252 research outputs found
Substrate surface patterning by optical near field modulation around colloidal particles immersed in a liquid
Optical near field enhancements in the vicinity of particles illuminated by laser light are increasingly recognized as a powerful tool for nanopatterning applications, but achieving sub-wavelength details from the near-field distribution remains a challenge. Here we present a quantitative analysis of the spatial modulation of the near optical fields generated using single 8 ps, 355 nm (and 532 nm) laser pulses around individual colloidal particles and small close packed arrays of such particles on silicon substrates. The analysis is presented for particles in air and, for the first time, when immersed in a range of liquid media. Immersion in a liquid allows detailed exploration of the effects on the near field of changing not just the magnitude but also the sign of the refractive index difference between the particle and the host medium. The level of agreement between the results of ray tracing and Mie scattering simulations, and the experimentally observed patterns on solid surfaces, should encourage further modelling, predictions and demonstrations of the rich palette of sub-wavelength surface profiles that can be achieved using colloidal particles immersed in liquids
Scanning the phases of QCD with BRAHMS
BRAHMS has the ability to study relativistic heavy ion collisions from the
final freeze-out of hadrons all the way back to the initial wave-function of
the gold nuclei. This is accomplished by studying hadrons with a very wide
range of momenta and angles. In doing so we can scan various phases of QCD,
from a hadron gas, to a quark gluon plasma and perhaps to a color glass
condensate.Comment: 8 pages, 6 figures, proceedings of plenary talk at Quark Matter 2004
conferenc
Centrality dependence of charged-particle pseudorapidity distributions from d+Au collisions at sqrt(s_{NN})=200 GeV
Charged-particle pseudorapidity densities are presented for the d+Au reaction
at sqrt{s_{NN}}=200 GeV with -4.2 <= eta <= 4.2$. The results, from the BRAHMS
experiment at RHIC, are shown for minimum-bias events and 0-30%, 30-60%, and
60-80% centrality classes. Models incorporating both soft physics and hard,
perturbative QCD-based scattering physics agree well with the experimental
results. The data do not support predictions based on strong-coupling,
semi-classical QCD. In the deuteron-fragmentation region the central 200 GeV
data show behavior similar to full-overlap d+Au results at sqrt{s_{NN}}=19.4
GeV.Comment: 4 pages, 3figures; expanded discussion of uncertainties; added 60-80%
centrality range; added additional discussion on centrality selection bia
High Pt Hadron Spectra at High Rapidity
We report the measurement of charged hadron production at different
pseudo-rapidity values in deuteron+gold as well as proton+proton collisions at
= 200GeV at RHIC. The nuclear modification factors and
are used to investigate new behaviors in the deuteron+gold system as
function of rapidity and the centrality of the collisions respectively.Comment: Nine pages 4 figures to be published in the QM2004 Proceedings, typos
corrected and one reference adde
Rapidity and centrality dependence of particle production for identified hadrons in Cu+Cu collisions at GeV
The BRAHMS collaboration has measured transverse momentum spectra of pions,
kaons, protons and antiprotons at rapidities 0 and 3 for Cu+Cu collisions at
GeV. As the collisions become more central the collective
radial flow increases while the temperature of kinetic freeze-out decreases.
The temperature is lower and the radial flow weaker at forward rapidity. Pion
and kaon yields with transverse momenta between 1.5 and 2.5 GeV/c are
suppressed for central collisions relative to scaled collisions. This
suppression, which increases as the collisions become more central is
consistent with jet quenching models and is also present with comparable
magnitude at forward rapidity. At such rapidities initial state effects may
also be present and persistence of the meson suppression to high rapidity may
reflect a combination of jet quenching and nuclear shadowing. The ratio of
protons to mesons increases as the collisions become more central and is
largest at forward rapidities.Comment: 19 pages, 11 figures and 6 table
Rapidity dependence of deuteron production in Au+Au collisions at = 200 GeV
We have measured the distributions of protons and deuterons produced in high
energy heavy ion Au+Au collisions at RHIC over a very wide range of transverse
and longitudinal momentum. Near mid-rapidity we have also measured the
distribution of anti-protons and anti-deuterons. We present our results in the
context of coalescence models. In particular we extract the "volume of
homogeneity" and the average phase-space density for protons and anti-protons.
Near central rapidity the coalescence parameter and the space
averaged phase-space density are very similar for both protons and
anti-protons. For protons we see little variation of either or the
space averaged phase-space density as the rapidity increases from 0 to 3.
However both these quantities depend strongly on at all rapidities. These
results are in contrast to lower energy data where the proton and anti-proton
phase-space densities are different at =0 and both and depend
strongly on rapidity.Comment: Document updated after proofs received from PR
Nuclear Stopping in Au+Au Collisions at sqrt(sNN) = 200 GeV
Transverse momentum spectra and rapidity densities, dN/dy, of protons,
anti-protons, and net--protons (p-pbar) from central (0-5%) Au+Au collisions at
sqrt(sNN) = 200 GeV were measured with the BRAHMS experiment within the
rapidity range 0 < y < 3. The proton and anti-proton dN/dy decrease from
mid-rapidity to y=3. The net-proton yield is roughly constant for y<1 at
dN/dy~7, and increases to dN/dy~12 at y~3. The data show that collisions at
this energy exhibit a high degree of transparency and that the linear scaling
of rapidity loss with rapidity observed at lower energies is broken. The energy
loss per participant nucleon is estimated to be 73 +- 6 GeV.Comment: 5 pages, 4 figure
Evolution of the nuclear modification factors with rapidity and centrality in d+Au collisions at $\sqrt{s_{NN}} = 200 GeV
We report on a study of the transverse momentum dependence of nuclear
modification factors for charged hadrons produced in deuteron + gold
collisions at GeV, as a function of collision centrality
and of the pseudorapidity () of the produced hadrons. We
find significant and systematic decrease of with increasing rapidity.
The midrapidity enhancement and the forward rapidity suppression are more
pronounced in central collisions relative to peripheral collisions. These
results are relevant to the study of the possible onset of gluon saturation at
RHIC energies.Comment: Four pages, four figures. Published in PRL. Figures 1 and 2 have been
updated, and several changes made to the tex
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