Antishadowing contribution to the pomeron structure function

In a particular model of the pomeron, its gluon and quark distributions are subject to anti-shadowing, as described by a modified GLR equation recently derived. Inclusion of antishadowing cures the problem of momentum nonconservation previously existing in the GLR equation. A possible outcome of a measurement of the pomeron structure function at HERA is shown in case of DGLAP, GLR and modified GLR parton dynamics.     

Triple pomeron vertex and complex pomeron trajectories

Complex Pomeron trajectories can give rise to (log s)²-shrinkage of the diffraction peak. With factorization, one obtains consistency with the energy momentum sum rule in the triple-Regge limit provided the total cross section approaches a constant or increases less fast than $\text{(}\text{log}\text{s)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$.     

The pomeron structure in DIS and gluon recombination effects

A formalism for diffractive deep inelastic scattering is presented based on the concept of a pomeron structure function. Assuming the pomeron to be essentially a gluonic object with the small size indicated by its small interaction cross-sections, we show that the QCD evolution of its structure function exhibits large screening corrections due to gluon recombinations at small-χ. With a gluon-to-quark conversion appropriate at small-χ, diffractiveep cross-sections are obtained giving large enough statistics at HERA to distinguish between QCD evolution governed by the conventional Altarelli-Parisi equations and by the non-linear Gribov-Levin-Ryskin equation.     

Hadron diffractive processes: the structure of soft pomeron and colour screening

On the basis of the experimental data on diffractive processes in πp, pp and pˉp collisions at intermediate, moderately high and high energies, we restore the scattering amplitude related to the t-channel exchange by vacuum quantum numbers by taking account of the diffractive s-channel rescatterings. At intermediate and moderately high energies, the t-channel exchange amplitude turns, with a good accuracy, into an effective pomeron which renders the results of the additive quark model. At superhigh energies the scattering amplitude provides a Froissart-type behaviour, with an asymptotic universality of cross sections such as σ^tot _πp/σ^tot _pp→ 1 at s→∞. The quark structure of hadrons being taken into account at the level of constituent quarks, the cross sections of pion and proton (antiproton) in the impact parameter space of quarks, σ_π(r _1⊥, r _2⊥; s) and σ_p(r _1⊥, r _2⊥, r _3⊥; s), are found as functions of s. These cross sections implicate the phenomenon of colour screening: they tend to zero at |r _i⊥−r _k⊥|→ 0. The effective colour screening radius for pion (proton) is found for different s. The predictions for the diffractive cross sections at superhigh energies are presented. Received: 15 December 1998     

Probing the halo structure

Halo nuclei are characterized by outer nucleons that reside mostly in the classically forbidden region. The large average distance of halo nucleons suggests the decoupling of core and halo degrees of freedom. This is the basis for the few-body structure models developed in the past decade. Few-body models have been the most frequent tool when probing the halo structure. Coulomb dissociation, Knock-out or Transfer reactions have provided detailed structure information for exotic nuclei. Nowadays, the accumulating data impose severe tests for the few-body models. We discuss the achievements of these models as well as their limitations. To cite this article: F.M. Nunes, C. R. Physique 4 (2003).     

The pomeron colour structure and hadron scattering on the six-quark deuteron configuration

The pomeron colour structure provides some contribution to the elastic hadron-deuteron amplitude not contained in the Glauber like approach. This contribution is very sensitive to the short distance behaviour of the deuteron wave function and poses one to fix with a high precision a size of the deuteron six-quark bag.     

Unitarity and the BFKL pomeron

High energy onium-onium scattering is calculated as a function of impact parameter in the one- and two-pomeron exchange approximation. Difficulties with using the multiple scattering series to unitarize single pomeron exchange at high energy are noted. An operator formalism which sums all numbers of pomeron exchange is given. A toy model which has a similar operator structure at high energy as QCD is presented and the S-matrix is evaluated. Estimates of the energies and impact parameters at which blackness occurs in onium-onium scattering are given. It is emphasized that the problem of unitarity in high energy onium-onium scattering can be solved in a purely perturbative context, with a non-running coupling if the onium is heavy enough.     

Proton-proton scattering and the pomeron

We describe various attempts to fit the data on pp and $\text{p}\text{p}$ elastic scattering and total cross sections, including the recent ISR data, with a variety of Regge pole and cut models. Only two types of model are able to explain the data over the whole energy range satisfactorily. The simplest of these has no cuts, but a pomeron pole with trajectory αp(0) = 1.06 + 0.25 t, and a non-shrinking “core” term at large |t|. The other has a flat trajectory, αp(t) = 1.06 for all t, and strong eikonal-model cuts which explain both the shrinkage at low |t|, and the lack of it at larger |t|. We also briefly review various speculations which relate the fact that αp(0) > 1 to the small triple-pomeron coupling.     

