arXiv subjects

Claudio Toni

Publications and source records attributed to Claudio Toni.

At least 19 recordsLinked to original sources

arXiv2026

The $B^+ \to K^+ \nu \bar \nu$ decay as a QCD axion search: comparing reinterpretation approaches

Two recent independent analyses of Belle II $B^+ \! \to \! K^+\nu\bar\nu$ data yield limits on ${\mathcal B}(B^+ \! \to \! K^+ a)$ -- the two-body mode to a light invisible particle such as the QCD axion -- differing by a factor of roughly four; we trace this to the choice of kinematic variable space. The central figure of merit is the resolution in the reconstructed di-neutrino invariant mass $q^2_{\rm rec}$: fine-grained binning resolves the narrow axion signal, while coarse binning dilutes it into a background-dominated range. A BDT axis trained on $B^+ \! \to \! K^+\nu\bar\nu$ adds little discriminating power for $B^+ \! \to \! K^+ a$, as this axis is largely uncorrelated with $q^2$. These expectations are confirmed by a set of numerical tests. The subleading shape systematics omitted from our $q^2_{\rm rec}$-based approach {\em lower}, not raise, the $B^+ \! \to \! K^+ a$ limit: by better accommodating the $B^+ \! \to \! K^+\nu\bar\nu$ shape, they leave less room for the axion signal, making our $q^2_{\rm rec}$-based bound conservative, if anything. A dedicated reanalysis confirms that the kinematic-axes choice alone accounts for the factor-of-four sensitivity difference, and that the $B^+ \! \to \! K^+ a$ bound varies sizeably within the $q^2_{\rm rec}\times\eta({\rm BDT}_2)$ space, depending on the SM-likeness of $B^+ \! \to \! K^+\nu\bar\nu$, thus losing the dual-probe feature of our $q^2_{\rm rec}$-based approach. These results point to a broader consideration: likelihoods dominated by BDT variables are of limited use for reinterpretations when the signal shape differs appreciably from the BDT's training signal. We therefore advocate that experimental collaborations publish likelihood projections in physical variable spaces alongside BDT-based likelihoods, to maximise the reinterpretability of their measurements.

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arXiv2026

Constraints on a Light Leptophilic Scalar from Dark-Sector Couplings

We study a minimal framework where a Majorana fermion dark matter particle interacts with a light scalar mediator coupled mainly to electrons. We examine both freeze-out and freeze-in production to determine the regions of parameter space that yield the correct relic abundance with particular emphasis on a detailed comparison with results in the recent literature. The analysis includes cosmological and astrophysical constraints as well as laboratory bounds from electron-recoil experiments, fixed target searches, and precision measurements. The combined results identify a narrow but viable parameter region, favoring sub-GeV dark matter, and define clear targets for future experimental tests. This highlights the strong complementarity between direct-detection experiments and collider searches. We additionally investigate the mediator-mass region around 17 MeV, motivated by the hints reported by the ATOMKI experiment and the PADME collaboration, including couplings between the mediator and light quarks. Direct searches already constrain a large region of the parameter space, even when dark matter is produced via freeze-in, pointing again to sub-GeV dark matter.

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arXiv2026

A framework for missing-energy searches with anomalous light vectors

We study light spin-1 gauge bosons coupled to electroweak-anomalous currents. For generic charge assignments, anomaly cancellation requires new fermions (anomalons) that are chiral under the new abelian symmetry and carry electroweak charges. If their masses arise from the breaking of the new gauge symmetry, integrating them out generates Wess-Zumino interactions fixed by mixed-anomaly matching, providing the infrared description of the theory. We classify minimal anomalon spectra, derive the corresponding effective interactions, and combine experimental constraints with finite-naturalness considerations to bound the UV completion scale. Motivated by recent NA62 and Belle II results, we then develop a unified phenomenological framework for the missing-energy signatures of these anomalous light vectors, focusing on scenarios where the new vector decays predominantly into neutrinos so that the leading probes are rare processes with invisible final states. As applications, we survey current and projected searches across flavour and electroweak observables, including $K\to\pi E_{\rm miss}$, $B\to K^{(*)}E_{\rm miss}$, and $Z\to\gamma E_{\rm miss}$, and discuss their interplay with direct searches for anomalons.

