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Carlo Ewerz

Publications and source records attributed to Carlo Ewerz.

At least 37 records · Page 2Linked to original sources
arXiv2009

Ioffe Times in DIS from a Dipole Model Fit

We present a study of Ioffe times in deep inelastic electron-proton scattering. We deduce 'experimental' Ioffe-time distributions from the small-x HERA data as described by a particular colour-dipole-model fit. We show distributions for three representative gamma*-proton c.m. energies W and various values of the photon virtuality Q^2. These distributions are rather broad for transversely and very narrow for longitudinally polarised virtual photons. The Ioffe times for W=150 GeV, for example, range from around 1000 fm for Q^2=1 GeV^2 to around 10 fm for Q^2=100 GeV^2. Based on our results we discuss consequences for the limitations of applicability of the dipole picture.

hep-ph↗
arXiv2008

On the Range of Validity of the Dipole Picture

We derive correlated bounds on ratios of deep inelastic structure functions from the dipole picture of photon-hadron scattering at high energies. In particular we consider ratios of the longitudinal structure function, the total structure function and the charm part of the latter. We also consider ratios of total structure functions taken at the same energy but at three different photon virtualities. It is shown that by confronting these bounds with experimental data we can significantly constrain the range of validity of the dipole picture.

hep-ph↗
arXiv2007

On the Limitations of the Color Dipole Picture

We discuss two aspects of the color dipole picture of high energy photon-proton scattering. First we present bounds on various ratios of deep inelastic structure functions resulting from the dipole picture that, together with the measured data, can be used to restrict the kinematical range of its applicability. The second issue that we address is the choice of energy variable in the dipole-proton cross section.

hep-ph↗
arXiv2007

Diffractive Neutral Pion Production, Chiral Symmetry and the Odderon

We discuss the diffractive photo- and electroproduction of a single neutral pion at high energies where it can occur due to odderon exchange. We show that this process is dynamically suppressed as a consequence of chiral symmetry. Our result reconciles earlier theoretical expectations with the non-observation of this reaction at HERA.

hep-ph↗
arXiv2007

Bounds on Ratios of DIS Structure Functions from the Color Dipole Picture

We derive bounds on ratios of deep inelastic nucleon structure functions from the color dipole picture of high energy photon-hadron scattering. We find an upper bound on the ratio R=sigma_L/sigma_T of the total cross sections for longitudinally and transversely polarized photons. We further obtain bounds on the ratio of deep inelastic structure functions F_2 taken at the same energy but at different photon virtualities. It is shown that these bounds can be used to constrain the range of applicability of the dipole picture.

hep-ph↗
arXiv2007

Towards a Nonperturbative Foundation of the Dipole Picture: I. Functional Methods

This is the first of two papers in which we study real and virtual photon-proton scattering in a nonperturbative framework. We classify different contributions to this process and identify the leading contributions at high energies. We then study the renormalisation of the photon-quark-antiquark vertex that occurs in the leading contributions. We find something like the dipole picture in one of these contributions but also find two correction terms which can potentially become large at small photon virtualities. In the second paper we will discuss the additional approximations and assumptions that are necessary to obtain the dipole model of high energy scattering from the results found here.

hep-ph↗
arXiv2007

Towards a Nonperturbative Foundation of the Dipole Picture: II. High Energy Limit

This is the second of two papers in which we study real and virtual photon-proton scattering in a nonperturbative framework. In the first paper we have identified the leading contributions to this process at high energies and have derived expressions for them which take into account the renormalisation of the photon-quark-antiquark vertex. In the present paper we investigate the approximations and assumptions that are necessary to obtain the dipole model of high energy scattering from the results derived in the first paper. We discuss the gauge invariance of different contributions to the scattering amplitude and point out some subtleties related to gauge invariance in the correct definition of a perturbative photon wave function. As a phenomenological consequence of the dipole picture we derive a bound on the ratio of the cross sections for longitudinally and transversely polarised photons. This bound is independent of any particular model for the dipole-proton cross section and allows one to test the validity of the assumptions leading to the dipole picture in particular at low photon virtualities. We conclude that the naive dipole model formula should be supplemented by two additional terms which can potentially become large at small photon virtualities.

