Determining the nature of phase transition from hadronic matter to the Quark-Gluon Plasma (QGP) phase of the Quantum Chromodynamics (QCD) phase diagram at finite net-baryon density has been the focus of the RHIC beam energy scan program. The directed flow (v₁) and elliptic flow (v₂) are excellent probes for studying properties of the nuclear matter created in high-energy nuclear collisions owing to their sensitivity to the expansion dynamics. On the other hand, v₁ and v₂ are particularly sensitive to the Equation-of-State (EoS) and degrees of freedom of nuclear matter. The large positive v₂ along with the observation of its number-of-constituent-quarks (NCQ) scaling are strong evidence for the formation of a hydrodynamically expanding QGP phase with partonic degrees of freedom. Flow measurements at high baryon density region, e.g. 3 GeV, provide information that the created nuclear matter is dominant by hadronic or partonic degrees of freedom, thus explore the QCD phase structure.

STAR collaboration recently published a new measurement of v₁ and v₂ for identified hadrons in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 3, 27, and 54.4 GeV (Phys. Lett. B 827 (2022) 137003). The lowest collision energy that RHIC has reached, 3 GeV, corresponds to a baryon chemical potential $\mu_{B} \sim$ 750 MeV on the QCD phase diagram. The data were taken under the fixed-target configuration with the STAR detector covering the full acceptance from mid-rapidity to the target rapidity region. In this paper, at the two higher collision energies, the observed v₂ values at mid-rapidity are positive and consistent with previous measurements. Meanwhile, the NCQ scaling of v₂ holds well indicating partonic collectivity has been built-up. Contrary to the results from higher collision energies, the measured v₂ values at mid-rapidity for all hadrons are negative and the NCQ scaling disappears for the positively charged particles in 3 GeV Au+Au collisions. In addition, the v₁ slopes at mid-rapidity for almost all observed hadrons are found to be positive, implying dominant repulsive baryonic interactions. Furthermore, calculations of hadronic transport models with a baryonic mean-field qualitatively reproduced the data. These observations imply the vanishing of partonic collectivity and a new EoS, likely dominated by baryonic interactions in the high baryon density region.

Figure: v₂ scaled by the number of constituent quarks, v₂/n_q, as a function of scaled transverse kinetic energy ((m_T-m₀)/n_q) for pions, kaons, and protons from Au+Au collisions in 10-40% centrality at $\sqrt{s_{\rm NN}}$ = 3, 27, and 54.4 GeV for positively charged particles (left panel) and negatively charged particles (right panel.) The measurements are in the rapidity range |y| < 0.5 at 27 and 54.4 GeV, and in $-$0.5 < y < 0 at 3 GeV. Color dashed lines represent the scaling fit to data from Au+Au collisions at 7.7, 14.5, 27, 54.4, and 200 GeV from STAR experiment at RHIC. Statistical and systematic uncertainties are shown as bars and gray bands, respectively.

Posted March 10, 2022

Previous STAR Focus Features

The STAR Collaboration recently published the first “Differential measurements of jet substructure and partonic energy loss in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}} =$ 200 GeV" in Phys Rev C 105 (2022) 4, 044906.

Parton energy loss serves as the earliest signature of the Quark-Gluon Plasma (QGP) in central heavy-ion collisions. Hard-scattered partons traverse through the QGP, and through their interactions with the medium, lose energy via many different channels. Thus it is crucial to study the degree to which the energy loss is dependent on the parton shower topology. In this measurement, for the first time, STAR has quantified an angular or resolution scale for a particular jet, and differentially measured the energy loss for two populations, one with wider opening angles and one with narrower angles.

Data from Au+Au collisions at $\sqrt{s_{\mathrm{NN}}} =$ 200 GeV, from 2007, with a single high energy calorimeter tower are selected and both trigger and recoil jets are reconstructed with charged particle tracks and towers with E_T > 2 GeV. These di-jets with a hard-constituent selection are termed Hard-Core di-jets, which are primarily free of any combinatorial background contribution. Once we find Hard-Core jets, we drop the 2 GeV threshold down to 0.2 GeV and run the jet finder over all the soft tracks and towers. The di-jets which geometrically match in η-φ space to our Hard-Core di-jets are now called the Matched di-jets. The advantage of such a procedure is to provide us with two collections of di-jets originating from a hard scattering, one with only high momentum particles which undergo energy loss, and the other including both high and low momentum particles, which can potentially include the recovery of the quenched energy. The reference dataset for this measurement is proton-proton collisions also at $\sqrt{s}$ = 200 GeV collected in 2006 and embedded into minimum-bias AuAu collisions from 2007. This embedded reference ensures that the effect of the heavy-ion background and the STAR detector are comparable and any potential differences between the datasets can be attributed to the effects of topology/substructure dependent energy loss.

