Weak gauge boson radiation in parton showers

The emission of W and Z gauge bosons off quarks is included in a traditional QCD + QED shower. The unitarity of the shower algorithm links the real radiation of the weak gauge bosons to the negative weak virtual corrections. The shower evolution process leads to a competition between QCD, QED and weak radiation, and allows for W and Z boson production inside jets. Various effects on LHC physics are studied, both at low and high transverse momenta, and effects at higher-energy hadron colliders are outlined

Cold Nuclear Matter Effects on J/psi Yields as a Function of Rapidity and Nuclear Geometry in Deuteron-Gold Collisions at sqrt(s_NN) = 200 GeV

We present measurements of J/psi yields in d+Au collisions at sqrt(s_NN) = 200 GeV recorded by the PHENIX experiment and compare with yields in p+p collisions at the same energy per nucleon-nucleon collision. The measurements cover a large kinematic range in J/psi rapidity (-2.2 < y < 2.4) with high statistical precision and are compared with two theoretical models: one with nuclear shadowing combined with final state breakup and one with coherent gluon saturation effects. To remove model dependent systematic uncertainties we also compare the data to a simple geometric model. We find that calculations where the nuclear modification is linear or exponential in the density weighted longitudinal thickness are difficult to reconcile with the forward rapidity data.Comment: 449 authors from 66 institutions, 6 pages, 3 figures. Submitted to Physical Review Letters. Plain text data tables for the points plotted in figures for this and previous PHENIX publications are (or will be) publicly available at http://www.phenix.bnl.gov/papers.htm

Colour Reconnection and Weak Showers

This thesis consider the improvement of the simulation tools used to describe high energy particle collisions. These simulation programs are normally referred to as event generators. The emphasis is on two specific parts of the event generation: the introduction of radiation of weak bosons associated with the collision and a new model for colour reconnection. Each collision contains significant additional radiation of particles, but earlier models have only included QCD and QED radiation. This thesis presents a first attempt at including also the weak radiation. The inclusion of weak emissions allows for a better understanding of already observed data. The weak radiation is also included in the common framework for including multi-jet matrix elements into the description of the radiation. Colour Reconnection (CR) address the question: between which partons do colour strings form? The new model incorporates three principles, the SU(3) colour rules from QCD, a space-time causality requirement, and a minimization of the potential energy. It also introduces a new type of reconnections producing junction structures. The new model is able to explain the lambda production simultaneously at both LEP and LHC. The effects of CR, both the new model as well as older models, are also considered at potential future electron-positron colliders. Comparisons between the new model and a string rope model is carried out, which include suggestions for relevant observables

Merging weak and QCD showers with matrix elements

We present a consistent way of combining associated weak boson radiation in hard dijet events with hard QCD radiation in Drell–Yan-like scatterings. This integrates multiple tree-level calculations with vastly different cross sections, QCD- and electroweak parton-shower resummation into a single framework. The new merging strategy is implemented in the Pythi

Effects of color reconnection on hadron flavor observables

We present a series of observables for soft inclusive physics, and utilize them for comparison between two recently developed color reconnection models: the new color reconnection model in Pythia and the DIPSY rope hadronization model. The observables are ratios of identified hadron yields as a function of the final-state activity, as measured by the charged multiplicity. Since both considered models have a nontrivial dependence on the final-state activity, the above observables serve as excellent probes to test the effect of these models. Both models show a clear baryon enhancement with increasing multiplicity, while only the DIPSY rope model leads to a strangeness enhancement. Flowlike patterns, previously found to be connected to color reconnection models, are investigated for the new models. Only Pythia shows a p(perpendicular to)-dependent enhancement of the Lambda/K ratio as the final-state activity increases, with the enhancement being largest in the mid-p(perpendicular to) region

Color reconnection at future e(+) e(-) colliders

The effects of color reconnection (CR) at e(+) e(-) colliders are revisited, with focus on recently developed CR models. The new models are compared with the LEP2 measurements for e(+) e(-) -> W+W- -> q(1)(q) over bar (2)q(3)(q) over bar4 and found to lie within their limits. Prospects for constraints from new high- luminosity e(+) e(-) colliders are discussed. The novel arena of CR in Higgs decays is introduced, and it is illustrated by shifts in angular correlations that would be used to set limits on a potential CP-odd admixture of the 125 GeV Higgs state