arXiv subjects

Lorenzo Calibbi

Publications and source records attributed to Lorenzo Calibbi.

At least 19 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

Phenomenology of Non-Abelian Gauge and Goldstone Bosons in a U(2) Flavor Model

We investigate the phenomenological implications of the bosons associated with the $SU(2)_F$ subgroup in a simple and realistic $U(2)_F$ flavor model. While the Nambu-Goldstone boson of the $U(1)_F$ factor behaves as a standard QCD axion (an axiflavon) with suppressed flavor-violating couplings, the three degrees of freedom from $SU(2)_F$ have not been studied before. This work focuses on these states, considering both the case where $SU(2)_F$ is a global symmetry, yielding pseudo-Nambu-Goldstone bosons (PNGBs), and the case where it is a gauge symmetry with a potentially small coupling, yielding a triplet of (possibly) light gauge bosons. In both scenarios, these new bosons naturally feature unsuppressed flavor-violating couplings to Standard Model fermions in the mass basis. We derive the resulting predictions for flavor-changing neutral currents and lepton flavor violation, including exotic decays of mesons and leptons. Our analysis shows that processes like $K \to \pi X$ and $\mu \to e X$ place the most stringent constraints, probing the flavor symmetry breaking scale $v_\phi$ up to $10^{11}-10^{12}$~GeV for light bosons, while heavier states are tested in $B$ and $\tau$ decays, as well as by $K-\bar K$ mixing and $\mu\to e \gamma$. We demonstrate that low-energy flavor experiments provide a powerful probe of this framework, capable of testing ultra-high symmetry breaking scales that surpass the limits set by astrophysical observations.

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arXiv2025

Hunting for Neutrino Texture Zeros with Muon and Tau Flavor Violation

We revisit the minimal type II seesaw mechanism generating the Majorana neutrino mass matrix $M^{\nu}$, under the assumption that two entries of $M^{\nu}$ vanish. Such flavor structures are known as two-zero textures. Processes with charged lepton flavor violation (CLFV), absent in the Standard Model (SM), can have sizable rates in this framework and are directly linked to the flavor structure of $M^{\nu}$. For each allowed two-zero texture, we quantify the predicted correlations among various CLFV observables using current neutrino oscillation data and show that they lead to distinctive patterns of CLFV processes that could be discriminated between at running and upcoming experiments. In addition, together with information from colliders, the sensitivity of these correlations to renormalization group (RG) effects could shed light on the potentially ultra-high scale where new dynamics (e.g. some underlying flavor symmetry) give rise to the two-zero texture. Furthermore, we find that certain zero textures, although not third-generation specific, can suppress $\mu\to e$ transitions while allowing the rate of the process $\tau\to \bar\mu ee$ to be within the future experimental sensitivity, even when the RG evolution is taken into account. The lowest possible cut-off scale of the effective theory, constructed by treating the two-zero flavor structure of $M^{\nu}$ as a CLFV spurion, can therefore reach $5-6$ TeV. Our results provide further motivation for searches for $\tau$ CLFV at Belle II, as probes of new physics complementary to MEG II and the upcoming Mu3e, COMET, and Mu2e experiments, as well as for collider searches for doubly charged scalar bosons.

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arXiv2025

Selection rules for charged lepton flavour violating processes from residual flavour groups

We systematically investigate the possible phenomenological impact of residual flavour groups in the charged lepton sector. We consider all possible flavour charge assignments for abelian residual symmetries up to Z8. The allowed flavour structures of operators in Standard Model Effective Field Theory (up to dimension six) lead to distinctive and observable patterns of charged lepton flavour violating processes. We illustrate the relevance of such selection rules displaying the current bounds on and the future sensitivities to the new physics scale. These results demonstrate, in particular, the importance and discriminating power of searches for lepton flavour violating tau lepton decays and muonium to antimuonium conversion.

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arXiv2025

New Physics Search at the CEPC: a General Perspective

The Circular Electron-Positron Collider (CEPC), a proposed next-generation Higgs factory, provides new opportunities to explore physics beyond the Standard Model (SM). With its clean electron-positron collision environment and the ability to collect large samples of Higgs, W, and Z bosons, the CEPC enables precision measurements and searches for new physics. This white paper outlines the CEPC's discovery potential, including studies of exotic decays of the Higgs, Z, and top quarks, dark matter and dark sector phenomena, long-lived particles, supersymmetry, and neutrino-related signatures. Advanced detector technologies and reconstruction techniques, such as one-to-one correspondence reconstruction and jet origin identification, significantly improve sensitivity to rare and weakly interacting processes. The CEPC is particularly well suited to probe the electroweak phase transition and test models of electroweak baryogenesis and dark sector interactions. In addition, global fit analyses highlight the CEPC's complementary role in constraining a wide range of new physics scenarios. These features position the CEPC as a powerful tool for exploring the next frontier in fundamental particle physics in the post-Higgs discovery era.

