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Abstract The pseudorapidity distribution of charged hadrons produced in Au+Au collisions at a center-of-mass energy of s NN$$ \\sqrt{{\\textrm{s}}_{\\textrm{NN}}} $$= 200 GeV is measured using data collected by the sPHENIX detector. Charged hadron yields are extracted by counting cluster pairs in the inner and outer layers of the Intermediate Silicon Tracker, with corrections applied for detector acceptance, reconstruction efficiency, combinatorial pairs, and contributions from secondary decays. The measured distributions cover |η| < 1.1 across various centralities, and the average pseudorapidity density of charged hadrons at mid-rapidity is compared to predictions from Monte Carlo heavy-ion event generators. This result, featuring full azimuthal coverage at mid-rapidity, is consistent with previous experimental measurements at the Relativistic Heavy Ion Collider, thereby supporting the broader sPHENIX physics program.

Reconstructed jets in heavy ion collisions are a crucial tool for understanding the quark-gluon plasma. The separation of jets from the underlying event is necessary particularly in central heavy ion reactions in order to quantify medium modifications of the parton shower and the response of the surrounding medium itself. There have been many methods proposed and implemented for studying the underlying event substructure in proton-proton and heavy ion collisions. In this paper, we detail a method for understanding underlying event contributions in Au+Au collisions at \\(\\sqrt{s_{NN}}\\) = 200 GeV utilizing the HIJING event generator. This method, extended from previous work by the ATLAS collaboration, provides a well-defined association of \"truth jets\" from the fragmentation of hard partons with \"reconstructed jets\" using the anti-\\(k_T\\) algorithm. Results presented here are based on an analysis of 750M minimum bias HIJING events. We find that there is a substantial range of jet energies and radius parameters where jets are well separated from the background fluctuations (often termed \"fake jets\") that make jet measurements at RHIC a compelling physics program.

The sPHENIX Time Projection Chamber Outer Tracker (TPOT) is a Micromegas based detector. It is a part of the sPHENIX experiment that aims to facilitate the calibration of the Time Projection Chamber, in particular the correction of the time-averaged and beam-induced distortions of the electron drift. This paper describes the detector mission, setup, construction, installation, commissioning and performance during the first year of sPHENIX data taking.

The super Pioneering High Energy Nuclear Interaction eXperiment (sPHENIX) at the Relativistic Heavy Ion Collider (RHIC) will perform high precision measurements of jets and heavy flavor observables for a wide selection of nuclear collision systems, elucidating the microscopic nature of strongly interacting matter ranging from nucleons to the strongly coupled quark-gluon plasma. A prototype of the sPHENIX calorimeter system was tested at the Fermilab Test Beam Facility as experiment T-1044 in the spring of 2016. The electromagnetic calorimeter (EMCal) prototype is composed of scintillating fibers embedded in a mixture of tungsten powder and epoxy. The hadronic calorimeter (HCal) prototype is composed of tilted steel plates alternating with plastic scintillator. Results of the test beam reveal the energy resolution for electrons in the EMCal is \\(2.8\\%\\oplus~15.5\\%/\\sqrt{E}\\) and the energy resolution for hadrons in the combined EMCal plus HCal system is \\(13.5\\%\\oplus 64.9\\%/\\sqrt{E}\\). These results demonstrate that the performance of the proposed calorimeter system satisfies the sPHENIX specifications.

The invariant differential cross section for inclusive electron production in \\(p + p\\) collisions at \\(\\sqrt{s} = 200\\)~GeV has been measured by the PHENIX experiment at the Relativistic Heavy Ion Collider over the transverse momentum range \\(0.4 \\le p_T \\le 5.0\\)~GeV/\\(c\\) in the central rapidity region (\\(|\\eta| \\le 0.35\\)). The contribution to the inclusive electron spectrum from semileptonic decays of hadrons carrying heavy flavor, {\\it i.e.} charm quarks or, at high \\(p_T\\), bottom quarks, is determined via three independent methods. The resulting electron spectrum from heavy flavor decays is compared to recent leading and next-to-leading order perturbative QCD calculations. The total cross section of charm quark-antiquark pair production is determined to be \\(\\sigma_{c\\bar{c}} = 0.92 \\pm 0.15 {\\rm (stat.)} \\pm 0.54 {\\rm (sys.)}\\)~mb.

Hot QCD physics studies the nuclear strong force under extreme temperature and densities. Experimentally these conditions are achieved via high-energy collisions of heavy ions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). In the past decade, a unique and substantial suite of data was collected at RHIC and the LHC, probing hydrodynamics at the nucleon scale, the temperature dependence of the transport properties of quark-gluon plasma, the phase diagram of nuclear matter, the interaction of quarks and gluons at different scales and much more. This document, as part of the 2023 nuclear science long range planning process, was written to review the progress in hot QCD since the 2015 Long Range Plan for Nuclear Science, as well as highlight the realization of previous recommendations, and present opportunities for the next decade, building on the accomplishments and investments made in theoretical developments and the construction of new detectors. Furthermore, this document provides additional context to support the recommendations voted on at the Joint Hot and Cold QCD Town Hall Meeting, which are reported in a separate document.

The PHENIX experiment at RHIC should be sensitive to decays of the the anti--pentaquark \\(\\bar{\\Theta}^-\\) via the K\\(^-\\) \\(\\bar{n}\\) channel. Charged kaons can be identified using the standard tracking and time of flight up to a momentum of 1.5 GeV/c. Anti--neutron candidates are detected via their annihilation signal in the highly segmented electromagnetic calorimeter (EMCal). In order to assess the quality of the anti--neutron identification we reconstruct the \\(\\bar{\\Sigma} \\to \\bar{n}\\pi\\). As an additional crosscheck the invariant mass of K\\(^+\\) \\(\\bar{n}\\) is reconstructed where no resonance in the pentaquark mass range is expected. At the present time no enhancement at the expected pentaquark mass is observed in dAu collisions at $\\sqrt{s_{NN}} = 200 GeV.

We have measured the proton elliptic flow excitation function for the Au + Au system spanning the beam energy range 2 -- 8 AGeV. The excitation function shows a transition from negative to positive elliptic flow at a beam energy, \\(E_{tr} \\sim\\) 4 AGeV. Detailed comparisons with calculations from a relativistic Boltzmann-equation are presented. The comparisons suggest a softening of the nuclear equation of state (EOS) from a stiff form (K \\sim 380 MeV) at low beam energies (E_{Beam} \\le 2 AGeV) to a softer form (K \\sim 210 MeV) at higher energies (E_{Beam} \\ge \\( 4 AGeV) where the calculated baryon density \\) \\rho \\sim 4 \\rho_0$.

The E895 experiment at the AGS measured strange particle production and collective behavior in Au+Au collisions between 2--8 AGeV. The production of \\(\\Lambda\\) Baryons and K\\(^0\\) Mesons as function of energy rises smoothly and exhibits a nonlinear impact parameter dependence. Neutral and positively charged Kaons exhibit a strong anti-flow behavior. \\(\\Lambda\\) Baryons show a smaller flow signal than protons.

Using a relativistic hadron transport model, we investigate the utility of the elliptic flow excitation function as a probe for the stiffness of nuclear matter and for the onset of a possible quark-gluon-plasma (QGP) phase-transition at AGS energies 1 < E_Beam < 11 AGeV. The excitation function shows a strong dependence on the nuclear equation of state, and exhibits characteristic signatures which could signal the onset of a phase transition to the QGP.