  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-dist ance 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. Several types of DEMP reactions can be
generated: $t$-channel $p(e,e^{\prime}\pi^+)n$,
$p(e,e^{\prime}K^+)\Lambda[\Sigma^0]$, and $\vec{n}(e,e^{\prime}\pi^-)p$ 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.
