Eur. Phys. J. Plus (2026) 141: 495
https://doi.org/10.1140/epjp/s13360-026-07673-x

Regular Article

Probing vector- versus scalar-mediator dark-matter scenarios in $B\to (K,K^{\ast })M_X$ decays

¹ D. V. Skobeltsyn Institute of Nuclear Physics, M. V. Lomonosov Moscow State University, 119991, Moscow, Russia
² Institute of High Energy Physics, Austrian Academy of Sciences, Nikolsdorfergasse 18, 1050, Vienna, Austria
³ Marietta Blau Institute (MBI) for Particle Physics, Austrian Academy of Sciences, Dominikanerbastei 16, 1010, Vienna, Austria
⁴ Joint Institute for Nuclear Research, 141980, Dubna, Russia
⁵ Faculty of Physics, University of Vienna, Boltzmanngasse 5, 1090, Vienna, Austria

^a This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 January 2026
Accepted: 9 April 2026
Published online: 4 May 2026

Abstract

Within the hypothesis of the dark-matter (DM) origin of the excess in $B\to K{M}_X$ decays over the standard-model expectation, observed by Belle II, we report the following results: (i) Combining the available data on the integrated decay rates $\mathrm{\Gamma }(B\to K{M}_X)$ and the upper limit on the integrated decay rates $\mathrm{\Gamma }(B\to K^{\ast }{M}_X)$ provides a tight constraint on the vector-mediator mass $M_V\lesssim 3$ GeV, whereas no constraints on the scalar-mediator mass are imposed by these data. (ii) Both scalar- and vector-mediator scenarios allow for a good description of the differential distributions in $B\to K{M}_X$ measured by Belle II and an extraction of dark-model parameters within both scenarios. Previously, for the scalar-mediator scenario, we extracted the scalar-mediator parameters $M_{\varphi }\sim 2.4$ GeV, ${\mathrm{\Gamma }}_{\varphi }\sim 2.9$ GeV, and the DM fermion mass $m_{\chi }\sim 0.42$ GeV. We now extract the parameters for the vector-mediator scenario: $M_V\sim 3.0$ GeV, ${\mathrm{\Gamma }}_V\sim 4.0$ GeV, and $m_{\chi }\sim 0.6$ GeV. (iii) In view of a nice description of the Belle II data in scalar- and vector-mediator scenarios, one faces the problem of discriminating between these scenarios. We propose a novel clear measurable signature to discriminate between two DM scenarios: The ratio of the differential distributions in $B\to K{M}_X$ and $B\to K^{\ast }{M}_X$ decays is shown to have qualitatively different shapes for scalar- and vector-mediator scenarios.