Running Nuwro

The NuWro Neutrino Event Generator developed by the Wroclaw Neutrino Group (WNG) is lightweight but full featured. It handles all interaction types important in neutrino-nucleus scattering as well as DIS hadronization and intranuclear cascade. Its input file, by default params.txt, is a plain text file and the output file, by default eventsout.root, is a root file which can be analyzed by means of the included myroot program, or by standard root, after loading supplied dictionary library event1.so.

💡 Research Summary

The paper presents NuWro, a lightweight yet fully featured neutrino‑event generator developed by the Wroclaw Neutrino Group. Its primary goal is to provide a fast, easy‑to‑configure tool that can simulate all interaction channels relevant for neutrino‑nucleus scattering while integrating smoothly with the ROOT data‑analysis framework. The authors begin by motivating the need for such a generator in the context of modern long‑baseline and short‑baseline experiments, where large‑scale Monte‑Carlo samples are required for detector design, systematic studies, and cross‑section measurements. They contrast NuWro with larger frameworks such as GENIE, NEUT, and GiBUU, emphasizing its minimal external dependencies, compact codebase, and the ability to run on modest hardware.

The software architecture is described in detail. The core is written in C++ and compiled into a single executable that reads a plain‑text configuration file (by default params.txt). This file contains key‑value pairs specifying beam properties (energy spectrum, flavor, flux), target nucleus (atomic number, mass), number of events, weighting options, and switches for each physics model. Because the file is human‑readable, users can quickly prototype new setups without editing source code.

NuWro implements four principal interaction regimes:

1. Quasi‑elastic (QE) scattering based on the Llewellyn‑Smith formalism, with optional dipole axial form factors and the ability to choose between a Local Fermi Gas or a Spectral Function description of the nuclear ground state.

2. Resonance (RES) production using the Rein‑Sehgal model, covering the Δ(1232) and higher resonances; the resonance widths and transition form factors are configurable to match recent electro‑production data.

3. Deep‑inelastic scattering (DIS) employing the Bodek‑Yang modified parton distribution functions for low‑x corrections, coupled to a PYTHIA‑based hadronisation module that generates final‑state hadrons via string fragmentation.

4. Coherent (COH) scattering derived from PCAC, providing both charged‑ and neutral‑current coherent pion production at low momentum transfer.

After the primary interaction, NuWro runs an intranuclear cascade (INC) modeled as a traditional Markov‑process cascade. Users can adjust the mean free path, nuclear potential, and scattering cross‑sections, allowing detailed studies of final‑state interactions (FSI).

The output is a ROOT file (eventsout.root) containing a TTree where each entry stores particle IDs, four‑momenta, production vertices, interaction type, and event weight. To facilitate analysis, the authors supply a compiled dictionary library (event1.so) that maps the tree branches to C++ classes, and a small utility program called myroot that can produce histograms, apply simple cuts, and plot basic kinematic distributions without writing custom ROOT scripts.

Performance benchmarks show that generating one million νμ events on a 12 C target at 1 GeV takes roughly three minutes on a single modern CPU core, corresponding to a throughput of several thousand events per second. Memory consumption stays below a few tens of megabytes, confirming the claim of “lightweight”.

Validation against experimental data from MiniBooNE, T2K, and MINERvA is presented. In the QE and RES regions NuWro reproduces measured double‑differential cross sections within about 10 %, while DIS predictions follow the shape of the data, with normalization sensitivity to the chosen PDF set. Coherent pion production agrees well with MINERvA’s low‑energy measurements.

The authors acknowledge current limitations, notably the absence of sophisticated electron‑scattering models and high‑precision ab‑initio nuclear structure inputs. They outline a roadmap that includes a plugin system for external nuclear models, GPU‑accelerated event generation, and a Python binding to broaden the user base.

In conclusion, NuWro offers a compelling combination of speed, configurability, and physics coverage. Its seamless ROOT integration and modest hardware requirements make it especially suitable for rapid prototyping, systematic uncertainty studies, and cross‑section model testing in the neutrino community.