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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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"

July 11, 2017
Congratulations to Chris Flores on successfully defended his PhD thesis at UC Davis. His thesis title is: "A Systematic, Large Phase Space Study of Pion, Kaon, and Proton Production in Au+Au Heavy-Ion Collisions from the RHIC Beam Energy Scan Program at STAR"

July 4, 2017
Congratulations to Kunsu Oh who successfully defended his PhD thesis at Pusan in May, 2017. His thesis title is: "Charm and Bottom Hadron Production via Semi-leptonic Decay Channel in Relativistic Heavy Ion Collisions".

June 30, 2017
We get to know one of our new collaborating institutions in the June 2017 edition of the STAR Newsletter, and are called to volunteer for the annual Summer Sunday at BNL. We also hear from our spokesperson about Collaboration topics, and our S&C Leader on computing efforts. The recent PAC and RHIC/AGS Users' Meetings make the news, along with recognitions for members of the STAR Collaboration.

June 23, 2017
Congratulations Prashanth Shanmuganathan (Ph.D. Supervisor Declan Keane, Kent State) and Zilong Chang (Ph.D. Supervisor Carl Gagliardi, Texas A&M University) for wining the RHIC/AGS Users Thesis awards. Their theses can be found on our web pages or via the links below:

June 18, 2017
Congratulations to Xu Wang who successfully defended his PhD thesis at SDU on May 26, 2017. His thesis title is: "Setup of a PMT Test System for LHAASO-KM2A and STAR-iTPC MWPC Prototype Design and Performance Measurement".

May 25, 2017
Congratulations to Long Ma who successfully defended his PhD thesis at SINAP on May 25, 2017. His thesis title is: "Measurement of D-meson triggered azimuthal correlations and study of anisotropic flow fluctuations at RHIC energies".

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