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
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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Previous STAR Focus Features
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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Previous STAR Focus Features
STAR Autumn 2017 Analysis Meeting
Nov. 2-4, 2017, BNL
The 2nd CBM-STAR joint Workshop
Sep. 23, 2017, CCNU, Wuhan
STAR Regional Meeting:
Heavy Quark Production,
Jets and Correlations
Jun. 27-29, 2017, Warsaw
STAR Collaboration Meeting
May 15-19, 2017, BNL
CBM-STAR joint Workshop
Mar. 18, 2017, TU Darmstadt
STAR Pre-QM Meeting
Feb. 1-3, 2017, BNL
TOF and MTD in STAR
Dec. 8-10, 2016, NCKU
STAR Analysis Meeting
Nov. 10-12, 2016, LBL
STAR Collaboration Meeting
Aug. 15-20, 2016, OSU
STAR Regional Meeting
Jun. 13-15, 2016, AGH, Cracow
STAR Computer Account
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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
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
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".