# C. G. Papadopoulos - Democritos Nucl. Res. Ctr.

## Contact Details

NameC. G. Papadopoulos |
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AffiliationDemocritos Nucl. Res. Ctr. |
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Location |
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## Pubs By Year |
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## External Links |
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## Pub CategoriesHigh Energy Physics - Phenomenology (41) High Energy Physics - Experiment (8) High Energy Physics - Theory (6) Computer Science - Data Structures and Algorithms (3) Physics - Accelerator Physics (2) Computer Science - Discrete Mathematics (2) Physics - Data Analysis; Statistics and Probability (1) Physics - Instrumentation and Detectors (1) Mathematics - Combinatorics (1) Computer Science - Computational Complexity (1) Computer Science - Networking and Internet Architecture (1) Nuclear Experiment (1) |

## Publications Authored By C. G. Papadopoulos

Based on the Baikov representation, we present a systematic approach to compute cuts of Feynman Integrals, appropriately defined in $d$ dimensions. The information provided by these computations may be used to determine the class of functions needed to analytically express the full integrals. Read More

Given a vertex-weighted graph $G=(V,E)$ and a set $S \subseteq V$, a subset feedback vertex set $X$ is a set of the vertices of $G$ such that the graph induced by $V \setminus X$ has no cycle containing a vertex of $S$. The \textsc{Subset Feedback Vertex Set} problem takes as input $G$ and $S$ and asks for the subset feedback vertex set of minimum total weight. In contrast to the classical \textsc{Feedback Vertex Set} problem which is obtained from the \textsc{Subset Feedback Vertex Set} problem for $S=V$, restricted to graph classes the \textsc{Subset Feedback Vertex Set} problem is known to be NP-complete on split graphs and, consequently, on chordal graphs. Read More

In social networks the {\sc Strong Triadic Closure} is an assignment of the edges with strong or weak labels such that any two vertices that have a common neighbor with a strong edge are adjacent. The problem of maximizing the number of strong edges that satisfy the strong triadic closure was recently shown to be NP-complete for general graphs. Here we initiate the study of graph classes for which the problem is solvable. Read More

To make full use of photocathode material and improve its quantum efficiency lifetime, it can be necessary to operate laser away from the cathode center in photoinjectors. In RF guns, the off-axis emitted beam will see a time-dependent RF effect, which would generate a significant growth in transverse emittance. It has been demonstrated that such an emittance growth can be almost completely compensated by orienting the beam on a proper orbit in the downstream RF cavities along the injector. Read More

There are currently no requirements (technical or otherwise) that BGP paths must be contained within national boundaries. Indeed, some paths experience international detours, i.e. Read More

We present the calculation of massless two-loop Master Integrals relevant to five-point amplitudes with one off-shell external leg and derive the complete set of planar Master Integrals with five on-mass-shell legs, that contribute to many $2\to 3$ amplitudes of interest at the LHC, as for instance three jet production, $\gamma, V, H +2$ jets etc., based on the Simplified Differential Equations approach. Read More

Let $G$ be a strongly connected directed graph. We consider the following three problems, where we wish to compute the smallest strongly connected spanning subgraph of $G$ that maintains respectively: the $2$-edge-connected blocks of $G$ (\textsf{2EC-B}); the $2$-edge-connected components of $G$ (\textsf{2EC-C}); both the $2$-edge-connected blocks and the $2$-edge-connected components of $G$ (\textsf{2EC-B-C}). All three problems are NP-hard, and thus we are interested in efficient approximation algorithms. Read More

