When energy goes missing: new physics in $$b\rightarrow s \nu \nu $$ with sterile neutrinos

We investigate the physics reach and potential for the study of various decays involving a $$ b\to s\nu \overline{\nu} $$ b → sν ν ¯ transition at the Future Circular Collider running electron-positron collisions at the Z-pole (FCC-ee). Signal and background candidates, which involve inclusive Z contributions from $$ b\overline{b} $$ b b ¯ , $$ c\overline{c} $$ c c ¯ and uds final states, are simulated for a proposed multi-purpose detector. Signal candidates are selected using two Boosted Decision Tree algorithms. We determine expected relative sensitivities of 0.53%, 1.20%, 3.37% and 9.86% for the branching fractions of the $$ {B}^0\to {K}^{\ast 0}\nu \overline{\nu} $$ B 0 → K ∗ 0 ν ν ¯ , $$ {B}_s^0\to \phi \nu \overline{\nu} $$ B s 0 → ϕν ν ¯ , $$ {B}^0\to {K}_S^0\nu \overline{\nu} $$ B 0 → K S 0 ν ν ¯ and $$ {\Lambda}_b^0\to \Lambda \nu \overline{\nu} $$ Λ b 0 → Λ ν ν ¯ decays, respectively. In addition, we investigate the impact of detector design choices related to particle-identification and vertex resolution. The phenomenological impact of such measurements on the extraction of Standard Model and new physics parameters is also studied.

Section: Jhep01(2024)144mentioning

confidence: 99%

Prospects for searches of $$ b\to s\nu \overline{\nu} $$ decays at FCC-ee

Amhis,

Kenzie,

Reboud

et al. 2024

Model independent bounds on heavy sterile neutrinos from the angular distribution of B → D*ℓν decays

Bernlochner,

Fedele,

Kretz

et al. 2025

In this paper we study the bounds that can be inferred on New Physics couplings to heavy sterile neutrinos N from the recent measurements performed by the Belle collaboration of the angular analysis of $$ B\to {D}^{\ast}\ell {\overline{\nu}}_{\ell } $$ B → D ∗ ℓ ν ¯ ℓ decays, with ℓ = e, μ. Indeed, a sterile neutrino N may lead to competing $$ B\to {D}^{\ast}\ell \overline{N} $$ B → D ∗ ℓ N ¯ decays and Belle might have measured an incoherent sum of these two independent channels. After reviewing the theoretical formalism required to describe this phenomenon in full generality, we first perform a bump hunt in the $$ {M}_{\textrm{miss}}^2 $$ M miss 2 Belle distribution to search for evidences of an additional massive neutrino. We found in such a way a small hint at $$ {M}_{\textrm{miss}}^2 $$ M miss 2 ∼ (354 MeV)2. However, the Belle angular analysis is sensitive to N masses up to $$ \mathcal{O} $$ O (50 MeV), preventing us to further inspect this hint. Nevertheless, we study the potential impact of this additional channel in the allowed mass range on the measured angular distributions and extract model-independent bounds on the new-physics couplings which could mediate such an interaction. In particular, in the mass window here inspected, we obtain the most stringent bounds for vector and left-handed scalar operators to date.

Deciphering the Belle II data on $$ B\to K\nu \overline{\nu} $$ decay in the (dark) SMEFT with minimal flavour violation

Hou,

Li,

Shen

et al. 2024

Recently, the Belle II collaboration announced the first measurement of the branching ratio $$ \mathcal{B}\left({B}^{+}\to {K}^{+}\nu \overline{\nu}\right) $$ B B + → K + ν ν ¯ , which is found to be about 2.7σ higher than the Standard Model (SM) prediction. We decipher the data with two new physics scenarios: the underlying quark-level $$ b\to s\nu \overline{\nu} $$ b → sν ν ¯ transition is, besides the SM contribution, further affected by heavy new mediators that are much heavier than the electroweak scale, or amended by an additional decay channel with undetected light final states like dark matter or axion-like particles. These two scenarios can be most conveniently analyzed in the SM effective field theory (SMEFT) and the dark SMEFT (DSMEFT) framework, respectively. We consider the flavour structures of the resulting effective operators to be either generic or satisfy the minimal flavour violation (MFV) hypothesis, both for the quark and lepton sectors. In the first scenario, once the MFV assumption is made, only one SM-like low-energy effective operator induced by the SMEFT dimension-six operators can account for the Belle II excess, whose parameter space is, however, excluded by the Belle upper bound of the branching ratio $$ \mathcal{B}\left({B}^0\to {K}^{\ast 0}\nu \overline{\nu}\right) $$ B B 0 → K ∗ 0 ν ν ¯ . In the second scenario, it is found that the Belle II excess can be accommodated by 22 of the DSMEFT operators involving one or two scalar, fermionic, or vector dark matters as well as axion-like particles. These operators also receive dominant constraints from the B0 → K*0 + inv and Bs → inv decays. Once the MFV hypothesis is assumed, the number of viable operators is reduced to 14, and the B+ → π+ + inv and K+ → π+ + inv decays start to put further constraints on them. Within the parameter space allowed by all the current experimental data, the q2 distributions of the B → K(*) + inv decays are then studied for each viable operator. We find that the resulting prediction of the operator $$ {\mathcal{Q}}_{q\chi}=\left({\overline{q}}_p{\gamma}_{\mu }{q}_r\right)\left(\overline{\chi}{\gamma}^{\mu}\chi \right) $$ Q qχ = q ¯ p γ μ q r χ ¯ γ μ χ with a fermionic dark matter mass mχ ≈ 700 MeV can closely match the Belle II event distribution in the bins 2 ≤ q2 ≤ 7 GeV2. In addition, we, for the first time, calculate systematically the longitudinal polarization fraction FL of K* in the B → K* + inv decays within the DLEFT. By combining the decay spectra and FL, almost all the DSMEFT operators are found to be distinguishable from each other. Finally, the future prospects at Belle II, CEPC and FCC-ee are also discussed for some of these FCNC processes.