W. A. Zajc’s research while affiliated with Columbia University and other places

We present the historical antecedents to the field of relativistic heavy ion physics, beginning with early attempts to model the strong interaction and ending with the endorsement of a relativistic heavy ion collider in the 1983 U.S. Long-Range Plan for Nuclear Science. Particular attention is paid to two major themes: 1) A program to study high density states of nuclear matter emerging from the 1974 Bear Mountain conference and 2) Efforts to understand the predictions of QCD for matter at high densities and/or temperatures.

The PHENIX experiment at the Relativistic Heavy Ion Collider measured the second Fourier component $v_2$ of the direct-photon azimuthal anisotropy at midrapidity in Au+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The results are presented in 10\% wide bins of collision centrality and cover the transverse-momentum range of $1<p_T<20$ GeV/c, and are in quantitative agreement with findings published earlier, but provide better granularity and higher $p_T$ reach. Above a $p_T$ of 8--10 GeV/c, where hard scattering dominates the direct-photon production, $v_2$ is consistent with zero. Below that in each centrality bin $v_2$ as a function of $p_T$ is comparable to the $\pi^0$ anisotropy albeit with a tendency of being somewhat smaller. The results are compared to recent theory calculations that include, in addition to thermal radiation from the quark-gluon plasma and hadron gas, sources of photons from pre-equilibrium, strong magnetic fields, or radiative hadronization. While the newer theoretical calculations describe the data better than previous models, none of them alone can fully explain the results, particularly in the region of $p_T=4$--8 GeV/c.

This paper reports measurements of the transverse energy per unit pseudorapidity ($dE_{T}/d\eta$) produced in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV, performed with the sPHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The results cover the pseudorapidity range $\left|\eta\right| < 1.1$ and constitute the first such measurement performed using a hadronic calorimeter at RHIC. Measurements of $dE_{T}/d\eta$ are presented for a range of centrality intervals and the average $dE_{T}/d\eta$ as a function of the number of participating nucleons, $N_{\mathrm{part}}$, is compared to a variety of Monte Carlo heavy-ion event generators. The results are in agreement with previous measurements at RHIC, and feature an improved granularity in $\eta$ and improved precision in low-$N_{\mathrm{part}}$ events.

The pseudorapidity distribution of charged hadrons produced in Au+Au collisions at a center-of-mass energy of $\sqrt{s_\mathrm{NN}} = 200$ 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 $|\eta| < 1.1$ 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.

This corrects the article DOI: 10.1103/PhysRevLett.116.122301.

PHENIX presents a simultaneous measurement of the production of direct γ and π^{0} in d+Au collisions at sqrt[s_{NN}]=200 GeV over a p_{T} range of 7.5 to 18 GeV/c for different event samples selected by event activity, i.e., charged-particle multiplicity detected at forward rapidity. Direct-photon yields are used to empirically estimate the contribution of hard-scattering processes in the different event samples. Using this estimate, the average nuclear-modification factor, R_{dAu,EXP}^{π^{0}}, is 0.925±0.023(stat)±0.15(scale), consistent with unity for minimum-bias (MB) d+Au collisions. For event classes with low and moderate event activity, R_{dAu,EXP}^{π^{0}} is consistent with the MB value within 5% uncertainty. This result confirms that the previously observed enhancement of high-p_{T} π^{0} production found in small-system collisions with low event activity is a result of a bias in interpreting event activity within the Glauber framework. In contrast, for the top 5% of events with the highest event activity, R_{dAu,EXP}^{π^{0}} is suppressed by 20% relative to the MB value with a significance of 4.5σ, which may be due to final-state effects. This suppression corresponds to a p_{T} shift of δp_{T}=0.213±0.055 Gev/c at 9 Gev/c.

We present the first forward-rapidity measurements of elliptic anisotropy of open-heavy-flavor muons at the BNL Relativistic Heavy Ion Collider. The measurements are based on data samples of Au+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV collected by the PHENIX experiment in 2014 and 2016 with integrated luminosity of 14.5~nb$^{-1}$. The measurements are performed in the pseudorapidity range $1.2<|\eta|<2$ and cover transverse momenta $1<p_T<4$~GeV/c. The elliptic flow of charged hadrons as a function of transverse momentum is also measured in the same kinematic range. We observe significant elliptic flow for both charged hadrons and heavy-flavor muons. The results show clear mass ordering of elliptic flow of light- and heavy-flavor particles. The magnitude of the measured $v_2$ is comparable to that in the midrapidity region. This indicates that there is no strong longitudinal dependence in the quark-gluon-plasma evolution between midrapidity and the rapidity range of this measurement at $\sqrt{s_{_{NN}}}=200$~GeV.