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STAR focus: Measurement of Groomed Jet Substructure Observables in pp Collisions at $\sqrt{s} = 200$ GeV with STAR
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The STAR collaboration has recently published the first "Measurements of Groomed Jet Substructure Observables in pp Collisions
at $\sqrt{s} = 200$ GeV with STAR" in Phys. Lett. B Volume 811.
This paper presents differential measurements of jets substructure via the SoftDrop momentum fraction ($z_{\rm{g}}$) and groomed jet radius
($R_{\rm{g}}$) for jets in the kinematic range $15 < p_{\rm{T}} < 60$ GeV/$c$ and for a variety of jet resolution parameters from $R=0.2$ to $R=0.6$.
These substructure measurements are expected to be sensitive to the modeling of jet evolution in vacuum, including both perturbative and
non-perturbative parts of the jet shower and serve as a baseline for future measurements in heavy ion collisions.
The measurements are fully unfolded and corrected to particle level in 2-dimensions i.e., $p_{\rm{T, jet}}$ and $z_{\rm{g}}$ or $R_{\rm{g}}$ via
bayesian unfolding as implemented in the RooUnfold package. We find the STAR tuned PYTHIA 6 model is able to quantitatively reproduce the trends of
both substructure observables in data whilst LHC tuned PYTHIA 8 and HERWIG 7 are unable to describe both measurements and end up predicting larger
opening angle for jets (PYTHIA 8) or more symmetric splittings (HERWIG 7), respectively. These comparisons highlight the need for further tuning of
MC models at varied center of mass energies and for understanding hadronization effects on jet evolution at RHIC kinematics.
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Figure: Radial scans of the SoftDrop $z_{\rm{g}}$ in pp collisions at $\sqrt{s} = 200$ GeV for
anti-k$_{\rm{T}}~R=0.2$ (left), $R=0.4$ (middle)
and $R=0.6$ (right) jets of varying transverse momenta ($15 < p_{\rm{T, jet}}< 20$ GeV/$c$ and $30 < p_{\rm{T, jet}} < 40$ GeV/$c$ in the top
and bottom rows respectively). The measurements are compared to various MC models shown in the colored lines.
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The differential measurements enable radial and $p_{\rm{T}}$ scans of the jet substructure which show significant modifications to
the $z_{\rm{g}}$ shape for jets with smaller resolution parameters and lower $p_{\rm{T, jet}}$ with respect to the ideal DGLAP splitting function,
and do not reproduce the characteristic $1/z$ shape seen at higher $p_{\rm{T, jet}}$. We understand this as a consequence of significantly
constricting the phase space for radiation within the reconstructed jets.
We also compared our measurements to recent calculations at next-to-leading-log accuracy for $R_{\rm{g}}$. These
predictions are for jets at the parton level without non-perturbative corrections, with large systematic uncertainties arising from scale
variations close to $\Lambda_{QCD}$. We see large discrepancies between the calculations and data for all of the jet resolution parameters and
momenta except at the largest resolution parameter and highest $p_{\rm{T, jet}}$ where the scales are strictly perturbative. These comparisons
highlight the need for more realistic calculations, including corrections arising from non-perturbative effects and higher-order corrections at
small jet scales to further quantitatively understand the jet substructure at RHIC energies.
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Posted December 1, 2020
Previous STAR Focus Features
Twenty Years of STAR Features
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STAR focus: Measurement of Inclusive Charged-Particle Jet Production in Au + Au collisions
at $\sqrt{s_{\rm{NN}}}=200$ GeV
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The STAR Collaboration has recently published "Measurement of inclusive charged-particle jet production in Au + Au collisions
at $\sqrt{s_{\rm{NN}}}=200$ GeV” in Physical Review C 102, 054913 (2020)
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Collisions of heavy atomic nuclei at the Relativistic Heavy Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN generate tiny
droplets of matter under conditions of extreme temperature and density, similar to those of the early universe a few microseconds after the
Big Bang, called the Quark-Gluon Plasma (QGP). The QGP, which has been studied at colliders for two decades, is a “perfect liquid,” with exotic
properties. Among the most important experimental tools to study the QGP are jets, from rare hard scatterings of quarks and gluons from the
colliding nuclei, and which are seen in the detectors as correlated sprays of particles. Jets generated in head-on (“central”) nuclear collisions
plough through the QGP and interact with it before flying off to the detectors. This interaction causes the jets to lose energy (“jet quenching”),
suppressing their production rate relative to that in proton-proton collisions and other simple systems, where a QGP is not expected to be
formed.
