Alessandro Cafarella

Helac-Phegas: A generator for all parton level processes

Abstract The updated version of the Helac-Phegas 1 event generator is presented. The matrix elements are calculated through Dyson–Schwinger recursive equations using color connection representation. Phase-space generation is based on a multichannel approach, including optimization. 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. New version program summary Program title: HELAC-PHEGAS Catalogue identifier: ADMS_v2_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADMS_v2_0.html Program obtainable from: CPC Program Library, Queens University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 35 986 No. of bytes in distributed program, including test data, etc.: 380 214 Distribution format: tar.gz Programming language: Fortran Computer: All Operating system: Linux Classification: 11.1, 11.2 External routines: Optionally Les Houches Accord (LHA) PDF Interface library ( http://projects.hepforge.org/lhapdf/ ) Catalogue identifier of previous version: ADMS_v1_0 Journal reference of previous version: Comput. Phys. Comm. 132 (2000) 306 Does the new version supersede the previous version?: Yes, partly Nature of problem: One of the most striking features of final states in current and future colliders is the large number of events with several jets. Being able to predict their features is essential. To achieve this, the calculations need to describe as accurately as possible the full matrix elements for the underlying hard processes. Even at leading order, perturbation theory based on Feynman graphs runs into computational problems, since the number of graphs contributing to the amplitude grows as n !. Solution method: Recursive algorithms based on Dyson–Schwinger equations have been developed recently in order to overcome the computational obstacles. The calculation of the amplitude, using Dyson–Schwinger recursive equations, results in a computational cost growing asymptotically as 3 n , where n is the number of particles involved in the process. Off-shell subamplitudes are introduced, for which a recursion relation has been obtained allowing to express an n -particle amplitude in terms of subamplitudes, with 1-, 2-, …  up to ( n − 1 ) particles. The color connection representation is used in order to treat amplitudes involving colored particles. In the present version HELAC-PHEGAS can be used to efficiently obtain helicity amplitudes, total cross sections, parton-level event samples in LHA format, for arbitrary multiparticle processes in the Standard Model in leptonic, p p ¯ and pp collisions. Reasons for new version: Substantial improvements, major functionality upgrade. Summary of revisions: Color connection representation, efficient integration over PDF via the PARNI algorithm, interface to LHAPDF, parton level events generated in the most recent LHA format, k ⊥ reweighting for Parton Shower matching, numerical predictions for amplitudes for arbitrary processes for phase-space points provided by the user, new user interface and the possibility to run over computer clusters. Running time: Depending on the process studied. Usually from seconds to hours. References: [1] A. Kanaki, C.G. Papadopoulos, Comput. Phys. Comm. 132 (2000) 306. [2] C.G. Papadopoulos, Comput. Phys. Comm. 137 (2001) 247.

NNLO logarithmic expansions and exact solutions of the DGLAP equations from x-space: New algorithms for precision studies at the LHC

Abstract A NNLO analysis of certain logarithmic expansions, developed for precision studies of the evolution of the QCD parton distributions (pdf) at the large hadron collider, is presented. We elaborate on their relations to all the solutions of the DGLAP equations that have been hitherto obtained from Mellin space, to which are equivalent. Exact expansions, equivalent to exact solutions of the equations, are constructed in the non-singlet sector. The algorithmic features of our approach are also emphasized, since this method allows to obtain numerical solutions of the evolution equations with the same accuracy of other methods, based on Mellin space, and of brute force methods, which solve the equations by finite differences. The implementation of our analysis allows to compare with existing benchmarks for the evolution of the pdfs, useful for applications at the LHC, and to extend them significantly in a systematic fashion, especially when solutions that retain logarithmic corrections only of a certain accuracy are searched for.

Precision studies of the NNLO DGLAP evolution at the LHC with Candia

Abstract We summarize the theoretical approach to the solution of the NNLO DGLAP equations using methods based on the logarithmic expansions in x-space and their implementation into the C program Candia 1.0 . 1 We present the various options implemented in the program and discuss the different solutions. The user can choose the order of the evolution, the type of the solution, which can be either exact or truncated, and the evolution either with a fixed or a varying flavor number, implemented in the varying-flavor-number scheme (VFNS). The renormalization and factorization scale dependencies are treated separately. In the non-singlet sector the program implements an exact NNLO solution. Program summary Program title: CANDIA Catalogue identifier: AEBK_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEBK_v1_0.html Program obtainable from: CPC Program Library, Queens University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 101 376 No. of bytes in distributed program, including test data, etc.: 5 865 234 Distribution format: tar.gz Programming language: C and Fortran Computer: All Operating system: Linux RAM: In the given examples, it ranges from 4 to 490 MB Classification: 11.1, 11.5 Nature of problem: The program provided here solves the DGLAP evolution equations for the parton distribution functions up to NNLO. Solution method: The algorithm implemented is based on the theory of the logarithmic expansions in Bjorken x-space. Additional comments: To be sure of getting the latest version of the program, the authors suggest downloading the code from their official CANDIA website ( http://www.le.infn.it/candia ). Running time: In the given examples, it ranges from 1 to 40 minutes. The jobs have been executed on an Intel Core 2 Duo T7250 CPU at 2 GHz with a 64 bit Linux kernel. The test run script included in the package contains 5 sample runs and may take a number of hours to process, depending on the speed of the processor used and the size of the available RAM.

