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STAR focus: Azimuthal transverse single-spin asymmetries of inclusive jets and identified hadrons
within jets from polarized pp collisions at √s = 200 GeV
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Recently, the STAR Collaboration published the most precise measurement of the transverse
single-spin asymmetries for charged hadrons inside jets in Phys. Rev. D 106, 072010.
Transverse spin experiments at STAR provide new ways to map out the three-dimensional
nature of quark fragmentation and illustrate the interplay between the structure of a hadron
and the color environment. The Collins effect describes the azimuthal distribution of hadron
fragments from a transversely polarized quark, and thus provides access to both the quark
transversity in the proton and the transverse momentum dependent Collins fragmentation
function. Transversity is one of the three leading twist parton distribution functions of the
nucleon. It describes the transverse spin structure of quarks in a transversely polarized proton.
The Collins fragmentation function is one of the most important fragmentation functions in the
transverse-momentum-dependent (TMD) formalism. This effect has only been studied in semi-inclusive deep-inelastic
scattering (SIDIS) and electron-positron annihilation before the STAR experiment.
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The figure presented above shows the first measurement of Collins asymmetry with jT
(momentum transverse to the jet axis) dependence in pp collisions at √s = 200 GeV. The results
here are divided into six different jet-pT bins that probe different hard scales in the experiment.
The DMP+2013 and KPRY model expectations are also presented in the plot. Both are based on fits
to experimental data from SIDIS and electron-positron processes. Our results slightly favor the
KPRY model that treats TMD evolution up to the next-to-leading logarithmic effects; however,
significant discrepancies exist between the data and both model calculations. These new results
will provide valuable new constraints on the kinematic dependence of the Collins
fragmentation function when included in future global analyses.
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Posted Oct 28, 2022
Previous STAR Focus Features
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STAR focus: Evidence for Nonlinear Gluon Effects in QCD and their A Dependence at STAR
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Extracting the data from previous experiments, gluon density grows rapidly towards
small momentum fraction (x) with respect to the nucleon. Under the color glass condensate
(CGC) framework, the growth is explained by gluon splitting. The nonlinear QCD effects
at small x should tame this growth by gluon recombination. The so called “gluon
saturation” is reached at the point when the splitting and recombination are balanced.
Understanding the nonlinear behavior of the gluon is one of the most important physics
goals for RHIC Cold QCD program and the future electron ion collider (EIC) project.
Back-to-back dihadron azimuthal angle correlation has been proposed to be a sensitive
probe to directly access the underlying gluon dynamics involved in hard scatterings.
With a high gluon density at the initial state, the product of back-to-back dihadron
modulation will be suppressed. It is predicted that the density of gluons per unit
transverse area is larger in nuclei than in nucleons and is amplified by a factor of A1/3 for a
nucleus with mass number A; thus, nuclei provide a natural environment to study
nonlinear gluon evolution.
The STAR Collaboration performed the measurements of back-to-back azimuthal
correlations of di-π0s produced at forward pseudorapidities (2.6 < η < 4.0) in p+p,
p+Al, and p+Au collisions at a center-of-mass energy of 200 GeV. The results have been
recently published in Phys. Rev. Lett. 129, 092501.
We observe a clear suppression of the correlated yields of
back-to-back π0 pairs in p+Al and p+Au collisions compared to the p+p data. The
observed suppression is larger at smaller transverse momentum, which indicates lower
x and Q2 . The larger suppression found in p+Au relative to p+Al collisions exhibits a
dependence of the saturation scale Q2s on the mass number A. A linear scaling of the
suppression with A1/3 is observed with a slope of −0.09±01.
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Figure: Relative area of back-to-back
di-π0 correlations at forward
pseudorapidities (2.6 < η < 4.0) in p+Au and p+Al with respect to p+p
collisions for $p_T^{\mathrm{trig}}$=1.5–2 GeV/c and $p_T^{\mathrm{asso}}$=1–1.5
GeV/c. The area is the integral of the
back-to-back correlation after pedestal
subtraction. The data points are fitted by
a linear function, whose slope (P) is
found to be −0.09±01.
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Posted August 22, 2022
Previous STAR Focus Features
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STAR focus: Pattern of Global Spin Alignment of φ and K*0 mesons in Heavy-Ion Collisions
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The STAR Collaboration recently published the “Pattern of Global Spin Alignment of φ and K*0 mesons in Heavy-
Ion Collisions” in Nature.
Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in
terms of meson exchange, a full understanding of the strong interaction remains a major challenge in modern
science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the
phenomenon of quark confinement at distances on the order of the size of the proton. Here we show that in
relativistic heavy-ion collisions, where quarks and gluons are set free over an extended volume, two species of
produced vector (spin-1) mesons, namely φ and K*0, emerge with a surprising pattern of global spin alignment. In
particular, the global spin alignment for φ is unexpectedly large, while that for K*0 is consistent with zero. The ρ00
for φ mesons, averaged over beam energies between 11.5 and 62 GeV is 0.3512 ± 0.0017 (stat.) ± 0.0017 (syst.).
Taking the total uncertainty as the sum in quadrature of statistical and systematic uncertainties, our results indicate
that the φ-meson ρ00 is above 1/3 with a significance of 7.4σ. Th ρ00 for K*0, averaged over beam energies of 54.4
GeV and below is 0.3356 ± 0.0034 (stat.) ± 0.0043 (syst.).
