The Linked Dipole Chain Monte Carlo

We present an implementation of the Linked Dipole Chain model for deeply inelastic ep scattering into the framework of the Ariadne event generator. Using this implementation we obtain results both for the inclusive structure function as well as for exclusive properties of the hadronic final state. (The original publication was based on results from an implementation containing an error. In this revised version this error has been corrected, some of the beyond leading-log assumptions have been revised and so have some of the results.)Comment: 26 page

Investigations into the BFKL Mechanism with a Running QCD Coupling

We present approximations of varying degree of sophistication to the integral equations for the (gluon) structure functions of a hadron (``the partonic flux factor'') in a model valid in the Leading Log Approximation with a running coupling constant. The results are all of the BFKL-type, i.e. a power in the Bjorken variable x_B^{-\lambda} with the parameter \lambda determined from the size \alpha_0 of the ``effective'' running coupling \bar{\alpha}\equiv 3\alpha_s/\pi= \alpha_0/\log(k_{\perp}^2) and varying depending upon the treatment of the transverse momentum pole. We also consider the implications for the transverse momentum (k_{\perp}) fluctuations along the emission chains and we obtain an exponential falloff in the relevant \kappa\equiv \log(k_{\perp}^2)-variable, i.e. an inverse power (k_{\perp}^2)^{-(2+\lambda)} with the same parameter \lambda. This is different from the BFKL-result for a fixed coupling, where the distributions are Gaussian in the \kappa-variable with a width as in a Brownian motion determined by ``the length'' of the emission chains, i.e. \log(1/x_B). The results are verified by a realistic Monte Carlo simulation and we provide a simple physics motivation for the change.Comment: 24 pages, 10 supplementary files, submitted to Physical Review

Colour: A Computer Program for QCD Colour Factor Calculations

A computer program for evaluating colour factors of QCD Feynman diagrams is presented, and illustrative examples on how to use the program to calculate non trivial colour factors are given. The program and the discussion in this paper is based on a diagrammatic approach to colour factors.Comment: 12 pages, Latex and postscript figures in an uuencode packag

Gluon Distribution Functions in the kT-factorization Approach

At small x, the effects of finite transverse momenta of partons inside a hadron become increasingly important, especially in analyses of jets and heavy-quark production. These effects can be systematically accounted for in a formalism based on kT-factorization and unintegrated distribution functions. We present results for the unintegrated distribution function, together with the corresponding integrated one, obtained within the framework of the Linked Dipole Chain model. Comparisons are made to results obtained within other approaches

BFKL and CCFM final states

I give a brief presentation of recent results on the equivalence of BFKL and CCFM small-x final states, and discuss their implications for phenomenology.Comment: 4 pages. Talk presented at 7th International Workshop on Deep Inelastic Scattering, Zeuthen, Germany, April 199

Effect of aging on cerebral tissue oxygenation in relation to reflex syncope

Background: There is an increased susceptibility to syncope with aging attributed to age-related physiological impairments. Cerebral oxime-try non-invasively measures cerebral tissue oxygenation (SctO2) and has been shown to be valuable in syncope evaluation. SctO2 has beenfound to decrease with aging but it is unknown whether the decrease in SctO2 is related to increased susceptibility to syncope during ortho-static provocation. By measuring SctO2 during head up tilt test (HUT) we can study age-related differences in SctO2 and their impact ondeveloping reflex syncope.Purpose: To investigate the effect of age on the cerebral tissue oxygenation threshold for syncope and presyncope among patients withvasovagal syncope.Methods: Non-invasive haemodynamic monitoring and near-infrared spectroscopy (NIRS) were applied during head-up tilt (HUT) in 139vasovagal syncope patients (mean [SD] 45[17] years, 60% female), and 82 control patients with a normal response to HUT (45[18] years,61% female). Group differences in SctO2 and systolic blood pressure (SBP) during HUT in supine position, after 3 and 10 min of HUT, 30seconds prior to syncope ("presyncopal phase") and during syncope in different age groups (60 years) were comparedusing one-way ANOVA and Tukey"s multiple comparison test. Associations between age and SctO2 were studied using linear regressionmodels adjusted for sex and concurrent SBP.Results: Lower SctO2 in supine position was associated with increasing age among controls (B=-0.085, p = 0.010) but not among VVS pa-tients (B=-0.036, p = 0.114). No age-related differences in SctO2 were found after 3 and 10 minutes of HUT and during syncope. MeanSctO2 (%) during the presyncopal phase decreased over the advancing age groups (60: 62.2 ± 5.8; p = 0.009 for inter-group comparison). In contrast, mean SBP during the presyncopal phase did not differ by age groups (60: 77.6 ± 20.8 mmHg, p = 0.133). Age was associated with lower SctO2 during the presyncopal phase after adjusting for sexand SBP (B = 0.096, p = 0.001).Conclusion: Older VVS patients have lower cerebral tissue oxygenation in the presyncopal phase compared with younger patients inde-pendently of systolic blood pressure. These results suggest either that with imminent reflex syncope cerebral tissue oxygenation diminishesmore with advancing age or that cerebral deoxygenation is better tolerated by older reflex syncope patients

Uncertainties on Central Exclusive Scalar Luminosities from the unintegrated gluon distributions

In a previous report we used the Linked Dipole Chain model unintegrated gluon densities to investigate the uncertainties in the predictions for central exclusive production of scalars at hadron colliders. Here we expand this investigation by also looking at other parameterizations of the unintegrated gluon density, and look in more detail on the behavior of these at small k_T. We confirm our conclusions that the luminosity function for central exclusive production is very sensitive to this behavior. However, we also conclude that the available densities based on the CCFM and LDC evolutions are not constrained enough to give reliable predictions even for inclusive Higgs production at the LHC

Small-x Dipole Evolution Beyond the Large-N_c Limit

We present a method to include colour-suppressed effects in the Mueller dipole picture. The model consistently includes saturation effects both in the evolution of dipoles and in the interactions of dipoles with a target in a frame-independent way. When implemented in a Monte Carlo simulation together with our previous model of energy--momentum conservation and a simple dipole description of initial state protons and virtual photons, the model is able to reproduce to a satisfactory degree both the gamma*-p cross sections as measured at HERA as well as the total p-p cross section all the way from ISR energies to the Tevatron and beyond

Energy Conservation and Saturation in Small-x Evolution

Important corrections to BFKL evolution are obtained from non-leading contributions and from non-linear effects due to unitarisation or saturation. It has been difficult to estimate the relative importance of these effects, as NLO effects are most easily accounted for in momentum space while unitarisation and saturation are easier in transverse coordinate space. An essential component of the NLO contributions is due to energy conservation effects, and in this paper we present a model for implementing such effects together with saturation in Mueller's dipole evolution formalism. We find that energy conservation severely dampens the small-x rise of the gluon density and, as a consequence, the onset of saturation is delayed. Using a simple model for the proton we obtain a reasonable qualitative description of the x-dependence of F2 at low Q^2 as measured at HERA even without saturation effects. We also give qualitative descriptions of the energy dependence of the cross section for gamma*-gamma* and gamma*-nucleus scattering