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.

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.

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.

Posted December 1, 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.

Figure 1. STAR event display of a central (head-on) Au + Au collision with back-to-back jets.

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.

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.

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.

Posted December 1, 2020

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Twenty Years of STAR Features

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.

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.

Posted November 30, 2020

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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.

Fig. 1: STAR $W^+/W^-$ cross-section ratio measurements as a function of decay lepton pseudorapidity. Curves show various PDF predictions.

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.

Posted January 8, 2021

Previous STAR Focus Features

Twenty Years of STAR Features