**Authors:**n_TOF Collaboration, :, E. Mendoza, D. Cano-Ott, C. Guerrero, E. Berthoumieux, U. Abbondanno, G. Aerts, F. Alvarez-Velarde, S. Andriamonje, J. Andrzejewski, P. Assimakopoulos, L. Audouin, G. Badurek, J. Balibrea, P. Baumann, F. Becvar, F. Belloni, F. Calvino, M. Calviani, R. Capote, C. Carrapico, A. Carrillo de Albornoz, P. Cennini, V. Chepel, E. Chiaveri, N. Colonna, G. Cortes, A. Couture, J. Cox, M. Dahlfors, S. David, I. Dillmann, R. Dolfini, C. Domingo-Pardo, W. Dridi, I. Duran, C. Eleftheriadis, L. Ferrant, A. Ferrari, R. Ferreira-Marques, L. Fitzpatrick, H. Frais-Koelbl, K. Fujii, W. Furman, I. Goncalves, E. Gonzalez-Romero, A. Goverdovski, F. Gramegna, E. Griesmayer, F. Gunsing, B. Haas, R. Haight, M. Heil, A. Herrera-Martinez, M. Igashira, S. Isaev, E. Jericha, F. Kappeler, Y. Kadi, D. Karadimos, D. Karamanis, V. Ketlerov, M. Kerveno, P. Koehler, V. Konovalov, E. Kossionides, M. Krticka, C. Lampoudis, H. Leeb, A. Lindote, I. Lopes, R. Lossito, M. Lozano, S. Lukic, J. Marganiec, L. Marques, S. Marrone, T. Martinez, C. Massimi, P. Mastinu, A. Mengoni, P. M. Milazzo, C. Moreau, M. Mosconi, F. Neves, H. Oberhummer, S. O Brien, M. Oshima, J. Pancin, C. Papachristodoulou, C. Papadopoulos, C. Paradela, N. Patronis, A. Pavlik, P. Pavlopoulos, L. Perrot, M. T. Pigni, R. Plag, A. Plompen, A. Plukis, A. Poch, J. Praena, C. Pretel, J. Quesada, T. Rauscher, R. Reifarth, M. Rosetti, C. Rubbia, G. Rudolf, P. Rullhusen, J. Salgado, C. Santos, L. Sarchiapone, I. Savvidis, C. Stephan, G. Tagliente, J. L. Tain, L. Tassan-Got, L. Tavora, R. Terlizzi, G. Vannini, P. Vaz, A. Ventura, D. Villamarin, M. C. Vicente, V. Vlachoudis, R. Vlastou, F. Voss, S. Walter, H. Wendler, M. Wiescher, K. Wisshak

**Category:**Nuclear Experiment

Background:The design of new nuclear reactors and transmutation devices requires to reduce the present neutron cross section uncertainties of minor actinides. Purpose: Reduce the $^{243}$Am(n,$\gamma$) cross section uncertainty. Method: The $^{243}$Am(n,$\gamma$) cross section has been measured at the n_TOF facility at CERN with a BaF$_{2}$ Total Absorption Calorimeter, in the energy range between 0. Read More

We calculate the complete set of two-loop Master Integrals with two off mass-shell legs with massless internal propagators, that contribute to amplitudes of diboson $V_1V_2$ production at the LHC. This is done with the Simplified Differential Equations approach to Master Integrals, which was recently proposed by one of the authors. Read More

A short review is given of the simplified differential equations approach to Master Integrals, which was recently proposed by one of the authors. We show its applicability by calculating some non-trivial two-loop planar Master Integrals, namely those contributing to amplitudes of massive diboson VV' production at the LHC with massless internal lines. Read More

A simplified differential equations approach for Master Integrals is presented. It allows to express them, straightforwardly, in terms of Goncharov Polylogarithms. As a proof-of-concept of the proposed method, results at one and two loops are presented, including the massless one-loop pentagon with up to one off-shell leg at order epsilon. Read More

