1,011 research outputs found
A Quantitative Investigation of the Pomeron
A comparative investigation of various Pomeron models is carried out through
the study of their predicted values of , B, and
in high energy pp and p scattering.
Our results strongly support a picture of the Pomeron in which we have both
moderate blackening and expansion of the p() - p amplitude in impact
parameter space as a function of energy in the ISR-SSC domain. In particular,
we obtain an excellent reproduction of the data with a hybrid eikonal model
which combines the hard Lipatov-like QCD Pomeron with the old fashioned soft
Pomeron and Regge terms. Our analysis shows that the additive quark model, at
least in the naive form, is not compatible with the data.Comment: pages 15, Late
Towards a new global QCD analysis: low x DIS data from non-linear evolution
A new approach to global QCD analysis is developed. The main ingredients are
two QCD-based evolution equations. The first one is the Balitsky-Kovchegov
nonlinear equation, which sums higher twists while preserving unitarity. The
second equation is linear and it is responsible for the correct short distance
behavior of the theory, namely it includes the DGLAP kernel. Our approach
allows extrapolation of the parton distributions to very high energies
available at the LHC as well as very low photon virtualities, .
All existing low data on the structure function is reproduced using
one fitting parameter. The resulting .
Analyzing the parameter at
very low and well below we find . A result which agrees with the "soft pomeron" intercept without
involving soft physics.Comment: 27 pages, 11 figure
Probing decisive answers to dark energy questions from cosmic complementarity and lensing tomography
We study future constraints on dark energy parameters determined from several
combinations of CMB experiments, supernova data, and weak lensing surveys with
and without tomography. In this analysis, we look in particular for
combinations that will bring the uncertainties to a level of precision tight
enough (a few percent) to answer decisively some of the dark energy questions.
We probe the dark energy using two variants of its equation of state, and its
energy density.We consider a set of 13 cosmological and systematic parameters,
and assume reasonable priors on the lensing and supernova systematics. We
consider various lensing surveys: a wide survey with f_{sky}=0.7, and with 2
(WLT2) and 5 (WLT5) tomographic bins; a deep survey with 10 bins (WLT10). The
constraints found from Planck, 2000 supernovae with z_max=0.8, and WLT2 are:
{sigma(w_0)=0.086, sigma(w_1)=0.069}, {sigma(w_0)=0.088, sigma(w_a)=0.11}, and
{sigma(E_1)=0.029, sigma(E_2)=0.065}. With 5 bins, we find {sigma(w_0)=0.04,
sigma(w_1)=0.034}, {sigma(w_0)=0.041, sigma(w_a)=0.056}, and {sigma(E_1)=0.012,
sigma(E_2)=0.049}. Finally, we find from Planck, 2000 supernovae with
z_max=1.5, and WLT10 with f_{sky}=0.1: {sigma(w_0)=0.032, sigma(w_1)=0.027},
{sigma(w_0)=0.033, sigma(w_a)=0.040}, and {sigma(E_1)=0.01, sigma(E_2)=0.04}.
Although some worries remain about other systematics, our study shows that
after the combination of the 3 probes, lensing tomography with many redshift
bins and large coverages of the sky has the potential to add key improvements
to the dark energy parameter constraints. However, the requirement for very
ambitious and sophisticated surveys in order to achieve some of the constraints
or to improve them suggests the need for new tests to probe the nature of dark
energy in addition to constraining its equation of state. (Abriged)Comment: 14 pages, 5 figures; matches MNRAS accepted versio
A late-time transition in the cosmic dark energy?
We study constraints from the latest CMB, large scale structure (2dF,
Abell/ACO, PSCz) and SN1a data on dark energy models with a sharp transition in
their equation of state, w(z). Such a transition is motivated by models like
vacuum metamorphosis where non-perturbative quantum effects are important at
late times. We allow the transition to occur at a specific redshift, z_t, to a
final negative pressure -1 < w_f < -1/3. We find that the CMB and supernovae
data, in particular, prefer a late-time transition due to the associated delay
in cosmic acceleration. The best fits (with 1 sigma errors) to all the data are
z_t = 2.0^{+2.2}_{-0.76}, \Omega_Q = 0.73^{+0.02}_{-0.04} and w_f = -1^{+0.2}.Comment: 6 Pages, 5 colour figures, MNRAS styl
Distinguishing among Scalar Field Models of Dark Energy
We show that various scalar field models of dark energy predict degenerate
luminosity distance history of the Universe and thus cannot be distinguished by
supernovae measurements alone. In particular, models with a vanishing
cosmological constant (the value of the potential at its minimum) are
degenerate with models with a positive or negative cosmological constant whose
magnitude can be as large as the critical density. Adding information from CMB
anisotropy measurements does reduce the degeneracy somewhat but not
significantly. Our results indicate that a theoretical prior on the preferred
form of the potential and the field's initial conditions may allow to
quantitatively estimate model parameters from data. Without such a theoretical
prior only limited qualitative information on the form and parameters of the
potential can be extracted even from very accurate data.Comment: 15 pages, 5 figure
Determining cosmological parameters from X-ray measurements of strong lensing clusters
We discuss a new method which is potentially capable of constraining
cosmological parameters using observations of giant luminous arcs in rich X-ray
clusters of galaxies. The mass profile and the mass normalization of the lenses
are determined from X-ray measurements. The method also allows to probe the
amount and equation of state of the dark energy in the universe. The analysis
of a preliminary sample of 6 luminous, relatively relaxed clusters of galaxies
strongly favours an accelerating expansion of the universe. Under the
assumption that the dark energy is in the form of a cosmological constant, the
data provide an estimate of with a statistical error of
. Including the constraint of a flat universe and an equation of state
for the dark energy , we obtain and
. Relaxing the prior on , we find that the null
energy condition is satisfied at the 3- confidence level.Comment: 9 pages; 5 figures; accepted for publication in MNRA
A unitarity bound and the components of photon - proton interactions
We show how and why the small distance ("hard") interaction, calculable in
perturbative QCD, provides a mass cutoff in Gribov's formula for photon-proton
collisions. This enables us to find a new and more restrictive bound for this
process, .A simple model the published
experimental data over a wide range of photon virtualities () and energies
(), is developed. This model provides a quantitative way of evaluating the
relative rate of the small and large distance contributions, in the different
kinematic regions. The main results of the analysis are (i) that even at and high energies the short distance contribution is not small, and it
provides a possible explanation for the experimental rise of the
photoproduction cross section; and (ii) at large values of , the long
distance processes still contribute to the total cross section.Comment: latex file with 8 figs. in eps files, small changes in text and in
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