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Figure 1. STAR event display of a central (head-on) Au + Au collision with back-to-back jets.
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Since the beginning of the RHIC program STAR has played a key role in the discovery and elucidation of jet quenching, and it continues to pioneer
in this area. Figure 1 shows a STAR event display of a central Au + Au collision, including a pair of energetic jets that are back-to-back in
azimuth at 90 degrees to the beam direction, as expected from the hard scattering of incoming quarks or gluons. While such jets are easy to see
when highlighted in color, finding and measuring them accurately in the complex environment of Au + Au collisions is very challenging. Solving
this problem has required the development of novel approaches to background suppression.
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Using these novel techniques, STAR recently reported the first measurement of jet yield suppression in central Au + Au collisions at RHIC,
opening up a new chapter in the study of jet quenching. Figure 2 shows the strong yield suppression of jets in central Au + Au collisions compared
to that in glancing (“peripheral”) collisions (filled blue points). The figure also shows a similar measurement by ALICE for jets at the LHC
(filled red) and for single charged particles at both RHIC and LHC (faded blue and red); such comparisons provide crucial constraints on
theoretical models. These new data are a significant step towards meeting the goal of the 2015 NSAC Long Range Plan to explore the inner workings
of the QGP using jet probes.
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Figure 2. New STAR measurement of the yield suppression of jets in head-on Au + Au collisions (filled blue points). Absence of suppression
corresponds to a value of unity. Also shown are similar measurements for jets at the LHC and single charged particles at both RHIC and LHC.
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Posted December 1, 2020
Previous STAR Focus Features
Twenty Years of STAR Features
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STAR focus: Measurement of Inclusive $J/\psi$ Polarization in $p+p$ collisions at $\sqrt{s}$ = 200 GeV
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The STAR Collaboration has recently published “Measurement of inclusive $J/\psi$ polarization in $p+p$ collisions at $\sqrt{s}$ = 200 GeV by the STAR
experiment” in Physical Review D 102, 092009.
The $J/\psi$ meson, a bound state of charm quark and its anti-quark, is one of the simplest systems in Quantum Chromodynamics (QCD). It was
discovered in 1974 but its production mechanism in elementary particle collisions is still not fully understood. One of the difficulties is that the
transition from charm and anti-charm quark interstate to the final-state color neutral meson involves soft processes, which cannot be calculated
perturbatively and has to rely on modeling. The most popular models on the market are the Color Singlet Model (CSM), Color Evaporation Model and
Non-relativistic QCD (NRQCD) framework. These models can describe the production yields measured from SPS to LHC energies reasonably well, but
could not match the measured polarization consistently. Measurements of $J/\psi$ polarization provide powerful tests and constraints on
modelling the $J/\psi$ production mechanism in vacuum.
The polarization parameters are measured via the angular distributions of the decayed leptons in the rest frame of the $J/\psi$ with respect to a
certain quantization axis (reference frame). This paper presents the first measurement of inclusive $J/\psi$ polarization parameters
$\lambda_\theta$, $\lambda_\phi$, $\lambda_{\theta\phi}$ in two reference frames (Helicity frame and Collis-Soper frame) via both di-electron and
di-muon decay channels. The results are shown as a function of transverse momentum in the figure and compared to calculations from various theoretical
models. The inclusive $J/\psi$’s do not exhibit significant transverse or longitudinal polarization. Among several model calculations, the NRQCD coupled
with Color-Glass-Condensate (CGC) implementation agrees the best overall with data. The data presented in this paper provide additional tests and
valuable guidance for theoretical efforts towards a complete understanding of the $J/\psi$ production mechanism in vacuum.
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Figure: $J/\psi$ polarization parameters $\lambda_\theta$, $\lambda_\phi$, $\lambda_{\theta\phi}$ as a function of transverse momentum in
Helicity frame (Left) and Collis-Soper frame (Right) in $p+p$ collisions at $\sqrt{s}$ = 200 GeV measured through di-electron and di-muon decay
channels. Results are compared to various theoretical model calculations.
