| 1. |
- Bergh, Magnus, et al.
(författare)
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Model for the Dynamics of a Water Cluster in an X-ray Free Electron Laser Beam
- 2004
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 70:5:1, s. 051904-
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Tidskriftsartikel (refereegranskat)abstract
- A microscopic sample placed into a focused x-ray free electron laser beam will explode due to strong ionization on a femtosecond time scale. The dynamics of this Coulomb explosion has been modeled by Neutze et al. [Nature (London) 406, 752 (2000)] for a protein, using computer simulations. The results suggest that by using ultrashort exposures, structural information may be collected before the sample is destroyed due to radiation damage. In this paper a method is presented to include the effect of screening by free electrons in the sample in a molecular dynamics simulation. The electrons are approximated by a classical gas, and the electron distribution is calculated iteratively from the Poisson-Boltzmann equation. Test simulations of water clusters reveal the details of the explosion dynamics, as well as the evolution of the free electron gas during the beam exposure. We find that inclusion of the electron gas in the model slows down the Coulomb explosion. The hydrogen atoms leave the sample faster than the oxygen atoms, leading to a double layer of positive ions. A considerable electron density is located between these two layers. The fact that the hydrogens are found to explode much faster than the oxygens means that the diffracting part of the sample stays intact somewhat longer than the sample as a whole.
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| 2. |
- Enberg, Rikard, et al.
(författare)
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Diffractive Higgs Boson Production at the Fermilab Tevatron and the CERN Large Hadron Collider
- 2002
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 89:8, s. 081801-
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Tidskriftsartikel (refereegranskat)abstract
- Improved possibilities to find the Higgs boson in diffractive events, having less hadronic activity, depend on whether the cross section is large enough. Based on the soft color interaction models that successfully describe diffractive hard scattering at DESY HERA and the Fermilab Tevatron, we find that only a few diffractive Higgs events may be produced at the Tevatron, but we predict a substantial rate at the CERN Large Hadron Collider.
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| 3. |
- Enberg, Rikard, et al.
(författare)
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Diffractive Higgs bosons and prompt photons at hadron colliders
- 2003
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Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 67, s. 011301-
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Tidskriftsartikel (refereegranskat)abstract
- Models for soft color interactions have been successful in describing and predicting diffractive hard scattering processes in ep collisions at DESY HERA and pp̅ at the Fermilab Tevatron. Here we present new comparisons of the model to recent diffractive dijet data, also showing good agreement. The topical issue of diffractive Higgs boson production at the Tevatron and CERN LHC hadron colliders is further investigated. For H⃗γγ the irreducible background of prompt photon pairs from qq̅ →γγ and gg⃗γγ is always dominating, implying that higher branching ratio decay modes of the Higgs boson have to be used. However, such prompt photons can be used to test the basic prediction for Higgs boson production since gg⃗γγ involves a quark loop diagram similar to gg⃗H.
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| 4. |
- Enberg, Rikard, et al.
(författare)
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Rapidity gaps at HERA and the Tevatron from soft colour exchanges
- 2000
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Ingår i: Journal of Physics G. - : IOP Publishing. - 0954-3899 .- 1361-6471. ; 26, s. 712-715
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Tidskriftsartikel (refereegranskat)abstract
- Models based on soft colour exchanges to rearrange colour strings in the final state provide a general framework for both diffractive and non-diffractive events in ep and hadron-hadron collisions. We study two such models and find that they can reproduce rapidity gap data from both HERA and the Tevatron. We also discuss the influence of parton cascades and multiple interactions on the results.
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| 5. |
- Enberg, Rikard, et al.
(författare)
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Soft color interactions and diffractive hard scattering at the Fermilab Tevatron
- 2001
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Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 64:11, s. 114015-
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Tidskriftsartikel (refereegranskat)abstract
- 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 pp̅ . Here we present a detailed study of such models (the soft color interaction model and the generalized area law model) applied to pp̅ , considering also the general problem of the underlying event including beam particle remnants. With models tuned to DESY HERA ep data, we find a good description also of Fermilab 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/ψ production where the soft exchange mechanism produces both a gap and a color singlet cc̅ 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.
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| 6. |
- Enberg, Rikard, et al.
(författare)
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Soft colour interactions and diffractive Higgs production
- 2004
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 33:Suppl. 1, s. 542-544
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Tidskriftsartikel (refereegranskat)abstract
- 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 DØ data on diffractive hard scattering, such as production of W/Z, dijets, beauty and $J/\psi$ is also given.
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| 7. |
- Gunnar, Ingelman, et al.
(författare)
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Soft remnant interactions and rapidity gaps
- 2000
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Soft colour exchange models give a unified description of both diffractive and non-diffractive events, such that e-p and p-pbar collider data with and without rapidity gaps are well reproduced. We show that these models also describe the new Tevatron data
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| 8. |
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| 9. |
- Kwiecinski, J., et al.
(författare)
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Two-photon cross-sections from the saturation model
- 2002
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Ingår i: Acta Physica Polonica B. - : Jagellonian University, Cracow. - 0587-4254 .- 1509-5770. ; B33:6, s. 1559-
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Tidskriftsartikel (refereegranskat)abstract
- A saturation model for the total \gamma \gamma and \gamma* \gamma* cross-sections and for the real photon structure function F2\gamma (x,Q2) is described. The model is based on a QCD dipole picture of high energy scattering. The two-dipole cross-section is assumed to satisfy the saturation property with the saturation radius taken from the GBW analysis of the \gamma*p interaction at HERA. The model is combined with the QPM and non-pomeron reggeon contributions and it gives a very good description of the data on the \gamma \gamma total cross-section, on the photon structure function F2\gamma (x,Q2) at low x and on the \gamma* \gamma* cross-section. Production of heavy quarks in \gamma \gamma collisions is also studied.
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| 10. |
- Timneanu, Nicusor, et al.
(författare)
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Auger electron cascades in water and ice
- 2004
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Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104 .- 1873-4421. ; 299, s. 277-283
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Tidskriftsartikel (refereegranskat)abstract
- Secondary electron cascades can induce significant ionisation in condensed matter due to electron–atom collisions. This is of interest in the context of diffraction and imaging using X-rays, where radiation damage is the main limiting factor for achieving high resolution data. Here we present new results on electron-induced damage on liquid water and ice, from the simulation of Auger electron cascades. We have compared our theoretical estimations to the available experimental data on elastic and inelastic electron–molecule interactions for water and found the theoretical results for elastic cross-sections to be in very good agreement with experiment. As a result of the cascade we find that the average number of secondary electrons after 100 fs in ice is about 25, slightly higher than in water, where it is about 20. The difference in damage between ice and water is discussed in the context of sample handling for biomolecular systems.
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