arXiv subjects

Marco Ardu

Publications and source records attributed to Marco Ardu.

14 recordsLinked to original sources

arXiv2026

ALP production in Lepton Flavour Violating meson, tau and gauge boson decays

In this paper we study axion-like particles (ALPs) with lepton-flavour-violating (LFV) couplings in the mass regime above the muon threshold, $m_a>m_\mu$, where the strong bound from the exotic muon decay $\mu\to ea$ no longer apply and the decay channel $a\to e\mu$ becomes kinematically accessible. In this region, the ALP typically decays promptly, motivating new search strategies based on its production in decays involving virtual muons. We analyse charged-meson and $W$ decays, neutral-current processes such as $Z$ and quarkonium decays, and, when couplings to the third generation are present, LFV $\tau$ decays. The subsequent decay $a\to e\mu$ leads to striking LFV signatures with negligible Standard Model backgrounds. Combining these production modes with current low-energy constraints, we assess the sensitivity of future high-energy $e^+e^-$ colliders, flavour factories such as Belle II and STCF, fixed-target experiments such as NA62, and proton beam-dump facilities such as SHiP. Overall, our results identify LFV ALP production in meson, gauge-boson, quarkonium and $\tau$ decays (with displaced vertices) as a promising and largely unexplored avenue to test ALP interactions with charged leptons above the muon mass threshold.

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arXiv2025

Left-Handed Physics is not right for EDMs

Heavy New Physics models with lepton flavour-changing interactions are motivated by neutrino masses, and generically induce dipole interactions for leptons, which can be flavour-changing ($l_j\to l_i \gamma$) or flavour-diagonal (magnetic and electric dipole moments(edms)). We focus on models with complex couplings, and where the singlet Standard Model leptons ($\{e_R^i\}$) do not interact with the New Physics. In such models, edms are calculated to arise at two loops, despite that complex amplitudes for $l_j\to l_i \gamma$ appear at one loop. We explore whether the extra loop suppression of edms survives flavour basis rotations that could be induced by flavour-changing NP contributions to the charged lepton mass matrix. We show that one-loop edms vanish in both the mass and Yukawa eigenstate bases.

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arXiv2025

The equivalent Electric Dipole Moment in SMEFT

The Electric Dipole Moment of the electron (eEDM) is typically investigated in experiments using paramagnetic molecules. However, the physical observable in these searches consists in a linear combination of CP-violating interactions, rather than the eEDM alone, which is commonly referred to as the equivalent EDM of the system. Assuming the presence of new CP-odd physics from heavy degrees of freedom, we parameterize its effects within the Standard Model Effective Field Theory (SMEFT) framework. We systematically compute the contributions to the full low-energy direction probed by EDM searches, focusing on leading-order effects at dimension six and one-loop level, while also discussing selected two-loop and dimension-eight contributions. We find that eEDM experiments are sensitive to a broader class of SMEFT operators than previously recognized.

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arXiv2025

Asymmetric Dark Matter in SUSY with approximate $R-$symmetry

We implement the asymmetric dark matter framework, linking the ordinary and dark matter abundances, within a supersymmetric context. We consider a supersymmetric model that respects an approximate $U(1)_R$ symmetry, which is broken in such a way that at high temperature the $R$ breaking sector mediate processes in equilibrium, but at the SUSY mass scale, the sparticles asymmetry is frozen. In this framework, the gravitino serves as the dark matter candidate, and its mass is predicted to be $\sim10$ GeV to match the observed relic abundance. We identify several realistic spectra; however, the requirement for the Next-to-Lightest Supersymmetric Particle (NLSP) to decay into the gravitino before Big Bang Nucleosynthesis constrains the viable spectrum to masses above 2 TeV.

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arXiv2024

Electric Dipole Moments as indirect probes of Dark Sectors

Dark sectors provide beyond Standard Model scenarios which can address unresolved puzzles, such as the observed dark matter abundance or the baryon asymmetry of the Universe. A naturally small portal to the dark sector is obtained if dark-sector interactions stem from a non-Abelian hidden gauge group that couples through kinetic mixing with the hypercharge boson. In this work, we investigate the phenomenology of such a portal of dimension five in the presence of CP violation, focusing on its signatures in fermion electric dipole moments. We show that, currently unbounded regions of the parameter space from dark photon searches can be indirectly probed with upcoming electron dipole moment experiments for dark boson masses in the range $1-100$ GeV. We also discuss two particular scenarios where a $SU(2)_D$ dark gauge group spontaneously breaks into either an Abelian $U(1)_D$ or nothing. In both cases, we show that potentially observable electron dipole moments can be produced in vast regions of the parameter space compatible with current experimental constraints and observed dark matter abundance.

