CepGen - A generic central exclusive processes event generator for hadron-hadron collisions

We present an event generator for the simulation of central exclusive processes in hadron-hadron reactions. Among others, it implements the two-photon production of lepton pairs previously introduced in LPAIR. As a proof of principle, we show that the two approaches are numerically consistent. The k(T)-factorized description of this process is also handled, along with the two-photon production of a quark, or a W-+/- gauge boson pair. This toolbox may be used as a common framework for the definition of many other processes following this approach. Additionally, photoproduction and other photon induced processes are also considered, or being implemented. Program summary Program title: CepGen CPC Library link to program files: https://doi.org/10.17632/24jg665g65.1 Developer's repository link: https://github.com/cepgen/cepgen Licensing provisions: GNU General Public License 3 Programming language: C++/Python External routines/libraries: GSL [1] for MC integration and histogramming, optional wrappers for LHAPDF [2] for the partonic proton structure functions evaluation, or ROOT [3], Delphes [4] for the output treatment. Nature of problem: The simulation of central exclusive, and in particular two-photon induced processes is becoming increasingly topical given its potential source of contamination for electroweak studies and resonance searches at LHC and future colliders. However, most of simulation tools available are only accounting for the production of photons collinear to the incoming proton beams. Legacy codes such as LPAIR, have however shown their effectiveness in predicting such processes at LHC energies. Unfortunately, they are barely maintained nor maintainable with modern computing infrastructures. Solution method: CepGen provides a modern implementation of legacy photon-induced matrix elements (gamma gamma -> l(+)l(-), and W+ W-, with more to be added), including standard e(+) e(-), or ppbeams (both elastic and dissociative final beam states for the latter). For the modern implementation of LPAIR, it inherits from the former fine treatment of the low-|t| region accounting for a large fraction of the cross section. It also introduces a general wrapping framework to define new photon-induced and diffractive processes, either in C++ or in Fortran. This wrapper provides the k(T) factorization procedure for 2 -> 4 process computation, and a highly flexible 2 -> N process placeholder. A user-defined taming of the matrix element is also included to study the effect of kinematic variables-dependent survival factors observed experimentally. Additional comments including restrictions and unusual features: Depending on the complexity of the central process, memory and CPU time. Currently event generation runs only in single-threaded mode, development ongoing to support multi-threading. (C) 2021 The Author. Published by Elsevier B.V.