Kinematic Fingerprints of a Nucleon-Triggered Vℬ in Rare η(′)→π0(η)γγ Decays on Nucleon Targets
Abstract
The nucleon-triggered vector boson Vℬ is a hypothetical particle motivated by a statistically significant discrepancy between the KLOE measurement, BRη→π0γγKLOE = (0.98±0.11stat±0.14syst)×10−4, and the current world average, (2.55±0.22)×10−4, dominated by MAMI photoproduction data. This hypothetical new interaction is activated by external nucleons, causing deviations from the Standard Model predictions, while leaving leptonic measurements, such as KLOE and BESIII, unaffected. We embed this mechanism in representative MAMI- and JEF-like photoproduction settings, deriving distinctive and directly testable kinematic and cut-dependent signatures to quantify small differences between the two regimes. One such signature is a small predicted population of events beyond the nominal on-shell boundaries of the γγ and π0γ spectra, containing 4--23 events in a sample of 1200, depending on the selection window. These events correlate with the recoil proton kinematics, and the polar angle relative to the on-shell hypothesis can reach ≲ −5∘ at MAMI energies, but only ≈ −0.2∘ at JEF. For identical selection windows, the JEF-like effective branching fraction exceeds the MAMI-like prediction by less than 1%, while widening the window increases both predictions by up to ∼4%. Both energy regimes produce a similar upward shift of the mean reconstructed mass, ⟨ Mπ0γγ⟩−mη≃ +2--7 MeV. Finally, we identify representative Vℬ-induced topologies that could contribute to recently reported discrepancies in the angular distributions of γd→π0ηd and γd→π0π0 d, leaving their quantitative investigation to future work.
Related
- Nucleon decays into three leptons plus a kaon
- Chiral matching of dimension-7 baryon-number-violating operators and application to ΔI=3/2 nucleon decays
- Baryon-number-violating nucleon decays into a dark photon particle
- Baryon-number-violating nucleon decays into a dark photon particle
- Sensitivity targets for dark photon searches in charged kaon decays
BibTeX
@misc{balytskyi2026kinematic,
title = {{Kinematic Fingerprints of a Nucleon-Triggered $V_\mathcalB$ in Rare $η^(\prime)\toπ^0(η)γγ$ Decays on Nucleon Targets}},
author = {Yaroslav Balytskyi},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2608.13537},
eprint = {2608.13537},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.13537},
}
RIS
TY - UNPB
TI - Kinematic Fingerprints of a Nucleon-Triggered V_ℬ in Rare η^(′)→π^0(η)γγ Decays on Nucleon Targets
AU - Yaroslav Balytskyi
PY - 2026
DA - 2026/09/07
PB - arXiv
DO - 10.48550/arxiv.2608.13537
UR - https://arxiv.org/abs/2608.13537
AB - The nucleon-triggered vector boson V_ℬ is a hypothetical particle motivated by a statistically significant discrepancy between the KLOE measurement, BR^η→π^0γγ_KLOE = (0.98±0.11_stat±0.14_syst)×10^−4, and the current world average, (2.55±0.22)×10^−4, dominated by MAMI photoproduction data. This hypothetical new interaction is activated by external nucleons, causing deviations from the Standard Model predictions, while leaving leptonic measurements, such as KLOE and BESIII, unaffected. We embed this mechanism in representative MAMI- and JEF-like photoproduction settings, deriving distinctive and directly testable kinematic and cut-dependent signatures to quantify small differences between the two regimes. One such signature is a small predicted population of events beyond the nominal on-shell boundaries of the γγ and π^0γ spectra, containing 4--23 events in a sample of 1200, depending on the selection window. These events correlate with the recoil proton kinematics, and the polar angle relative to the on-shell hypothesis can reach ≲ −5^∘ at MAMI energies, but only ≈ −0.2^∘ at JEF. For identical selection windows, the JEF-like effective branching fraction exceeds the MAMI-like prediction by less than 1%, while widening the window increases both predictions by up to ∼4%. Both energy regimes produce a similar upward shift of the mean reconstructed mass, ⟨ M_π^0γγ⟩−m_η≃ +2--7 MeV. Finally, we identify representative V_ℬ-induced topologies that could contribute to recently reported discrepancies in the angular distributions of γd→π^0ηd and γd→π^0π^0 d, leaving their quantitative investigation to future work.
ER -