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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: Beam Energy Dependence of Triton Production and Yield Ratio (Nt × Np/Nd2) in Au + Au Collisions at RHIC
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Relativistic heavy-ion collision experiments serve as one of the primary avenues for
studying strong interactions, aiming to explore the properties of Quark-Gluon Plasma
(QGP) and the phase structure of Quantum Chromodynamics (QCD), including the
search for first-order phase transitions in nuclear matter and the QCD critical point,
which marks the endpoint of the phase boundary. Light nuclei are among the main
products in the final stages of heavy-ion collision experiments, characterized by a
small binding energy and finite radius, making them effective probes for exploring
phase boundaries and critical points in the QCD phase diagram. Theoretical studies
propose that the compound yield ratio of light nuclei based on the nucleon
coalescence model (Nt × Np / Nd2) is directly correlated with local neutron density fluctuations in
the system, providing a sensitive measurement for the search of the QCD critical point
and first-order phase transitions.
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Figure: Collision energy, centrality, and pT dependence of the yield ratio Nt × Np / Nd2 in Au + Au collisions
at RHIC. Solid circles are the results from 0%–10% central (left panel) and 40%–80%
peripheral (right panel) collisions. Colored bands in panel (a) denote pT acceptance
dependence, for which the statistical and systematic uncertainties are added in quadrature.
Red solid circles are the final results with extrapolation to the full pT range. Statistical and
systematic uncertainties are shown as bars and brackets, respectively. Red vertical bands on
the right side of panels represent the common systematic uncertainties. Dashed lines are the
coalescence baselines obtained from the coalescence-inspired fit. Shaded areas denote the
calculations from hadronic transport AMPT and MUSIC + UrQMD hybrid models.
Recently, the STAR Collaboration published significant results titled "Beam Energy
Dependence of Triton Production and Yield Ratio (Nt × Np / Nd2) in Au + Au Collisions at RHIC"
in PHYSICAL REVIEW LETTERS 130, 202301 (2023). The paper focuses on the
measurement of triton yields, the extraction of primordial proton yields, and the
results of light nuclei compound yield ratio in the STAR BES-I. We observed that the     Nt × Np / Nd2
ratio exhibits scaling behavior with the system volume, and most importantly, a
significant deviation from the model baseline at 4.1σ was observed in the 0%-10%
central collisions at $\sqrt{s_{\mathrm{NN}}}$ = 19.6 and 27 GeV, which may be due to the enhanced baryon
density fluctuations induced by the critical point or first-order phase transition in
heavy-ion collisions. These systematic measurements of triton yields and yield ratios
over a broad energy range provide important insights into the production dynamics of
light nuclei and our understanding of the QCD phase diagram.
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Posted June 17, 2023
Previous STAR Focus Features
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STAR focus: Measurement of electrons from open heavy-flavor hadron decays in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV with the STAR detector
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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 lose energy through interactions with the QGP via both collisional and radiative processes. These interactions modify the momentum distributions of heavy quarks in heavy-ion collisions compared to that in p+p collisions, and measurements of such modifications provide important insights into the properties of the QGP. Experimentally, nuclear modification factor, $R_{\rm{AA}}$, is measured to study the parton energy loss.
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Figure: HFE $R_{\rm{AA}}$ (red circles) as a function of $p_{\rm{T}}$ in different centrality intervals of Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV, compared with STAR (yellow stars) and PHENIX (green squares) published results, and Duke (blue line) and PHSD (orange line) model calculations. Vertical bars and boxes around data points represent combined statistical and systematic uncertainties from both Au+Au and p+p measurements, respectively. Boxes at unity show the global uncertainties, which for this analysis include the 8% global uncertainty on p+p reference and the $N_{coll.}$ uncertainties. The left box is for PHENIX and the right one for STAR.
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Recently, the STAR Collaboration published "Measurement of electrons from open heavy-flavor hadron decays in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV with the STAR detector " in JHEP 06 (2023) 176. In this publication, the new measurements of $R_{\rm AA}$ for inclusive heavy flavor-decay electrons in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV are reported. We find the HFE yields in head-on Au+Au collisions are suppressed by approximately a factor of 2 compared to that in p + p collisions scaled by the average number of binary collisions ($N_{coll.}$), indicating strong interactions between heavy quarks and the hot and dense medium created in heavy-ion collisions. Comparison of these results with models provides additional tests of theoretical calculations of heavy quark energy loss in the QGP. Furthermore, these results provide an improved reference for $R_{\rm{AA}}$ measurements of charm- and bottom-hadron decayed electrons in heavy-ion collisions.
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Posted Jul 7, 2023
Previous STAR Focus Features
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STAR Collaboration Meeting Fall 2023 Oct. 16 - 20, 2023, AUC
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recent news
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August 29, 2023
Please join us in congratulating Dr. Yang Li from USTC who has successfully
defended his Ph.D. thesis titled “Measurement of identified particle
spectra and tracking the baryon number in Ru+Ru and Zr+Zr collisions at 200
GeV.” His supervisors were Prof. Zebo Tang, Dr. Rongrong Ma, and Dr.
Lijuan Ruan in a collaboration between USTC and BNL.
June 30, 2023
Please join us in congratulating Dr. Daniel Kincses from Eotvos Lorand
University, who has successfully defended his Ph.D. thesis titled
“Experimental and phenomenological investigations of femtoscopic
correlation functions in heavy-ion collisions” which includes both PHENIX
and STAR results. His supervisor was Prof. Mate Csanad.
June 29, 2023
Please join us in congratulating Dr. Sebastian Siejka from Warsaw
University of Technology, who has successfully defended his Ph.D. thesis
titled “Proton’s and antiproton’s femtoscopy at the Beam Energy Scan
Program at the STAR experiment.” His supervisor was Prof. Hanna
Zbroszczyk.
June 27, 2023
Congratulations to Dr. Debasish Mallick from NISER, who successfully
defended on March 31st his Ph.D. thesis titled “Probing thermalization
and deuteron production mechanism via fluctuations in heavy-ion collisions
in STAR at RHIC.". His supervisor was Prof. Bedangadas Mohanty.
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