MCAmbulance

MCAmbulance is a small python package to correct Monte-Carlo samples of $B \rightarrow D_0^\ast \ell \nu$ and $B \rightarrow D_1^\prime \ell \nu$ decays produced by the basf2 version of EvtGen for missing phase-space factors.

MCAmbulance relies on

• numpy
• scipy

The missing phase-space factors and the reweighting strategy are described in detail in the preprint arXiv:2602.18378. MCAmbulance uses masses and widths based on the default values in basf2, taken from evt.pdl.

The following decays are supported:

• $B \rightarrow D_0^\ast(\rightarrow D \pi) \ell \nu$, based on DECAY_BELLE2.DEC
• $B \rightarrow D_0^\ast(\rightarrow D \eta) \ell \nu$, based on 1196708001.dec and 1296708001.dec
• $B \rightarrow D_0^\ast(\rightarrow D \pi \pi) \ell \nu$, based on 1196700003.dec and 1296700003.dec (Note, this decay violates parity)
• $B \rightarrow D_0^\ast(\rightarrow D^\ast \pi \pi) \ell \nu$, based on 1196700004.dec and 1296700004.dec
• $B \rightarrow D_1^\prime(\rightarrow D^\ast \pi) \ell \nu$, based on DECAY_BELLE2.DEC
• $B \rightarrow D_1^\prime(\rightarrow D^\ast \eta) \ell \nu$, based on 1196708000.dec and 1296708000.dec
• $B \rightarrow D_1^\prime(\rightarrow D \pi \pi) \ell \nu$, based on 1196700001.dec and 1296700001.dec
• $B \rightarrow D_1^\prime(\rightarrow D^\ast \pi \pi) \ell \nu$, based on 1196700002.dec and 1296700002.dec

In addition, we provide ISGW2 implementations of

• $B \rightarrow D_0^\ast(\rightarrow D \pi) \ell \nu$
• $B \rightarrow D_1^\prime(\rightarrow D^\ast \pi) \ell \nu$

Note that decays simulated with the PHSP model are not affected, and so we do not provide code to reweight them.

Installation

1. Clone this repository into a local directory.

   git clone <repo-url>

2. Add the repository root (the directory that contains the mcambulance/ folder) to your PYTHONPATH:

   export PYTHONPATH="/path/to/MCAmbulance:${PYTHONPATH}"

Alternative (script/notebook only)

If you don't want to set environment variables, you can add the repo to sys.path at runtime:

import sys
sys.path.insert(0, "/path/to/MCAmbulance")  # repo root containing `mcambulance/`
import mcambulance

Usage

Correcting $B^\pm \rightarrow D_0^\ast(\rightarrow D \pi) \mu \nu_\mu$ samples is achieved through:

from mcambulance import MCAmbulance

d0_d_pi_fix = MCAmbulance('bp', 'd0_d_pi', 'mu')

weights = d0_d_pi_fix.CorrectionWeight(dpi_invM)

Here, dpi_invM is a numpy array containing the truth-level $D\pi$ invariant mass for each affected event. The numpy array weights now contains the correction weights for each event.

The constructor of MCAmbulance takes three arguments:

• bmeson: bp or b0 for charged or neutral $B$ mesons, respectively
• channel: d0_d_pi, d0_d_eta, d0_d_pipi, d0_dstar_pipi, d1p_dstar_pi, ...
• lepton: e, mu or tau

To select the ISGW2 implementations, channel is either d0_d_pi_isgw2 or d1p_dstar_pi_isgw2.

In addition, we support the BCL model for $B^+\rightarrow\rho^0\mu^+\nu_\mu$ decays. This EvtGen model has an additional problem, a division by zero in a basis change, which we reproduce as closely as feasible.

References

If you use MCAmbulance, you should cite the following references:

• MCAmbulance: Florian Herren and Raynette van Tonder, arXiv:2602.18378
• EvtGen: David J. Lange, Nucl.Instrum.Meth.A 462 (2001) 152-155
• basf2: Belle-II Framework Software Group, Comput.Softw.Big Sci. 3 (2019) 1, 1
• LLSW parametrisation: Adam K. Leibovich, Zoltan Ligeti, Ian M. Stewart and Mark B. Wise, Phys.Rev.Lett. 78 (1997) 3995-3998 & Phys.Rev.D 57 (1998) 308-330
• Default form factor values: Florian U. Bernlochner and Zoltan Ligeti, Phys.Rev.D 95 (2017) 1, 014022
• If using the ISGW2 form factors: Daryl Scora and Nathan Isgur, Phys.Rev.D 52 (1992) 2783-2812

Authors

• Florian Herren florian.s.herren@gmail.com
• Raynette van Tonder raynette.vantonder@kit.edu

Contributors

• Ilias Tsaklidis