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A bstract We present a novel method to combine QCD calculations at next-to-next-to-leading order (NNLO) with parton shower (PS) simulations, that can be applied to the production of heavy systems in hadronic collisions, such as colour singlets or a t t ¯ pair. The NNLO corrections are included by connecting the M i NLO′ method with transverse- momentum resummation, and they are calculated at generation time without any additional reweighting, making the algorithm considerably efficient. Moreover, the combination of different jet multiplicities does not require any unphysical merging scale, and the matching preserves the structure of the leading logarithmic corrections of the Monte Carlo simulation for parton showers ordered in transverse momentum. We present proof-of-concept applications to hadronic Higgs production and the Drell-Yan process at the LHC.

A bstract In the present work we consider the assignment of the factorization and renormalization scales in hadron collider processes with associated jet production, at next-to-leading order (NLO) in perturbation theory. We propose a simple, definite prescription to this end, including Sudakov form factors to consistently account for the distinct kinematic scales occuring in such collisions. The scheme yields results that are accurate at NLO and, for a large class of observables, it resums to all orders the large logarithms that arise from kinematic configurations involving disparate scales. In practical terms the method is most simply understood as an NLO extension of the matrix element reweighting procedure employed in tree level matrix element-parton shower merging algorithms. By way of a proof-of-concept, we apply the method to Higgs and Z boson production in association with up to two jets.

A bstract In this work we study the impact of recent findings regarding non-perturbative corrections in the three-jet region to e + e − hadronic observables, by performing a simultaneous fit of the strong coupling constant α s and the non-perturbative parameter α 0 . We extend the calculation of these power corrections, already known for thrust and C-parameter, to other e + e − hadronic observables. We find that for some observables the non-perturbative corrections are reasonably well behaved in the two-jet limit, while for others they have a more problematic behaviour. If one limits the fit to the three-jet region and to the well-behaved observables, one finds in general very good results, with the extracted value of α s agreeing well with the world average. This is the case in particular for the thrust and C -parameter for which notably small values of α s have been reported when non-perturbative corrections have been computed using analytic methods. Furthermore, the more problematic variables are also well described provided one stays far enough from the two-jet limit, while in this same region they cannot be described using the traditional implementation of power-corrections based on two-jet kinematics.

We present a calculation of inclusive diboson ( WZ, ZZ, WW ) processes at the LHC in the presence of intermediate polarised weak bosons decaying leptonically, matching next-to-leading-order accuracy in QCD with parton-shower effects. Starting from recent developments in polarised-boson simulation based on the helicity selection at the amplitude level, we have carried out the implementation in the Powheg-Box-Res framework, and validated it against independent fixed-order calculations. A phenomenological analysis in realistic LHC setups, as well as a comparison with recent ATLAS measurements, are presented.

A bstract As is the case for all light coloured Standard Model particles, also photons and charged leptons appear as constituents in ultrarelativistic hadron beams, and admit a parton density function (PDF). It has been shown recently that the photon PDF can be given in terms of the structure functions and form factors for electron-proton scattering. The same holds for lepton PDFs. In the present work we set up a calculation of the lepton PDFs at next-to-leading order, using the same data input needed in the photon case. A precise knowledge of the lepton densities allows us to study lepton-initiated processes even at a hadron collider, with all possible combinations of same-charge, opposite-charge, same-flavour, different-flavour leptons and leptons-quarks, most of which cannot be realized in any other forseable experiment. The lepton densities in the proton are extremely small, so that their contribution to Standard Model processes is generally shadowed by processes initiated by coloured partons. We will show, however, that there are cases where these processes can be relevant, giving rise to rare Standard Model signatures and to new production channels, that can enlarge the discovery potential of New Physics at the LHC and future high energy colliders with hadrons in the initial state.

A bstract We consider W + W − production in hadronic collisions and present the computation of next-to-next-to-leading order accurate predictions consistently matched to parton showers (NNLO+PS) using the M i NNLO PS method. Spin correlations, interferences and off-shell effects are included by calculating the full process pp → e + ν e μ − ν ¯ μ . This is the first NNLO+PS calculation for W + W − production that does not require an a-posteriori multi-differential reweighting. The evaluation time of the two-loop contribution has been reduced by more than one order of magnitude through a four-dimensional cubic spline interpolation. We find good agreement with the inclusive and fiducial cross sections measured by ATLAS and CMS. Both NNLO corrections and matching to parton showers are important for an accurate simulation of the W + W − signal, and their matching provides the best description of fully exclusive W + W − events to date.

A bstract This work is an extension of a previous publication [1] where we fitted the strong coupling α s together with the non-perturbative parameter α 0 from event-shape and jet-shape distributions using power corrections computed in the three-jet region. In ref. [1] only ALEPH data at the Z -pole were used in the fit. Here, instead, we include a large data sample from various e + e − experiments at energies ranging from 22 to 207 GeV and revisited the treatment of theoretical uncertainties. We find that the inclusion of different energies, while not changing the central fit result considerably, helps to disentangle the dependence of perturbative and non-perturbative corrections. Our best fit result is α s ( M Z ) = 0.1181 − 0.0005 − 0.0021 + 0.0002 + 0.0018 , where the first error includes experimental uncertainties and the second one includes uncertainties associated with scale variation, mass effects, fit limits, non-perturbative schemes and non- perturbative uncertainties.

A bstract We consider Zγ production in hadronic collisions and present the first computation of next-to-next-to-leading order accurate predictions consistently matched to parton showers (NNLO+PS). Spin correlations, interferences and off-shell effects are included by calculating the full process pp → ℓ + ℓ − γ . We extend the recently developed M i NNLO PS method to genuine 2 → 2 hard scattering processes at the LHC, which paves the way for NNLO+PS simulations of all diboson processes. This is the first 2 → 2 NNLO+PS calculation that does not require an a-posteriori multi-differential reweighting. We find that both NNLO corrections and matching to parton showers are crucial for an accurate simulation of the Zγ process. Our predictions are in very good agreement with recent ATLAS data.

A bstract We detail a simulation of Higgs boson production via gluon fusion, accurate at next-to-next-to-leading order in the strong coupling, including matching to a parton shower, yielding a fully exclusive, hadron-level description of the final-state. The approach relies on the P owheg method for merging the NLO Higgs plus jet cross-section with the parton shower, and on the M inlo method to simultaneously achieve NLO accuracy for inclusive Higgs boson production. The NNLO accuracy is reached by a reweighting procedure making use of the H nnlo program.

A bstract The photon PDF of the proton is needed for precision comparisons of LHC cross sections with theoretical predictions. In a recent paper, we showed how the photon PDF could be determined in terms of the electromagnetic proton structure functions F 2 and F L measured in electron-proton scattering experiments, and gave an explicit formula for the PDF including all terms up to next-to-leading order. In this paper we give details of the derivation. We obtain the photon PDF using the factorisation theorem and applying it to suitable BSM hard scattering processes. We also obtain the same PDF in a process-independent manner using the usual definition of PDFs in terms of light-cone Fourier transforms of products of operators. We show how our method gives an exact representation for the photon PDF in terms of F 2 and F L , valid to all orders in QED and QCD, and including all non-perturbative corrections. This representation is then used to give an explicit formula for the photon PDF to one order higher than our previous result. We also generalise our results to obtain formulæ for the polarised photon PDF, as well as the photon TMDPDF. Using our formula, we derive the P γ i subset of DGLAP splitting functions to order αα s and α 2 , which agree with known results. We give a detailed explanation of the approach that we follow to determine a photon PDF and its uncertainty within the above framework.