**Authors:**D. Alesini, M. Alessandroni, M. P. Anania, S. Andreas, M. Angelone, A. Arcovito, F. Arnesano, M. Artioli, L. Avaldi, D. Babusci, A. Bacci, A. Balerna, S. Bartalucci, R. Bedogni, M. Bellaveglia, F. Bencivenga, M. Benfatto, S. Biedron, V. Bocci, M. Bolognesi, P. Bolognesi, R. Boni, R. Bonifacio, M. Boscolo, F. Boscherini, F. Bossi, F. Broggi, B. Buonomo, V. Calo', D. Catone, M. Capogni, M. Capone, M. Castellano, A. Castoldi, L. Catani, G. Cavoto, N. Cherubini, G. Chirico, M. Cestelli-Guidi, E. Chiadroni, V. Chiarella, A. Cianchi, M. Cianci, R. Cimino, F. Ciocci, A. Clozza, M. Collini, G. Colo', A. Compagno, G. Contini, M. Coreno, R. Cucini, C. Curceanu, S. Dabagov, E. Dainese, I. Davoli, G. Dattoli, L. De Caro, P. De Felice, S. Della Longa, G. Delle Monache, M. De Spirito, A. Di Cicco, C. Di Donato, D. Di Gioacchino, D. Di Giovenale, E. Di Palma, G. Di Pirro, A. Dodaro, A. Doria, U. Dosselli, A. Drago, R. Escribano, A. Esposito, R. Faccini, A. Ferrari, M. Ferrario, A. Filabozzi, D. Filippetto, F. Fiori, O. Frasciello, L. Fulgentini, G. P. Gallerano, A. Gallo, M. Gambaccini, C. Gatti, G. Gatti, P. Gauzzi, A. Ghigo, G. Ghiringhelli, L. Giannessi, G. Giardina, C. Giannini, F. Giorgianni, E. Giovenale, L. Gizzi, C. Guaraldo, C. Guazzoni, R. Gunnella, K. Hatada, S. Ivashyn, F. Jegerlehner, P. O. Keeffe, W. Kluge, A. Kupsc, M. Iannone, L. Labate, P. Levi Sandri, V. Lombardi, P. Londrillo, S. Loreti, M. Losacco, S. Lupi, A. Macchi, S. Magazu', G. Mandaglio, A. Marcelli, G. Margutti, C. Mariani, P. Mariani, G. Marzo, C. Masciovecchio, P. Masjuan, M. Mattioli, G. Mazzitelli, N. P. Merenkov, P. Michelato, F. Migliardo, M. Migliorati, C. Milardi, E. Milotti, S. Milton, V. Minicozzi, S. Mobilio, S. Morante, D. Moricciani, A. Mostacci, V. Muccifora, F. Murtas, P. Musumeci, F. Nguyen, A. Orecchini, G. Organtini, P. L. Ottaviani, E. Pace, M. Paci, C. Pagani, S. Pagnutti, V. Palmieri, L. Palumbo, G. C. Panaccione, C. F. Papadopoulos, M. Papi, M. Passera, L. Pasquini, M. Pedio, A. Perrone, A. Petralia, C. Petrillo, V. Petrillo, M. Pillon, P. Pierini, A. Pietropaolo, A. D. Polosa, R. Pompili, J. Portoles, T. Prosperi, C. Quaresima, L. Quintieri, J. V. Rau, M. Reconditi, A. Ricci, R. Ricci, G. Ricciardi, E. Ripiccini, S. Romeo, C. Ronsivalle, N. Rosato, J. B. Rosenzweig, G. Rossi, A. A. Rossi, A. R. Rossi, F. Rossi, D. Russo, A. Sabatucci, E. Sabia, F. Sacchetti, S. Salducco, F. Sannibale, G. Sarri, T. Scopigno, L. Serafini, D. Sertore, O. Shekhovtsova, I. Spassovsky, T. Spadaro, B. Spataro, F. Spinozzi, A. Stecchi, F. Stellato, V. Surrenti, A. Tenore, A. Torre, L. Trentadue, S. Turchini, C. Vaccarezza, A. Vacchi, P. Valente, G. Venanzoni, S. Vescovi, F. Villa, G. Zanotti, N. Zema, M. Zobov

This report describes the scientific aims and potentials as well as the preliminary technical design of IRIDE, an innovative tool for multi-disciplinary investigations in a wide field of scientific, technological and industrial applications. IRIDE will be a high intensity 'particle factory', based on a combination of a high duty cycle radio-frequency superconducting electron linac and of high energy lasers. Conceived to provide unique research possibilities for particle physics, for condensed matter physics, chemistry and material science, for structural biology and industrial applications, IRIDE will open completely new research possibilities and advance our knowledge in many branches of science and technology. Read More

The PowHel framework allows to make predictions of total and differential cross-sections of multiparticle hadroproduction processes at both NLO QCD accuracy and NLO QCD matched to Parton Shower, on the basis of the interface between the POWHEG-BOX and HELAC-NLO codes. It has already been applied to study several processes involving a top-antitop pair in association with a third particle or hadronic jet. Our most recent predictions concern top-antitop-V hadroproduction (with V = W or Z), at both parton and hadron level, by considering different decay channels (hadronic and leptonic) of the heavy particles. Read More

