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Name
P. Nason
Affiliation
conveners
City
Brechin
Country
United Kingdom

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High Energy Physics - Phenomenology (50)
 
High Energy Physics - Experiment (24)

Publications Authored By P. Nason

We present a Monte Carlo generator that implements significant theoretical improvements in the simulation of top-quark pair production and decay at the LHC. Spin correlations and off-shell effects in top-decay chains are described in terms of exact matrix elements for $p p \to \ell^+\nu_{\scriptscriptstyle\ell}\, l^-\bar{\nu}_{\scriptscriptstyle l} b \bar{b}$ at NLO QCD, where the leptons $\ell$ and $l$ belong to different families, and $b$ quarks are massive. Thus, the contributions from $t\bar{t}$ and $Wt$ single-top production as well as their quantum interference are fully included. Read More

It has become apparent in recent years that it is important, notably for a range of physics studies at the Large Hadron Collider, to have accurate knowledge on the distribution of photons in the proton. We show how the photon parton distribution function (PDF) can be determined in a model-independent manner, using electron-proton ($ep$) scattering data, in effect viewing the $ep\to e+X$ process as an electron scattering off the photon field of the proton. To this end, we consider an imaginary, beyond Standard Model process with a flavour changing photon-lepton vertex. Read More

This report summarises the properties of Standard Model processes at the 100 TeV pp collider. We document the production rates and typical distributions for a number of benchmark Standard Model processes, and discuss new dynamical phenomena arising at the highest energies available at this collider. We discuss the intrinsic physics interest in the measurement of these Standard Model processes, as well as their role as backgrounds for New Physics searches. Read More

This report was prepared in the context of the LPCC "Electroweak Precision Measurements at the LHC WG" and summarizes the activity of a subgroup dedicated to the systematic comparison of public Monte Carlo codes, which describe the Drell-Yan processes at hadron colliders, in particular at the CERN Large Hadron Collider (LHC). This work represents an important step towards the definition of an accurate simulation framework necessary for very high-precision measurements of electroweak (EW) observables such as the $W$ boson mass and the weak mixing angle. All the codes considered in this report share at least next-to-leading-order (NLO) accuracy in the prediction of the total cross sections in an expansion either in the strong or in the EW coupling constant. Read More

We combine the known asymptotic behaviour of the QCD perturbation series expansion, which relates the pole mass of a heavy quark to the MSbar mass, with the exact series coefficients up to the four-loop order to determine the ultimate uncertainty of the top-quark pole mass due to the renormalon divergence. We perform extensive tests of our procedure by varying the number of colours and flavours, as well as the scale of the strong coupling and the MSbar mass, and conclude that this uncertainty is around 70 MeV. We further estimate the additional contribution to the mass relation from the five-loop correction and beyond to be 250 MeV. Read More

I review few selected topics on recent theoretical progress in top physics. In particular I will discuss recent progress in the computation of the relationship between the MS-bar and pole top mass, in the NNLO calculation of top differential distributions, and in the simulation of top production and decays. Implications for top mass measurements will be discussed. Read More

In this work we present a new subtraction method for next-to-leading order calculations that is particularly convenient even when narrow resonances are present. The method is particularly suitable for the implementation of next-to-leading order calculations matched to parton shower generators. It allows at the same time for the inclusion of all finite width effects, including interferences, and for a consistent treatment of resonances in the shower approach, preserving the mass of resonances near their peak. Read More

We discuss production of charm and bottom quarks at forward rapidity in pp collisions at the LHC, updating the QCD predictions for the run at \sqrt{S}=13 TeV. We show that, while the absolute rates suffer from large theoretical systematics, dominated by scale uncertainties, the increase relative to the rates precisely measured at 7 TeV can be predicted with an accuracy of a few percent, sufficient to highlight the sensitivity to the gluon distribution function. Read More

This Report summarizes the results of the activities in 2014 of the Standard Model Working Group within the workshop "What Next" of INFN. We present a framework, general questions, and some indications of possible answers on the main issue for Standard Model physics in the LHC era and in view of possible future accelerators. Read More