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arXiv2026

RadioAxion results on the search for axion dark matter under Gran Sasso

We report first results from RadioAxion, an underground experiment searching for axion dark matter through periodic modulations of radioisotope decays. We monitor the $\alpha$ decay of ${^{241}}$Am via its $59.5$ keV $\gamma$ line using a NaI detector installed at the Gran Sasso Laboratory, where cosmic-ray-induced systematics are strongly suppressed. We present the measured spectra and the corresponding time-series analysis. No evidence for a periodic modulation is observed. From these data we derive constraints on the axion decay constant in the axion mass range from $10^{-21}$ to $10^{-9}$ eV.

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arXiv2026

Dark Matter emission at Belle II and NA62 in Minimal Flavor Violation framework

Minimal Flavor Violation (MFV) provides a compelling framework for exploring physics beyond the Standard Model, in which new QCD-singlet fields transforming under the global $\mathrm{SU}(3)^3$ quark flavor symmetry can naturally be stable and act as dark matter (DM) candidates. We show that the DM-MFV framework naturally accommodates the excess in either $K^+ \to \pi^+ \nu \bar{\nu}$ or $B^+ \to K^+ \nu \bar{\nu}$, while a unified explanation of both channels simultaneously cannot be achieved within a minimal setup containing only a single dark matter multiplet with nearly degenerate masses. Overall, our findings underscore the intricate interplay between MFV-based model building, flavored dark matter scenarios, and precision flavor experiments, highlighting flavored dark matter as a framework that is both theoretically robust and experimentally testable.

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arXiv2025

The $B^+ \to K^+ \nu \bar \nu$ decay as a search for the QCD axion

We introduce a model-independent framework to reinterpret Belle II results using only public data, analytically reconstructing the mapping between true and reconstructed kinematic variables within the statistically dominant Inclusive Tagging Analysis. This enables rare-decay measurements to probe light invisible particles -- such as the QCD axion or axion-like particles, collectively denoted $a$ -- without relying on internal simulations. Applying the method to $B^+ \! \to \! K^+ \nu \bar\nu$ yields the strongest bound on the branching fraction for $B^+ \! \to \! K^+ a$, improving existing limits by about a factor of nine and constraining the axion's fundamental flavour-changing coupling to $b$ and $s$ quarks. The approach establishes $B^+ \! \to \! K^+ \nu \bar\nu$ as a dual probe -- simultaneously testing short-distance new physics and light invisible states, the two probes working independently to an excellent approximation -- and provides a general strategy for model-independent reinterpretation of collider data.

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arXiv2025

Probing a Light Scalar Boson with a few-MeV Proton Beam Deep Underground

We propose to investigate the production of a light scalar boson $\phi$ in low-energy proton-nucleus interactions using the 3.5 MV accelerator of the Bellotti Ion Beam Facility, located in the underground Gran Sasso National Laboratory. Nuclear reactions induced by a few-MeV proton beam on suitable target materials can act as a controlled source of $\phi$ particles. Owing to the deep-underground location, the facility benefits from substantial cosmic-ray shielding, enabling searches for rare processes with minimal background. The produced $\phi$ particles will be sought with large-volume, low-background detectors already operating or currently under construction at the Gran Sasso National Laboratory. This approach combines a tunable accelerator-based production mechanism with the exceptional sensitivity of underground rare-event searches, offering a novel avenue to probe light scalar bosons beyond the Standard Model.

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arXiv2025

Gauged $\tau$-lepton chiral currents and $B \to K^{(*)} E_{\rm miss}$

We consider a class of theories with a $U(1)_X$ gauge symmetry associated with leptonic chiral currents. The low-energy effective field theory includes a light spin-$1$ boson coupled to the electroweak gauge sector via a Wess-Zumino term, which ensures anomaly cancellation in the infrared. As a concrete application, we show that a light vector boson with mass $m_X \simeq 2.1\,\text{GeV}$, coupled to a $\tau$-lepton chiral current, can readily account for the recent $3\sigma$ excess observed in $B \to K^{(*)} E_{\rm miss}$ at Belle II, while remaining consistent with existing constraints from $Z \to \gamma E_{\rm miss}$ and direct searches for anomalon fields responsible for anomaly cancellation in the ultraviolet. After classifying phenomenologically viable models, we explore in greater detail two concrete realizations which give rise to distinctive phenomenological signatures, potentially accessible at future experiments at the high-energy and intensity frontiers.

