Missing Mass Matters: Neutrinophilic Scalar in D-Meson Decays

@misc{cai2026missing,
  title = {{Missing Mass Matters: Neutrinophilic Scalar in $D$-Meson Decays}},
  author = {Jin-Man Cai and Gang Li},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.08606},
  eprint = {2610.08606},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.08606},
}

TY  - UNPB
TI  - Missing Mass Matters: Neutrinophilic Scalar in D-Meson Decays
AU  - Jin-Man Cai
AU  - Gang Li
PY  - 2026
DA  - 2026/10/06
PB  - arXiv
DO  - 10.48550/arxiv.2610.08606
UR  - https://arxiv.org/abs/2610.08606
AB  - A light neutrinophilic scalar ϕ, which couples predominantly to neutrinos, can be probed through leptonic meson decays for scalar masses 10 MeV≲ m_ϕ≲𝒪(100) MeV. We study neutrinophilic-scalar emission in D-meson decays at BESIII and the future Super Tau-Charm Facility (STCF), focusing on D_(s)^+→ e^+ν̅_αϕ. Accounting for the m_ϕ-dependent missing-mass squared (M_miss^2) distribution within the experimentally selected region, we find the resulting D^+ constraint to be substantially weaker than previous estimates based on the integrated three-body decay rate. Extending the M_miss^2 coverage provides sensitivity to heavier scalars. With the full kinematic range of M_miss^2 and an integrated luminosity of 1 ab^−1, the projected STCF sensitivities surpass the existing kaon bound for m_ϕ≳ 380 MeV in the D^+ decay and m_ϕ≳ 310 MeV in the D_s^+ decay. We further discuss direct searches for D_(s)^+→μ^+ν̅_αϕ and their potential for probing the neutrinophilic scalar. A complete analytic expression for the three-body decay width is also derived with full dependence on the scalar and charged-lepton masses, including its behavior in the m_ϕ→ 0 and m_ℓ→ 0 limits.
ER  -