look for this effect, STAR measured the collective motion of positively and negatively charged pions, the most abundant particles produced in RHIC collisions. It is found that the collective elliptic flow of negatively charged pions—their tendency to flow out along the equator—was enhanced, while the elliptic flow of the positive pions was suppressed, resulting in a higher abundance of positive particles at the poles. Importantly, the difference in elliptic flow between positive and negative pions increased with the net charge density produced in RHIC collisions. This is exactly what is expected from calculations using the theory predicting the existence of the chiral magnetic wave. The results hold out for almost all energies at which a quark-gluon plasma is believed to be created at RHIC, and, so far, no other model can explain them.

Left: Positive (negative) pion v2 linearly decreases (increases) with the increasing charge assymetry, which is expected by a CMW model. Right: The slope parameter, r, as a function of centrality for Au+Au collisions at 200 GeV. Also shown is the UrQMD simulation, and the calculation with CMW with different duration times. See paper for explanation for grey bands and cross-hatched band.

The presence of this wave is one of the inevitable consequences scientists were expecting to observe in the quark-gluon plasma. The discovery, if confirmed, would also provide circumstantial evidence in support of a long-debated quantum phenomenon required for wave’s existence.

First, the process resulting in propagation of the chiral magnetic wave requires that “chiral symmetry”—the independent identities of left- and right-handed particles—be “restored.” The restoration of the chiral symmetry has been long sought after at RHIC as a key signature of the QGP.

Second, seeing evidence for the chiral magnetic wave means the elements required to create the wave must also exist in the quark-gluon plasma. One of these is the chiral magnetic effect—the quantum physics phenomenon that causes the electric charge separation along the axis of the magnetic field—which has been a hotly debated topic in physics. Evidence of the wave is evidence that the chiral magnetic effect also exists.

The chiral magnetic effect is also required as an ingredient to see tiny bubbles of “broken symmetry” in a quark-gluon plasma—hints of which were observed in other analyses at RHIC. So this new evidence of the wave provides circumstantial support for those earlier intriguing findings.

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Posted June 21, 2015



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