Heavy Neutral Lepton simulation

Developer team

Komninos-John Plows

Target GENIE product releases

v3.4.0?

Scope of project

Develop a complete simulation of long-lived Heavy Neutral Leptons (HNL) in GENIE.

Deliverables

1. Codes for calculating HNL fluxes, based on a standardised output for neutrino beamline simulations (dk2nu)
2. Codes for simulating the propagation and decay of HNL to configurable final states
3. A specialised HNL event generation app with hooks for detector geometry and flux descriptions
   • User-input flux description
     • Flux in histograms
     • Flux in tuples (dk2nu-like)
     • Other flux formats?
   • User-input ROOT geometry (other formats? gdml, for instance?)
4. A new section for the Non-Neutrino Event Generation Modes of the GENIE Physics and User Manual

Milestones

1. Implementation and validation of HNL flux generation routines
2. Validation of ROOT geometry routines with custom transformation between “beam” and “detector” frames
3. Implementation of polarisation

Requirements

HNL generation package is standalone - just enable it in configure and go.

Schematic outline of program of work

1. Framework consolidation - pass as many physics variables (such as HNL parameter space & Dirac vs Majorana nature) to user, allow user-input detector geometry, user input fluxes
2. POT counting: maps N(HNL events in channel) |--> N'(exp. POT delivered) writes weight = POT expected for event
3. Flux generation - implementation of dk2nu ==> flux
   • Currently flux prediction requires dk2nu ==> flat ROOT trees ==> flux
4. HNL polarisation effects with hook for switching on or off (user may have unpolarised HNL beam)
5. Majorana HNL implementation
6. Polarisation effect inclusion for two- and three- body decays of HNL.
7. Heavy HNL - this is going to need a custom flux calculation for heavy particles such as Ds that can decay into HNL with M > m_K+. These are important for probing channels that contain tau in final state
8. Multiple Lagrangians and their associated decay widths. TBD.
9. Multi-HNL hypothesis - see the relevant paper

Details on validation plan

1. Test POT map with wide selection of couplings / ROOT geometry files / volumes
   • Ensure output matches analytical calculation for “particle-gun”-like HNL (user-input energy + momentum direction)
   • Ensure match for specific flux inputs (i.e. sampled energy + momentum)
2. Flux prediction should be understood / (match literature if using same dk2nu flux files?) - do we want to ask theorists for input?
   • Energy spectrum weighted for HNL collimation efffect <==> acceptance increase
   • Production point
   • User <–> beam transformation
   • HNL production rates per parent
3. Validate HNL decay rates match literature, (& that polarisation effects are taken into account correctly depending on user hook)
4. Make sure HNL final state kinematics are sensible for different channels
5. Validate geometry integration for decay-in-flight weight calculation

Documentation and publications

• Contribution to the GENIE User and Physics manual

-

Anticipated review points

Dependencies with other projects

N/A

References

Code repository:

HNL fork of Generator

Documents from first review:

https://genie-docdb.pp.rl.ac.uk/cgi-bin/private/ShowDocument?docid=295

Description of HNL decay channels & rates:

P. Coloma et al, EPJ C 81 (2021) 78

Description of HNL fluxes & production branching ratios:

P. Ballett et al, JHEP 2020 (2020) 111

Polarisation of HNL:

J.-M. Levy, 1805.06419 A. de Gouvea et al, Phys. Rev. D (2022) 105

Treatment of multi-HNL physics case:

J.-L. Tastet et al, JHEP 2021 (2021) 182

Other relevant documents:

• HNL search at T2K, K. Abe et al, Phys. Rev. D 100 (2019) 052006