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arXiv2024

Completing the one-loop $\nu$SMEFT Renormalization Group Evolution

In this work we consider the Standard Model Effective Field Theory extended with right-handed neutrinos, the $\nu$SMEFT, and calculate the full set of one-loop anomalous dimensions that are proportional to Yukawa couplings. These contributions are particularly relevant when symmetry-protected low scale seesaw models are embeded in the SMEFT, since large neutrino Yukawa couplings are expected. By combining our results with the already available gauge anomalous dimensions, we provide the complete set of one-loop renormalization group evolution equations for the dimension six $\nu$SMEFT. As a possible phenomenological implication of our results, we discuss the sensitivity of lepton flavor-violating observables to $\nu$SMEFT operators, focusing on the more sensitive $\mu\to e$ transitions.

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arXiv2024

Constraining New Physics models from $\mu\to e$ observables in bottom-up EFT

Upcoming experiments will improve the sensitivity to $\mu\to e$ processes by several orders of magnitude, and could observe lepton flavour-changing contact interactions for the first time. In this paper, we investigate what could be learned about New Physics from the measurements of these $\mu\to e$ observables, using a bottom-up effective field theory (EFT) approach and focusing on three popular models with new particles around the TeV scale (the type II seesaw, the inverse seesaw and a scalar leptoquark). We showed in a previous publication that $\mu\to e$ observables have the ability to rule out these models because none can fill the whole experimentally accessible parameter space. In this work, we give more details on our EFT formalism and present more complete results. We discuss the impact of some observables complementary to $\mu\to e$ transitions (such as the neutrino mass scale and ordering, and LFV $\tau$ decays) and draw attention to the interesting appearance of Jarlskog-like invariants in our expressions for the low-energy Wilson coefficients.

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arXiv2023

Distinguishing models with $\mu \to e $ observables

Upcoming experiments will improve the reach for the lepton flavour violating (LFV) processes $\mu \to e \gamma$, $\mu \to e \bar{e} e$ and $\mu A \to e A$ by orders of magnitude. We investigate whether this upcoming data could rule out some popular TeV-scale LFV models (the type II seesaw, the inverse seesaw and a scalar leptoquark) using a bottom-up EFT approach involving twelve Wilson coefficients that can in principle all be determined by experimental measurements. In this 12-dimensional coefficient space, each model can only predict points in a specific subspace; for instance, flavour change involving singlet electrons is suppressed in the seesaw models, and the leptoquark induces negligible coefficients for 4-lepton scalar operators. Using the fact that none of these models can populate the whole region accessible to upcoming experiments, we show that $\mu \to e$ experiments have the ability to rule them out.

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arXiv2022

Semi-inclusive Lepton Flavor Universality ratio in $b\to s\ell^+\ell^-$ transitions

We construct a semi-inclusive Lepton Flavour Universality (LFU) ratio, $R_Σ$, to test $μ/e$ universality in $b\to s\ell^+\ell^-$ transitions at $e^+e^-$ $B$-meson factories. Combining different decay channels, this observable maximises the sensitivity to possible LFU violations of short distance origin, yet preserving a clean theoretical interpretation in case of a deviation from its Standard Model prediction, $R_Σ^{\rm SM}=1$.

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arXiv2022

A viable $L_e-L_\mu$ model with $\mu\to e$ violation

We extend the Standard Model gauge group by $U(1)_{L_e-L_\mu}$ and introduce two scalars, a doublet and a singlet, that are charged under this new group and have lepton flavour violating couplings. Since in this model $\mu \to e$ processes can only be mediated by $\mu\to \tau\times \tau\to e$ interactions, bounds from $\mu\to e$ transitions can be avoided while allowing for accessible new physics. We consider the case of a $Z'$ boson with a mass of $M_{Z'}\simeq10~$GeV and a gauge coupling $g'\simeq 10^{-4}$, which is in reach of Belle-II, and a long-lived $Z'$ boson with a mass of $\text{MeV}\lesssim M_{Z'}\lesssim m_\mu-m_e$ which can be probed by searching for $\ell\to \ell'+\text{inv.}$. Neutrino masses and mixing angles can also be accounted for if sterile neutrinos are added to the spectrum.

