The AcerMC Monte Carlo Event Generator is dedicated for the generation of Standard Model background processes at LHC collisions. The program itself provides a library of the massive matrix elements and phase space modules for generation of a set of selected processes: , , , , and complete electroweak process. The hard process event, generated with one of these modules, can be completed by the initial and final state radiation, hadronization and decays, simulated with either PYTHIA or HERWIG Monte Carlo event generator. Interfaces to both of these generators are provided in the distribution version. The matrix element codes have been derived with the help of the MADGRAPH package. The phase-space generation is based on the multi-channel self-optimizing approach as proposed in NEXTCALIBUR event generator. Eventually, additional smoothing of the phase space was obtained by using a modified ac-VEGAS routine in order to improve the generation efficiency.
References (49)
G. Marchesini
Comput. Phys. Comm.
(1992)
G. Corcella
JHEP
(2001)
B. Kersevan, E. Richter-Was, ATLAS Internal Note, ATL-PHYS-2001-020,...
A measurement of the mass difference between top and anti-top quarks is presented. In a data sample of proton–proton collisions at recorded with the ATLAS detector at the LHC, events consistent with production and decay into a single charged lepton final state are reconstructed. For each event, the mass difference between the top and anti-top quark candidate is calculated. A two b-tag requirement is used in order to reduce the background contribution. A maximum likelihood fit to these per-event mass differences yields , consistent with CPT invariance.
Nevertheless, in complex automated setups it is sometimes hard to achieve optimal phase space sampling and the user interfaces are necessarily generic, thus for now dedicated matrix-element-based generators like AcerMC, with standardised interfaces (defined e.g. in [11]), still can play a visible role. This paper supersedes the first version of the manual, published in [18]. The outline of this paper is as follows.
The AcerMC Monte Carlo generator is dedicated to the generation of Standard Model background processes which were recognised as critical for the searches at LHC, and generation of which was either unavailable or not straightforward so far. The program itself provides a library of the massive matrix elements (coded by MADGRAPH) and native phase space modules for generation of a set of selected processes. The hard process event can be completed by the initial and the final state radiation, hadronisation and decays through the existing interface with either PYTHIA, HERWIG or ARIADNE event generators and (optionally) TAUOLA and PHOTOS. Interfaces to all these packages are provided in the distribution version. The phase-space generation is based on the multi-channel self-optimising approach using the modified Kajantie–Byckling formalism for phase space construction and further smoothing of the phase space was obtained by using a modified ac-VEGAS algorithm. An additional improvement in the recent versions is the inclusion of the consistent prescription for matching the matrix element calculations with parton showering for a select list of processes.
Catalogue identifier of previous version: ADQQ_v1_0
Journal reference of previous version: Comput. Phys. Comm. 149(2003)142
Does the new version supersede the previous version?: Yes
Nature of problem: Despite a large repertoire of processes implemented for generation in event generators like PYTHIA [1] or HERWIG [2] a number of background processes, crucial for studying the expected physics of the LHC experiments, is missing. For some of these processes the matrix element expressions are rather lengthy and/or to achieve a reasonable generation efficiency it is necessary to tailor the phase space selection procedure to the dynamics of the process. That is why it is not practical to imagine that any of the above general purpose generators will contain every, or even only observable, processes which will occur at LHC collisions. A more practical solution can be found in a library of dedicated matrix-element-based generators, with the standardised interfaces like that proposed in [3], to the more universal one which is used to complete the event generation.
Solution method: The AcerMC EventGenerator provides a library of the matrix-element-based generators for several processes. The initial- and final-state showers, beam remnants and underlying events, fragmentation and remaining decays are supposed to be performed by the other universal generator to which this one is interfaced. We will call it a supervising generator. The interfaces to PYTHIA 6.4, ARIADNE 4.1 and HERWIG 6.5, as such generators, are provided. Provided is also an interface to TAUOLA [4] and PHOTOS [5] packages for -lepton decays (including spin correlations treatment) and QED radiations in decays of particles. At present, the following matrix-element-based processes have been implemented: , ; ; ; ; complete EW ; ; ; . Both interfaces allow the use of the LHAPDF/LHAGLUE library of parton density functions. Provided is also a set of control processes: ; ; and ;
Reasons for new version: Implementation of several new processes and methods.
Summary of revisions: Each version added new processes or functionalities, a detailed list is given in the section “Changes since AcerMC 1.0”.
Restrictions: The package is optimised for the 14 TeV collision simulated in the LHC environment and also works at the achieved LHC energies of 7 TeV and 8 TeV. The consistency between results of the complete generation using PYTHIA 6.4 or HERWIG 6.5 interfaces is technically limited by the different approaches taken in both these generators for evaluating and couplings and by the different models for fragmentation/hadronisation. For the consistency check, in the AcerMC library contains native coded definitions of the QCD and . Using these native definitions leads to the same total cross-sections both with PYTHIA 6.4 or HERWIG 6.5 interfaces.
Additional comments: !!!!! The distribution file for this program is over 67 Mbytes and therefore is not delivered directly when download or Email is requested. Instead an html file giving details of how the program can be obtained is sent. !!!!!
Running time: On an PIII 800 MHz PC it amounts to events/sec, depending on the choice of process.
References:
[1]
T. Sjostrand et al., High energy physics generation with PYTHIA 6.2, eprint hep-ph/0108264, LU-TP 01-21, August 2001.
[2]
G. Julyesini et al., Comp. Phys. Commun. 67 (1992) 465, G. Corcella et al., JHEP 0101 (2001) 010.
[3]
E. Boos at al., Generic user process interface for event generators, hepph /0109068.
[4]
S. Jadach, J. H. Kuhn, Z. Was, Comput. Phys. Commun. 64 (1990) 275; M. Jezabek, Z. Was, S. Jadach, J. H. Kuhn, Comput. Phys. Commun. 70 (1992) 69; R. Decker, S. Jadach, J. H. Kuhn, Z. Was, Comput. Phys. Commun. 76 (1993) 361.
[5]
E. Barberio and Z. Was, Comp. Phys. Commun. 79 (1994) 291.
We summarise predictions for production at the LHC in next-to-leading order QCD. The precise description of this background process is a prerequisite to observe associated production in the decay channel. The one-loop amplitudes are computed using Feynman diagrams and numerical tensor reduction. This approach provides very high numerical stability and CPU efficiency. We find that the scale choice adopted in ATLAS simulations underestimates the background by a factor two and introduce a new dynamical scale that stabilises the perturbative predictions. In the regime of highly boosted Higgs bosons, which offers better perspectives to observe the signal, the corrections induce significant distortions in the kinematic distributions.
Supported in part by Polish Government grant KBN 2P03B11819, by the European Commission 5th framework contract HPRN-CT-2000-00149 and by Polish–French Collaboration with IN2P3.