J-plane structure of the “cylinder”; slope and intercept of the bare pomeron

We derive and investigate an integral equation for the “cylinder” with full t-dependence. In contrast to the pure pole planar model, the “cylinder” contains, in addition to the bare pomeron pole, also a Reggeon-Reggeon cut. The strongly correlated slope and intercept of the bare pomeron have the correct observed values. We also briefly comment on the concept of “asymptotic planarity”.     

Topological pomeron andA-universality

The nuclear mass number dependence of inclusive spectra of secondaries with different quantum numbers in the projectile fragmentation region is analysed. We note that in models with topological pomeron, all the particle spectra fall into two main categories. The first one comprises particles which have a common “valence” quark with the projectile, the second one comprises all the other particles built of “sea” quarks. Thus, in the parameterization \(x\frac{{d\sigma }}{{dx}} \propto A^{\alpha (x)} \) the spectra of all “valence” hadrons (p, n, Λ, π^+,0,?,K ⁺, ... in thepA-interaction) atx→1 can be characterized by the single exponent α_υ =α(x?1) which differs slightly from α _s characterizing the spectra of “sea” hadrons ( \(\bar p, \bar \Lambda \) ,K ^?, ... forpA-interactions). This observation is essentially modelindependent and follows only from the topological structure of the pomeron and Gribov's space-time picture of soft hadronic interactions. Deviations from universality due to preasymptotic corrections and coherent particle production processes are estimated.     

How to cut the pomeron

We show how to make s-channel unitarity cuts through an arbitrary graph involving bare pomerons.     

Diffusion and the BFKL pomeron

We study the high energy behaviour of elastic scattering amplitudes within the leading logarithm approximation. In particular, we cast the amplitude in a form which allows us to study the internal dynamics of the BFKL Pomeron for general momentum transfer. We demonstrate that the momentum transfer acts as an effective infrared cut-off which ensures that the dominant contribution arises from short distance physics.     

Search for glue balls in a pomeron pomeron scattering experiment

The exclusive reactionpp→ppπ⁺ π^? has been studied using the SFM spectrometer at ISR energies of \(\sqrt s = 24\) and 63 GeV. The cross section in the double pomeron exchange region defined by |x _p |>0.9, |y _π |<1 is 28±6, 23(+13, ?6) and 29(+29, ?10)μb, respectively. The ππ mass spectrum is dominated by an 1/m _ππ ² part, as expected for pomeron pomeron scattering. In addition, thef(1, 270) resonance is clearly distinguished. There are no further resonance signals, which could be assigned to glueball candidates expected in the 1,000 MeV region. Compared to photon photon scattering, the ratio of continuum tof-resonance production cross sections is significantly larger in pomeron pomeron scattering.     

Absence of ghost in the dual pomeron

We show that the ghost state found recently by Cremmer and Scherk in the dual pomeron is due to an incorrect treatment of massless spin-two states.     

QCD instantons and the soft pomeron

We study the rôle of semiclassical QCD vacuum solutions in high-energy scattering by considering the instanton contribution to hadronic cross sections. We propose a new type of instanton-induced interactions (“instanton ladder”) that leads to the rising with energy cross section σs^Δ of Regge type (the pomeron). We argue that this interaction may be responsible for the structure of the soft pomeron. The intercept Δ>0 is calculated. It has a nonanalytic dependence on the strong coupling constant, allowing a nonsingular continuation into the nonperturbative region. We derive the pomeron trajectory, which appears to be approximately linear in some range of (negative) momentum transfer t, but exhibits a curvature at small t. Possible rôle of instantons in multiparticle production is also discussed.     

Probing lepton structure at the SSC

If quarks and leptons are composite objects with common constituents, then there exist two-quark-two-lepton contact interactions which could affect lepton pair production at the SSC. We show that for a range of compositeness scales and lepton pair invariant masses, the contact interaction contributions dominate the standard model contributions to the cross section. In this regime, we show that there should be a measurable and statistically significant front-back asymmetry.     

One-dimensional quantum theory of the pomeron

The scattering amplitude, originated by the sum over all multi-pomeron exchanges, is evaluated in the zero-slope limit by solving a one-dimensional quantum field theory. In the classical limit, the theory gives back the tree-diagram results obtained in a preceding paper. The constant cross sections obtained in that case for a pomeron with intercept larger than one, are modified only at ultra-asymptotic energies by quantum effects. In the one-dimensional theory, this modification gives rise to cross sections behaving as s^?ΔE where ΔE is a small quantity generated by a tunnel effect.     

A quantitative investigation of the pomeron

A comparative investigation of various pomeron models is carried out through the study of their predicted values of _tot,B, and in high energypp and scattering. Our results strongly support a picture of the pomeron in which we have both moderate blackening and expansion of the amplitude in impact parameter space as a function of energy in the ISR-SSC domain. Our analysis shows the data is compatible with a hybrid eikonal model which combines the hard Lipatovlike QCD pomeron with the old fashioned soft pomeron and Regge terms. We show that the additive quark model, at least in the naive form, is not compatible with the data.Work supported in part by Israel Academy of Sciences and Humanities