A Quantitative Investigation of the Pomeron

A comparative investigation of various Pomeron models is carried out through the study of their predicted values of $\sigma_{tot}$, B, and $\frac{\sigma_{el}}{\sigma_{tot}}$ in high energy pp and p$\bar{p}$ scattering. Our results strongly support a picture of the Pomeron in which we have both moderate blackening and expansion of the p($\bar{p}$) - 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, $Q^2\ll 1 {\rm GeV^2}$. All existing low $x$ data on the $F_2$ structure function is reproduced using one fitting parameter. The resulting $\chi^2/df=1$. Analyzing the parameter $\lambda\equiv \partial\ln F_2/\partial(\ln 1/x)$ at very low $x$ and $Q^2$ well below $1 {\rm GeV^2}$ we find $\lambda \simeq 0.08 - 0.1$. 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 $\Omega_\Lambda = 1.1$ with a statistical error of $\pm 0.2$. Including the constraint of a flat universe and an equation of state for the dark energy $w_X \geq -1$, we obtain $\Omega_M = 0.10 \pm 0.10$ and $w_X = -0.84 \pm 0.14$. Relaxing the prior on $w_X$, we find that the null energy condition is satisfied at the 3-$\sigma$ 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, $\sigma(\gamma^{*}p)\leq C(ln 1/x)^{5/2}$.A simple model the published experimental data over a wide range of photon virtualities ($Q^2$) and energies ($W$), 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 $Q^2 =0$ 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 $Q^2$, 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 Fig.