Diffractive Cross Sections Research Articles

Structural studies of confined liquids: The case of water confined in MCM-41

We outline the experimental procedure and data analysis required to study the microscopic structure of confined liquids, looking in particular at the case of water confined in MCM-41-S-15. The structural results are shown and commented at two temperatures, namely T = 300 K and the proposed temperature of minimum water density ( T = 210 K). Our analysis shows that at both temperatures water does not uniformly occupy the available volume and manifests effects due to cohesive failure. This phenomenon has the same effect of density fluctuations on the neutron diffraction cross section and its temperature dependence determines the decrease of Bragg peak intensity seen in the Small Angle Neutron Scattering experiments. The radial distribution functions of water are strongly different from those of bulk water at both temperatures investigated. Moreover water layers closer to the substrate surface strongly interact with the confining walls, and their relative orientations are quite distinct from those of molecules in the middle of the confining volume.

Unitarized model of inclusive and diffractive DIS withQ2evolution

We discuss the interplay of low-x physics and QCD scaling violations by extending the unified approach describing inclusive structure functions and diffractive production in $\gamma* p$ interactions proposed in previous papers, to large values of Q2. We describe the procedure of extracting, from the non-perturbative model, initial conditions for the QCD evolution that respect unitarity. Assuming Regge factorization of the diffractive structure function, a similar procedure is proposed for the calculation of hard diffraction. The results are in good agreement with experimental data on the proton structure function $F_2$ and the most recent data on the reduced diffractive cross section, $x_P \sigma_r^{\D(3)}$. Predictions for both $F_2$ and $F_L$ are presented in a wide kinematical range and compared to calculations within high-energy QCD.

Estimate of the single diffractive heavy quark production in heavy ion collisions at the CERN LHC

The single diffractive cross section for heavy quarks production is calculated at next-to-leading order (for nucleus-nucleus collisions. Such processes are expected to occur at the LHC, where the nuclei involved are lead at $\sqrt{s}=5.5\text{ }\text{ }\mathrm{TeV}$ and calcium at $\sqrt{s}=6.3\text{ }\text{ }\mathrm{TeV}$. We start using the hard diffractive factorization formalism, taking into account a recent experimental parametrization for the Pomeron structure function (DPDF). Absorptive corrections are accounted for by the multiple Pomeron contributions considering a gap survival probability, where its theoretical uncertainty for nuclear collisions is discussed. We estimate the diffractive ratios for the single diffraction process in nuclear coherent/incoherent collisions at the LHC.

Diffractive quarkonium production in association with a photon at the LHC

The rates for diffractive quarkonium plus prompt-photon associated production at the LHC are estimated. The theoretical framework considered in the J/Ψ (or ϒ) production associated with a direct photon at the hadron collider is the non-relativistic QCD (NRQCD) factorization formalism. The corresponding single diffractive cross section is computed based on the hard diffractive scattering factorization supplemented by absorptive corrections. Such processes are sensitive to the gluon content of the Pomeron at small Bjorken-x and they may also be a good place to test the different available mechanisms for quarkonium production at hadron colliders.

Multiple parton interactions in hadron collisions and diffraction

Hadrons are composite objects made of quarks and gluons, and during a collision one can have several elementary interactions between the constituents. These elementary interactions, using an appropriate theoretical framework, can be related to the total and elastic cross sections. At high c.m. energy it also becomes possible to identify experimentally a high ${p}_{\ensuremath{\perp}}$ subset of the parton interactions and to study their multiplicity distribution. Predictions of the multiple interaction rates are difficult because in principle one needs to have a knowledge of the correlated parton distribution functions that describe the probability to find simultaneously different partons in different elements of phase space. In this work we address this question and suggest a method to describe effectively the fluctuations in the instantaneous configuration of a colliding hadron. This problem is intimately related to the origin of the inelastic diffractive processes. We present a new method to include the diffractive cross section in an eikonal formalism that is equivalent to a multichannel eikonal. We compare with data and present an extrapolation to higher energy.

