[Paper Review] BC_NPI module for the analysis of Bc -> J/psi +n pi and Bc -> B_s +n pi decays within the EvtGen package

A. Berezhnoy, А. К. Лиходед+1 more|arXiv (Cornell University)|Apr 5, 2011

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents the BC NPI module for the EvtGen event generator, enabling detailed simulation of Bc → J/ψ + nπ and Bc → Bs + nπ decays (n ≤ 4) using a factorization model where Bc decays into a heavy hadron and a virtual W∗, followed by W∗ → nπ. The model adopts form factors and transition parameters from τ → ντ + nπ and e⁺e⁻ → 4π data, allowing accurate Monte Carlo generation for LHC Bc physics analysis with validated agreement to theoretical predictions.

ABSTRACT

The module for the generation of $B_c$ meson decays into $J/\psi + n\pi$ and $B_s^{(*)} + n\pi$ ($n\le 4$) is implemented into EvtGen program package. The decay amplitudes are calculated in the frame work of factorization model. Within this approach the decay can be represented as $B_c$ decay into $J/\psi (B_s)+W^*$ followed by the virtual $W^*$-boson decay into the final set of $\pi$-mesons. The described calculation technique allows to adopt the parameters of $W^* o n\pi$ transition from the analysis of $ au$ decay into $ u_ au+n\pi$. Comparison with available theoretical predictions is performed.

Motivation & Objective

• To enable precise Monte Carlo simulation of exclusive Bc decays into J/ψ + nπ and Bs + nπ final states with up to four pions.
• To implement a factorization-based model in the EvtGen event generator for realistic simulation of Bc decays under LHC kinematic conditions.
• To adopt W∗ → nπ transition parameters from experimental τ → ντ + nπ and e⁺e⁻ → 4π data to improve theoretical accuracy.
• To provide a tool for future LHC experiments to analyze branching fractions, invariant mass spectra, and transverse momentum distributions of decay products.

Proposed method

• The decay amplitude is modeled as Bc → heavy hadron + W∗ followed by W∗ → nπ, using factorization to separate weak and strong interaction contributions.
• The Bc → J/ψ + W∗ transition is described by form factors FV, FA₀, FA₊, FA₋, parameterized via exponential fits: Fi(q²) = Fi(0) exp(c₁q² + c₂q⁴).
• W∗ → nπ transition amplitudes are derived from experimental data on τ → ντ + nπ (n ≤ 3) and e⁺e⁻ → 4π processes.
• The model is implemented as a C++ module within the EvtGen package, interfacing with ROOT and CLHEP for event generation.
• The renormalization scale dependence is included via a1(mb) = 1.4, with higher-order corrections accounted for in the factorization approach.
• The module supports full kinematic reconstruction, including transverse momentum cuts and invariant mass spectra for ππ systems.

Experimental results

Research questions

• RQ1 How can exclusive Bc decays into J/ψ + nπ and Bs + nπ final states be accurately simulated using factorization and known hadronic decays of virtual W∗?
• RQ2 To what extent can parameters from τ → ντ + nπ and e⁺e⁻ → 4π decays be reliably transferred to model W∗ → nπ transitions in Bc decays?
• RQ3 What is the impact of different form-factor parametrizations (e.g., exponential fits) on the predicted branching fractions and invariant mass spectra?
• RQ4 How well does the implemented model reproduce theoretical predictions for Bc → J/ψ + 2π decay distributions?
• RQ5 Can the EvtGen-based simulation tool enable detailed analysis of kinematic distributions such as transverse momentum and q² spectra in realistic LHC conditions?

Key findings

• The BC NPI module successfully implements Bc → J/ψ + nπ and Bc → Bs + nπ decays (n ≤ 4) in EvtGen, enabling full event-level simulation with realistic kinematics.
• The model achieves good agreement with theoretical predictions, particularly in the q² distribution of ππ systems in Bc → J/ψ + 2π decay, matching results from [13] within statistical uncertainties.
• Form-factor parameters for Bc → J/ψ + W∗ are provided for multiple sets: FV(0) = 0.11, FA₀(0) = 5.9, FA₊(0) = -0.074, FA₋(0) = 0.11, with c₁ = 0.049, c₂ = 0.0015.
• For Bc → Bs + nπ, the form-factor parameters are FV(0) = 1.08, FA₀(0) = 8.1, FA₊(0) = 0.15, FA₋(0) = 1.08, with c₁ = 0.30, c₂ = 0.069.
• The simulation includes transverse momentum distributions with pT² > 5 GeV² cuts, showing that such observables cannot be captured by spectral function formalism alone.
• The implementation supports experimental analysis needs at the LHC, where ~10¹⁰ Bc events per year are expected, enabling precision studies of branching fractions and dynamics.