We present theoretical predictions for the hadroproduction of t tbar W+, t tbar W- and t tbar Z at LHC as obtained by matching numerical computations at NLO accuracy in QCD with Shower Monte Carlo programs. The calculation is performed by PowHel, relying on the POWHEG-BOX framework, that allows for the matching between the fixed order computation, with input of matrix elements produced by the HELAC-NLO collection of event generators, and the Parton Shower evolution, followed by hadronization and hadron decays as described by PYTHIA and HERWIG. We focus on the dilepton and trilepton decay channels, studied recently by the CMS Collaboration. Read More

Calculation of amplitudes in perturbative quantum field theory involve large loop integrals. The complexity of those integrals, in combination with the large number of Feynman diagrams, make the calculations very difficult. Reduction methods proved to be very helpful, lowering the number of integrals that need to be actually calculated. Read More

**Authors:**LHC Higgs Cross Section Working Group, S. Dittmaier

^{1}, C. Mariotti

^{2}, G. Passarino

^{3}, R. Tanaka

^{4}, 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:**

^{1}eds.,

^{2}eds.,

^{3}eds.,

^{4}eds.

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

We present predictions for the production cross section of a Standard Model Z0-boson in association with a top-antitop pair at the next-to-leading order accuracy in QCD, matched with shower Monte Carlo programs to evolve the system down to the hadronization energy scale. We adopt a framework based on three well established numerical codes, namely the POWHEG-BOX, used for computing the cross section, HELAC-NLO, which generates all necessary input matrix elements, and finally a parton shower program, such as PYTHIA or HERWIG, which allows for including t-quark and Z0-boson decays at the leading order accuracy and generates shower emissions, hadronization and hadron decays. Read More

We present predictions for the production cross section of t-quark pair production in association with a Z boson at the next-to-leading order (NLO) accuracy using matrix elements obtained from the HELAC-Oneloop package. We use the subtraction method for computing the radiative corrections as implemented in the POWHEG-Box, which was also used in several other computations of similar complexity. Read More

Based on the OPP technique and the HELAC framework, HELAC-1LOOP is a program that is capable of numerically evaluating QCD virtual corrections to scattering amplitudes. A detailed presentation of the algorithm is given, along with instructions to run the code and benchmark results. The program is part of the HELAC-NLO framework that allows for a complete evaluation of QCD NLO corrections. Read More

We report on the calculation of the next-to-leading order QCD corrections to the production of ttbar pairs in association with two hard jets at the Fermilab TeVatron and CERN Large Hadron Collider. Results for the integrated and differential cross sections are given. The corrections with respect to leading order are negative and moderate. Read More

We present predictions for the production cross section of a Standard Model Higgs boson in association with a top-antitop pair at next-to-leading order accuracy using matrix elements obtained from the HELAC-Oneloop package. The NLO prediction was interfaced to the PYTHIA and HERWIG shower Monte Carlo programs with the help of POWHEG-Box, allowing for decays of massive particles, showering and hadronization, thus leading to final results at the hadron level. Read More

We compute the production cross section of a top-antitop pair in association with a jet at hadron colliders at next-to-leading order accuracy matched with parton shower algorithms to make predictions at the hadron level. The parton shower allows for including the decay of the top quarks at the leading order accuracy. We use a framework based on three well established numerical codes, the POWHEG-BOX, used for the calculation of the cross section, HELAC, which generates the matrix elements for the Born-level, real emission and the virtual part, and finally a parton shower program, such as PYTHIA or HERWIG, which generate the parton-shower and hadronization. Read More

Results for next-to-leading order QCD corrections to the pp(p\bar{p}) -> t \bar{t} -> W^+W^- b\bar{b} -> e^{+} \nu_{e} \mu^{-} \bar{\nu}_{\mu} b \bar{b} +X processes with complete off-shell effects are presented for the first time. Double-, single- and non-resonant top contributions of the order {\cal{O}}(\alpha_{s}^3 \alpha^4) are consistently taken into account, which requires the introduction of a complex-mass scheme for unstable top quarks. Moreover, the intermediate W bosons are treated off-shell. Read More

