MARLEY v2¶
I am pleased to announce version 2.0.0 of MARLEY, the first official update to the code since November 2023. This major release implements an improved model of the charged-current cross section that is described in arXiv:2604.26801 [hep-ph]. The previous MARLEY output format has been entirely replaced by the HepMC3 event record. The representation of neutrino interactions within HepMC3 data structures follows the recommendations of version 1.0.0 of the NuHepMC standard.
Physics updates¶
The headline change in v2.0.0 is an overhaul of MARLEY’s neutrino cross-section calculation that removes the rough allowed approximation (AA) treatment used in the v1 release series. In MARLEY v2, neutrino-induced transitions to high-lying nuclear excitation energies are modeled using a Hartree-Fock Continuum Random Phase Approximation (HF-CRPA) approach, including forbidden nuclear matrix elements. The data-driven strategy from v1 is retained for transitions to discrete nuclear levels, but approximate corrections for momentum-transfer dependence are introduced to avoid taking the limit from the AA that was used previously. The updated hybrid model of the charged-current interaction is discussed at length in arXiv:2604.26801 [hep-ph].
Hauser-Feshbach statistical de-excitation calculations in MARLEY v2 still use the 46-parameter nuclear optical potential developed for the 2003 Koning-Delaroche global fit. However, the default parameter values have now been updated to match the “federal” KDUQ re-evaluation published in 2023. All parameters used in the nuclear optical model are now configurable via the JSON job configuration file used to run MARLEY.
Output events now contain the full nuclear de-excitation history rather than merely vectors of the initial- and final-state particles. Each binary decay vertex in the de-excitation cascade is explicitly recorded. Metadata fields in the event record are used to store properties of the intermediate nuclear states (excitation energy, spin, parity) as well as the total and partial decay widths associated with each sampled transition.
Rather than treating nuclear de-excitations as instantaneous (as was done in v1), MARLEY v2 adds tracking of particle emission times to the simulation of continuum decays and (where half-life data are available) γ-ray transitions between discrete levels. This functionality had to be introduced externally to support an exciting recent analysis by DEAP-3600 that reported first evidence for charged-current absorption of solar neutrinos on an argon target: arXiv:2605.12769 [hep-ex]. It is now available “out of the box” for all users.
New code infrastructure for event reweighting has been added that enables evaluation of systematic uncertainties without re-running the simulation. A full uncertainty budget for MARLEY predictions has not yet been developed, but weight calculators for two classes of uncertainties (related to discrete transition matrix elements and the KDUQ nuclear optical potential) are available for the first time in version 2.0.0.
Technical updates¶
The MARLEY-specific output format used in v1 has been deprecated in favor of HepMC3, a highly-flexible event record definition that is widely used for collider physics simulations. Recent community discussions about neutrino event generator data formats led to creation of NuHepMC, a set of conventions for describing simulated neutrino reactions within the HepMC3 framework. The updated MARLEY event format follows version 1.0.0 of the NuHepMC standard. This is a breaking change from v1 that will typically require updates to later stages of experimental simulation chains and downstream analysis code. The interpreting the output webpage provides details about the new format and MARLEY’s built-in capabilities for conversions to other representations of the simulation results.
The marley executable now provides a unified command-line interface that
accepts multiple commands: generate, decay, convert, print,
reweight, summarize, xsec, help, and version. These
commands replace the previous system of several different executables and
helper scripts. For backwards compatibility, marley CONFIG_FILE still works
as shorthand for marley generate CONFIG_FILE. A new command reference webpage provides documentation for all of the new
commands.
MARLEY can now be built using CMake in addition to
plain GNU Make. For convenience, the top-level Makefile will delegate to
CMake automatically if it is available and will fall back to a standalone GNU
Make recipe otherwise; users who prefer to invoke CMake directly are free to
follow the usual CMake build procedure instead. Note that a C++17-compliant
compiler (GCC ≥ 9.1.0 or Clang ≥ 9.0.0) is now required to build MARLEY.
The JSON-like job configuration file format has been extended to support an
#include:"file" directive, allowing MARLEY’s runtime configuration to be
split across multiple files. Error messages produced by the JSON parser now
report the file name and the line/column position at which a problem was
encountered, making configuration mistakes easier to diagnose and fix.
Outlook¶
Thanks to a 2025 Early Career award from the U.S. Department of Energy, MARLEY has recently returned to a state of active development. Further details about physics model improvements in MARLEY v2 can be found in my Neutrino 2026 poster presentation and this talk from the July 2026 APS DPF meeting.
For the v2.0.0 release, I thank all the co-authors of arXiv:2604.26801 [hep-ph] for their scientific collaboration. I am also grateful for help with the implementation of new MARLEY features from Pablo Barham Alzás and Luca Abu El-Haj (cross-section improvements, optical potential reweighting) as well as Everardo Granados Vázquez (γ-ray emission timing).
The MARLEY code remains enthusiastically open-source. Feedback and contributions from the community are both valued and welcomed. I look forward with excitement as you give MARLEY v2.0.0 a try!