We include finite top- and bottom-mass effects in the next-to-next-to-leading order parton shower (NNLOPS) event generator for inclusive Higgs boson production in gluon fusion based upon the POWHEG+MiNLO approach. Since fixed-order results for quark-mass effects only reach NLO accuracy, we add them to the NNLOPS generator at that accuracy. We explore uncertainties related to the unknown all-order logarithmic structure of bottom-mass effects by comparing the assumption of full exponentiation to no exponentiation at all. Read More

We present a next-to-leading order (NLO) calculation of $t\bar{t}$ production in hadronic collisions interfaced to shower generators according to the POWHEG method. We start from an NLO result from previous work, obtained in the zero width limit, where radiative corrections to both production and decays are included. The POWHEG interface required an extension of the POWHEG BOX framework, in order to deal with radiation from the decay of resonances. Read More

We detail a calculation of W gamma production in hadronic collision, at Next-to-Leading Order (NLO) QCD interfaced to a shower generator according to the POWHEG prescription supplemented with the MiNLO procedure. The fixed order result is matched to an interleaved QCD+QED parton shower, in such a way that the contribution arising from hadron fragmentation into photons is fully modeled. In general, our calculation illustrates a new approach to the fully exclusive simulation of prompt photon production processes accurate at the NLO level in QCD. Read More

2014May
Affiliations: 1conveners, 2conveners, 3conveners, 4conveners, 5conveners, 6conveners, 7conveners, 8conveners, 9conveners, 10conveners, 11conveners, 12conveners, 13conveners, 14conveners, 15conveners, 16conveners, 17conveners, 18conveners, 19conveners, 20conveners, 21conveners, 22conveners, 23conveners, 24conveners, 25conveners, 26conveners, 27conveners, 28conveners, 29conveners, 30conveners, 31conveners, 32conveners, 33conveners, 34conveners, 35conveners, 36conveners, 37conveners, 38conveners, 39conveners, 40conveners, 41conveners, 42conveners, 43conveners, 44conveners, 45conveners, 46conveners, 47conveners, 48conveners, 49conveners, 50conveners, 51conveners, 52conveners, 53conveners, 54conveners, 55conveners, 56conveners, 57conveners, 58conveners, 59conveners, 60conveners, 61conveners, 62conveners, 63conveners, 64conveners, 65conveners, 66conveners, 67conveners, 68conveners, 69conveners, 70conveners, 71conveners, 72conveners, 73conveners, 74conveners, 75conveners, 76conveners, 77conveners, 78conveners, 79conveners, 80conveners

This Report summarizes the proceedings of the 2013 Les Houches workshop on Physics at TeV Colliders. Session 1 dealt primarily with (1) the techniques for calculating standard model multi-leg NLO and NNLO QCD and NLO EW cross sections and (2) the comparison of those cross sections with LHC data from Run 1, and projections for future measurements in Run 2. Read More

We present an implementation of the production of three jets at NLO plus parton-shower effects in the POWHEG BOX. Using the recently introduced MiNLO procedure for setting the renormalization and factorization scales, we are able to obtain a generator that is also well behaved when the third jet becomes unresolved. We compare key distributions computed at the NLO level, at the level of the POWHEG hard emission and after full shower by PYTHIA, Pythia8 and HERWIG6. Read More

We present an implementation of the vector boson pair production processes ZZ, W+W- and WZ within the POWHEG BOX V2. This implementation, derived from the POWHEG BOX version, has several improvements over the old one, among which the inclusion of all decay modes of the vector bosons, the possibility to generate different decay modes in the same run, speed optimization and phase space improvements in the handling of interference and singly resonant contributions. Read More

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 POWHEG method for merging the NLO Higgs plus jet cross-section with the parton shower, and on the MINLO method to simultaneously achieve NLO accuracy for inclusive Higgs boson production. The NNLO accuracy is reached by a reweighting procedure making use of the HNNLO program. Read More