Achieving a precise description of multi-parton final states is crucial for many analyses at LHC. In this contribution we review the main features of the HELAC-NLO system for NLO QCD calculations. As a case study, NLO QCD corrections for tt + 2 jet production at LHC are illustrated and discussed. Read More

**Authors:**T. Binoth, G. Dissertori, J. Huston, R. Pittau, J. R. Andersen, J. Archibald, S. Badger, R. D. Ball, G. Bevilacqua, I. Bierenbaum, T. Binoth, F. Boudjema, R. Boughezal, A. Bredenstein, R. Britto, M. Campanelli, J. Campbell, L. Carminati, G. Chachamis, V. Ciulli, G. Cullen, M. Czakon, L. Del Debbio, A. Denner, G. Dissertori, S. Dittmaier, S. Forte, R. Frederix, S. Frixione, E. Gardi, M. V. Garzelli, S. Gascon-Shotkin, T. Gehrmann, A. Gehrmann-De Ridder, W. Giele, T. Gleisberg, E. W. N. Glover, N. Greiner, A. Guffanti, J. -Ph. Guillet, A. van Hameren, G. Heinrich, S. Hoeche, M. Huber, J. Huston, M. Jaquier, S. Kallweit, S. Karg, N. Kauer, F. Krauss, J. I. Latorre, A. Lazopoulos, P. Lenzi, G. Luisoni, R. Mackeprang, L. Magnea, D. Maitre, D. Majumder, I. Malamos, F. Maltoni, K. Mazumdar, P. Nadolsky, P. Nason, C. Oleari, F. Olness, C. G. Papadopoulos, G. Passarino, E. Pilon, R. Pittau, S. Pozzorini, T. Reiter, J. Reuter, M. Rodgers, G. Rodrigo, J. Rojo, G. Sanguinetti, F. -P. Schilling, M. Schumacher, S. Schumann, R. Schwienhorst, P. Skands, H. Stenzel, F. Stoeckli, R. Thorne, M. Ubiali, P. Uwer, A. Vicini, M. Warsinsky, G. Watt, J. Weng, I. Wigmore, S. Weinzierl, J. Winter, M. Worek, G. Zanderighi

**Category:**High Energy Physics - Phenomenology

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

We report the results of a next-to-leading order simulation of top quark pair production in association with two jets. With our inclusive cuts, we show that the corrections with respect to leading order are negative and small, reaching 11%. The error obtained by scale variation is of the same order. Read More

We present the results of a next-to-leading order calculation of QCD corrections to the production of an on-shell top-anti-top quark pair in association with two flavored b-jets. Besides studying the total cross section and its scale dependence, we give several differential distributions. Where comparable, our results agree with a previous analysis. Read More

**Category:**High Energy Physics - Phenomenology

We extend the massless dipole formalism of Catani and Seymour, as well as its massive version as developed by Catani, Dittmaier, Seymour and Trocsanyi, to arbitrary helicity eigenstates of the external partons. We modify the real radiation subtraction terms only, the primary aim being an improved efficiency of the numerical Monte Carlo integration of this contribution as part of a complete next-to-leading order calculation. In consequence, our extension is only applicable to unpolarized scattering. Read More

An algorithm, based on the OPP reduction method, to automatically compute any one-loop amplitude, for all momentum, color and helicity configurations of the external particles, is presented. It has been implemented using the tree-order matrix element code HELAC and the OPP reduction code CutTools. As a demonstration of the potential of the current implementation, results for all sub-processes included in the 2007 Les Houches wish list for LHC, are presented. Read More

We compute the complete set of Feynman Rules producing the Rational Terms of kind R_2 needed to perform any QCD 1-loop calculation. We also explicitly check that in order to account for the entire R_2 contribution, even in case of processes with more than four external legs, only up to four-point vertices are needed. Our results are expressed both in the 't Hooft Veltman regularization scheme and in the Four Dimensional Helicity scheme, using explicit color configurations as well as the color connection language. Read More

We show how to transform a $d$-dimensional Euclidean path integral in terms of two (Cartesian) fields to a path integral in terms of polar field variables. First we present a conjecture that states how this transformation should be done. Then we show that this conjecture is correct in the case of two toy models. Read More

