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STAR focus: Evidence of Mass Ordering of Charm and Bottom Quark Energy Loss in Au+Au Collisions at RHIC
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Studying the properties of the Quark Gluon Plasma (QGP) created in heavy-ion collisions
is a main goal of the RHIC physics program. Heavy quarks, i.e., charm and bottom
quarks, have emerged as essential probes of the QGP as they are produced
predominantly at the initial stage of the heavy-ion collisions and subsequently experience
the entire evolution of the QGP. In particular, heavy quarks are expected lose energy in
the QGP via (quasi-)elastic scatterings with the medium constituents and induced gluon
radiation, and QCD predicts that heavy quarks lose less energy than light quarks due to
the so-called “dead cone” effect. Therefore, parton energy loss in the QGP is expected to
follow a hierarchy ordered by parton color charge and mass. Experimentally, nuclear
modification factors, RAA and RCP, are measured to study the parton energy loss.
Recently, the STAR Collaboration published "Evidence of Mass Ordering of Charm and
Bottom Quark Energy Loss in Au+Au Collisions at RHIC" in
Eur. Phys. J. C 82
(2022)1150. In this publication, the new measurements of RAA for inclusive heavy
flavor-decay electrons and separately for bottom- and charm-decay electrons, as well as double
ratios of bottom- and charm-decay electron RAA and RCP in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV are
reported. We find the bottom-decay electron RAA and RCP to be significantly higher than those of
charm-decay electrons. Duke and PHSD model calculations, including mass-dependent
parton energy loss in a strongly coupled medium, are found to be consistent with data.
These observations provide strong evidence of the mass ordering of charm and bottom
quark energy loss in the QGP.
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Figure: (a) The RAA ratio of bottom- to
charm-decay electrons in intervals
of electron pT in Au+Au collisions at
$\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. (b) The RCP ratios of
bottom-decay electrons to that of
charm-decay electrons in intervals
of electron pT in Au+Au collisions at
$\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. The red diamonds
show the ratios of RCP
(0−20%/40−80%), and the blue
circles show the ratios of RCP
(0−20%/20−40%). In all panels the
error bars and the brackets show
statistical and systematic
uncertainties, respectively. The
Duke and PHSD models are
shown as the various lines. The
null hypothesis calculations are
shown as the shaded bands.
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Posted Mar 8, 2023
Previous STAR Focus Features
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STAR focus: Measurement of sequential Upsilon suppression in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV with
the STAR experiment
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A primary goal of the RHIC physics program is to study the properties of the Quark-Gluon Plasma (QGP),
a novel state of QCD matter consisting of deconfined quarks and gluons. Among various probes used, Υ mesons,
bound states of bottom and anti-bottom quark pairs, play a unique role as they are believed to be produced before the QGP formation,
and then get destroyed in the QGP due to the color-screening of the potential between the bottom and anti-bottom quarks as well as scatterings
with medium constituents. There are three Υ states (Υ(1S), Υ(2S), Υ(3S)), which possess different
amounts of binding energies, with Υ(1S) being bounded the strongest while Υ(3S) the weakest.
Consequently, they are expected to experience different levels of yield suppression in the QGP depending
on the interplay between the medium temperature and their binding energies. Measurement of such sequential suppression for the three Y
states can be used to study the modification of the QCD force in the medium and the QGP’s thermodynamic properties.
Recently, the STAR experiment published the first measurement of yield suppression for the three Y states separately
in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV, as shown in the figure below (PRL 130 (2023) 112301).
The level of suppression is quantified using
the nuclear modification factor (RAA), a ratio between the Y yield in Au+Au collisions to that in p+p collisions scaled properly
to account for the trivial geometric difference between nucleon+nucleon and nucleus+nucleus collisions.
In the 0-60% centrality class, RAA for all three Y states are much smaller than 1, indicating large losses of yields in the QGP.
It is worth noting that a large fraction of the measured Y(1S) suppression arises from the decreased Y(2S) and Y(3S) yields that contribute
to the Y(1S) production through decays. Comparing among different Y states, Y(3S) is significantly more suppressed
than Y(1S) while Y(2S) lies in between. The result is consistent with the expected sequential suppression for the Y family in
heavy-ion collisions, and can be used to provide additional constraints to model calculations, a necessary step to infer
QGP properties from these measurements. With the large sample of Au+Au collisions to be collected by the STAR experiment
in 2023 and 2025, a significant improvement in the measurement precision, especially for the excited Y(2S) and Y(3S) states, is foreseen.
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Figure: Left: Y(1S) (circles) and Y(2S)
(squares) RAA as a function of Npart for pT < 10
GeV/c. Data points for Y(2S) are displaced
horizontally for better visibility. The vertical
bars on data points indicate statistical errors,
while the systematic uncertainties are shown
as boxes. Shadowed bands around each
marker depict the systematic uncertainties
from Ncoll . The bands at unity indicate the
global uncertainties. Right: RAA for various Y
states, including the 95% upper limit for
Y(3S), in 0-60% Au+Au collisions.
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Posted Mar 15, 2023
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: Higher-order cumulants and correlation functions of proton multiplicity distributions
in $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV Au+Au collisions at the RHIC STAR experiment
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Experimental observations at the Relativistic Heavy Ion Collider (RHIC) and the
Large Hadron Collider (LHC) have confirmed the existence of Quark-Gluon Plasma
(QGP) in high-energy heavy-ion collisions at μB ~ 0. The transition from hadronic matter
to QGP is believed to be a crossover based on Lattice QCD calculations, but there
may be a first-order phase transition and a critical point at finite baryon chemical
potential in the QCD phase diagram. Higher-order cumulants of conserved quantities
(Baryon number, Electric charge, and Strangeness) have been suggested as sensitive
observables to search for the critical point and phase boundary in the QCD phase
diagram. The fourth-order cumulant ratio (C4/C2) of conserved quantities is
predicted to exhibit a non-monotonic energy dependence due the critical point. A
recent paper on net-proton cumulants from the RHIC beam energy scan program has
demonstrated a non-monotonic trend, but with large statistical uncertainty.
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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 $\sqrt{s_{\rm NN}}$ = 3 GeV collisions is shown as a cyan
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 transport model UrQMD are presented.
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As a longer version of the recent published paper Phys. Rev. Lett. 128, 202303,
the paper Phys. Rev. C 107, 024908 [Editors’ Suggestion] summarizes in detail the
analysis of proton cumulant and correlation function measurements of STAR fixed-target
$\sqrt{s_{\rm NN}}$ = 3 GeV Au+Au collision data. The paper discusses the
effect of pileup which is important in fixed-target data analysis and gives a correction
method in higher-moments measurements. In addition, at low collision energy, due
to limited reference multiplicity to define centrality, a large initial volume fluctuation
effect is seen. A model dependent method to suppress the volume fluctuation effect
is tested in this work and shows that the most central centrality is least affected by
the effect. However the method is not ideal due to its model dependency.
The above figure shows the net-proton and proton fourth-order cumulant ratio
C4/C2 from the top 5% most central Au+Au collisions. As one can see there is a
clear non-monotonic energy dependence in the data but a monotonic dependence in the model
calculation. The cyan filled square at $\sqrt{s_{\rm NN}}$ = 3 GeV represents the
new measurement. The consistency of data with UrQMD calculation (yellow cross)
implies that the collisions occurred at 3 GeV are dominated by hadronic
interactions. It is expected that with more data from the RHIC beam energy scan
phase II, the uncertainty will be reduced.
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Posted Feb 23, 2023
Previous STAR Focus Features
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