DEMPgen: Physics event generator for Deep Exclusive Meson Production at Jefferson Lab and the EIC

There is increasing interest in deep exclusive meson production (DEMP) reactions, as they provide access to Generalized Parton Distributions over a broad kinematic range, and are the only means of measuring pion and kaon charged electric form factors at high Q 2 . Such investigations are a particularly useful tool in the study of hadronic structure in QCD's transition regime from long-distance interactions described in terms of meson-nucleon degrees of freedom, to short-distance interactions governed by hard quark-gluon degrees of freedom. To assist the planning of future experimental investigations of DEMP reactions in this transition regime, such as at Jefferson Lab and the Electron-Ion Collider (EIC), we have written a special purpose event generator, DEMPgen. Currently, DEMPgen can generate the following reactions: t -channel p ( e , e ′ π + ) n , p ( e , e ′ K + ) Λ [ Σ 0 ] , and n → ( e , e ′ π − ) p from a polarized 3 He target. DEMPgen is modular in form, so that additional reactions can be added over time. The generator produces kinematically-complete reaction events which are absolutely-normalized, so that projected event rates can be predicted, and detector resolution requirements studied. The event normalization is based on parameterizations of theoretical models, appropriate to the kinematic regime under study. Both fixed target modes and collider beam modes are supported. This paper presents the structure of the generator, the model parameterizations used for absolute event weighting, the kinematic distributions of the generated particles, some initial results using the generator, and instructions for its use. PROGRAM SUMMARY Program title: DEMPgen CPC Library link to program files: https://doi.org/10.17632/3nxcm5fsdz.1 Developer's repository link: https://github.com/JeffersonLab/DEMPgen/releases Licensing provisions: GNU General Public License 3 Programming language: C++ External package: ROOT Nature of problem: Despite the increasing interest in deep exclusive meson production (DEMP) reactions, no physics event generator currently exists for these types of reactions. Such a generator is needed for the planning of future experimental investigations at Jefferson Lab and the Electron-Ion Collider (EIC), so we have undertaken to write a special purpose event generator, DEMPgen. Solution method: DEMPgen produces kinematically-complete reaction events which are absolutely-normalized, so that projected event rates can be predicted, and detector resolution requirements studied. The event normalization is based on parameterizations of theoretical models, appropriate to the kinematic regime under study. The following reactions can be generated: t -channel p ( e , e ′ π + ) n , p ( e , e ′ K + ) Λ [ Σ 0 ] , and n → ( e , e ′ π − ) p from a polarized 3 He target. These reactions have been parameterized for kinematic regimes relevant to experiments at SoLID and the EIC (as described in Secs. 3.1 , 3.2 ). DEMPgen is modular in form, so that additional reactions can be added over time. Both fixed target modes and collider beam modes are supported. Additional comments including restrictions and unusual features: DEMPgen events have variable weights, which must be included to yield realistic distributions and rate projections, see Secs. 3.1.2 , 3.3.6 for more details. DEMPgen does not yet include the radiative tail contributions for exclusive lepton-hadron reactions. This is a planned future upgrade, along with other upgrades described in Sec. 5 . Developer's repository maintained by Department of Physics, University of Regina.

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