Fraunhofer diffraction by arbitrary-shaped obstacles

We consider Fraunhofer diffraction by an ensemble of large arbitrary-shaped screens that are randomly oriented in the plane of a wavefront and have edges of arbitrary shape. It is shown that far outside the main diffraction peak the differential scattering cross section behaves asymptotically as theta(-3), where theta is the diffraction angle. Moreover, the differential scattering cross section depends only on the length of the contours bordering the screens and does not depend on the shape of the obstacles. As both strictly forward and total diffraction cross sections are specified by obstacle area only, the differential cross section of size-distributed obstacles is expected to be nearly independent of obstacle shape over the entire region of the diffraction angles.

Soft interaction at high energy andN= 4 SYM

In this paper we show that the N=4 SYM total cross section violates the Froissart theorem, and in the huge range of energy this cross section is proportional to s1/3. The graviton reggeization will change this increase to the normal logarithmic behavior σ ln2s. However, we demonstrated that this happens at ultra high energy, much higher than the LHC energy. In the region of accessible energy we need to assume that there is a different source for the total cross section, with the value of the cross section about 40 mb. With this assumption we successfully describe σtot,σel and σdiff for the accessible range of energy from the fixed target Fermilab to the Tevatron energies. It turns out that the N=4 SYM mechanism can be responsible only for a small part of the inelastic cross section for this energy region (about 2 mb). However, at the LHC energy the N=4 SYM theory can describe the multiparticle production with σin ≈ 30 mb. The second surprise is the fact that the total cross section and the diffraction cross section can increase considerably from the Tevatron to the LHC energy. The bad description of Bel gives the strong argument that the non N=4 SYM background should depend on energy. We believe that we have a dilemma: to find a new mechanism for the inelastic production in the framework of N=4 SYM other than the reggeized graviton interaction, or to accept that N=4 SYM is irrelevant to any experimental data that has been measured before the LHC era.

Diffractive cross sections at HERA and diffractive PDFs

A large collection of results for the diffractive dissociation of virtual photons, $\gamma^{\star}p \to Xp$, have been obtained with the H1 and ZEUS detectors at HERA. Different experimental techniques have been used, by requiring a large rapidity gap between $X$ and the outgoing proton, by analyzing the mass distribution, $M_X$, of the hadronic final state, as well as by directly tagging the proton. A reasonable compatibility between those techniques and between H1 and ZEUS results have been observed. Some common fundamental features in the measurements are also present in all data sets. They are detailed in this document. Diffractive PDFs can give a good account of those features. Ideas and results are discussed in the following.

Growth of high quality N-polar AlN(0001¯) on Si(111) by plasma assisted molecular beam epitaxy

High quality N-polar AlN epilayers were grown and characterized on Si(111) substrates by plasma assisted molecular beam epitaxy as a first step toward growth of N-polar nitrides on Si(111). Polarity inversion to N-face by an optimized predeposition of Al adatoms on the reconstructed 7×7 Si(111) surface was investigated. Al adatoms can saturate the dangling bonds of Si atoms, resulting in growth of AlN in (0001¯) direction on subsequent exposure to N2 plasma. N-polarity was confirmed by observing strong 3×3 and 6×6 reflection high-energy electron diffraction reconstructions, convergent beam electron diffraction imaging and KOH etching studies. The structural properties were investigated by x-ray diffraction measurements, cross section and plan-view TEM studies.

Elastic and quasi-elastic pp and γ ⋆ p scattering in the dipole model

We have in earlier papers presented an extension of Mueller’s dipole cascade model, which includes sub-leading effects from energy conservation and running coupling as well as color-suppressed saturation effects from pomeron loops via a “dipole swing”. The model was applied to the description of the total and diffractive cross sections in pp and γ * p collisions, and also the elastic cross section in pp scattering. In this paper we extend the model to the description of the corresponding quasi-elastic cross sections in γ * p, namely the exclusive production of vector mesons and deeply virtual Compton scattering. Also for these reactions we find good agreement with measured cross sections. In addition, we obtain a reasonable description of the t-dependence of the elastic pp and quasi-elastic γ ⋆ p cross sections.

The Photon in Diffraction at HERA

The universality of the diffractive proton structure is investigated at HERA using structure function measurements in diffractive deep-inelastic scattering (DIS) and diffractive jet and charm production in DIS and photoproduction. While all point-like photon scattering processes can be described using the same parton densities (factorisation), the situation is different in real photon scattering where the diffractive dijet cross section measured by H1 for ETjet(1,2)>5,4 GeV is a factor of approximately 0.5 lower than expectations based on the DIS parton densities (factorisation breaking). Strikingly, the suppression factor (or ‘gap survival probability’) is independent of the photon energy that manifests itself outside the jet system—direct and resolved photon processes are equally suppressed. This can be understood qualitatively in the dipole picture. A reduced suppression with respect to theoretical predictions is found in a ZEUS analysis of diffractive photoproduction of dijets with higher transverse momenta (ETjet(1,2)>7.5,6.5 GeV).