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Figure: Global spin alignment of φ and K*0 vector mesons in
heavy-ion collisions. The measured matrix element ρ00 as a
function of beam energy for the φ and K*0 vector mesons
within the indicated windows of centrality, transverse
momentum (pT ) and rapidity (y). The open symbols indicate
ALICE results for Pb+Pb collisions at 2.76 TeV. The red solid
curve is a fit to data in the range of $\sqrt{s_{\mathrm{NN}}}$ = 19.6 to 200 GeV,
based on a theoretical calculation with a φ-meson field. Th
red dashed line is an extension of the solid curve with the
fitted parameter Gs(y). The black dashed line represents ρ00 =
1/3.
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Measurements of the global spin alignment of vector mesons is argued to provide new knowledge about the vector
meson fields. The vector meson fields are an essential part of the nuclear force that binds nucleons inside atomic
nuclei, and are also pivotal in describing properties of nuclear structure and nuclear matter. The ρ00 for the φ meson
has a desirable feature in that all contributions depend on squares of field amplitudes; it can be regarded as a field
analyzer which makes it possible to extract the imprint of the φ-meson field even if the field fluctuates strongly in
space-time. Another important feature worthy of mention is that an essential contribution to the φ-meson ρ00 is from
the term $~S\cdot(\mathrm{E}_{\phi}\times \mathrm{p})$, where Eφ is the electric part of the φ-meson field induced by the local, net strangenes
current density, and S and p are the spin and momentum of the strange (anti)quarks, respectively. Such a term is
nothing but the quark version of the spin-orbit force which, at the nucleon level, plays a key role in the nuclear shell
structure. Our measurements of a signal based on global spin alignment for vector mesons reveal a surprising pattern
and a value for φ meson that is orders of magnitude larger than can be explained by conventional effects. This work
provides a potential new avenue for understanding the strong interaction at work at the sub-nucleon level.
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Posted January 19, 2023
Previous STAR Focus Features
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STAR focus: Measurements of proton high order cumulants in $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV Au+Au
collisions and implications for the QCD critical point
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With the discovery of the quark-gluon plasma (QGP) at the Relativistic Heavy Ion Collider
(RHIC), physicists are starting to investigate the phase structure of the QCD matter, especially in
the high baryon density region. The stark differences between the properties of QGP and lower
energy nuclear matter draw interest to the thermodynamic processes, specifically those related to
the nature of phase transitions. Experimenters can access the QCD phase diagram, expressed in
temperature (T) and baryonic chemical potential (μB), and search for phase boundaries by varying
the heavy-ion collision energy. At regions of equal baryon and anti-baryon density, μB = 0,
theoretical approaches work well, with lattice QCD calculations predicting a smooth cross-over
transition from hadronic matter to a QGP. In addition, Lattice QCD calculations have predicted a
positive cumulant ratio of net-proton (proton minus anti-proton) C4/C2 for the formation of QGP
matter at μB = 200 MeV [1]. However, at finite μB, the existence and the nature of the phase transition
are not well understood.
Recent reports on net-proton fluctuation measurements from RHIC's Beam Energy Scan program
(BES-I) have demonstrated the non-monotonic collision energy dependence in the 4TH order net-proton
cumulant ratio C4/C2 from the top 5% central Au+Au collisions of the range of 7.7 – 200
GeV [2].
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Figure: Collision energy dependence of the ratios of
cumulants, C4 /C2 , for proton (squares) and
net-proton (red circles) from top 5% Au+Au collisions
at RHIC. The points for protons are shifted
horizontally for clarity. The new result for proton
from the $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV collisions is shown as a filled
square. HADES data of 2.4 GeV top 10% collisions
is also shown. The vertical black and gray bars are
the statistical and systematic uncertainties,
respectively. In addition, results from the HRG
model, based on both Canonical Ensemble (CE) and
Grand-Canonical Ensemble (GCE), and transport
model UrQMD are presented.
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In this Letter [3], we report cumulants and their ratios of proton
multiplicity distribution from $\sqrt{s_{\mathrm{NN}}}$ =3 GeV
Au+Au collisions. The new data are measured by the STAR experiment configured in
fixed-target mode. At this collision energy, the corresponding baryonic chemical potential μB ~ 750 MeV,
close to the largest value ever reached in heavy ion collisions. Protons are measured with the
acceptance (-0.5 < y < 0 and 0.4 < pT < 2.0 GeV/c). The rapidity and transverse momentum
dependencies of the cumulant ratios C2/C1, C3/C2 , and C4/C2 are discussed. As shown in the
figure, a suppression with respect to the Poisson baseline is observed in proton C4/C2 = -0.85
0.09 (stat) 0.82 (syst) in the most central collisions at $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV. The hadronic transport model
UrQMD reproduces the observed trend in the centrality dependence of the cumulant ratios. This
new result is consistent with fluctuations driven by baryon number conservation at the high
baryon density region. These data imply that the QCD critical region, if created in heavy-ion
collisions, could only exist at energies higher than $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV.
References:
[1] A. Bazavov et al. (HotQCD), Phys. Rev. D96, 074510 (2017) and R. Bellwied et al., Phys.
Rev. D104, 094508 (2021).
[2] (STAR Collaboration), Phys. Rev. Lett. 126 (2021) 92301.
[3] (STAR Collaboration), Phys. Rev. Lett. 128, (2022) 202303.
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Posted May 29, 2022
Previous STAR Focus Features
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