2013Jul
Authors: The LHC Higgs Cross Section Working Group, S. Heinemeyer1, C. Mariotti2, G. Passarino3, R. Tanaka4, J. R. Andersen, P. Artoisenet, E. A. Bagnaschi, A. Banfi, T. Becher, F. U. Bernlochner, S. Bolognesi, P. Bolzoni, R. Boughezal, D. Buarque, J. Campbell, F. Caola, M. Carena, F. Cascioli, N. Chanon, T. Cheng, S. Y. Choi, A. David, P. de Aquino, G. Degrassi, D. Del Re, A. Denner, H. van Deurzen, S. Diglio, B. Di Micco, R. Di Nardo, S. Dittmaier, M. Duhrssen, R. K. Ellis, G. Ferrera, N. Fidanza, M. Flechl, D. de Florian, S. Forte, R. Frederix, S. Frixione, S. Gangal, Y. Gao, M. V. Garzelli, D. Gillberg, P. Govoni, M. Grazzini, N. Greiner, J. Griffiths, A . V. Gritsan, C. Grojean, D. C. Hall, C. Hays, R. Harlander, R. Hernandez-Pinto, S. Hoche, J. Huston, T. Jubb, M. Kadastik, S. Kallweit, A. Kardos, L. Kashif, N. Kauer, H. Kim, R. Klees, M. Kramer, F. Krauss, A. Laureys, S. Laurila, S. Lehti, Q. Li, S. Liebler, X. Liu, H. E. Logan, G. Luisoni, M. Malberti, F. Maltoni, K. Mawatari, F. Maierhofer, H. Mantler, S. Martin, P. Mastrolia, O. Mattelaer, J. Mazzitelli, B. Mellado, K. Melnikov, P. Meridiani, D. J. Miller, E. Mirabella, S. O. Moch, P. Monni, N. Moretti, A. Muck, M. Muhlleitner, P. Musella, P. Nason, C. Neu, M. Neubert, C. Oleari, J. Olsen, G. Ossola, T. Peraro, K. Peters, F. Petriello, G. Piacquadio, C. T. Potter, S. Pozzorini, K. Prokofiev, I. Puljak, M. Rauch, D. Rebuzzi, L. Reina, R. Rietkerk, A. Rizzi, Y. Rotstein-Habarnau, G. P. Salam, G. Sborlini, F. Schissler, M. Schonherr, M. Schulze, M. Schumacher, F. Siegert, P. Slavich, J. M. Smillie, O. Stal, J. F. von Soden-Fraunhofen, M. Spira, I. W. Stewart, F. J. Tackmann, P. T. E. Taylor, D. Tommasini, J. Thompson, R. S. Thorne, P. Torrielli, F. Tramontano, N. V. Tran, Z. Trocsanyi, M. Ubiali, P. Vanlaer, M. Vazquez Acosta, T. Vickey, A. Vicini, W. J. Waalewijn, D. Wackeroth, C. Wagner, J. R. Walsh, J. Wang, G. Weiglein, A. Whitbeck, C. Williams, J. Yu, G. Zanderighi, M. Zanetti, M. Zaro, P. M. Zerwas, C. Zhang, T. J . E. Zirke, S. Zuberi
Affiliations: 1eds., 2eds., 3eds., 4eds.

This Report summarizes the results of the activities in 2012 and the first half of 2013 of the LHC Higgs Cross Section Working Group. The main goal of the working group was to present the state of the art of Higgs Physics at the LHC, integrating all new results that have appeared in the last few years. This report follows the first working group report Handbook of LHC Higgs Cross Sections: 1. Read More

We present a generator for the production of a Higgs boson H in association with a vector boson V=W or Z (including subsequent V decay) plus zero and one jet, that can be used in conjunction with general-purpose shower Monte Carlo generators, according to the POWHEG method, as implemented within the POWHEG BOX framework. We have computed the virtual corrections using GoSam, a program for the automatic construction of virtual amplitudes. In order to do so, we have built a general interface of the POWHEG BOX to the GoSam package. Read More

