24,624 research outputs found
Gluon Distribution Functions for Very Large Nuclei at Small Transverse Momentum
We show that the gluon distribution function for very large nuclei may be
computed for small transverse momentum as correlation functions of an
ultraviolet finite two dimensional Euclidean field theory. This computation is
valid to all orders in the density of partons per unit area, but to lowest
order in . The gluon distribution function is proportional to ,
and the effect of the finite density of partons is to modify the dependence on
transverse momentum for small transverse momentum.Comment: TPI--MINN--93--52/T, NUC--MINN--93--28/T, UMN--TH--1224/93, LaTex, 11
page
Detailed design of a resonantly-enhanced axion-photon regeneration experiment
A resonantly-enhanced photon-regeneration experiment to search for the axion
or axion-like particles is described. This experiment is a shining light
through walls study, where photons travelling through a strong magnetic field
are (in part) converted to axions; the axions can pass through an opaque wall
and convert (in part) back to photons in a second region of strong magnetic
field. The photon regeneration is enhanced by employing matched Fabry-Perot
optical cavities, with one cavity within the axion generation magnet and the
second within the photon regeneration magnet. Compared to simple single-pass
photon regeneration, this technique would result in a gain of (F/pi)^2, where F
is the finesse of each cavity. This gain could feasibly be as high as 10^(10),
corresponding to an improvement in the sensitivity to the axion-photon
coupling, g_(agg), of order (F/pi)^(1/2) ~ 300. This improvement would enable,
for the first time, a purely laboratory experiment to probe axion-photon
couplings at a level competitive with, or superior to, limits from stellar
evolution or solar axion searches. This report gives a detailed discussion of
the scheme for actively controlling the two Fabry-Perot cavities and the laser
frequencies, and describes the heterodyne signal detection system, with limits
ultimately imposed by shot noise.Comment: 10 pages, 5 figure
Random walks of partons in SU(N_c) and classical representations of color charges in QCD at small x
The effective action for wee partons in large nuclei includes a sum over
static color sources distributed in a wide range of representations of the
SU(N_c) color group. The problem can be formulated as a random walk of partons
in the N_c-1 dimensional space spanned by the Casimirs of SU(N_c). For a large
number of sources, k >> 1, we show explicitly that the most likely
representation is a classical representation of order O(\sqrt{k}). The quantum
sum over representations is well approximated by a path integral over classical
sources with an exponential weight whose argument is the quadratic Casimir
operator of the group. The contributions of the higher N_c-2 Casimir operators
are suppressed by powers of k. Other applications of the techniques developed
here are discussed briefly.Comment: 51 pages, includes 3 eps file
A phenomenological theory giving the full statistics of the position of fluctuating pulled fronts
We propose a phenomenological description for the effect of a weak noise on
the position of a front described by the Fisher-Kolmogorov-Petrovsky-Piscounov
equation or any other travelling wave equation in the same class. Our scenario
is based on four hypotheses on the relevant mechanism for the diffusion of the
front. Our parameter-free analytical predictions for the velocity of the front,
its diffusion constant and higher cumulants of its position agree with
numerical simulations.Comment: 10 pages, 3 figure
Dilepton low suppression as an evidence of the Color Glass Condensate
The dilepton production is investigated in proton-nucleus collisions in the
forward region using the Color Glass Condensate approach. The transverse
momentum distribution (), more precisely the low region, where the
saturation effects are expected to increase, is analyzed. The ratio between
proton-nucleus and proton-proton differential cross section for RHIC and LHC
energies is evaluated, showing the effects of saturation at small , and
presenting a Cronin type peak at moderate . These features indicate the
dilepton as a most suitable probe to study the properties of the saturated
regime and the Cronin effect.Comment: 10 pages, 8 figures, replaced with the version to appear in Physical
Review
A Survey for Massive Giant Planets in Debris Disks with Evacuated Inner Cavities
The commonality of collisionally replenished debris around main sequence
stars suggests that minor bodies are frequent around Sun-like stars. Whether or
not debris disks in general are accompanied by planets is yet unknown, but
debris disks with large inner cavities - perhaps dynamically cleared - are
considered to be prime candidates for hosting large-separation massive giant
planets. We present here a high-contrast VLT/NACO angular differential imaging
survey for eight such cold debris disks. We investigated the presence of
massive giant planets in the range of orbital radii where the inner edge of the
dust debris is expected. Our observations are sensitive to planets and brown
dwarfs with masses >3 to 7 Jupiter mass, depending on the age and distance of
the target star. Our observations did not identify any planet candidates. We
compare the derived planet mass upper limits to the minimum planet mass
required to dynamically clear the inner disks. While we cannot exclude that
single giant planets are responsible for clearing out the inner debris disks,
our observations constrain the parameter space available for such planets. The
non-detection of massive planets in these evacuated debris disks further
reinforces the notion that the giant planet population is confined to the inner
disk (<15 AU).Comment: Accepted for publication in Ap
SN 2005hj: Evidence for Two Classes of Normal-Bright SNe Ia and Implications for Cosmology
HET Optical spectra covering the evolution from about 6 days before to about
5 weeks after maximum light and the ROTSE-IIIb unfiltered light curve of the
"Branch-normal" Type Ia Supernova SN 2005hj are presented. The host galaxy
shows HII region lines at redshift of z=0.0574, which puts the peak unfiltered
absolute magnitude at a somewhat over-luminous -19.6. The spectra show weak and
narrow SiII lines, and for a period of at least 10 days beginning around
maximum light these profiles do not change in width or depth and they indicate
a constant expansion velocity of ~10,600 km/s. We analyzed the observations
based on detailed radiation dynamical models in the literature. Whereas delayed
detonation and deflagration models have been used to explain the majority of
SNe Ia, they do not predict a long velocity plateau in the SiII minimum with an
unvarying line profile. Pulsating delayed detonations and merger scenarios form
shell-like density structures with properties mostly related to the mass of the
shell, M_shell, and we discuss how these models may explain the observed SiII
line evolution; however, these models are based on spherical calculations and
other possibilities may exist. SN 2005hj is consistent with respect to the
onset, duration, and velocity of the plateau, the peak luminosity and, within
the uncertainties, with the intrinsic colors for models with M_shell=0.2 M_sun.
Our analysis suggests a distinct class of events hidden within the
Branch-normal SNe Ia. If the predicted relations between observables are
confirmed, they may provide a way to separate these two groups. We discuss the
implications of two distinct progenitor classes on cosmological studies
employing SNe Ia, including possible differences in the peak luminosity to
light curve width relation.Comment: ApJ accepted, 31 page
Geometric scaling as traveling waves
We show the relevance of the nonlinear Fisher and Kolmogorov-Petrovsky-
Piscounov (KPP) equation to the problem of high energy evolution of the QCD
amplitudes. We explain how the traveling wave solutions of this equation are
related to geometric scaling, a phenomenon observed in deep-inelastic
scattering experiments. Geometric scaling is for the first time shown to result
from an exact solution of nonlinear QCD evolution equations. Using general
results on the KPP equation, we compute the velocity of the wave front, which
gives the full high energy dependence of the saturation scale.Comment: 4 pages, 1 figure. v2: references adde
- …