Merge New Decfiles for Run 3 Inclusive T-track HNL Search
This merge request aims to add new decfiles allowing the simulation of the complete Heavy Neutral Lepton (HNL) phenomenology when produced by B mesons. They model decays of the form: B_{u,d,s,c} \rightarrow \mu^+ \{X\} (N \rightarrow \mu^\mp X^\pm \{Neutrino\}). All of these decays are products of the same Feynman diagram, below:
Central simulation is essential for the Bq_muXN_muX analysis, discussed, e.g., here. Three HLT2 lines have been implemented as of 2025 to trigger on HNL decays of this form.
The merge request adds 17 new decfiles as follows:
| Eventtype | Mother B meson | HNL mass (GeV) | HNL lifetime (ns) |
|---|---|---|---|
| 11973005 | B^0 | 1.6 | 3 |
| 11973006 | B^0 | 2.0 | 3 |
| 11973007 | B^0 | 2.5 | 3 |
| 11973008 | B^0 | 3.0 | 3 |
| 11973009 | B^0 | 3.5 | 3 |
| 12973005 | B^+ | 1.6 | 3 |
| 12973006 | B^+ | 2.0 | 3 |
| 12973007 | B^+ | 2.5 | 3 |
| 12973008 | B^+ | 3.0 | 3 |
| 12973009 | B^+ | 3.5 | 3 |
| 13973005 | B_s^0 | 1.6 | 3 |
| 13973006 | B_s^0 | 2.0 | 3 |
| 13973007 | B_s^0 | 2.5 | 3 |
| 13973008 | B_s^0 | 3.0 | 3 |
| 13973009 | B_s^0 | 3.5 | 3 |
| 14713000 | B_c^+ | 3.0 | 3 |
| 14713001 | B_c^+ | 3.5 | 3 |
Each decfile describes a complex cocktail, as follows:
- The production of the HNL is simulated as phase space only (typical for these studies), and with the branching ratios following the below plot based off data from Bondarenko et al. 2018 (the standard reference) HNL_Production.pdf
- The decay is again simulated as phase space only, with 25 % branching ratios to each of: N\rightarrow \mu \pi, N \rightarrow \mu \pi X, N \rightarrow \mu \mu \nu and N \rightarrow \mu e \nu. Within N \rightarrow \mu \pi X, the branching ratios follow the below plot, again based off Bondarenko et al.. In addition, 2/3 of HNL decays are forced to have two opposite sign muons, 1/3 to have same sign muons. This is to allow ~2/3 of the data to be used for both the Dirac (lepton-number conserving) and Majorana (lepton-number violating) HNL interpretations. HNL_Decay.pdf
HNls are modelled by changing the properties, in particular, the mass and lifetime of the H_10 particle.
To increase the statistics generated, several non-standard generator-level cuts are applied. This is necessary as the standard daughters_in_LHCb cuts do not function for highly displaced particles. The cuts are as follows:
| Quantity | Cut |
|---|---|
| HNL daughter linearly extrapolated position at SciFi (y) | -2500 < SFY < 2500 |
| HNL daughter linearly extrapolated position at SciFi (x) | -3500 < SFX < 3500 |
| HNL daughter P | 2/3 of HLT2 requirement |
| HNL daughter P_T | 2/3 of HLT2 requirement |
| HNL P_T | 2/3 of HLT2 requirement |
| HNL endvertex position | END_VZ < 8000 |
A discussion for the rationale of these cuts and confirmation that signal is not lost is given here.
The correct application of these cuts has been confirmed at generator level. E.g. see the following:
The warnings due to the decfile parsing are expected and understood.
There are two types of warning:
- The reconstructed decay descriptor does not match the stated descriptor. This is because the parser only reconstructs the most representative decay, not the inclusive decay as is stated in the top-level decay descriptor.
- The eventtype flags are wrong. This is associated with the above issue, and the eventtype numbers are understood to be correct.
CPUTime tests were conducted on the local EPFL cluster. The total time to generate and simulate the interaction of 25 events using a single CPU core (1 thread) was recorded. The log states: "This machine has a speed about 4.17 times the speed of a 2.8 GHz Xeon". The exact breakdown is given below. All events had a CPUTime between 2 and 3 minutes, thus a CPUTime estimate of 3 minutes is given
| Eventtype | Mother B meson | HNL mass (GeV) | Test runtime (s) |
|---|---|---|---|
| 11973005 | B^0 | 1.6 | 3846 |
| 11973006 | B^0 | 2.0 | 4060 |
| 11973007 | B^0 | 2.5 | 3674 |
| 11973008 | B^0 | 3.0 | 3640 |
| 11973009 | B^0 | 3.5 | 3542 |
| 12973005 | B^+ | 1.6 | 3776 |
| 12973006 | B^+ | 2.0 | 3725 |
| 12973007 | B^+ | 2.5 | 3740 |
| 12973008 | B^+ | 3.0 | 3769 |
| 12973009 | B^+ | 3.5 | 3396 |
| 13973005 | B_s^0 | 1.6 | 4241 |
| 13973006 | B_s^0 | 2.0 | 3922 |
| 13973007 | B_s^0 | 2.5 | 3985 |
| 13973008 | B_s^0 | 3.0 | 3884 |
| 13973009 | B_s^0 | 3.5 | 3848 |
| 14713000 | B_c^+ | 3.0 | 3220 |
| 14713001 | B_c^+ | 3.5 | 3337 |