In this work we present the implementation of generators for W and Z bosons in association with two jets interfaced to parton showers using the POWHEG BOX. We incorporate matrix elements from the parton-level Monte Carlo program MCFM in the POWHEG BOX, allowing for a considerable improvement in speed compared to previous implementations. We address certain problems that arise when processes that are singular at the Born level are implemented in a shower framework using either a generation cut or a Born suppression factor to yield weighted events. Read More

In this short note we discuss the origin of spikes in physical distributions in dijet production, originating after the Monte Carlo shower of POWHEG Les Houches events. We introduce a refinement of the way in which the POWHEG BOX separates the singular regions that reduces the appearance of spikes in the computed distributions. We also study the sensitivity of the results due to a modification of the way the POWHEG events are passed to the shower Monte Carlo program. Read More

Following recent work on the combination of electroweak and strong radiative corrections to single W-boson hadroproduction in the POWHEG BOX framework, we generalize the above treatment to cover the neutral current Drell-Yan process. According to the POWHEG method, we combine both the next-to-leading order (NLO) electroweak and QED multiple photon corrections with the native NLO and Parton Shower QCD contributions. We show comparisons with the predictions of the electroweak generator HORACE, to validate the reliability and accuracy of the approach. Read More

We consider the POWHEG generator for a H/W/Z boson plus one jet, augmented with the recently proposed MiNLO method for the choice of scales and the inclusion of Sudakov form factors. Within this framework, the generator covers all the transverse-momentum region of the H/W/Z boson, i.e. Read More

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. Read More

We present predictions for a variety of single-inclusive observables that stem from the production of charm and bottom quark pairs at the 7 TeV LHC. They are obtained within the FONLL semi-analytical framework, and with two "Monte Carlo + NLO" approaches, MC@NLO and POWHEG. Results are given for final states and acceptance cuts that are as close as possible to those used by experimental collaborations and, where feasible, are compared to LHC data. Read More

The associated production of charged Higgs bosons and top quarks at hadron colliders is an important discovery channel to establish the existence of a non-minimal Higgs sector. Here, we present details of a next-to-leading order (NLO) calculation of this process using the Catani-Seymour dipole formalism and describe its implementation in POWHEG, which allows to match NLO calculations to parton showers. Numerical predictions are presented using the PYTHIA parton shower and are compared to those obtained previously at fixed order, to a leading order calculation matched to the PYTHIA parton shower, and to a different NLO calculation matched to the HERWIG parton shower with MC@NLO. Read More

We present a next-to-leading order calculation of Higgs boson production plus one and two jets via gluon fusion interfaced to shower Monte Carlo programs, implemented according to the POWHEG method. For this implementation we have used a new interface of the POWHEG BOX with MadGraph4, that generates the codes for generic Born and real processes automatically. The virtual corrections have been taken from the MCFM code. Read More

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. Read More

We present a fully consistent implementation of electroweak and strong radiative corrections to single W hadroproduction in the POWHEG BOX framework, treating soft and collinear photon emissions on the same ground as coloured parton emissions. This framework can be easily extended to more complex electroweak processes. We describe how next-to-leading order (NLO) electroweak corrections are combined with the NLO QCD calculation, and show how they are interfaced to QCD and QED shower Monte Carlo. Read More

