STAR focus: Global Λ hyperon polarization in nuclear collisions

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 √sNN=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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STAR focus: A new approach to jet quenching measurements

The STAR Collaboration has recently published a paper, Phys. Rev. C 96, 024905, presenting a novel approach to measurements of jet quenching, one of the most important ways to study the Quark-Gluon Plasma (QGP) generated in nuclear collisions at RHIC and the LHC.

High energy collisions generate "jets", which are correlated sprays of particles arising from the decay of energetic quarks and gluons. In heavy ion physics, jets provide self-generated tomographic probes of the QGP; they are produced in the collisions itself and interact with the surrounding matter before flying off to be observed in the detectors. This interaction between a jet and the QGP modifies jet properties dramatically relative to those in vacuum (“jet quenching”), and has produced some of the most striking measurements of the QGP. Such jet measurements are challenging, however. Jet quenching was initially discovered by STAR and PHENIX, in Au+Au collisions at RHIC, by studying distributions of single high-momentum particles and their correlations, which are indirect jet messengers.

Left Figure: Measured jet rate (signal plus background) in head-on Au+Au collisions (red points) and the "mixed event" background (grey distribution). Right Figure: Azimuthal deflection of jets recoiling from a trigger particle (data points), and a calculation including QCD effects but not scattering in the QGP (red curve).

The new STAR paper utilizes the distribution of charged-particle jets recoiling from a high momentum single-hadron trigger to study jet quenching. The huge backgrounds underlying the reconstructed jet signal in Au+Au collisions are measured using a sophisticated event-mixing technique. The contribution of uncorrelated background is then corrected "statistically", i.e. on the measured jet spectrum averaged over the entire ensemble of events, rather than attempting to correct for background on an event-by-event basis. This statistical-correction method enables jet measurements at RHIC over the complete range of jet momenta - including very low momentum - in all collision systems, for large jet-cone radius. This approach enables qualitatively new ways of studying jet quenching.

Posted Sep. 12, 2017

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December 9, 2017
Congratulations to Dr. Ting Lin on successfully defended his PhD thesis at Indiana University. His thesis title is: "Longitudinal double-spin asymmetries for di-jet production at intermediate pseudorapidity in polarized pp collisions at √s = 200 GeV".

December 8, 2017
Congratulations to Dr. Xinjie Huang on successfully defended his PhD thesis at Tsinghua University. His thesis title is: “Study of the Moun Telescope Detector at RHIC and the Upsilon production in Au+Au collisions at 200GeV".

November 30, 2017
Congratulations to Dr. Jan Rusnak on successfully defended his PhD thesis at the Nuclear Physics Institute of the Czech Academy of Sciences. His thesis title is: "Jet Reconstruction in Au+Au collisions at RHIC".

October 31, 2017
Interesting low pT di-leption physics from juniors Shuai Yang & Wangmei Zha kick off the October 2017 edition of the STAR Newsletter. We also hear from our Spokespersons and Computing Leadership on hardware and software activities, about the Blind Analysis Committee & CME Focus Group, and also the 2nd CBM-STAR workshop and the fall DNP meeting.

August 31, 2017
We get to know another of our new collaborating institutions in the August 2017 edition of the STAR Newsletter, and hear about programatic directions of our Czech collaborating institutions. Also, we toast to 20 years of seeing particles in the STAR TPC, as well as a successful Summer Sunday of public outreach.

August 1, 2017
Congratulations to Dr. Kevin Adkins on successfully defended his Ph.D. thesis at University of Kentucky his thesis title is: "Studying Transverse Momentum Dependent Distributions in Polarized Proton Collisions via Azimuthal Single Spin Asymmetries of Charged Pions in Jets"

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