Vertex reconstruction algorithms in the PHOBOS experiment at RHIC

The PHOBOS experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is studying interactions of heavy nuclei at the largest energies available in the laboratory. The high multiplicity of particles created in heavy ion collisions makes precise vertex reconstruction possible using information from a spectrometer and a specialized vertex detector with relatively small acceptances. For lower multiplicity events, a large acceptance, single layer multiplicity detector is used and special algorithms are developed to reconstruct the vertex, resulting in high efficiency at the expense of poorer resolution. The algorithms used in the PHOBOS experiment and their performance are presented.Comment: presented at the Workshop on Tracking In high Multiplicity Environments, TIME0

EUV and HXR Signatures of Electron Acceleration During the Failed Eruption of a Filament

We search for EUV brightenings in TRACE 171 {\AA} images and HXR bursts observed during failed eruptions. We expect that if an eruption is confined due to interaction with overlying magnetic structures then we should observe effects connected with reconnection between magnetic structures and acceleration of particles. We utilized TRACE observations of three well observed failed eruptions. EUV images were compared to HXR spatial distribution reconstructed from Yohkoh/HXT and RHESSI data. The EUV light curves of a selected area were compared to height profiles of eruption, HXR emission and HXR photon spectral index of power-law fit to HXR data. We have found that EUV brightenings are closely related to the eruption velocity decrease, to HXR bursts and to episodes of hardening of HXR spectra. The EUV brightened areas are observed far from the flaring structure, in footpoints of large systems of loops observed 30-60 minutes after the maximum of a flare. These are not `post-flare' loops that are also observed but at significantly lower heights. The high lying systems of loops are observed at heights equal to height, at which eruption was observed to stop. We observed HXR source spatially correlated with EUV brightening only once. For other EUV brightened areas we estimated the expected brightness of HXR sources. We find that EUV brightenings are produced due to interaction between the erupting structure with overlying loops. The interaction is strong enough to heat the system of high loops. These loops cool down and are visible in EUV range about 30-60 minutes later. The estimated brightness of HXR sources associated with EUV brightenings shows that they are too weak to be detected with present instruments. However, next generation instruments will have enough dynamic range and sensitivity to enable such observations.Comment: A&A accepte

Latest results from the PHOBOS experiment

Over the past years PHOBOS has continued to analyze the large datasets obtained from the first five runs of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory. The two main analysis streams have been pursued. The first one aims to obtain a broad and systematic survey of global properties of particle production in heavy ion collisions. The second class includes the study of fluctuations and correlations in particle production. Both type of studies have been performed for a variety of the collision systems, covering a wide range in collision energy and centrality. The uniquely large angular coverage of the PHOBOS detector and its ability to measure charged particles down to very low transverse momentum is exploited. The latest physics results from PHOBOS, as presented at Quark Matter 2008 Conference, are contained in this report.Comment: 9 pages, 9 figures, presented at the 20th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions, "Quark Matter 2008", Jaipur, India, Feb.4-10, 200

Exploring impulsive solar magnetic energy release and particle acceleration with focused hard X-ray imaging spectroscopy

How impulsive magnetic energy release leads to solar eruptions and how those eruptions are energized and evolve are vital unsolved problems in Heliophysics. The standard model for solar eruptions summarizes our current understanding of these events. Magnetic energy in the corona is released through drastic restructuring of the magnetic field via reconnection. Electrons and ions are then accelerated by poorly understood processes. Theories include contracting loops, merging magnetic islands, stochastic acceleration, and turbulence at shocks, among others. Although this basic model is well established, the fundamental physics is poorly understood. HXR observations using grazing-incidence focusing optics can now probe all of the key regions of the standard model. These include two above-the-looptop (ALT) sources which bookend the reconnection region and are likely the sites of particle acceleration and direct heating. The science achievable by a direct HXR imaging instrument can be summarized by the following science questions and objectives which are some of the most outstanding issues in solar physics (1) How are particles accelerated at the Sun? (1a) Where are electrons accelerated and on what time scales? (1b) What fraction of electrons is accelerated out of the ambient medium? (2) How does magnetic energy release on the Sun lead to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI) instrument, which can be built now using proven technology and at modest cost, would enable revolutionary advancements in our understanding of impulsive magnetic energy release and particle acceleration, a process which is known to occur at the Sun but also throughout the Universe

The Importance of Correlations and Fluctuations on the Initial Source Eccentricity in High-Energy Nucleus-Nucleus Collisions

In this paper, we investigate various ways of defining the initial source eccentricity using the Monte Carlo Glauber (MCG) approach. In particular, we examine the participant eccentricity, which quantifies the eccentricity of the initial source shape by the major axes of the ellipse formed by the interaction points of the participating nucleons. We show that reasonable variation of the density parameters in the Glauber calculation, as well as variations in how matter production is modeled, do not significantly modify the already established behavior of the participant eccentricity as a function of collision centrality. Focusing on event-by-event fluctuations and correlations of the distributions of participating nucleons we demonstrate that, depending on the achieved event-plane resolution, fluctuations in the elliptic flow magnitude $v_2$ lead to most measurements being sensitive to the root-mean-square, rather than the mean of the $v_2$ distribution. Neglecting correlations among participants, we derive analytical expressions for the participant eccentricity cumulants as a function of the number of participating nucleons, \Npart,keeping non-negligible contributions up to \ordof{1/\Npart^3}. We find that the derived expressions yield the same results as obtained from mixed-event MCG calculations which remove the correlations stemming from the nuclear collision process. Most importantly, we conclude from the comparison with MCG calculations that the fourth order participant eccentricity cumulant does not approach the spatial anisotropy obtained assuming a smooth nuclear matter distribution. In particular, for the Cu+Cu system, these quantities deviate from each other by almost a factor of two over a wide range in centrality.Comment: 18 pages, 10 figures, submitted to PR