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arXiv2022

Introduction to Charged Lepton Flavour Violation

Neutrino masses provide evidence of lepton flavour violation, but no violation in the interactions among the charged leptons has been observed yet. Many models of Physics Beyond the Standard Model (BSM) predict Charged Lepton Flavour Violation (CLFV) in a wide spectrum of processes with rates in reach of upcoming experiments. The experimental searches that provide the current best limits on the CLFV searches are reviewed, with a particular emphasis on the muon-based experiments that give the most stringent constraints on BSM parameter space. The next generation of muon-based experiments (MEG-II, Mu2e, COMET, Mu3e) aim to reach improvements by many orders of magnitude w.r.t. the current best limits, thanks to several technological advancements. We review popular heavy BSM theories, and we present the calculations of the predicted CLFV branching ratios, focusing on the more sensitive $\mu\to e$ sector.

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arXiv2022

The sensitivity of $μ\to e$ processes to $τ$ flavour change

Transforming a $μ$ to a $τ$, then the $τ$ to to an $e$, results in $μ\to e$. In an EFT framework, we explore the sensitivity of $μ\to e$ observables to products of $(μ\to τ)\times (τ\to e)$ interactions, and show that the exceptional sensitivity of upcoming $μ\to e$ experiments could allow to probe parameter space beyond the reach of upcoming $τ\to l$ searches in Higgs, $τ$ and $B$ decays. We describe the $τ\to l$ interactions as dimension six operators in the SM EFT, identify pairs of them giving interesting contributions to $μ\to e$ processes, and obtain the anomalous dimensions mixing those pairs into dimension eight $μ\to e$ operators. We find that $μ\to e$ processes are sensitive to $τ$ flavour-changing $B$ decays at rates comparable to current $B$ anomalies, but cannot reduce rates -- as appropriate in many current $B$ anomalies -- because they do not interfere with the SM.

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arXiv2021

What is Leading Order for LFV in SMEFT?

Upcoming searches for lepton flavour change (LFV) aim to probe New Physics(NP) scales up to $ \sim 10^4$ TeV, implying that they will be sensitive to NP at lower scales that is suppressed by loops or small couplings. We suppose that the NP responsable for LFV is beyond the reach of the LHC and can be parametrised in Effective Field Theory, introduce a small power-counting parameter à la Cabibbo-Wolfenstein, and assess whether the existing dimension six operator basis and one-loop RGEs provide a good approximation for LFV. We find that mu to e flavour-changing observables can be sensitive to a few dozen dimension eight operators, and to some effects of two-loop anomalous dimensions, for NP scales below 20-100 TeV. We also explore the effect of some simplifying assumptions in the one-loop RGEs, such as neglecting flavour-changing effects.

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arXiv2021

Axion quality from the (anti)symmetric of SU(N)

We propose two models where a U(1) Peccei-Quinn global symmetry arises accidentally and is respected up to high-dimensional operators, so that the axion solution to the strong CP problem is successful even in the presence of Planck-suppressed operators. One model is SU$(N)$ gauge interactions with fermions in the fundamental and a scalar in the symmetric. The axion arises from spontaneous symmetry breaking to SO$(N)$, that confines at a lower energy scale. Axion quality in the model needs $N \gtrsim 10$. SO bound states and possibly monopoles provide extra Dark Matter candidates beyond the axion. In the second model the scalar is in the anti-symmetric: SU$(N)$ broken to Sp$(N)$ needs even $N \gtrsim 20$. The cosmological DM abundance, consisting of axions and/or super-heavy relics, can be reproduced if the PQ symmetry is broken before inflation (Boltzmann-suppressed production of super-heavy relics) or after (super-heavy relics in thermal equilibrium get partially diluted by dark glue-ball decays).

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