hep-ph↗
arXiv2006

Chiral Symmetry and Diffractive Neutral Pion Photo- and Electroproduction

We show that diffractive production of a single neutral pion in photon-induced reactions at high energy is dynamically suppressed due to the approximate chiral symmetry of QCD. These reactions have been proposed as a test of the odderon exchange mechanism. We show that the odderon contribution to the amplitude for such reactions vanishes exactly in the chiral limit. This result is obtained in a nonperturbative framework and by using PCAC relations between the amplitudes for neutral pion and axial vector current production.

hep-ph↗
arXiv2005

The C-Odd Four-Gluon State in the Color Glass Condensate

The study of the perturbative Odderon at high gluon densities in the color glass condensate requires to take into account states with more than three gluons. We formulate evolution equations for these states in which the number of gluons is not fixed during the evolution. We further determine the coupling of these Odderon states to the gamma to eta_c impact factor for arbitrary numbers of gluons. We find an exact solution of the evolution equation for the four-gluon Odderon state in terms of the three-gluon Odderon state. It is shown that a similar solution exists also for a different class of Odderon solutions which does not couple to the gamma to eta_c impact factor. We discuss the implications of our result from the perspective of constructing an effective field theory of reggeized gluons for the color glass condensate.

hep-ph↗
arXiv2005

Diffraction, the Color Glass Condensate and String Theory

We explain the main ideas of the color glass condensate in high energy collisions. Different approaches to the problem are outlined with emphasis on the resummation approach. We present evidence that the color glass condensate can be described by an effective conformal field theory or even by a string theory.

hep-ph↗
arXiv2004

Pion Corrections in Gribov's Approach to the Dyson-Schwinger Equation

Chiral symmetry breaking in QCD leads to the emergence of pions as Goldstone bosons. Their existence in turn affects the Green functions of the theory. Here we study the effect of pion corrections on the light quark's Green function in Feynman gauge using the framework of Gribov's approach to the Dyson-Schwinger equation.

hep-ph↗
arXiv2004

How Pomerons Meet in Coloured Glass

We compute the perturbative one-to-three Pomeron vertex in the colour glass condensate using the extended generalized leading logarithmic approximation in high energy QCD. The vertex is shown to be a conformal four-point function in two-dimensional impact parameter space. Our result can be used to compare different theoretical descriptions of the colour glass condensate.

hep-ph↗
arXiv2004

The Perturbative Odderon in Quasidiffractive Photon-Photon Scattering

We study the perturbative Odderon in the quasidiffractive process gamma(*) gamma(*) to eta_c eta_c. At high energies this process is dominated by Odderon exchange and can be viewed as the theoretically cleanest test of the perturbative Odderon. We calculate the differential and total cross section, as well as the dependence on the energy and on the photon virtualities taking into account the effects of resummation of logarithms of the energy. The results are compared with those obtained with a simple exchange of three noninteracting gluons. We present the expected cross section for this process at a future Linear Collider and discuss implications for other processes involving the perturbative Odderon.

hep-ph↗
arXiv2004

The Perturbative Pomeron and the Odderon: Where can we find them?

QCD predicts the existence of the perturbative Pomeron and of the Odderon. But both of them appear to be rather difficult to observe experimentally. We describe the experimental status of these two objects, discuss possible reasons for their elusive behavior, and point out promising search strategies.

hep-ph↗
arXiv2003

The Odderon in Quantum Chromodynamics

The Odderon is the leading exchange in hadronic scattering processes at high energies in which negative charge conjugation and parity quantum numbers are transferred in the t-channel. We review the origin of the Odderon in Regge theory, its status in perturbative and nonperturbative Quantum Chromodynamics, as well as its phenomenology.

hep-ph↗