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Posted November 30, 2020
Previous STAR Focus Features
Twenty Years of STAR Features
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STAR focus: Measurements of $W$ and $Z/\gamma^*$ cross sections and their ratios in $p+p$ collisions at RHIC
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The STAR Collaboration has recently published “Measurements of $W$ and $Z/\gamma^*$ cross sections and their ratios in $p+p$ collisions at RHIC” in
Phys. Rev. D 103, 012001.
One of the fundamental goals of nuclear physics is to understand the proton’s structure and
dynamics. Parton distribution functions (PDFs) of the proton account for the probability of
finding a parton at a given fraction of the proton’s momentum, $x$, and four-momentum
transfer, $Q^2$ . Although PDFs have become more precise, there are still kinematic regions where
more data are needed to help constrain global PDF extractions, such as the ratio of the sea
quark distributions $\bar{d}/\bar{u}$ near the valence region. Furthermore, different measurements
appear to suggest different high-$x$ behaviors of this ratio. The $W$ boson cross-section ratio
($W^+/W^-$) is sensitive to the $\bar{d}/\bar{u}$ distributions at large $Q^2$ . Such a measurement can be
used to help constrain the $\bar{d}/\bar{u}$ ratio.
Through $W$ and $Z$ boson production in $p+p$ collisions at a center-of-mass energy of 510 GeV,
STAR has measured $W$ and $Z$ cross sections via the boson's leptonic decay channel from the
2011, 2012, and 2013 RHIC data sets. The combined result for the $W$ cross-section ratio is
shown in Fig. 1, along with comparisons to several PDF predictions. A PDF reweighting study,
using the new $W^+/W^-$ measurement, was done to provide an initial assessment of the data's
sensitivity for $\bar{d}$, $\bar{u}$, $\bar{u}-\bar{d}$, and $\bar{d}/\bar{u}$ PDF distributions. The reweighting
study shows modest constraining power on the PDFs. However, a proper assessment of the
data's impact on PDF distributions requires a full global PDF analysis, including this STAR data in
the fits used to extract the PDFs.
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Fig. 1: STAR $W^+/W^-$ cross-section ratio measurements as a function of decay lepton
pseudorapidity. Curves show various PDF predictions.
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In addition to the $W$ cross-section ratio, STAR also reports on the measured $W/Z$ cross-section
ratio, differential, and total $W$ and $Z$ cross sections, which are also sensitive to the proton's
quark and antiquark distributions and can constrain proton PDFs further when used in a global
PDF analysis.
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Posted January 8, 2021
Previous STAR Focus Features
Twenty Years of STAR Features
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code of conduct
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The STAR Collaboration
believes that our scientific
mission is best achieved by
building a culture of
inclusion...
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community links
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collaboration links
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Incoming events
Online STAR Collaboration Meeting Mar. 1-12, 2021, Hosted by CTU in Prague and NPI of Czech Academy of Sciences
Past events
Online STAR Analysis Meeting Jan. 6-8, 2021
Online STAR Collaboration Meeting Sept. 14-25, 2020, Hosted by the Indian Institute of Science Education and Research (IISER)
STAR Collaboration Meeting Mar. 11-15, 2020, Berkeley, UC Clark Kerr Campus (Online)
STAR Pre-QM Meeting Oct. 29 - Nov. 1, 2019, Qingdao
STAR Collaboration Meeting Aug. 19, 2019, Cracow
STAR Collaboration Meeting Mar. 29 - Apr 2, 2019, BNL
STAR Analysis Meeting Dec. 12-14, 2018, BNL
ALICE-STAR Indian Meeting Sep. 17-21, 2018, NISER
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recent news
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December 15, 2020
Congratulations to Dr. Rafal Sikora who just successfully defended his
Ph.D. thesis at the AGH UST. His thesis is titled "Measurement of the
diffractive central exclusive production in the STAR experiment at RHIC and
the ATLAS experiment at LHC”.
October 20, 2020
Congratulations to Dr. Yuanjing Ji from USTC who successfully defended her
thesis titled "Measurements of elliptic flow of heavy flavor electrons and
$D^{*+}$ production in Au+Au collisions at RHIC”.
July 15, 2020
Congratulations to Dr. Lukasz Fulek from the AGH UST, who has successfully
defended his PhD thesis titled "Charged particle production in diffractive
proton-proton scattering at the RHIC and LHC energies”.
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