Vacuum Polarization Effects from Dark Fermion Loops in Z' Mediator Models and their Impact on Angular Observables in B → K* μ+μ- Decays
Abstract
Rare flavour changing neutral current decays such as B → K*μ+μ− provide sensitive probes of physics beyond the Standard Model (SM), being forbidden at tree level and arising only through loop induced processes. Angular analyses yield observables sensitive to short distance dynamics. The optimized observable P'5 shows a persistent ∼ 3σ deviation from SM predictions in q2∈[1.1,6.0] GeV2, motivating modified Wilson coefficients. A theoretically motivated framework is investigated in which vacuum polarization of a Dirac dark fermion χ modifies the propagator of a heavy Z' boson, inducing a momentum dependent contribution to C9(q2). The loop corrected propagator is incorporated into the effective weak Hamiltonian for b→ sℓ+ℓ−. Bin averaged angular observables are computed with \texttt{flavio}, and 34,997 parameter points are scanned over different chiral structures of the Z' couplings. A χ2 analysis identifies preferred regions that are vector dominated and chirally asymmetric. The best fit solution significantly alleviates the P'5 tension, reducing the deviation in the central q2 bin to 0.12σ, an improvement of approximately 96% relative to the SM. This improvement is driven by tree level Z' exchange, which induces ΔC9≃−1.2, consistent with global fits and compatible with other angular observables and Bs→μ+μ−. Dark fermion vacuum polarization adds a subleading q2 dependence to C9; it does not dominate the fit, but supplies a distinctive threshold structure testable with finer binning. These results show that a Z' portal to a dark fermion provides a phenomenologically consistent description of the P'5 anomaly and a concrete benchmark for LHCb Run~3 analyses.
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BibTeX
@misc{gupta2026vacuum,
title = {{Vacuum Polarization Effects from Dark Fermion Loops in $Z'$ Mediator Models and their Impact on Angular Observables in $B \to K^* μ^+μ^-$ Decays}},
author = {Suhani Gupta and Sunil Dogra},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.15026},
eprint = {2609.15026},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.15026},
}
RIS
TY - UNPB
TI - Vacuum Polarization Effects from Dark Fermion Loops in Z' Mediator Models and their Impact on Angular Observables in B → K^* μ^+μ^- Decays
AU - Suhani Gupta
AU - Sunil Dogra
PY - 2026
DA - 2026/09/14
PB - arXiv
DO - 10.48550/arxiv.2609.15026
UR - https://arxiv.org/abs/2609.15026
AB - Rare flavour changing neutral current decays such as B → K^*μ^+μ^− provide sensitive probes of physics beyond the Standard Model (SM), being forbidden at tree level and arising only through loop induced processes. Angular analyses yield observables sensitive to short distance dynamics. The optimized observable P'_5 shows a persistent ∼ 3σ deviation from SM predictions in q^2∈[1.1,6.0] GeV^2, motivating modified Wilson coefficients. A theoretically motivated framework is investigated in which vacuum polarization of a Dirac dark fermion χ modifies the propagator of a heavy Z' boson, inducing a momentum dependent contribution to C_9(q^2). The loop corrected propagator is incorporated into the effective weak Hamiltonian for b→ sℓ^+ℓ^−. Bin averaged angular observables are computed with \texttt{flavio}, and 34,997 parameter points are scanned over different chiral structures of the Z' couplings. A χ^2 analysis identifies preferred regions that are vector dominated and chirally asymmetric. The best fit solution significantly alleviates the P'_5 tension, reducing the deviation in the central q^2 bin to 0.12σ, an improvement of approximately 96% relative to the SM. This improvement is driven by tree level Z' exchange, which induces ΔC_9≃−1.2, consistent with global fits and compatible with other angular observables and B_s→μ^+μ^−. Dark fermion vacuum polarization adds a subleading q^2 dependence to C_9; it does not dominate the fit, but supplies a distinctive threshold structure testable with finer binning. These results show that a Z' portal to a dark fermion provides a phenomenologically consistent description of the P'_5 anomaly and a concrete benchmark for LHCb Run~3 analyses.
ER -