2012Jan
Authors: LHC Higgs Cross Section Working Group, S. Dittmaier1, C. Mariotti2, G. Passarino3, R. Tanaka4, S. Alekhin, J. Alwall, E. A. Bagnaschi, A. Banfi, J. Blumlein, S. Bolognesi, N. Chanon, T. Cheng, L. Cieri, A. M. Cooper-Sarkar, M. Cutajar, S. Dawson, G. Davies, N. De Filippis, G. Degrassi, A. Denner, D. D'Enterria, S. Diglio, B. Di Micco, R. Di Nardo, R. K. Ellis, A. Farilla, S. Farrington, M. Felcini, G. Ferrera, M. Flechl, D. de Florian, S. Forte, S. Ganjour, M. V. Garzelli, S. Gascon-Shotkin, S. Glazov, S. Goria, M. Grazzini, J. -Ph. Guillet, C. Hackstein, K. Hamilton, R. Harlander, M. Hauru, S. Heinemeyer, S. Hoche, J. Huston, C. Jackson, P. Jimenez-Delgado, M. D. Jorgensen, M. Kado, S. Kallweit, A. Kardos, N. Kauer, H. Kim, M. Kovac, M. Kramer, F. Krauss, C. -M. Kuo, S. Lehti, Q. Li, N. Lorenzo, F. Maltoni, B. Mellado, S. O. Moch, A. Muck, M. Muhlleitner, P. Nadolsky, P. Nason, C. Neu, A. Nikitenko, C. Oleari, J. Olsen, S. Palmer, S. Paganis, C. G. Papadopoulos, T . C. Petersen, F. Petriello, F. Petrucci, G. Piacquadio, E. Pilon, C. T. Potter, J. Price, I. Puljak, W. Quayle, V. Radescu, D. Rebuzzi, L. Reina, J. Rojo, D. Rosco, G. P. Salam, A. Sapronov, J. Schaarschmidt, M. Schonherr, M. Schumacher, F. Siegert, P. Slavich, M. Spira, I. W. Stewart, W. J. Stirling, F. Stockli, C. Sturm, F. J. Tackmann, R. S. Thorne, D. Tommasini, P. Torrielli, F. Tramontano, Z. Trocsanyi, M. Ubiali, S. Uccirati, M. Vazquez Acosta, T. Vickey, A. Vicini, W. J. Waalewijn, D. Wackeroth, M. Warsinsky, M. Weber, M. Wiesemann, G. Weiglein, J. Yu, G. Zanderighi
Affiliations: 1eds., 2eds., 3eds., 4eds.

This Report summarises the results of the second year's activities of the LHC Higgs Cross Section Working Group. The main goal of the working group was to present the state of the art of Higgs Physics at the LHC, integrating all new results that have appeared in the last few years. The first working group report Handbook of LHC Higgs Cross Sections: 1. Read More

Incorporating all recent theoretical advances, we resum soft-gluon corrections to the total $t\bar t$ cross-section at hadron colliders at the next-to-next-to-leading logarithmic (NNLL) order. We perform the resummation in the well established framework of Mellin $N$-space resummation. We exhaustively study the sources of systematic uncertainty like renormalization and factorization scale variation, power suppressed effects and missing two- and higher-loop corrections. Read More

We present an implementation of the vector boson pair production processes ZZ, W+W- and W Z within the POWHEG framework, which is a method that allows the interfacing of NLO calculations to shower Monte Carlo programs. The implementation is built within the POWHEG BOX package. The Z/\gamma^* interference, as well as singly resonant contributions, are properly included. Read More

We present an implementation of the calculation of the production of W^+W^+ plus two jets at hadron colliders, at next-to-leading order (NLO) in QCD, in the POWHEG framework, which is a method that allows the interfacing of NLO calculations to shower Monte Carlo programs. This is the first 2 -> 4 process to be described to NLO accuracy within a shower Monte Carlo framework. The implementation was built within the POWHEGBOX package. Read More

We present an implementation of the next-to-leading order dijet production process in hadronic collisions in the framework of POWHEG, which is a method to implement NLO calculations within a shower Monte Carlo context. In constructing the simulation, we have made use of the POWHEG BOX toolkit, which makes light of many of the most technical steps. The majority of this article is concerned with the study of the predictions of the Monte Carlo simulation. Read More

We present an implementation of the next-to-leading order vector boson plus one jet production process in hadronic collision in the framework of POWHEG, which is a method to implement NLO calculations within a Shower Monte Carlo context. All spin correlations in the vector boson decay products have been taken into account. The process has been implemented in the framework of the POWHEG-BOX, an automated computer code for turning a NLO calculation into a shower Monte Carlo program. Read More

In recent times the algorithms for the simulation of hadronic collisions have been subject to two substantial improvements: the inclusion, within parton showering, of exact higher order tree level matrix elements (MEPS) and, separately, next-to-leading order corrections (NLOPS). In this work we examine the key criteria to be met in merging the two approaches in such a way that the accuracy of both is preserved, in the framework of the POWHEG approach to NLOPS. We then ask to what extent these requirements may be fulfilled using existing simulations, without modifications. Read More

