STAR has recently reported the first observation of global polarization of Lambda hyperons in heavy ion collisions. The discovery has been published in Nature 548, 62 (2017) as a cover story.

Due to the parity-violating nature of their weak decay, Lambdas reveal the direction of their spin by preferentially emitting the daughter proton along that direction. The average spin direction of a population of Lambdas is the polarization. Lambdas at midrapidity were topologically reconstructed in the STAR TPC, and the Beam-Beam Counters (BBC) at forward and backward rapidity were used to estimate the direction of the total angular momentum of the collision. We discovered that the polarization direction of the Lambdas was correlated at the level of several percent with the direction of the system angular momentum in non-central collisions at √s_NN=7.7-32 GeV.

It has been well-established that the hot system created at midrapidity in the system may be considered a fluid, and hydrodynamic calculations relate the polarization of emitted particles is directly related to the vorticity - the curl of the flow field - of the fluid. Using this relation, we estimate that the curl of the fluid created at RHIC is about 9×10²¹ s^-1, 14 orders of magnitude higher than any fluid ever observed. Previous results have established the system at RHIC to be the hottest and the least viscous (relative to entropy density) fluid ever created. Our new result adds another record - collisions at RHIC produce the most vortical fluid.

This first view of the rotational substructure of the fluid at RHIC represents an entirely new direction in hot QCD research. It has generated considerable theoretical activity in the field, and may have important connections with the Chiral Magnetic and Chiral Vortical Effects (CME and CVE). With increased statistics, there may even be the opportunity to probe the magnetic field produced in heavy ion collisions by measuring the difference in polarization of Lambda and AntiLambda hyperons. Such studies are planned for the future.

Posted Aug. 16, 2017

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The STAR Collaboration has recently published “Coherent diffractive photoproduction of ρ⁰ mesons on gold nuclei at 200 GeV/nucleon-pair at the Relativistic Heavy Ion Collider,” in Physical Review C 96, 054904 (2017).

This paper reports on a special type of heavy-ion interaction, where the ions do not physically collide, but interact via a long-range electromagnetic interaction, whereby photons emitted by one nucleus probe the structure of the other nucleus. The photons come from the electric and magnetic fields carried by the highly charged nuclei. The electric fields radiate radially outward, while magnetic fields circle the ion’s trajectory. The two fields are perpendicular, just like those of a photon, and they can be treated as such.

In the reaction considered here, the photon may be thought of as briefly fluctuating to a quark-antiquark pair, as allowed by the Heisenberg uncertainty principle. Quark-antiquark pairs are mesons; this photon fluctuation acts like a meson with the same quantum numbers (spin one and negative parity) as the photon. These virtual (short-lived) mesons can scatter from the target nucleus, and emerge as real mesons.

Left: The cross-section as a function of t, the squared momentum transfer to the nucleus. The dips and peaks are a diffraction pattern, akin to the pattern made by a 2-slit interferometer. ‘XnXn’ and ‘1n1n’ are two different STAR data samples.The inset shows the distribution for very small momentum transfers. Right: The two-dimensional Fourier transform of the left panel, showing the density of the interaction sites in the nucleus, as a function of transverse distance from its center. This is a map of where the mesons interacted in the target. Although there is considerable systematic uncertainty (the blue region) near the center of the target, the edges of the nuclei are well defined.

The photons scatter equally from protons and neutrons. But, we can’t tell which proton or neutron an individual meson scattered from. In quantum mechanics, we add the amplitudes to scatter from each target meson. The amplitude is a complex number with a phase which depends on the meson momentum and the position of the target nucleon. By studying how the scattering probability varies with the momentum transfer to the nucleus, we can image the matter distribution in the target. The left panel shows the scattering probability as a function of the square of the momentum transfer (‘t’) for two different STAR data samples. The dips are due to diffraction, like the fringes seen in the classic two-slit diffraction pattern, but with a circular target.

The right panel show the two-dimensional Fourier-Bessel (Henckel) transform of the left panel, mapping the interaction density within the target. The transform converts a function of momentum to a function of position. The FWHM of the distribution is 12.34 ± 0.24 fm. Because of nuclear shadowing, this is not just the nuclear density distribution; shadowing will alter the distribution from that of the density of a gold nucleus; these corrections may also alter the apparent size of the nucleus. Unlike electron scattering measurements, this analysis is sensitive to both protons and neutrons.

Posted Jan. 9, 2018

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