107,067 research outputs found
Parton distributions
Full text linkParton distributions, α(χ,μ(^₂) are essential ingredients for almost all theoretical calculations at hadron colliders. They give the number densities of the colliding par- tons (quarks and gluons) inside their parent hadrons at a given momentum fraction χand scale μ(^₂). The scale dependence of the parton distributions is given by DGLAP evolution, while the X dependence must be determined from a global analysis of deep-inelastic scattering (DIS) and related hard-scattering data. In Part I we introduce ‘doubly-unintegrateď parton distributions, fa(x, z, k(^₂),μ(^₂)), which additionally depend on the splitting fraction z and the transverse momentum (k) associated with the last evolution step. We show how these distributions can be used to calculate cross sections for inclusive jet production in DIS and compare the predictions to data taken at the HERA ep collider. We then calculate the transverse momentum distributions of พ and z bosons at the Tevatron pp collider and of Standard Model Higgs bosons at the forthcoming LHC. In Part II we study diffractive DIS, which is characterised by a large rapidity gap between the slightly deflected proton and the products of the virtual photon dissociation. We perform a novel QCD analysis of recent HERA data and extract diffractive parton distributions. The results of this analysis are used to investigate the effect of absorptive corrections in inclusive DIS. These absorptive corrections are due to the recombination of partons within the proton and are found to enhance the size of the gluon distribution at small X. We discuss the problem that the gluon distribution decreases with decreasing X at low scales while the sea quark distribution increases with decreasing X, whereas Regge theory predicts that both should have the same small-X behaviour. Our study hints at the possible importance of power corrections at low scales of around 1 GeV
Parton Correlations in Double Parton Scattering
No full textDouble parton scattering events are directly sensitive to the correlations between two partons inside a proton and can answer fundamental questions on the connections between the proton constituents. In this chapter, the different types of possible correlations, our present knowledge of them, and the processes where they are likely to be important, are introduced and explained. The increasing integrated luminosity at the LHC and the refinements of the theory of double parton scattering, lead to interesting prospects for measuring, or severely constraining, two-parton correlations in the near future.</p
Measurement of associated W+ charm production in pp collisions at √s=7 TeV
Full text linkOpen Access, Copyright CERN, for the benefit of the CMS Collaboration.Measurements are presented of the associated production of a W boson and a charm-quark jet (W + c) in pp collisions at a center-of-mass energy of 7 TeV. The analysis is conducted with a data sample corresponding to a total integrated luminosity of 5 fb−1, collected by the CMS detector at the LHC. W boson candidates are identified by their decay into a charged lepton (muon or electron) and a neutrino. The W + c measurements are performed for charm-quark jets in the kinematic region p jet T > 25 GeV, |ηjet| 25 GeV) and σ(pp → W + c + X)× B (W → ℓν) = 84.1 ± 2.0 (stat.) ± 4.9 (syst.) pb ( p ℓ T > 35 GeV), and the cross section ratios σ(pp → W+ + c ¯ + X)/σ(pp → W− + c + X) = 0.954 ± 0.025 (stat.) ± 0.004 (syst.) ( p ℓ T > 25 GeV) and σ(pp → W+ + c ¯ + X)/σ(pp → W− + c + X) = 0.938 ± 0.019 (stat.) ± 0.006 (syst.) ( p ℓ T > 35 GeV). Cross sections and cross section ratios are also measured differentially with respect to the absolute value of the pseudorapidity of the lepton from the W-boson decay. These are the first measurements from the LHC directly sensitive to the strange quark and antiquark content of the proton. Results are compared with theoretical predictions and are consistent with the predictions based on global fits of parton distribution functions
HERWIG 6: an event generator for Hadron emission reactions with interfering gluons (including supersymmetric processes)
Full text linkHERWIG is a general-purpose Monte Carlo event generator, which includes the simulation of hard lepton-lepton, lepton-hadron and hadron-hadron scattering and soft hadron-hadron collisions in one package. It uses the parton-shower approach for initial- and final-state QCD radiation, including colour coherence effects and azimuthal correlations both within and between jets. This article updates the description of HERWIG published in 1992, emphasising the new features incorporated since then. These include, in particular, the matching of first-order matrix elements with parton showers, a more correct treatment of heavy quark decays, and a wide range of new processes, including many predicted by the Minimal Supersymmetric Standard Model, with the option of R-parity violation. At the same time we offer a brief review of the physics underlying HERWIG, together with details of the input and control parameters and the output data, to provide a self-contained guide for prospective users of the program
Monte Carlo simulations of hard QCD radiation
Full text linkMonte Carlo event generators, such as Herwig++, provide a full simulation of events at collider experiments. They give a fully exclusive description of hadronic final states and are therefore crucial tools for the planning of future experiments and analysing of data from existing experiments.