This report summarizes the activities of the SM and NLO Multileg Working Group of the Workshop "Physics at TeV Colliders", Les Houches, France 8-26 June, 2009. Read More

In this work we illustrate the POWHEG BOX, a general computer code framework for implementing NLO calculations in shower Monte Carlo programs according to the POWHEG method. Aim of this work is to provide an illustration of the needed theoretical ingredients, a view of how the code is organized and a description of what a user should provide in order to use it. Read More

I review recent developments in POWHEG, a method for interfacing parton-shower generators with NLO QCD computations. I illustrate recent progress in understanding several features of the method, and in clarifying similarity and differences with respect to MC@NLO. Furthermore, I briefly describe a recently introduced framework, the POWHEG BOX, that allows the automatic POWHEG implementation of any given NLO calculation, and has been recently applied to Z+jet production and to Higgs production via vector-boson fusion. Read More

We discuss a general framework for the inclusion of heavy quark mass contributions to deep-inelastic structure functions and their perturbative matching to structure functions computed in variable-mass schemes. Our approach is based on the so-called FONLL method, previously introduced and applied to heavy quark hadroproduction and photoproduction. We define our framework, provide expressions up to second order in the strong coupling, and use them to construct matched expressions for structure functions up to NNLO. Read More

We present a next-to-leading order calculation of Higgs boson production in vector-boson fusion processes interfaced to shower Monte Carlo programs, implemented according to the POWHEG method. Read More

We present a next-to-leading order calculation of single-top production interfaced to Shower Monte Carlo programs, implemented according to the POWHEG method. A detailed comparison with MC@NLO and PYTHIA is carried out for several observables, for the Tevatron and LHC colliders. Read More

These proceedings collect the presentations given at the first three meetings of the INFN "Workshop on Monte Carlo's, Physics and Simulations at the LHC", held at the Frascati National Laboratories in 2006. The first part of these proceedings contains pedagogical introductions to several basic topics of both theoretical and experimental high pT LHC physics. The second part collects more specialised presentations. Read More

These proceedings collect the presentations given at the first three meetings of the INFN "Workshop on Monte Carlo's, Physics and Simulations at the LHC", held at the Frascati National Laboratories in 2006. The first part of these proceedings contains pedagogical introductions to several basic topics of both theoretical and experimental high pT LHC physics. The second part collects more specialised presentations. Read More

We present a next-to-leading order calculation of Higgs boson production via gluon fusion interfaced to shower Monte Carlo programs, implemented according to the POWHEG method. A detailed comparison with MC@NLO and PYTHIA is carried out for several observables, for the Tevatron and LHC colliders. Comparisons with next-to-next-to-leading order results and with resummed ones are also presented. Read More

We present a next-to-leading-order calculation of W/Z production interfaced to shower Monte Carlo, implemented according to the POWHEG method. Finite width effects, Z/gamma interference and angular correlations of decay products are included. A detailed comparison with MC@NLO and PYTHIA is carried out. Read More

We present updated predictions for the total production cross section of top-quark pairs at the Tevatron and at the LHC, and, at the LHC, of heavy-quark pairs with mass in the range 0.5-2 TeV. For t\bar{t} production at the LHC we also present results at \sqrt{S}= 10 TeV, in view of the expected accelerator conditions during the forthcoming 2008 run. Read More

In this note I illustrate the program MINT, a FORTRAN program for Monte Carlo adaptive integration and generation of unweighted distributions. Read More

The aim of this work is to describe in detail the POWHEG method, first suggested by one of the authors, for interfacing parton-shower generators with NLO QCD computations. We describe the method in its full generality, and then specify its features in two subtraction frameworks for NLO calculations: the Catani-Seymour and the Frixione-Kunszt-Signer approach. Two examples are discussed in detail in both approaches: the production of hadrons in e+e- collisions, and the Drell-Yan vector-boson production in hadronic collisions. Read More