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arXiv2025

Combined Evidence for the $X_{17}$ Boson After PADME Results on Resonant Production in Positron Annihilation

The Positron Annihilation into Dark Matter Experiment at the Laboratori Nazionali di Frascati has reported an excess of $e^+e^-$ final-state events from positron annihilation on fixed-target atomic electrons. While the global significance remains at the $(1.77\pm 0.15)\,\sigma$ level, the excess is centered around $\sqrt{s} \sim 17\,\text{MeV}$, coinciding with the invariant mass at which anomalous $e^+e^-$ pair production has previously been observed in nuclear transitions from excited to ground states in $^8$Be, $^4$He and $^{12}$C, thereby strengthening the case for a common underlying origin, possibly involving a hypothetical new $X_{17}$ boson. We discuss the significance of this independent accelerator-based evidence. Combining it with existing nuclear physics results, we obtain a value for the $X_{17}$ mass of $m_{X_{17}} = 16.88 \pm 0.05\,\text{MeV}$, reducing the uncertainty from nuclear physics determinations by more than a factor of two, and mitigating the impact of poorly known correlations among their systematic errors.

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arXiv2025

New bound on the vectorial axion-down-strange coupling from $K^+ \to \pi^+ \nu \bar \nu$ data

We reinterpret publicly available $K^+ \to \pi^+ \nu \bar{\nu}$ data collected by NA62 from 2016 to 2024 to constrain the fundamental vectorial coupling of the QCD axion to down and strange quarks. Using a fully reproducible likelihood analysis and a complete renormalization-group evolution of the axion couplings from the Peccei-Quinn (PQ) scale to the kaon scale, we translate the experimental limit into bounds on both the low-energy flavour-violating coupling and the fundamental UV parameters. In the generic regime where strong contributions dominate the decay amplitude, we obtain $|(F_V)_{sd}(\mu_K)| > 1.6 \times 10^{12}\,\text{GeV}$. The coexistence of parametrically suppressed weak contributions implies a second, conceptually distinct constraint: the fact that weak-amplitude dominance arises from highly tuned UV coupling configurations, translates into a conservative general lower limit on the PQ scale, $f_a > 4.9 \times 10^4\,\text{GeV}$. These results provide the strongest accelerator-based constraints on axion-induced $d \leftrightarrow s$ transitions and establish a robust lower bound on $f_a$, complementary to astrophysical limits.

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arXiv2025

On the Atomki nuclear anomaly after the MEG-II result

Recent experimental results from the Atomki collaboration have reported the observation of anomalous effects in Beryllium, Helium and Carbon nuclear transitions that could hint at physics beyond the Standard Model. However, the MEG-II experiment has recently found no significant anomalous signal in the Beryllium transition ${^8}\text{Be}^\star\to{^8}\text{Be}+e^+e^-$. In view of this result, we critically re-examine the possible theoretical interpretations of the anomalies observed by the Atomki experiment in terms of a new boson $X$ with mass around $17\;$MeV. The present work aims to study the phenomenology of a spin-2 state and revisit the possibility of a pure CP-even scalar, which was initially dismissed due to its inability to explain the Beryllium anomalous signal. Our analysis shows that a spin-2 state is highly disfavoured by the SINDRUM constraint while a scalar boson could explain the Helium and Carbon anomalies while being compatible with other experimental constraints.

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arXiv2024

Weak nuclear decays deep-underground as a probe of axion dark matter

We investigate the time modulation of weak nuclear decays as a method to probe axion dark matter. To this end, we develop a theoretical framework to compute the $\theta$-dependence of weak nuclear decays, including electron capture and $\beta$ decay, which enables us to predict the time variation of weak radioactivity in response to an oscillating axion dark matter background. As an application, we recast old data sets, from the weak nuclear decays of ${^{40}\text{K}}$ and ${^{137}\text{Cs}}$ taken at the underground Gran Sasso Laboratory, in order to set constraints on the axion decay constant, specifically in the axion mass range from few $10^{-23}\;$eV up to $10^{-19}\;$eV. We finally propose a new measurement at the Gran Sasso Laboratory, based on the weak nuclear decay of ${^{40}\text{K}}$ via electron capture, in order to explore even shorter timescales, thus reaching sensitivities to axion masses up to $10^{-9}\;$eV.

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arXiv2024

Alpha radioactivity deep-underground as a probe of axion dark matter

We propose to investigate the time modulation of radioisotope decays deep underground as a method to explore axion dark matter. In this work, we focus on the $\alpha$-decay of heavy isotopes and develop a theoretical description for the $\theta$-dependence of $\alpha$-decay half-lives, which enables us to predict the time variation of $\alpha$-radioactivity in response to an oscillating axion dark matter background. To probe this scenario, we have recently constructed and installed a setup deep underground at the Gran Sasso Laboratory, based on the $\alpha$-decay of Americium-241. This prototype experiment, named RadioAxion-$\alpha$, will allow us to explore a broad range of oscillation's periods, from a micro-second up to one year, thus providing competitive limits on the axion decay constant across 13 orders of magnitude in the axion mass, ranging from $10^{-9}$ eV to $10^{-20}$ eV after one month of data collection, and down to $10^{-22}$ eV after three years.