Cosmic ray signals from mini black holes in models with extra dimensions: an analytical/Monte Carlo study

It is shown that the partial and total multiplicities and the lateral distributions of the shower particles can distinguish between events produced by the evaporation of mini black holes from normal ones with the same energy due, for instance, to a primary-proton - nucleon collision. The analysis is performed via a large scale simulation of the resulting cascades in the two cases over the entire range (1015−1019 eV) of ultra high energies, for several values of the number of large extra dimensions, for a variety of altitudes of the initial interaction and with the energy losses in the bulk taken into account. The electromagnetic/hadronic ratio is strongly fluctuating and, thus, not conclusive for the altitudes considered. The relevance of these results to a possible black hole interpretation of the Centauro events is commented upon.

NNLO logarithmic expansions and precise determinations of the neutral currents near the Z resonance at the LHC: the Drell-Yan case

We present a comparative study of the invariant mass and rapidity distributions in Drell-Yan lepton pair production, with particular emphasis on the role played by the QCD evolution. We focus our study around the Z resonance (50 < Q < 200 GeV) and perform a general analysis of the factorization/renormalization scale dependence of the cross sections, with the two scales included both in the evolution and in the hard scatterings. We also present the variations of the cross sections due to the errors on the parton distributions (pdfs) and an analysis of the corresponding K-factors. Predictions from several sets of pdfs, evolved by MRST and Alekhin are compared with those generated using CANDIA, a NNLO evolution program that implements the theory of the logarithmic expansions, developed in a previous work. These expansions allow to select truncated solutions of varying accuracy using the method of the x-space iterates. The evolved parton distributions are in good agreement with other approaches. The study can be generalized for high precision searches of extra neutral gauge interactions at the LHC.

Double transverse-spin asymmetries in Drell-Yan processes with antiprotons

Abstract We present next-to-leading order predictions for double transverse-spin asymmetries in Drell–Yan dilepton production initiated by proton–antiproton scattering. The kinematic region of the proposed PAX experiment at GSI: 30 ≲ s ≲ 200 GeV 2 and 2 ≲ M ≲ 7 GeV is examined. The Drell–Yan asymmetries turn out to be large, in the range 20–40%. Measuring these asymmetries would provide the cleanest determination of the quark transversity distributions.

On the scale variation of the total cross section for Higgs production at the LHC and at the Tevatron

We present a detailed study of the total pp cross section for scalar Higgs production to next-to-next-to-leading order in αs at LHC energies, and of the pp̄ cross section at the Tevatron, combining an implementation of the solutions of the parton evolution equations at the three-loop order with the corresponding hard scatterings, evaluated at the same perturbative order. Our solutions of the DGLAP equations are implemented directly in x-space and allow for the study of the dependence of the results on the factorization (μF) and renormalization scales (μR) typical of a given process, together with the stability of the perturbative expansion. The input sets for the parton evolutions are those given by Martin, Roberts, Stirling and Thorne and by Alekhin. Results for K-factors are also presented. The NNLO corrections can be quite sizeable at typical collider energies. The stability region of the perturbative expansion is found when μR>mH∼μF.

LARGE SCALE AIR SHOWER SIMULATIONS AND THE SEARCH FOR NEW PHYSICS AT AUGER

Large scale airshower simulations around the GZK cutoff are performed. An extensive analysis of the behaviour of the various subcomponents of the cascade is presented. We focus our investigation both on the study of total and partial multiplicities along the entire atmosphere and on the geometrical structure of the various cascades, in particular on the lateral distributions. The possibility of detecting new physics in Ultra High Energy Cosmic Rays (UHECR) at AUGER is also investigated. We try to disentangle effects due to standard statistical fluctuations in the first proton impact in the shower formation from the underlying interaction and comment on these points. We argue that theoretical models predicting large missing energy may have a chance to be identified, once the calibration errors in the energy measurements are resolved by the experimental collaborations, in measurements of inclusive multiplicities.

Searching for extra dimensions in high energy cosmic rays

We present a study of the decay of an intermediate mini black hole at the first impact of a cosmic ray particle with the atmosphere, in the context of D-brane world scenarios with TeV scale gravity and large extra dimensions. We model the decay of the black hole using the semi-classical approximation and include the corrections coming from energy loss into the bulk. Extensive simulations show that mini black hole events are characterized by essentially different multiplicities and wider lateral distributions of the air showers as a function of the energy of the incoming primary, as compared to standard events. Implications for their detection and some open issues on their possible discovery are also briefly addressed.

NNLO Logarithmic Expansions and High Precision Determinations of the QCD background at the LHC: The case of the Z resonance

New methods of solutions of the DGLAP equation and their implementation through NNLO in QCD are briefly reviewed. We organize the perturbative expansion that describes in x‐space the evolved parton distributions in terms of scale invariant functions, which are determined recursively, and logarithms of the ratio of the running couplings at the initial and final evolution scales. Resummed solutions are constructed within the same approach and involve logarithms of more complex functions, which are given in the non‐singlet case. Differences in the evolution schemes are shown to be numerically sizeable and intrinsic to perturbation theory. We illustrate these points in the case of Drell‐Yan lepton pair production near the Z resonance, analysis that can be extended to searches of extra Z′. We show that the reduction of the NNLO cross section compared to the NLO prediction may be attributed to the NNLO evolution.