For the differential measurement, the di-jet pairs are tagged based on the recoil jet’s opening angle defined using the subjets. This is a new substructure observable introduced in this publication which re-clusters the tracks and tower constituents of the anti-k_T R=0.4 jet into subjets with the same anti-k_T algorithm but a smaller jet radius of R=0.1. The leading and sub-leading subjets are then selected and the η-φ distance between the subjet axes is taken as the opening angle observable θ_SJ. The figure presented above shows both the di-jet asymmetry for Matched di-jets on left and a cartoon showing the subjet opening angle on the right for the recoil jets. The blue markers represent narrow recoil jets and the red markers include the wide subjets. For both, wide and narrow jet populations, we find that the energy loss experienced by high p_T particles is fully recovered within the jet cone included in soft particles as shown by a balanced A_J of Matched di-jets for both Au+Au (solid markers) and the embedded reference (open markers). This shows the first evidence for energy loss being independent on the jet topology, i.e. its opening angle. With such differential measurements, we can now quantify the mechanism of energy loss for specially selected di-jets are due to soft-gluon radiation off a single color charge undergoing the QCD equivalent of the LPM effect.

Posted May 4, 2022

Previous STAR Focus Features

Figure: (a) The HFE cross section at STAR in p+p collisions at $\sqrt{s}$ = 200 GeV from 2012 (filled circles) and the FONLL calculation (curves). (b) Ratio of data over FONLL calculation. The vertical bars and the boxes represent statistical and systematic uncertainties, respectively.

Posted March 8, 2022

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In a recent measurement published in Phys. Rev. Lett. 128, 122303, the STAR Collaboration has reported a result on colliding high-energy photons with gluons inside the deuteron. It has provided a first glimpse of the gluonic structure of the simplest atomic nucleus. The momentum distribution of gluons, measured through the J/ψ particle’s momenta shown in the figure, reflects the spatial distribution of gluons inside the nucleus. In addition, the breakup of the deuteron associated with the J/ψ particle probes the gluon dynamics of the nucleon-nucleon interaction, laying the foundation for its precision measurement at the upcoming Electron-Ion Collider.

Figure: Upper: differential cross section as a function of p²_T,J/ψ of J/ψ photoproduction in UPCs at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. Data for the total diffractive process are shown with solid markers, while data with neutron tagging in the deuteron-going ZDC are shown with open markers. Theoretical predictions based on the saturation model (CGC) and the nuclear shadowing model (LTA) are compared with data, shown as lines. Statistical uncertainty is represented by the error bars, and the systematic uncertainty is denoted by the shaded box. Lower: ratios of total data and models are presented as a function of −t ≈ p²_T,J/ψ. Color bands are statistical uncertainty based on the data only, while systematic uncertainty is indicated by the gray box.

Measuring diffractive Vector-Meson production, e.g. J/ψ, has been one of the most powerful tools in studying the nucleon and nucleus structure in high energy particle collisions. Instead of using a hadron projectile, a high-energy photon emitted by the gold nucleus has been used to probe the inner structure of the deuteron target, with its advantage of being a clean probe. This is type of collision is known as an “Ultra-Peripheral Collision” (UPC). A naïve picture of this process is the following. An incoming photon fluctuates into a quark-antiquark pair and forms a J/ψ particle with close to zero transverse momentum. The gluons jiggling inside the deuteron, although happening very rarely, can kick the J/ψ particle and it would deflect with some momentum. This momentum kick is a Fourier transform of the position of gluons, such that the position and momentum of gluons are the two sides of the same coin. Knowing one side would imply the understanding of the other.

The Zero-Degree-Calorimeter (ZDC), a detector 18 meters away from the center of the STAR main detector, can detect breakup neutrons from the deuteron. Understanding the nuclear breakup has been one of the challenges in Ultra-Peripheral Collisions, as well as at the Electron-Ion Collider. The reported data based on the deuteron has provided an essential experimental baseline on how this system breaks apart and provide quantitative constraints to leading theoretical models.

Posted March 24, 2022

Previous STAR Focus Features