The Monte Carlo Event Generator AcerMC 2.0 with Interfaces to PYTHIA 6.2 and HERWIG 6.5

TL;DR

AcerMC 3.8 presents a library of massive matrix-element generators for key Standard Model backgrounds at the LHC, interfaced to general-purpose shower/hadronisation programs (PYTHIA/HERWIG/ARIADNE) and to TAUOLA/PHOTOS. The core contribution is a multi-channel phase-space framework based on the Kajantie–Byckling formalism, enhanced by ac-VEGAS to efficiently sample high-dimensional topologies (including 2→4 and 2→6 processes) and to produce unweighted events with consistent running of $\alpha_s(Q^2)$ and $\alpha_{QED}(Q^2)$. AcerMC provides extensive process coverage (ttbb, Z/gamma* + bb, multi-top, single top, Z′ to tt, and related control channels), with detailed cross-section studies under multiple scale and coupling definitions and a robust Les Houches interface for integration with external generators. The package delivers practical tools for LHC background studies, enabling precise ME+PS matching and facilitating systematic comparisons across different shower models, while offering configurable parameters for phase-space training, color-flow handling, and tau/photon radiation. Together, these features advance the reliability and efficiency of background modeling in high-energy collider analyses and lay groundwork for further extensions toward broader process coverage and higher-order corrections.

Abstract

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 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 matrix element codes have been derived with the help of the MADGRAPH package. 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.

Figures (37)

• Figure 1: The calling sequence of the main event generation routine acevent_xx. The routine is called either through demo_py $\to$ acermc_py sequence when interfacing the PYTHIA 6.4 generator or demo_hw $\to$ acermc_hw sequence when the HERWIG 6.5 is linked. When the ARIADNE 4.1 setup is called via acermc_ar calls it still relies on PYTHIA 6.4 for hadronisation and particle(resonance) decays. The structure of the interface subroutines and relations with the corresponding ones from supervising generators and/or external libraries is also evident.
• Figure 2: The event generation sequence controlled by acevent_gen subroutine. Phase space generation is sequenced by calling the ackinch_gen2 routine to obtain the incoming gluon and the outgoing four-momenta of the participating particles. The latter routine handles the possible momenta permutations and calls the explicit four-momenta generation (and PS weight calculation) in the sequence prescribed by the event topology. These (channel-specific) routines are constructed from common building blocks listed in the next two columns. The acevent_gen routine also initialises MADGRAPH/HELAS package and retrieves the matrix element values. All the generated four-momenta, as well as the event weight are finally passed back to the supervising generator via the acevent_dump call.
• Figure 3: The structure of the AcerMC directories.
• Figure 4: The reading sequence of the input files and the performing subroutines in the AcerMC code.
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