STAR focus: Flow and interferometry results from Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 4.5 GeV

The STAR Collaboration has recently published “Flow and interferometry results from Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 4.5 GeV” in Physical Review C 103, 034908.

A large 1-kg block of VERY cold (-100℃=-148℉) ice is placed on a kitchen stove. It takes an hour for the temperature to change by 300℃. But 40 of those minutes are spent at a single temperature, in the phase transition from liquid to gaseous H2O.

If you heat water at a constant rate, the temperature rises at a nearly constant rate, until it reaches 100 degrees Celsius, at which point the temperature remains constant for some time, while the water vaporizes. During this process, all of the energy goes into the so-called Latent Heat of Vaporization, rather than increasing the temperature. After the phase of the system has changed from liquid to gas, the temperature again increases at a nearly constant rate. The same process, in reverse, occurs when the system cools as energy leaves the system—the temperature drops in one phase, then remains constant for some time, then continues to drop. Latent Heat is a feature of all First Order Phase Transitions (1oPT), and its magnitude is driven by the underlying physics of the system—in this case, the interactions between H2O molecules. Scientists often focus intensely on phase transitions to understand the fundamental nature of a system.

It is currently believed that the transition from the Quark Gluon Plasma (QGP) phase to the hadronic phase is a 1oPT with a Latent Heat driven by the interaction between quarks and gluons. Identifying and quantifying this transition has been one of the driving motivations behind experiments at RHIC. Since at least 30 years ago, theoretical modeling has suggested that the system created in the collision will have a “long” lifetime if the system is near the transition temperature. However the QGP only lives for about a billionth of a trillionth of a second—about the time required for light to cross an atomic nucleus! How can we measure those timescales? Nuclear physicists use quantum correlations between subatomic particles, called pions, emitted from the QGP to measure spatial and temporal scales, a technique known as “femtoscopy”. By systematically changing the initial temperature of the QGP—at RHIC, the beam energy is systematically varied—we look for the difference between the “out” and the “side” radius to first rise, and then fall.

The difference between the "out" and "side" radii is a measure of the lifetime of QGP. It is seen to grow, and then fall, as the collision energy is increased, in agreement with theoretical expectations for a first-order phase transition.

In a recent publication, the STAR Collaboration reports a compelling observation of this long-sought signal that supports the phase transition picture. This observation was possible due to several key features of the experiment. The unprecedented flexibility of the RHIC facility allowed high-statistics, precision measurements over a wide range of closely spaced energies. This included running the experiment in a novel configuration, in which only one nuclear beam impinges on a stationary gold foil. This so-called “fixed target” measurement is the focus of the recent STAR paper. The result at the low collision energy accessible in this mode was crucial, as it defines the left side of the peak structure with much smaller uncertainty than was possible in earlier measurements at similar energies. These earlier results are shown in the figure as open points; their error bars are clearly too large to allow detection of the underlying peak.

After years of systematic experimentation, we now have experimental observation and measurement of the timescale signature consistent with a first-order phase transition in heavy ion collisions. While systematic theoretical study is needed to understand the signal and its physical implications, STAR’s recent publication represents a big step towards mapping the phase structure of hot QCD matter.

Posted May 8, 2021

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STAR focus: Longitudinal Double-Spin Asymmetry for Inclusive Jet and Dijet Production in Polarized Proton Collisions at √s = 200 GeV

How is the spin of the proton distributed among its quark, anti-quark, and gluon constituents? The STAR experiment addressed this fundamental question using collisions of high-energy polarized protons.

We recently published results on the “Longitudinal Double-Spin Asymmetry for Inclusive Jet and Dijet Production in Polarized Proton Collisions at √s = 200 GeV” in Phys. Rev. D 103, L091103 (2021) highlighted as PRD Editors' Suggestion. The data give insight in the gluon spin contribution to the proton spin.

We measured the asymmetries in the differential production cross sections of inclusive jet and dijet probes for different longitudinal spin configurations of the colliding proton beams as a function of jet transverse momentum and dijet invariant mass. Since gluon-gluon and gluon-quark scattering contributions dominate the production of these probes in the STAR environment, the data provide sensitivity to the gluon spin contribution to the proton spin.

Left Figure: Results of the double-spin asymmetry for dijets as a function of dijet invariant mass for two different event topologies. The topologies probe complementary gluon fractional momenta. The green square markers show the new results from data collected in 2015. The blue triangle markers show our prior results based on data collected in 2009 and are seen to be in good agreement. The curves show dijet asymmetry expectations from the DSSV14 and NNPDFpol1.1 theory collaborations and the band indicates the size of the uncertainty in the NNPDFpol1.1 expectation.

Our prior results provided first evidence for a positive polarization of the gluons in the polarized nucleon for gluon fractional momenta larger than 0.05 upon their inclusion in global analyses. Our new results have an approximately twice larger figure of merit, with improved systematic uncertainties, and thus considerably strengthen this evidence. Since we concluded our data taking with longitudinally polarized protons in 2015, these data are anticipated to provide the most precise insights in gluon polarization well into the future, likely until the future Electron-Ion Collider comes online.

Posted May 27, 2021

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STAR focus: Transverse Single-Spin Asymmetries of π0 and Electromagnetic Jets at Forward Rapidity

The STAR Collaboration has recently published an article titled "Measurement of transverse single-spin asymmetries of π0 and electromagnetic jets at forward rapidity in 200 and 500 GeV transversely polarized proton-proton collisions” in Physical Review D 103 092009 (2021).

