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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STAR focus: STAR uses photons to probe the structure of gold nuclei
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,”
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
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
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
Posted Jan. 9, 2018
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August 13, 2018
Congratulations to James Daniel Brandenburg who successfully defended his
PhD thesis at Rice University. His thesis title is: "Systematic
Measurements of Dimuon Production in p+p and p+Au Collisions at
√sNN=200 GeV with the STAR Detector".
July 6, 2018
Read the June 2018 edition of the STAR Newsletter to find out more about the
goings on of STAR, including the recognition of numerous STAR
Collaborators, a personal recap of Quark Matter from Daniel Brandenburg,
and some good news from the S&C team.
May 30, 2018
Congratulations to Shenghui Zhang who successfully defended her PhD thesis
at USTC. Her thesis title is: "Measurements of electrons from heavy flavor
hadron decays in p+p and Au+Au collisions at 200 GeV by the STAR
May 30, 2018
Congratulations to Zhen Liu who successfully defended her PhD thesis at
USTC. Her thesis title is: "Measurements of J/Ψ polarization in p+p
collisions at 200 GeV with the STAR experiment".
May 24, 2018
Congratulations to Maowu Nie who successfully defended his PhD thesis at
SINAP. His thesis title is: "Investigation of Anisotropic Flow in
Relativistic Heavy-ion Collisions".
May 9, 2018
The April 2018 edition of the STAR Newsletter is available now with some
timely news. including a summary of Run 18 progress, a review of
preparations for next week's Quark Matter, a highlight of some recent
efforts in S&C, and an overview of the newly released BUR.
April 6, 2018
Congratulations to Dr. Isaac Upsal who successfully defended his Ph.D.
thesis at the Ohio State University on April 4th. His thesis title is:
"Global Polarization of Lambda Baryons in the STAR Beam Energy Scan"
March 30, 2018
Congratulations to Dr. John Campbell who successfully defended his Ph.D.
thesis at the Ohio State University. His thesis title is: "Azimuthally
sensitive pion femtoscopy in ultra-central U+U and Au+Au collisions at