The exotic structure of heavy nuclei Buck, B; Merchant, A C; Perez, S M ...
Journal of physics. G, Nuclear and particle physics,
12/2005, Letnik:
31, Številka:
12
Journal Article
This paper describes the conclusions that can be drawn from the data taken thus far with the PHOBOS detector at RHIC. In the most central Au
+
Au collisions at the highest beam energy, evidence is ...found for the formation of a very high energy density system whose description in terms of simple hadronic degrees of freedom is inappropriate. Furthermore, the constituents of this novel system are found to undergo a significant level of interaction. The properties of particle production at RHIC energies are shown to follow a number of simple scaling behaviors, some of which continue trends found at lower energies or in simpler systems. As a function of centrality, the total number of charged particles scales with the number of participating nucleons. When comparing Au
+
Au at different centralities, the dependence of the yield on the number of participants at higher
p
T
(
∼
4
GeV
/
c
) is very similar to that at low transverse momentum. The measured values of charged particle pseudorapidity density and elliptic flow were found to be independent of energy over a broad range of pseudorapidities when effectively viewed in the rest frame of one of the colliding nuclei, a property we describe as “extended longitudinal scaling”. Finally, the centrality and energy dependences of several observables were found to factorize to a surprising degree.
This Letter presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and 200 GeV using the PHOBOS detector at the ...Relativistic Heavy Ion Collider. The elliptic flow in Cu-Cu collisions is found to be significant even for the most central events. For comparison with the Au-Au results, it is found that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects. A new form of eccentricity, called the participant eccentricity, is introduced which yields a scaled elliptic flow in the Cu-Cu system that has the same relative magnitude and qualitative features as that in the Au-Au system.
A selection of experimental results from the PHOBOS Collaboration relevant for probing high-energy nuclear collisions with high transverse momentum particles is presented. The inclusive yields of ...charged particles and comparisons between nuclear and elementary collisions already reveal a large amount of parton energy loss in the hot and dense medium created in heavy ion collisions. Remarkable scaling and factorization features are observed, unifying the data taken at various collision energies, centralities and nuclear sizes. To further analyze the nature of the energy loss, a measurement of pseudorapidity (Δη) and azimuthal angle (Δφ) correlations between high transverse momentum charged hadrons (pT>2.5 GeV/c) and all associated charged particles is presented at both short-range (small Δη) and long-range (large Δη) over a continuous detector acceptance covering −4<Δη<2. Various near- and away-side features of the correlation structure are discussed as a function of centrality in Au + Au collisions at \(\sqrt{s_{NN}}=200\) GeV. The results provide new information about the longitudinal (Δη) extent of the near-side ‘ridge’ structure, first observed by the STAR Collaboration over a narrower η range. In central Au + Au collisions the ridge structure extends to at least Δη=4, and its strength completely diminishes as collisions become more peripheral.
Chromatograms of hydrophilic, hydrophobic, and intermediate-polarity analytes were developed in 50-μm capillaries by micellar electrokinetic chromatography at field strengths less than 31 kV/m. The ...analytes were solubilized by phosphate/borate buffers containing 15, 50, and 100 mM sodium dodecyl sulfate (SDS). The plate numbers N of the analytes, as well as those of the electroosmotic flow and micellar markers, were compared to predictions of N estimated by a simple model based on longitudinal diffusion and plug size. Good to fair agreement between theory and experiment was obtained for the hydrophilic and intermediate-polarity analytes in all buffers over the entire field strength range. Good agreement between theory and experiment was obtained for the hydrophobic analyte and micellar marker in all buffers at low field strengths; however, these compounds were subject to dispersion at higher field strengths by what appears to be Joule heating. The magnitudes of other, closely related Joule heating losses are quantified here using temperature profile measurements by Morris and co-workers and Taylor dispersion calculations. In contrast to the commonly reported increase of N with media concentration, the Ns of the hydrophilic and intermediate-polarity analytes were found to be essentially independent of SDS concentration over the investigated SDS range, and the Ns of the hydrophobic species were found to be independent of SDS concentration until (what appears to be) Joule heating became significant. These results were compared to those of Sepaniak and Cole. A critique of some previous studies of N vs SDS concentration is presented, in which quantitative explanations for some dispersions are offered as alternatives to surfactant concentration effects.
This paper establishes a physicochemical basis for the efficiency losses in micellar electrokinetic chromatography in buffers containing sodium dodecyl sulfate (SDS) and 2-propanol (2PN). Weakly, ...intermediately, and strongly retained analytes were separated in phosphate/borate buffers containing 50 mM SDS and from 0 to 10% 2PN by volume. Their plate numbers N generally agreed well with predictions of a theory for N based on longitudinal diffusion and instrumental contributions to dispersion. The N's of weakly and intermediately retained analytes were not affected strongly by 2PN over this concentration range, because their diffusion coefficients varied inversely with buffer viscosity and their retention times largely varied directly with viscosity. These combined effects on dispersion almost canceled. However, the N's of strongly retained analytes decreased with increasing 2PN, because their diffusion coefficients varied inversely with viscosity but their retention times increased more rapidly than did viscosity. These combined effects on dispersion did not cancel. These differences occurred because 2PN penetrated the micelles, caused bound counterions to be released, and increased the micellar charge and electrophoretic mobility. As 2PN concentration increased, the micelles electrophoresced increasingly rapidly against the electroosmotic flow. Consequently, strongly retained compounds required increasingly long times to elute.
We report on measurements of directed flow as a function of pseudorapidity in Au + Au collisions at energies of square root of SNN = 19.6, 62.4, 130 and 200 GeV as measured by the PHOBOS detector at ...the BNL Relativistic Heavy Ion Collider. These results are particularly valuable because of the extensive, continuous pseudorapidity coverage of the PHOBOS detector. There is no significant indication of structure near midrapidity and the data surprisingly exhibit extended longitudinal scaling similar to that seen for elliptic flow and charged particle pseudorapidity density.
PDF
