arXiv subjects

Alexander Berezhnoy

Publications and source records attributed to Alexander Berezhnoy.

8 recordsLinked to original sources

arXiv2026

$B_{(s)}\to V M_X$ decays as probes of dark-matter scenarios of Belle II enhancement in $B\to KM_X$ decays

Recently, we have shown that the hypothesis of the dark-matter (DM) origin of the Belle II excess events in $B\to K M_X$ decays allows for a successful description of the data. The DM parameters (masses and couplings) in two DM scenarios with either scalar (S-scenario) or vector (V-scenario) mediator have been extracted with rather small uncertainty from fits to the $B\to K M_X$ data. The same mechanism leads to a similar enhancement of the $B_s\to (η,η',ϕ) M_X$ decay rates. We present a detailed analysis of $B\to K^* M_X$ and $B_s\to ϕM_X$ decays and show that these decays provide a clear probe of the S- and V-scenarios: Compared to the Standard Model, in the S-scenario an enhancement by a factor $\sim 1.6$ is expected, whereas the V-scenario leads to a much larger enhancement by a factor $\sim 3.5$. For the $B\to K^* M_X$ decays, we provide the expected number of events at Belle II for the sample for which a rather large enhancement of $B\to K M_X$ decays compared to the Standard Model expectations has been reported.

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arXiv2025

Scrutinizing dark-matter scenarios with $B\to(K,K^*)\barνν$ decays

Conceivable explanations of Belle-II measurements of a (surprising) excess of missing-energy decays of the $B$ meson to the $K$ meson not covered by standard-model neutrino-antineutrino pairs might be offered by additional contributions of dark-matter fermion-antifermion pairs. Assuming the excessive missing-energy events to be mediated by a (generic) scalar or vector boson, a simultaneous inspection of both of the missing-energy $B$ decays into a pseudoscalar $K$ meson or a vector $K^*$ meson allows to gain information on the nature of bosons relating standard-model and dark-matter sectors, irrespective of any (unknown) dark-sector details. Upon availability of indispensable experimental data, most prominent among such insights might be the identification of the mediator spin from the differential $B$-meson decay widths.

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arXiv2025

Probing vector- vs scalar-mediator dark-matter scenarios in $B\to (K,K^*) M_X$ decays

Within the hypothesis of the dark-matter origin of the excess in $B\to K M_X$ decays over the standard-model expectation, observed by Belle-II, we show that: (i) Scalar- and vector-medator scenarios may be unambiguously discriminated by measuring the differential distributions in $B\to K M_X$ and $B\to K^* M_X$ decays. (ii) Combining the available data on $Γ(B\to K M_X)$ and the upper limit on $Γ(B\to K^* M_X)$ provides a tight constraint on the vector mediator mass $M_V\lesssim 3$ GeV. At the same time, no constraints on the scalar-mediator mass are imposed by these data. (iii) Both scalar- and vector-mediator scenarios allow 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.

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arXiv2025

Analysis of $B\to KM_X$ and $B\to K^* M_X$ decays in scalar- and vector-mediator dark-matter scenarios

The surprising excess of missing-energy events far beyond all standard-model expectations in the weak decays of the charged ground-state $B^+$ meson into some charged strange meson, rather recently observed by the Belle-II experiment, may (easily) be explained by the decay of the $B$ meson into the strange meson and a pair of dark-matter fermion and antifermion, mediated by an (intermediate) scalar or vector boson. Thorough inspections of both the total and the differential widths of these decays provide, among others, a simple means for the (straightforward) discrimination of such mediator boson's scalar or vector nature.

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arXiv2025

Analysis of $q_\mathrm{rec}^2$-distribution for $B\to K M_X$ and $B\to K^* M_X$ decays in a scalar-mediator dark-matter scenario

We demonstrate that the scalar-mediator dark-matter scenario is consistent with the experimental data on the decay $B\to K M_X$ and provides a good description of the shape of the observed excess. Within this scenario, the interaction with dark-matter particles leads to approximately the same excess in $Γ(B\to K^* M_X)$ and $Γ(B\to K M_X)$ compared to the Standard Model; also the differential distributions of the excess events are similar in shape in the variable $q_\mathrm{rec}^2$ measured by experiment.