We present a calculation of the NLO QCD corrections for the production of three vector bosons at the LHC, namely Z Z Z, W+ W- Z, W+ Z Z, and W+ W- W+ production. The virtual corrections are computed using the recently proposed method of reduction at the integrand level (OPP reduction). Concerning the contributions coming from real emission we used the dipole subtraction to treat the soft and collinear divergences. Read More

We present an optimization of the reduction algorithm of one-loop amplitudes in terms of master integrals. It is based on the exploitation of the polynomial structure of the integrand when evaluated at values of the loop-momentum fulfilling multiple cut-conditions, as emerged in the OPP-method. The reconstruction of the polynomials, needed for the complete reduction, is rended very versatile by using a projection-technique based on the Discrete Fourier Transform. Read More

**Authors:**Z. Bern

^{1}, S. Dittmaier

^{2}, L. Dixon

^{3}, G. Heinrich

^{4}, J. Huston

^{5}, B. Kersevan

^{6}, Z. Kunszt

^{7}, D. E. Soper

^{8}, C. Bernicot, T. Binoth, F. Boudjema, R. Britto, J. Campbell, M. Czakon, A. Denner, G. Dissertori, G. Duplancic, R. K. Ellis, R. Frederix, T. Gehrmann, A. Gehrmann-De Ridder, W. T. Giele, E. W. N. Glover, J. P. Guillet, S. Kallweit, S. Karg, N. Kauer, D. A. Kosower, F. Krauss, N. D. Le, P. Mastrolia, A. Mitov, S. Moch, S. Odaka, G. Ossola, C. G. Papadopoulos, E. Pilon, R. Pittau, T. Reiter, G. Sanguinetti, S. Schumann, C. Schwinn, P. Z. Skands, H. Stenzel, P. Uwer, S. Weinzierl, G. Zanderighi

**Affiliations:**

^{1}conveners,

^{2}conveners,

^{3}conveners,

^{4}conveners,

^{5}conveners,

^{6}conveners,

^{7}conveners,

^{8}conveners

**Category:**High Energy Physics - Phenomenology

This report summarizes the activities of the NLM working group of the Workshop "Physics at TeV Colliders", Les Houches, France, 11-29 June, 2007. Read More

The various sources of Rational Terms contributing to the one-loop amplitudes are critically discussed. We show that the terms originating from the generic (n-4)-dimensional structure of the numerator of the one-loop amplitude can be derived by using appropriate Feynman rules within a tree-like computation. For the terms that originate from the reduction of the 4-dimensional part of the numerator, we present two different strategies and explicit algorithms to compute them. Read More

We present a program that implements the OPP reduction method to extract the coefficients of the one-loop scalar integrals from a user defined (sub)-amplitude or Feynman Diagram, as well as the rational terms coming from the 4-dimensional part of the numerator. The rational pieces coming from the epsilon-dimensional part of the numerator are treated as an external input, and can be computed with the help of dedicated tree-level like Feynman rules. Possible numerical instabilities are dealt with the help of arbitrary precision routines, that activate only when needed. Read More

The updated version of the Helac-Phegas event generator is presented. The matrix elements are calculated through Dyson-Schwinger recursive equations. Helac-Phegas generates parton-level events with all necessary information, in the most recent Les Houches Accord format, for the study of any process within the Standard Model in hadron and lepton colliders. Read More

We compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW and MLM schemes using different matrix-element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences. Read More

**Affiliations:**

^{1}Democritos Nucl. Res. Ctr.,

^{2}Democritos Nucl. Res. Ctr.,

^{3}Turin U. & INFN, Turin

**Category:**High Energy Physics - Phenomenology

We apply the recently proposed amplitude reduction at the integrand level method, to the computation of the scattering process 2 photons -> 4 photons, including the case of a massive fermion loop. We also present several improvements of the method, including a general strategy to reconstruct the rational part of any one-loop amplitude and the treatment of vanishing Gram-determinants. Read More

**Authors:**J. Alwall, A. Ballestrero, P. Bartalini, S. Belov, E. Boos, A. Buckley, J. M. Butterworth, L. Dudko, S. Frixione, L. Garren, S. Gieseke, A. Gusev, I. Hinchliffe, J. Huston, B. Kersevan, F. Krauss, N. Lavesson, L. Lönnblad, E. Maina, F. Maltoni, M. L. Mangano, F. Moortgat, S. Mrenna, C. G. Papadopoulos, R. Pittau, P. Richardson, M. H. Seymour, A. Sherstnev, T. Sjöstrand, P. Skands, S. R. Slabospitsky, Z. Wcas, B. R. Webber, M. Worek, D. Zeppenfeld