Systematic analysis of scaling properties in deep inelastic scattering

Using the ``quality factor'' method, we analyze the scaling properties of deep inelastic processes at the accelerator HERA and fixed target experiments for $x\ensuremath{\le}{10}^{\ensuremath{-}2}$. We look for scaling formulas of the form ${\ensuremath{\sigma}}^{{\ensuremath{\gamma}}^{*}p}(\ensuremath{\tau})$, where $\ensuremath{\tau}(L=\mathrm{log}{Q}^{2},Y)$ is a scaling variable suggested by the asymptotic properties of QCD evolution equations with rapidity $Y$. We consider four cases: ``fixed coupling,'' corresponding to the original geometric scaling proposal and motivated by the asymptotic properties of the Balitsky-Kovchegov equation with fixed QCD coupling constant; two versions, ``running coupling I, II,'' of the scaling suggested by the Balitsky-Kovchegov equation with running coupling; and ``diffusive scaling'' suggested by the QCD evolution equation with Pomeron loops. The quality factors, quantifying the phenomenological validity of the candidate scaling variables, are fitted on the total and deeply virtual Compton scattering cross-section data from HERA and predictions are made for the elastic vector meson and for the diffractive cross sections at fixed small ${x}_{\mathbb{P}}$ or $\ensuremath{\beta}$. The first three scaling formulas have comparably good quality factors while the fourth one is disfavored. Adjusting initial conditions gives a significant improvement of the running coupling II scaling.

A QCD motivated model for soft interactions at high energies

In this paper we develop an approach to soft scattering processes at high energies which is based on two elements: the Good–Walker mechanism for low mass diffraction and multi-pomeron interactions for high mass diffraction. The principal idea, which allows us to specify the theory for pomeron interactions, is that the so called soft processes occur at rather short distances (r 2 ∝1/〈p t 〉2 ∝ α′ℙ≈0.01 GeV−2), where perturbative QCD is valid. The value of the pomeron slope α′ℙ is obtained from a fit to the experimental data. Using this theoretical approach, we suggest a model that fits all soft data in the ISR-Tevatron energy range: total, elastic, single and double diffractive cross sections, as well as the t dependence of the differential elastic cross section, and the mass dependence of single diffraction. In this model we calculate the survival probability of diffractive Higgs production, and we obtain a value for this observable that is smaller than 1% at the LHC energy range.

A Study of Soft Interactions at Ultra High Energies

We present and discuss our recent study of an eikonal two channel model, in which we reproduce the soft total, integrated elastic and diffractive cross sections, and the corresponding forward differential slopes in the ISR-Tevatron energy range. Our study is extended to provide predictions at the LHC and Cosmic Rays energies. These are utilized to assess the role of unitarity at ultra high energies, as well as predict the implied survival probability of exclusive diffractive central production of a light Higgs. Our approach is critically examined so as to estimate the margins of error of the calculated survival probability for diffractive Higgs production.

Probing the low-xstructure of nuclear matter with diffractive hadron production inpAcollisions

We argue that hadron production in coherent diffraction of protons on a heavy nucleus provides a very sensitive probe of the low-$x$ QCD dynamics. This process probes the BFKL dynamics in protons and the nonlinear gluon evolution in nuclei. We calculate the diffractive hadron production cross sections in the BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) kinematic regions. To study the nuclear effects we introduce the diffractive nuclear modification factor. We show that, unlike the nuclear modification factor for inclusive hadron production that has very interesting dynamics at RHIC but is expected to be almost completely saturated at the LHC, the nuclear modification factor for diffractive production exhibits a nontrivial behavior at both RHIC and LHC.