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arXiv2024

Nonfactorizable charming-loop contribution to FCNC $B_s\to γl^+l^-$ decay

We present the first theoretical calculation of nonfactorizable charm-quark loop contributions to the $B_s\to γl^+l^-$ amplitude. We calculate the relevant form factors, $H_{A,V}^{\rm NF}(k'^2,k^2)$, and provide convenient parametrizations of our results in the form of fit functions of two variables, $k'^2$ and $k^2$, applicable in the region below hadron resonances, $k'^2 < M_{J/ψ}^2$ and $k^2 < M_ϕ^2$. We report that factorizable and nonfactorizable charm contributions to the $B_s\toγl^+l^-$ amplitude have opposite signs. To compare the charm and the top contributions, it is convenient to express the NF charming loop contribution as a non-universal (i.e., dependent on the reaction) $q^2$-dependent correction $Δ^{\rm NF}C_7(q^2)$ to the Wilson coefficient $C_7$. For the $B_s\toγl^+l^-$ amplitude, the correction is found to be positive, $Δ^{\rm NF} C_7(q^2)/C_7 > 0$.

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arXiv2024

$B\to K^* M_X$ vs $B\to K M_X$ as a probe of a scalar-mediator dark matter scenario

Recently, Belle II reported the observation of the decay $B\to K M_X$, $M_X$ the missing mass, with the branching ratio much exceeding ${\cal B}(B\to K ν\barν)$ which is the only Standard Model (SM) process contributing to this reaction. If confirmed, this might be an indication of new nonSM particles produced in this decay. One of the possible explanations of the observed effect could be light dark-matter (DM) particles produced via a scalar mediator field. We give simple arguments, that a combined analysis of the $B\to K M_X$ and $B\to K^* M_X$ reactions would be a clean probe of the scalar mediator scenario: (i) making use of an observed value ${\cal B}(B\to K M_X)\simeq 5.4\, {\cal B}(B\to K ν\barν)_{\rm SM}$ and (ii) assuming that the effect is due to the light dark matter coupling to the top quark via a {\it scalar} mediator field, one finds an upper limit ${\cal B}(B\to K^* M_X) < 2.8 \, {\cal B}(B\to K^* ν\barν)_{\rm SM}$. Within the discussed scenario, this upper limit does not depend on the mass of the scalar mediator nor on the specific details of the unobserved dark-matter particles in the final state.

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arXiv2023

Charming-loop contribution to $B_s\to γγ$ decay

We present a detailed theoretical study of nonfactorizable contributions of the charm-quark loop to the amplitude of the $B_s\to γ\,γ$ decay. This contribution involves the $B$-meson three-particle Bethe-Salpeter amplitude, $\langle 0|\bar s(y)G_{μν}(x)b(0)|\bar B_s(p)\rangle$, for which we take into account constraints from analyticity and continuity. The charming-loop contribution of interest may be described as a correction to the Wilson coefficient $C_{7γ}$, $C_{7γ}\to C_{7γ}(1+δC_{7γ})$. We calculate an explicit dependence of $δC_{7γ}$ on the parameter $λ_{B_s}$. Taking into account all theoretical uncertainties, $δC_{7γ}$ may be predicted with better than 10\% accuracy for any given value of $λ_{B_s}$. For our benchmark point $λ_{B_s}=0.45$ GeV, we obtain $δC_{7γ}=0.045\pm 0.004$. Presently, $λ_{B_s}$ is not known with high accuracy, but its value is expected to lie in the range $0.3\le λ_{B_s}({\rm GeV})\le 0.6$. The corresponding range of $δC_{7γ}$ is found to be $0.02\le δC_{7γ}\le 0.1$. One therefore expects the correction given by charming loops at the level of at least a few percent.

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