Recent results in d+Au and p+Pb collisions at RHIC and the LHC provide evidence for collective expansion and flow of the created medium. We propose a control set of experiments to directly compare ...particle emission patterns from p+Pb, d+Au, and ^{3}He+Au or t+Au collisions at the same sqrts_{NN} . Using a Monte Carlo Glauber simulation we find that a ^{3}He or triton projectile, with a realistic wave function description, induces a significant intrinsic triangular shape to the initial medium. If the system lives long enough, this survives into a significant third-order flow moment v_{3} even with viscous damping. By comparing systems with one, two, and three initial hot spots, one could disentangle the effects from the initial spatial distribution of the deposited energy and viscous damping. These are key tools for answering the question of how small a droplet of matter is necessary to form a quark-gluon plasma described by nearly inviscid hydrodynamics.
We have used the Wide Field Planetary Camera 2 on the Hubble Space Telescope to image a star field in the wing of the Small Magellanic Cloud (SMC), near the H II region N81. The images were taken in ...the F336W, F547M, F675W, and F814W filters. From photometry of stars in this field, we construct color-magnitude diagrams (CMDs) for about 4200 stars and compare them with theoretical isochrones. In one CCD frame, we identify an open cluster with an approximate age between 400 and 560 Myr. After statistically subtracting the cluster stars, the remaining field star CMD shows both a strong upper main sequence and a well-developed red giant branch. The brightest main-sequence stars correspond to at most an age of 100 Myr. We also see a substantial number of turnoff stars with V magnitudes between 20 and 22, corresponding to an age range of 1.3 Gyr to at most 12 Gyr. We discuss the effects of the SMC's extended depth on the analysis. From a comparison of the observed CMD with Monte Carlo simulations, we find that the star formation history for this field is not consistent with a constant rate over the last 12 Gyr. We find that the CMD is consistent with increased star formation from 4-12 Gyr ago and over the past 1.7 Gyr. However, we find reduced star formation rates during the period 1.7-4 Gyr ago, in contrast to studies of star clusters that have suggested that the SMC experienced a burst of star formation 2 Gyr ago.
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.
Fluctuations in the initial conditions for relativistic heavy ion collisions are proving to be crucial to understanding final state flow and jet quenching observables. The initial geometry has been ...parametrized in terms of moments in the spatial anisotropy (i.e., {epsilon}{sub 2},{epsilon}{sub 3},{epsilon}{sub 4},{epsilon}{sub 5},...), and it has been stated in multiple published articles that the vector directions of odd moments are uncorrelated with the even moments and the reaction place angle. In this article, we demonstrate that this is incorrect and that a substantial nonzero correlation exists between the even and odd moments in peripheral Au+Au collisions. These correlations persist for all centralities, though at a very small level for the 0-55% most central collisions.
A novel Hadron Blind Detector (HBD) has been developed for an upgrade of the PHENIX experiment at RHIC. The HBD will allow a precise measurement of electron-positron pairs from the decay of the light ...vector mesons and the low-mass pair continuum in heavy ion collisions. The detector consists of a 50 cm long radiator filled with pure CF
4 and directly coupled in a windowless configuration to a triple Gas Electron Multiplier (GEM) detector with a CsI photocathode evaporated on the top face of the first GEM foil.
A numerical cloud model is used to evaluate the performance of several ice parameterizations. Results from simulations using these schemes are contrasted with each other, with an ice-free control ...simulation, and with observations to determine to what extent ice physics affect the realism of these results. Two different types of tropical convection are simulated. Tropical squall-type systems are simulated in two dimensions so that a large domain can be used to incorporate a complete anvil. Nonsquall-type convective lines are simulated in three dimensions owing to their smaller horizontal scale. The inclusion of ice processes enhances the agreement of the simulated convection with some features of observed convection, including the proportion of surface rainfall in the anvil region, and the intensity and structure of the radar brightband near the melting level in the anvil. In the context of our experimental design, the use of three ice classes produces better results than two ice classes or ice-free conditions, and for the tropical cumuli, the optimal mix of the bulk ice hydrometeors is cloud ice–snow–graupel. We infer from our modeling results that application of bulk ice microphysics in cloud models might be case specific, which is a significant limitation. This can have serious ramifications for microwave interpretation of cloud microphysical properties. Generalization of ice processes may require a larger number of ice categories than we have evaluated and/or the prediction of hydrometeor concentrations or particle-size spectra.
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