Soft Color Interaction Model Research Articles

Electroweak vector boson production at the LHC as a probe of mechanisms of diffraction

We show that the double diffractive electroweak vector boson production in the $pp$ collisions at the LHC is an ideal probe of QCD based mechanisms of diffraction. Assuming the resolved Pomeron model with flavor symmetric parton distributions, the $W$ production asymmetry in rapidity equals exactly zero. In other approaches, like the soft color interaction model, in which soft gluon exchanges are responsible for diffraction, the asymmetry is non-zero and equal to that in the inclusive $W$ production. In the same way, the ratio of the $W$ to $Z$ boson production is independent of rapidity in the models with resolved Pomeron in contrast to the predictions of the soft color interaction model.

Theoretical review of diffractive phenomena

We review QCD based descriptions of diffractive deep inelastic scattering emphasising the role of models with parton saturation. These models provide natural explanation of such experimentally observed facts as the constant ratio of σ diff / σ tot as a function of the Bjorken variable x, and Regge factorization of diffractive parton distributions. The Ingelman-Schlein model and the soft color interaction model are also presented.

Hard diffraction from parton rescattering in QCD

We analyze the QCD dynamics of diffractive deep inelastic scattering. The presence of a rapidity gap between the target and diffractive system requires that the target remnant emerges in a color singlet state, which we show is made possible by the soft rescattering of the struck quark. This rescattering is described by the path-ordered exponential (Wilson line) in the expression for the parton distribution function of the target. The multiple scattering of the struck parton via instantaneous interactions in the target generates dominantly imaginary diffractive amplitudes, giving rise to an "effective pomeron" exchange. The pomeron is not an intrinsic part of the proton but a dynamical effect of the interaction. This picture also applies to diffraction in hadron-initiated processes. Due to the different color environment the rescattering is different in virtual photon- and hadron-induced processes, explaining the observed non-universality of diffractive parton distributions. This framework provides a theoretical basis for the phenomenologically successful Soft Color Interaction model which includes rescattering effects and thus generates a variety of final states with rapidity gaps. We discuss developments of the SCI model to account for the color coherence features of the underlying subprocesses.

Soft colour interactions and diffractive Higgs production

The topical subject of Higgs production in diffractive hard scattering events at the Tevatron and LHC is discussed. This has been proposed as a Higgs discovery channel with appealing experimental features. Predictions are obtained from the Soft Colour Interaction model, where rapidity gaps are created by a new soft interaction added to the normal hard scattering processes, implemented in the Monte Carlo event generator PYTHIA. A brief review of the successful application of the model to describe all CDF and DO data on diffractive hard scattering, such as production of W/Z, dijets, beauty and J/psi is also given.

Soft and hard QCD dynamics in hadroproduction of charmonium

Both hard and soft QCD dynamics are important in charmonium production, as presented here through a next-to-leading order QCD matrix element calculation combined with the colour evaporation model. Observed $x_F$ and $p_\perp$ distributions of $J/\psi$ in hadroproduction at fixed target and $p\bar{p}$ collider energies are reproduced. Quite similar results can also be obtained in a more phenomenologically useful Monte Carlo event generator where the perturbative production of \ccbar pairs is instead obtained through leading order matrix elements and the parton shower approximation of the higher order processes. The soft dynamics may alternatively be described by the soft colour interaction model, originally introduced in connection with rapidity gaps. We also discuss the relative rates of different charmonium states and introduce an improved model for mapping the continuous \ccbar mass spectrum on the physical charmonium resonances.

Soft color interactions and diffractive hard scattering at the Fermilab Tevatron

An improved understanding of nonperturbative QCD can be obtained by the recently developed soft color interaction models. Their essence is the variation of color string-field topologies, giving a unified description of final states in high energy interactions, e.g., diffractive and nondiffractive events in ep and ppbar. Here we present a detailed study of such models (the soft color interaction model and the generalized area law model) applied to ppbar, considering also the general problem of the underlying event including beam particle remnants. With models tuned to HERA ep data, we find a good description also of Tevatron data on production of W, beauty and jets in diffractive events defined either by leading antiprotons or by one or two rapidity gaps in the forward or backward regions. We also give predictions for diffractive J/psi production where the soft exchange mechanism produces both a gap and a color singlet ccbar state in the same event. This soft color interaction approach is also compared with Pomeron-based models for diffraction, and some possibilities to experimentally discriminate between these different approaches are discussed.

Production of D ∗±(2010) mesons in DIS diffractive interactions at HERA

The production of diffractive open charm in DIS at HERA probes the gluonic content of the pomeron. The measured cross section σ(ep → e(D ∗± X)Y) is presented differentially as a function of observables exploring the dynamics of the process. The results are compared with a model based on a partonic pomeron as determined in a QCD analysis of the diffractive structure function, with perturbative QCD calculations of two gluon exchange, with a semi-classical model and with a soft colour interaction model.

Diffractive open charm production in DIS at H1

The rate of diffractive open charm production in DIS at HERA probes the gluonic content of the pomeron. The measured cross section σ(ep → e (D ∗±X)Y) is presented differentially as a function of observables exploring the dynamics of the process. The results are compared with a model based on a partonic pomeron as determined in a QCD analysis of the diffractive structure function, with perturbative QCD calculations of two gluon exchange and with a soft color interaction model.