Large transverse single-spin asymmetries (TSSA) had been observed for hadrons produced in transversely polarized hadron-hadron collisions. Based on the QCD framework, the TSSAs can originate from two possible sources. One being an initial state effect which is correlated to the parton distribution functions and another being a final state effect related to the fragmentation process. The classical measurement of a π0 TSSA usually mixes the two effects. This paper reports measurements of the π0 TSSA together with the TSSA for EM-jets and a TSSA sensitive to the Collins mechanism. The latter two results are sensitive to either the initial or final state effect respectively. The π0 TSSA result shows a weak energy dependence. The jet TSSA is small but non-zero, and the Collins asymmetry is consistent with zero.

In this paper, we also present a novel phenomenon of the π0 TSSA. In the analysis, it was found that the π0 TSSA is not only governed by its kinematics (energy and transverse momentum) but also by the event topology. The π0s which have no other energy around them are defined as “isolated π0s”, which tend to have a larger TSSA than “non-isolated π0s”. The figure below shows that both for √s = 200 GeV and 500 GeV, the π0 TSSAs for the isolated π0s are significantly larger than for non-isolated π0s. This difference suggests different underlying mechanisms for the two types of π0, which challenges our understanding to the origin of the π0 TSSA. Diffractive production may be a suspect for the source of isolated π0, this result certainly needs further efforts from both theorists and experimentalists.

Figure: The transverse single-spin asymmetry as a function of xF for the isolated and non-isolated π0 in transversely polarized proton-proton collisions at √s = 200 and 500 GeV. The error bars are statistical uncertainties only. A systematic uncertainty up to 5.8% of AN for each point is smaller than the size of the markers. Theory curves based on a recent global fit are also shown. The average pT of the π0 for each xF bin is shown in the lower panel. The isolated π0 TSSAs are significantly larger than those for non-isolated π0s.

Posted June 3, 2021

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STAR focus: Methods for a blind analysis of isobar data collected by the STAR collaboration

The STAR Collaboration has recently published “Methods for a blind analysis of isobar data collected by the STAR collaboration” in Nuclear Science and Techniques.

For more than a decade, STAR has searched for evidence of chiral magnetic effects (CME), which refer to induction of an electric current by the magnetic field in a chiral system. In 2018 STAR collected data from isobaric nuclei, ${^{96}_{44}Ru}+{^{96}_{44}Ru}$ and ${^{96}_{40}Zr}+{^{96}_{40}Zr}$. Varying the number of protons while maintaining a consistent number of nucleons presents an opportunity to vary the initial magnetic field while keeping background contributions approximately the same.

For the first time, STAR has implemented blind analyses of these data for CME-related studies. Many of the typical blind analysis methods are not well-suited to the specific needs of the isobar CME analyses. For example, many blind analysis methods “hide” or “offset” variables or information needed to gain sensitivity to signals. For the isobar analysis, randomizing the sign of charged-particle tracks or randomizing particle azimuthal angle would blind information needed for the CME signal. However, so doing would also prevent charge-dependent efficiency corrections and destroy correlations from secondary decays, respectively. Consequently, STAR developed a new method for blind analysis of isobar collision data.

The STAR isobar blind analysis method is a three-step process. In the first step, analysts are provided output files composed of events from a mix of the two isobar species. To the extent possible, the order of events respects temporal changes in running conditions. Events are randomly rejected at the level of ~10% to prevent determining the species by simply counting the number of events associated with a particular run or event trigger.

Analysts use this mixed data sample to tune analysis code and time-dependent Q/A. In the second step, analysts are provided an “unmixed-blind” sample of data comprised of files that obscure the true run number—hence, the isobar species—but do not mix events across different runs. This sample enables species-blind run-by-run Q/A and only run-by-run corrections and code alteration directly resulting from these corrections are allowed at this stage. Once Q/A is complete and analysis of the run-by-run Q/A data are final, full un-blinding proceeds. In this third stage, physics results are produced with the previously tuned, vetted, and fixed analysis codes.

Work toward the blind analysis began, even before the isobar data were collected. To the extent possible, information pertaining to the isobar species was restricted during the run. In assembling the computing machinery for data production, information such as the data-taking run, RHIC fill, event timestamp, collision species, ZDC hit rates, etc., were obfuscated to blind the collision species. Physics analysts participated in a “mock-data challenge.” In this study, analysts were presented data from collisions at GeV, also collected in 2018, produced in the same three-step manner as intended for the blind isobar productions. This exercise served as an opportunity for the software and computing team to develop, tune, and test the machinery necessary for the blind isobar data samples. Furthermore, the mock-data challenge served as an opportunity for analysts to test the feasibility of the methods to enable a robust data analysis for their particular physics observables. An example of a quality-assurance plot from the mock data challenge is shown, above.

The procedure described in our paper was accepted by the STAR institutional council in January 2018, prior to the start of the isobar collision runs. The isobar analysis is well underway following the procedure outlined in the published manuscript.

Posted May 17, 2021

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May 24, 2021
Congratulations to Dr. Pengfei Wang who successfully defended his PhD thesis at University of Science and Technology of China. Dr. Wang’s thesis was titled "Upsilon Measurement in Au+Au Collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV from the STAR Experiment".

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”.

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