The key component that allows this description of high-multiplicity final states is the parton shower. There has been much recent progress improving the parton shower description of hard radiation using exact matrix elements. This thesis describes research into implementing and improving such methods within the Herwig++ event generator.
In Chapter 1, the parton-shower formalism is reviewed and the structure of event generators described. Chapter 2 details the specifics of the \textsf{Herwig++} parton shower.
In Chapters 3 and 4, the POWHEG next-to-leading-order matching procedure is described, and work implementing the scheme within Herwig++ is presented. The method is implemented for the processes e+e- to hadrons and Drell-Yan vector boson production and the results are compared to experimental data from LEP and the Tevatron. This work includes the first full implementation of the truncated shower.
A description of the development and implementation of a modified matrix-element merging scheme is presented in Chapter 5. This scheme is based on CKKW merging but uses an extension of the POWHEG idea to improve the method using truncated showers. The method is implemented first for final-state radiation in e+e- to hadrons and then, in Chapter 6, extended to include initial-state radiation in Drell-Yan vector boson production
Quark--parton model with large parton k/sub T/
No full textThe quark--parton model is generalized to allow for arbitrarily large parton k/sub T/. Since it is expected that will rise with Q/sup 2/ in (highly virtual) photon mediated processes, this generalization is necessary to restore the applicability of the quark-parton model. By treating k/sub T/ as an essential kinematical variable, the introduction of a new scaling variable z is considered. Together with Bjorken's x variable, a unified kinematical description is given of the four distribution functions: hadron structure functions and jet decay functions for spacelike and timelike photons. The possibility of a simple interpolating universal function is considered. Phenomenological determination of that function is examined in detail. Predictions on R, parton , hadron in jets, etc., are made with the dimuon being used as an input. The usual relation = ..sqrt..2 is shown to be false in the region where k/sub T/ is not small compared to k/sub L/, a situation which prevails in the production of dileptons recently measured. The k/sub T/ distributions for timelike and spacelike cases are shown to be not identical. The model is consistent with nearly all relevant data on virtual photon processes. 38 reference
Measurement of the production cross section of prompt J/ψ mesons in association with a W± boson in pp collisions at p s = 7 TeV with the ATLAS detector
Full text linkThe process pp → W±J/ψ provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb-1 of p s = 7TeV pp collisions at the LHC, the first observation is made of the production of W± + prompt J/ events in hadronic collisions, using W± → μ and J/ψ → μ+μ-. A yield of 27.4±7.5 -6.5 W± + prompt J/ψ events is observed, with a statistical significance of 5.1. The production rate as a ratio to the inclusive W± boson production rate is measured, and the double parton scattering contribution to the cross section is estimated
Correlations with a Large p(T) Trigger, Parton Parton Scattering, and the Transverse Momentum of Partons
No full textPetersson B. Correlations with a Large p(T) Trigger, Parton Parton Scattering, and the Transverse Momentum of Partons. 1977
Measurement of the production of a W boson in association with a charm quark in pp collisions at s√ = 7 TeV with the ATLAS detector
Full text linkThe production of a W boson in association with a single charm quark is studied using 4.6 fb−1 of pp collision data at s√ = 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be 0.96+0.26−0.30 at Q 2 = 1.9 GeV2, which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio σ(W + + c¯)/σ(W − + c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the s−s¯ quark asymmetry
Le travail social constructif: sources philosophiques et principes de pratique (Constructive social work: Philosophical roots and practice principles)
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