QCD Jets and Parton Showers

I discuss the calculation of QCD jet rates in e+e- annihilation as a testing ground for parton shower simulations and jet finding algorithms.Comment: 13 pages, 5 figures, contribution to Proceedings of Gribov-80 Memorial Workshop on Quantum Chromodynamic and Beyond, ICTP, Trieste, Italy, 26-28 May, 201

Two-loop enhancement factor for 1/Q corrections to event shapes in deep inelastic scattering

We compute the two-loop enhancement factors for our earlier one-loop calculations of leading (1/Q) power corrections to the mean values of some event shape variables in deep inelastic lepton scattering. The enhancement is found to be equal to the universal ``Milan factor'' for those shape variables considered, provided the one-loop calculation is performed in a particular way. As a result, the phenomenology of power corrections to DIS event shapes remains largely unaffected.Comment: 15 pages, 1 figure, uses JHEP.cls. This revised version corrects an error in the calculation of the enhancement factor. The main conclusions remain unchange

The MC@NLO 2.2 Event Generator

This is the user's manual of MC@NLO 2.2. This package is a practical implementation, based upon the HERWIG event generator, of the MC@NLO formalism, which allows one to incorporate NLO QCD matrix elements consistently into a parton shower framework. Processes available in this version include the hadroproduction of Higgs bosons, single vector bosons, vector boson pairs, and heavy quark-antiquark pairs. This document is self-contained, but we emphasise the main differences with respect to previous versions.Comment: 17 pages, no figures. Version 2 has more details on some features. Program available at http://www.hep.phy.cam.ac.uk/theory/webber/MCatNLO

Jet Rates at Small x to Single-Logarithmic Accuracy

We present predictions of jet rates in deep inelastic scattering at small x to leading-logarithmic order in x, including all sub-leading logarithms of Q^2/m_R^2 where m_R is the transverse momentum scale at which jets are resolved. We give explicit results for up to three jets, and a perturbative expansion for multi-jet rates and jet multiplicities.Comment: 16 pages, 4 figure

The MC@NLO 3.4 Event Generator

This is the user's manual of MC@NLO 3.4. This package is a practical implementation, based upon the HERWIG event generator, of the MC@NLO formalism, which allows one to incorporate NLO QCD matrix elements consistently into a parton shower framework. Processes available in this version include the hadroproduction of single vector and Higgs bosons, vector boson pairs, heavy quark pairs, single top, single top in association with a W, lepton pairs, and Higgs bosons in association with a W or Z. Spin correlations are included for all processes except ZZ and WZ production. This document is self-contained, but we emphasise the main differences with respect to previous versions.Comment: 30 page

Multiplicity of (Mini-)Jets at Small x

We derive closed expressions for the mean and variance of the (mini-)jet multiplicity distribution in hard scattering processes at low x. Here (mini-)jets are defined as those due to initial-state radiation of gluons with transverse momenta greater than some resolution scale m_R, where Lambda^2 << m_R^2 << Q^2, Lambda being the intrinsic QCD scale and Q the momentum transfer scale of the hard scattering. Our results are valid to leading order in log(1/x) but include all sub-leading logarithms of Q^2/m_R^2. As an illustration, we predict the mini-jet multiplicity in Higgs boson production at the Large Hadron Collider.Comment: 14 pages, 5 figure

Next-to-Leading-Order Event Generators

We review the methods developed for combining the parton shower approximation to QCD with fixed-order perturbation theory, in such a way as to achieve next-to-leading-order (NLO) accuracy for inclusive observables. This has made it possible to generate fully-simulated hadronic final states with the precision and stability of NLO calculations. We explain the underlying theory of the existing methods, MC@NLO and POWHEG, together with their similarities, differences, achievements and limitations. For illustration we mainly compare results on Higgs boson production at the LHC, with particular emphasis on the residual uncertainties arising from the different treatment of effects beyond NLO. We also briefly summarize the difference between these NLO + parton shower methods and matrix-element + parton shower matching, and current efforts to combine the two approaches.Comment: 27 pages, 20 figure

Polarization Effects in Standard Model Parton Distributions at Very High Energies

We update the earlier work of Refs. arXiv:1703.08562 and arXiv:1712.07147 on parton distribution functions in the full Standard Model to include gauge boson polarization, non-zero input electroweak boson PDFs and next-to-leading-order resummation of large logarithms.Comment: 24 pages, 7 Figures. arXiv admin note: text overlap with arXiv:1703.08562, arXiv:1806.1015

Limits on Electroweak Instanton-Induced Processes with Multiple Boson Production

Recently, the CMS collaboration has reported their search for electroweak instanton-like processes with anomalous $B+L$ violation assuming multi-fermion but zero-boson final states. On the other hand, many theoretical studies suggest that anomalous $B+L$ processes may have an observably large production rate only if their final state contains a large number of electroweak gauge bosons. In this paper, we compare collider signatures of zero- and multi-boson events of anomalous $B+L$ violation at the LHC and derive an upper limit on the cross-section for the multi-boson process by recasting the CMS analysis.Comment: 14 pages, 6 figures; JHEP version with typographic errors fixe