87 research outputs found
Conditional probabilities in Ponzano-Regge minisuperspace
We examine the Hartle-Hawking no-boundary initial state for the Ponzano-Regge
formulation of gravity in three dimensions. We consider the behavior of
conditional probabilities and expectation values for geometrical quantities in
this initial state for a simple minisuperspace model consisting of a
two-parameter set of anisotropic geometries on a 2-sphere boundary. We find
dependence on the cutoff used in the construction of Ponzano-Regge amplitudes
for expectation values of edge lengths. However, these expectation values are
cutoff independent when computed in certain, but not all, conditional
probability distributions. Conditions that yield cutoff independent expectation
values are those that constrain the boundary geometry to a finite range of edge
lengths. We argue that such conditions have a correspondence to fixing a range
of local time, as classically associated with the area of a surface for
spatially closed cosmologies. Thus these results may hint at how classical
spacetime emerges from quantum amplitudes.Comment: 26 pages including 10 figures, some reorganization in the
presentation of results, expanded discussion of results in the context of 2+1
gravity in the Witten variables, 3 new reference
Shape Space Methods for Quantum Cosmological Triangleland
With toy modelling of conceptual aspects of quantum cosmology and the problem
of time in quantum gravity in mind, I study the classical and quantum dynamics
of the pure-shape (i.e. scale-free) triangle formed by 3 particles in 2-d. I do
so by importing techniques to the triangle model from the corresponding 4
particles in 1-d model, using the fact that both have 2-spheres for shape
spaces, though the latter has a trivial realization whilst the former has a
more involved Hopf (or Dragt) type realization. I furthermore interpret the
ensuing Dragt-type coordinates as shape quantities: a measure of
anisoscelesness, the ellipticity of the base and apex's moments of inertia, and
a quantity proportional to the area of the triangle. I promote these quantities
at the quantum level to operators whose expectation and spread are then useful
in understanding the quantum states of the system. Additionally, I tessellate
the 2-sphere by its physical interpretation as the shape space of triangles,
and then use this as a back-cloth from which to read off the interpretation of
dynamical trajectories, potentials and wavefunctions. I include applications to
timeless approaches to the problem of time and to the role of uniform states in
quantum cosmological modelling.Comment: A shorter version, as per the first stage in the refereeing process,
and containing some new reference
Quantum Cosmological Multidimensional Einstein-Yang-Mills Model in a Topology
The quantum cosmological version of the multidimensional Einstein-Yang-Mills
model in a topology is studied in the framework of
the Hartle-Hawking proposal. In contrast to previous work in the literature, we
consider Yang-Mills field configurations with non-vanishing time-dependent
components in both and spaces. We obtain stable compactifying
solutions that do correspond to extrema of the Hartle-Hawking wave function of
the Universe. Subsequently, we also show that the regions where 4-dimensional
metric behaves classically or quantum mechanically (i.e. regions where the
metric is Lorentzian or Euclidean) will depend on the number, , of compact
space dimensions.Comment: Plain Latex. Version that appeared in the October 15th, 1997 issue of
Physical Review
Evidence of Final-State Suppression of High-p_T Hadrons in Au + Au Collisions Using d + Au Measurements at RHIC
Transverse momentum spectra of charged hadrons with 6 GeV/c have
been measured near mid-rapidity (0.2 1.4) by the PHOBOS experiment
at RHIC in Au + Au and d + Au collisions at . The spectra for different collision centralities are compared to collisions at the same energy. The resulting nuclear modification
factor for central Au + Au collisions shows evidence of strong suppression of
charged hadrons in the high- region ( GeV/c). In contrast, the d +
Au nuclear modification factor exhibits no suppression of the high-
yields. These measurements suggest a large energy loss of the high-
particles in the highly interacting medium created in the central Au + Au
collisions. The lack of suppression in d + Au collisions suggests that it is
unlikely that initial state effects can explain the suppression in the central
Au + Au collisions.Comment: 3 pages, 4 figures, International Europhysics Conference on High
Energy Physics EPS (July 17th-23rd 2003) in Aachen, German
The PHENIX Experiment at RHIC
The physics emphases of the PHENIX collaboration and the design and current
status of the PHENIX detector are discussed. The plan of the collaboration for
making the most effective use of the available luminosity in the first years of
RHIC operation is also presented.Comment: 5 pages, 1 figure. Further details of the PHENIX physics program
available at http://www.rhic.bnl.gov/phenix
Charged hadron transverse momentum distributions in Au+Au collisions at sqrt(s_NN) = 200 GeV
We present transverse momentum distributions of charged hadrons produced in
Au+Au collisions at sqrt(s_NN) = 200 GeV. The spectra were measured for
transverse momenta p_T from 0.25 to 4.5 GeV/c in a rapidity range of 0.2 < y_pi
< 1.4. The evolution of the spectra is studied as a function of collision
centrality, from 65 to 344 participating nucleons. The results are compared to
data from proton-antiproton collisions and Au+Au collisions at lower RHIC
energies. We find a significant change of the spectral shape between
proton-antiproton and peripheral Au+Au collisions. Comparing peripheral to
central Au+Au collisions, we find that the yields at high p_T exhibit
approximate scaling with the number of participating nucleons, rather than
scaling with the number of binary collisions.Comment: 5 pages, 4 figures, submitted to Phys.Lett.
Nuclear microscopy: A novel technique for quantitative imaging of gadolinium Distribution within Tissue Sections
10.1017/S1431927609090813Microscopy and Microanalysis154338-344MIMI
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