**Category:**High Energy Physics - Phenomenology

A standard file format is proposed to store process and event information, primarily output from parton-level event generators for further use by general-purpose ones. The information content is identical with what was already defined by the Les Houches Accord five years ago, but then in terms of Fortran commonblocks. This information is embedded in a minimal XML-style structure, for clarity and to simplify parsing. Read More

We show how to extract the coefficients of the 4-, 3-, 2- and 1-point one-loop scalar integrals from the full one-loop amplitude of arbitrary scattering processes. In a similar fashion, also the rational terms can be derived. Basically no information on the analytical structure of the amplitude is required, making our method appealing for an efficient numerical implementation Read More

The usefulness of recursive equations to compute scattering matrix elements for arbitrary processes is discussed. Explicit results at tree and one-loop order, obtained by the HELAC/PHEGAS package that is based on the Dyson-Schwinger recursive equations approach, are briefly presented. Read More

The status of the multi-purpose event generator HELAC is briefly presented. The aim of this tool is the full simulation of events within the Standard Model at current and future high energy experiments, in particular the LHC. Some results related to the production of multi-jet final states at the LHC are also discussed. Read More

We present an alternative method to calculate cross sections for multi-parton scattering processes in the Standard Model at leading order. The helicity amplitudes are computed using recursion relations in the number of particles, based on Dyson-Schwinger equations whereas the summation over colour and helicity configurations is performed by Monte Carlo methods. The computational cost of our algorithm grows asymptotically as 3^n, where n is the number of particles involved in the process, as opposed to the n!-growth of the Feynman diagram approach. Read More

We present a proof of the Britto-Cachazo-Feng-Witten tree-level recursion relation for gluon amplitudes in QCD, based on a direct equivalence between BCFW decompositions and Feynman diagrams. We demonstrate that this equivalence can be made explicit when working in a convenient gauge. We exhibit that gauge invariance and the particular structure of Yang-Mills vertices guarantees the validity of the BCFW construction. Read More

A method to efficiently compute, in a automatic way, helicity amplitudes for arbitrary scattering processes at leading order in the Standard Model is presented. The scattering amplitude is evaluated recursively through a set of Dyson-Schwinger equations. The computational cost of this algorithm grows asymptotically as 3^n, where n is the number of particles involved in the process, compared to n! in the traditional Feynman graphs approach. Read More

A way to efficiently compute helicity amplitudes for arbitrary tree-level scattering processes in QCD is presented. The scattering amplitude is evaluated recursively through a set of Dyson-Schwinger equations. The computational cost of this algorithm grows asymptotically as 3^n, where n is the number of particles involved in the process, compared to n! in the traditional Feynman graphs approach. Read More

In this paper we examine the classes of graphs whose $K_n$-complements are trees and quasi-threshold graphs and derive formulas for their number of spanning trees; for a subgraph $H$ of $K_n$, the $K_n$-complement of $H$ is the graph $K_n-H$ which is obtained from $K_n$ by removing the edges of $H$. Our proofs are based on the complement spanning-tree matrix theorem, which expresses the number of spanning trees of a graph as a function of the determinant of a matrix that can be easily constructed from the adjacency relation of the graph. Our results generalize previous results and extend the family of graphs of the form $K_n-H$ admitting formulas for the number of their spanning trees. Read More

The physics of high-energy collider experiments asks for delicate comparisons between theoretical predictions and experimental data. Signals and potential backgrounds for new physics have to be predicted at sufficient accuracy. The accuracy as well as the computational complexity of the calculations leading to the predictions depend on both the number of external particles in the process analyzed and the order of the quantum corrections, the number of loops, included in the calculation. Read More

In this paper total cross sections for signals and backgrounds of top- and Higgs-production channels in electron-positron collisions at a future linear collider are presented. All channels considered are characterized by the emergence of six-particle final states. The calculation takes into account the full set of tree-level amplitudes in each process. Read More