Physics description

Danton implements an original backward Monte Carlo technique (see [NBCL18], [NM18] and [Nie23]), which requires special (backward-compliant) Monte Carlo engines, as described below.

Validation

The Danton physics implementation has been validated, e.g. by comparisons with NuTauSim (see [NM18]) as well as with NuPropEarth (see [AAA+22], sec. 6.6, fig. 63).

Neutrino interactions

The simulation of high-energy neutrino interactions is carried out with Ent. The dominant process is Deep Inelastic Scattering (DIS) on nuclei partons, through the exchange of a virtual W or Z boson. Additionally, collisions with atomic electrons are also considered, including the Glashow Resonant (GR) process.

DIS collisions

DIS collisions are randomised from the Doubly Differential Cross-Section (DDCS) in Bjorken-\(x\) and \(Q^2\), using Leading Order (LO) expressions with configurable Parton Distribution Functions (PDFs). However, the total DIS cross-sections are rescaled to more detailed computations (e.g. the [CMS11] or [BGR18] cross-sections).

Tau interactions

Tau interactions are simulated with Pumas, a dedicated heavy leptons transport engine, initially developed for muography applications. Pumas has been extensively validated, by comparisons to other Monte Carlo transport engines, including Geant4 and PROPOSAL (see [Nie22]).

High-energy tau interactions are typically divided into two categories: ionisation (resulting from inelastic collisions with atomic electrons) and radiative processes (dominant above ~10 TeV). Radiative processes encompasses bremsstrahlung, \(e^+e^-\) pair-production and photonuclear interactions (i.e. DIS through the exchange of a virtual photon). Additionally, Pumas can also account for the multiple scattering off nuclei through Coulomb collisions.

Cross-sections

Pumas implements a variety of cross-section models for radiative processes (see e.g. the Physics class). By default, the PROPOSAL cross-sections are utilised for the bremsstrahlung and pair-production processes (see e.g. [SSR19]). Ionisation and multiple scattering processes are modelled following [Sal13], which is comparable to the Penelope implementation. For further information on Pumas, refer to [Nie22].

Tau decays

Tau decays are simulated with Alouette, which is a thin wrapper around TAUOLA (C++ release), allowing for backward decays. This is essentially achieved by a relativistic change of frame (see [Nie23]), starting from TAUOLA-generated Centre-of-Mass (CM) decays.

Polarisation

The angular distribution of the decays products (in the CM) is dependent on the tau polarisation at decay. Danton’s assumption is that tau leptons are 100% anti-polarised at production, and that this polarisation is retained until the decay.