Measurements of Thermal Neutron Diffraction and Inelastic Scattering in Reactor-Grade Graphite

High-resolution Bragg-edge transmission measurements were conducted on granular as well as solid samples of graphite to understand the basis for a bulk measurement of the diffusion length 24% larger than predicted by MCNP5 for bulk reactor-grade graphite. High resolution enabled a measurement of the total diffraction cross section from 1 to 200 meV. This was subtracted from the total cross section to find the inelastic cross section in the same energy range. Small-angle scattering, which has been thought to contribute to the total cross section in the region of the lowest Bragg edge, is shown not to be present in our measurement or in those of others claiming to find it. Instead, neutron total reflection from the surface of graphite microcrystals is shown to contribute to the cross section at low energies. Reactor-grade graphite is shown to have an inelastic scattering cross section over most of the energy range larger by at least 10 than the nearly perfect crystal structure of pyrolytic graphite. The ratio of inelastic scattering to diffraction at 25 meV for our graphite is inferred to be twice as large as that of graphite manufactured 50 yr ago, and we believe that our larger diffusion coefficient is rooted in this difference. The distortions in the microcrystalline structure introduced in the manufacturing of the graphite studied here at 24°C are found to be equivalent to the uncertainty in atom positions seen in heating perfect crystal graphite to a temperature of ~1800°C.

Deep inelastic inclusive and diffractive scattering at [formula omitted] values from 25 to 320 GeV 2 with the ZEUS forward plug calorimeter

Deep inelastic scattering and its diffractive component, e p → e ′ γ ∗ p → e ′ X N , have been studied at HERA with the ZEUS detector using an integrated luminosity of 52.4 pb −1. The M X method has been used to extract the diffractive contribution. A wide range in the centre-of-mass energy W (37–245 GeV), photon virtuality Q 2 (20–450 GeV 2) and mass M X (0.28–35 GeV) is covered. The diffractive cross section for 2 < M X < 15 GeV rises strongly with W, the rise becoming steeper as Q 2 increases. The data are also presented in terms of the diffractive structure function, F 2 D ( 3 ) , of the proton. For fixed Q 2 and fixed M X , x P F 2 D ( 3 ) shows a strong rise as x P → 0 , where x P is the fraction of the proton momentum carried by the pomeron. For Bjorken- x < 1 × 10 −3 , x P F 2 D ( 3 ) shows positive log Q 2 scaling violations, while for x ⩾ 5 × 10 −3 negative scaling violations are observed. The diffractive structure function is compatible with being leading twist. The data show that Regge factorisation is broken.

Dijet cross sections and parton densities in diffractive DIS at HERA

Differential dijet cross sections in diffractive deep-inelastic scattering are measured with the H1 detector at HERA using an integrated luminosity of 51.5 pb-1. The selected events are of the type ep --> eXY, where the system X contains at least two jets and is well separated in rapidity from the low mass proton dissociation system Y. The dijet data are compared with QCD predictions at next-to-leading order based on diffractive parton distribution functions previously extracted from measurements of inclusive diffractive deep-inelastic scattering. The prediction describes the dijet data well at low and intermediate zpom (the fraction of the momentum of the diffractive exchange carried by the parton entering the hard interaction) where the gluon density is well determined from the inclusive diffractive data, supporting QCD factorisation. A new set of diffractive parton distribution functions is obtained through a simultaneous fit to the diffractive inclusive and dijet cross sections. This allows for a precise determination of both the diffractive quark and gluon distributions in the range 0.05<zpom<0.9. In particular, the precision on the gluon density at high momentum fractions is improved compared to previous extractions.

Diffractive excitation of heavy flavors: Leading twist mechanisms

Diffractive production of heavy flavors is calculated within the light-cone dipole approach. Novel leading twist mechanisms are proposed, which involve both short and long transverse distances inside the incoming hadron. Nevertheless, the diffractive cross section turns out to be sensitive to the primordial transverse momenta of projectile gluons, rather than to the hadronic size. Our calculations agree with the available data for diffractive production of charm and beauty, and with the observed weak variation of the diffraction-to-inclusive cross section ratios as function of the hard scale.

SATURATION PHYSICS AND THE ELECTRON-ION COLLIDER

Using an extension of the Iancu-Itakura-Munier model to nuclear targets we look for saturation effects in electron-ion collisions. In previous publications we have made definite predictions. Here we try to compare our results with already existing experimental data on structure functions and on total and diffractive cross sections. Strictly speaking such a comparison is not well justified because the present data are not yet in the saturation domain. Nevertheless our results agree qualitatively with data and, in some cases, even quantitatively.