System size and centrality dependence of charged hadron transverse momentum spectra in Au+Au and Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV

We present transverse momentum distributions of charged hadrons produced in Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV. The spectra are measured for transverse momenta of 0.25 < p_T < 5.0 GeV/c at sqrt(s) = 62.4 GeV and 0.25 < p_T < 7.0 GeV/c at sqrt(s) = 200 GeV, in a pseudo-rapidity range of 0.2 < eta < 1.4. The nuclear modification factor R_AA is calculated relative to p+p data at both collision energies as a function of collision centrality. At a given collision energy and fractional cross-section, R_AA is observed to be systematically larger in Cu+Cu collisions compared to Au+Au. However, for the same number of participating nucleons, R_AA is essentially the same in both systems over the measured range of p_T, in spite of the significantly different geometries of the Cu+Cu and Au+Au systems.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let

System Size, Energy and Centrality Dependence of Pseudorapidity Distributions of Charged Particles in Relativistic Heavy Ion Collisions

We present the first measurements of the pseudorapidity distribution of primary charged particles in Cu+Cu collisions as a function of collision centrality and energy, \sqrtsnn = 22.4, 62.4 and 200 GeV, over a wide range of pseudorapidity, using the PHOBOS detector. Making a global comparison of Cu+Cu and Au+Au results, we find that the total number of produced charged particles and the rough shape (height and width) of the pseudorapidity distributions are determined by the number of nucleon participants. More detailed studies reveal that a more precise matching of the shape of the Cu+Cu and Au+Au pseudorapidity distributions over the full range of pseudorapidity occurs for the same Npart/2A value rather than the same Npart value. In other words, it is the collision geometry rather than just the number of nucleon participants that drives the detailed shape of the pseudorapidity distribution and its centrality dependence at RHIC energies.Comment: Submitted to Physical Review Letter

Identified charged antiparticle to particle ratios near midrapidity in Cu+Cu collisions at sqrt(s) = 62.4 and 200 GeV

Antiparticle to particle ratios for identified protons, kaons and pions at sqrt(s) = 62.4 and 200 GeV in Cu+Cu collisions are presented as a function of centrality for the midrapidity region of 0.2 < eta < 1.4. No strong dependence on centrality is observed. For the / ratio at ~ 0.51 GeV/c, we observe an average value of 0.50 +/- 0.003_(stat) +/- 0.04_(syst) and 0.77 +/- 0.008_(stat) +/- 0.05_(syst) for the 10% most central collisions of 62.4 and 200 GeV Cu+Cu, respectively. The values for all three particle species measured at sqrt(s) = 200 GeV are in agreement within systematic uncertainties with that seen in both heavier and lighter systems measured at the same RHIC energy. This indicates that system size does not appear to play a strong role in determining the midrapidity chemical freeze-out properties affecting the antiparticle to particle ratios of the three most abundant particle species produced in these collisions.Comment: 5 Pages, 4 figures Made changes to the figures to include the panel numbers. Slight changes to the text. Updated data points from other experiment

Event-by-event fluctuations of azimuthal particle anisotropy in Au+Au collisions at sqrt(s_NN) = 200 GeV

This paper presents the first measurement of event-by-event fluctuations of the elliptic flow parameter v_2 in Au+Au collisions at sqrt(s_NN) = 200GeV as a function of collision centrality. The relative non-statistical fluctuations of the v_2 parameter are found to be approximately 40%. The results, including contributions from event-by-event elliptic flow fluctuations and from azimuthal correlations that are unrelated to the reaction plane (non-flow correlations), establish an upper limit on the magnitude of underlying elliptic flow fluctuations. This limit is consistent with predictions based on spatial fluctuations of the participating nucleons in the initial nuclear overlap region. These results provide important constraints on models of the initial state and hydrodynamic evolution of relativistic heavy ion collisions.Comment: 5 pages, 2 figures, Published in Phys. Rev. Lett

Charged-Particle Pseudorapidity Distributions in Au+Au Collisions at sqrt(s_NN)=62.4 GeV

The charged-particle pseudorapidity density for Au+Au collisions at sqrt(s_NN)=62.4 GeV has been measured over a wide range of impact parameters and compared to results obtained at other energies. As a function of collision energy, the pseudorapidity distribution grows systematically both in height and width. The mid-rapidity density is found to grow approximately logarithmically between AGS energies and the top RHIC energy. As a function of centrality, there is an approximate factorization of the centrality dependence of the mid-rapidity yields and the overall multiplicity scale. The new results at sqrt(s_NN)=62.4 GeV confirm the previously observed phenomenon of ``extended longitudinal scaling'' in the pseudorapidity distributions when viewed in the rest frame of one of the colliding nuclei. It is also found that the evolution of the shape of the distribution with centrality is energy independent, when viewed in this reference frame. As a function of centrality, the total charged particle multiplicity scales linearly with the number of participant pairs as it was observed at other energies.Comment: 6 pages, 7 figures, submitted to Phys. Rev. C - Rapid Communication