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arXiv2024

Implications of $B \to K \nu \bar{\nu}$ under Rank-One Flavor Violation hypothesis

We study the implications of the observed excess in $B^+ \to K^+ \nu \bar{\nu}$ under the assumption of Rank-One Flavour Violation, i.e. that New Physics couples to a single specific direction in flavour space. By varying this direction we perform analyses at the level of the low-energy EFT, the SMEFT, and with explicit mediators such as leptoquarks and colorless vectors ($Z^\prime$ and $V^\prime$). We study correlations with other flavour, electroweak and collider observables, finding that the most interesting ones are with $K \to \pi \nu \bar{\nu}$, $B_s \to \mu^+ \mu^-$, meson mixing and the LHC searches in $\tau^+ \tau^-$ high-energy tails. Among the various mediators, the scalar leptoquarks $\tilde{R}_2$ and $S_1$ offer the best fits of the Belle-II excess, while being consistent with the other bounds. On the other hand, colorless vectors are strongly constrained by meson mixing and resonance searches in $p p \to \tau^+ \tau^-$. In all cases we find that a flavour alignment close to the third generation is generically preferred.

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arXiv2023

Closing in on new chiral leptons at the LHC

We study the phenomenological viability of chiral extensions of the Standard Model, with new chiral fermions acquiring their mass through interactions with a single Higgs. We examine constraints from electroweak precision tests, Higgs physics and direct searches at the LHC. Our analysis indicates that purely chiral scenarios are perturbatively excluded by the combination of Higgs coupling measurements and LHC direct searches. However, allowing for a partial contribution from vector-like masses opens up the parameter space and non-decoupled exotic leptons could account for the observed 2$\sigma$ deviation in $h \to Z\gamma$. This scenario will be further tested in the high-luminosity phase of the LHC.

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arXiv2023

Perturbative unitarity constraints on generic vector interactions

We study perturbative unitarity constraints on generic interactions between fermion and vector fields, which are allowed to have generic quantum numbers under a $\prod_i SU(N_i) \otimes U(1)$ group. We derive compact expressions for the bounds on the couplings for the cases where the fields transform under the trivial, fundamental or adjoint representation of the various, considering both the case of a complex vector arbitrary interactions with fermionic current and also the case of vectors arising as gauge fields. We apply our results to some specific NP models showing the constraints that can be derived using the tool of perturbative unitarity.

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arXiv2022

An updated view on the ATOMKI nuclear anomalies

In view of the latest experimental results recently released by the ATOMKI collaboration, we critically re-examine the possible theoretical interpretation of the observed anomalies in terms of a new BSM boson $X$ with mass $\sim17\;$MeV. To this end we employ a multipole expansion method and give an estimate for the range of values of the nucleon couplings to the new light state in order to match the experimental observations. Our conclusions identify the axial vector state as the most promising candidate, while other spin/parity assignments seems disfavored for a combined explanation. This results is however based on an order of magnitude estimate for the, currently unknown, axial nuclear matrix element of the $^{12}$C transition, that needs then to be evaluated before being able to draw a definite conclusion. Intriguingly, an axial vector state can also simultaneously accommodate other experimental anomalies, {\emph{i.e.}} the KTeV anomaly in $\pi^0 \to e^+ e^-$ decay while being compatible with the conflicting measurements of the anomalous magnetic moment of the electron $(g-2)_e$ and other constraints on the electron couplings of the $X$ boson. The PADME experiment will completely cover the relevant region of the parameter space, thus allowing for a strong test of the existence of the $X$ particle.

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arXiv2022

Probing right-handed neutrinos dipole operators

We consider the minimal see-saw extension of the Standard Model with two right-handed singlet fermions $N_{1,2}$ with mass at the GeV scale, augmented by an effective dipole operator between the sterile states. We firstly review current bounds on this effective interaction from fixed-target and collider experiments as well as from astrophysical and cosmological observations. We then highlight the prospects for testing the decay $N_2 \to N_1 \gamma$ induced by the dipole at future facilities targeting long lived particles such as ANUBIS, CODEX-b, FACET, FASER 2, MAPP and SHiP.

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