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arXiv2025

t-channel dark matter at the LHC -- a whitepaper

This report, summarising work achieved in the context of the LHC Dark Matter Working Group, investigates the phenomenology of $t$-channel dark matter models, spanning minimal setups with a single dark matter candidate and mediator to more complex constructions closer to UV-complete models. For each considered class of models, we examine collider, cosmological and astrophysical implications. In addition, we explore scenarios with either promptly decaying or long-lived particles, as well as featuring diverse dark matter production mechanisms in the early universe. By providing a unified analysis framework, numerical tools and guidelines, this work aims to support future experimental and theoretical efforts in exploring $t$-channel dark matter models at colliders and in cosmology.

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arXiv2025

Is Dark Matter the origin of the $B\to K \nu\bar\nu$ excess at Belle II?

We present two models of dark matter (DM) that can provide a natural explanation of the excess of $B^+\to K^+ +\,\text{invisible}$ events with respect to the Standard Model (SM) prediction for $B^+\to K^+ \nu\bar\nu$, which has been reported by the Belle II collaboration. Interactions between the dark and the visible sector are mediated by an axion-like particle (ALP) in one case, by the kinetic mixing between a dark photon and the SM photon in the second case. Both models encompass a light fermion singlet as the DM candidate and can account for the observed DM relic abundance through, respectively, the freeze-in and the freeze-out production mechanism, while simultaneously explaining the Belle II excess.

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arXiv2024

Flavor Physics at the CEPC: a General Perspective

We discuss the landscape of flavor physics at the Circular Electron-Positron Collider (CEPC), based on the nominal luminosity outlined in its Technical Design Report. The CEPC is designed to operate in multiple modes to address a variety of tasks. At the $Z$ pole, the expected production of 4 Tera $Z$ bosons will provide unique and highly precise measurements of $Z$ boson couplings, while the substantial number of boosted heavy-flavored quarks and leptons produced in clean $Z$ decays will facilitate investigations into their flavor physics with unprecedented precision. We investigate the prospects of measuring various physics benchmarks and discuss their implications for particle theories and phenomenological models. Our studies indicate that, with its highlighted advantages and anticipated excellent detector performance, the CEPC can explore beauty and $\tau$ physics in ways that are superior to or complementary with the Belle II and Large-Hadron-Collider-beauty experiments, potentially enabling the detection of new physics at energy scales of 10 TeV and above. This potential also extends to the observation of yet-to-be-discovered rare and exotic processes, as well as testing fundamental principles such as lepton flavor universality, lepton and baryon number conservation, etc., making the CEPC a vibrant platform for flavor physics research. The $WW$ threshold scan, Higgs-factory operation and top-pair productions of the CEPC further enhance its merits in this regard, especially for measuring the Cabibbo-Kobayashi-Maskawa matrix elements, and Flavor-Changing-Neutral-Current physics of Higgs boson and top quarks. We outline the requirements for detector performance and considerations for future development to achieve the anticipated scientific goals.

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arXiv2024

Probing ALP Lepton Flavour Violation at $μ$TRISTAN

Axion-like particles (ALPs) with lepton flavour violating (LFV) interactions are predicted within a wide range of flavoured ALP models. The proposed $μ$TRISTAN high-energy $e^-μ^+$ and $μ^+μ^+$ collider will provide a good opportunity to explore flavour physics in the charged lepton sector. In this work, based on a model-independent effective Lagrangian describing the ALP leptonic interactions, we investigate the potential of $μ$TRISTAN to probe ALP LFV couplings. We analyse the testability of selected ALP production channels with potential sensitivity at $μ$TRISTAN, considering different beams and collision energies, including $e^- μ^+ \to a γ$, $e^- μ^+ \to e^- τ^+ a$, $μ^+ μ^+ \to μ^+ τ^+ a$, and $e^- μ^+ \to τ^- μ^+ a$. The produced ALP $a$ is either long-lived or can promptly decay to flavour violating or conserving charged lepton final states. In particular, combining the above LFV ALP production modes with a suitable LFV decay mode, one can identify signatures that are virtually free of Standard Model background. We show the resulting sensitivity of $μ$TRISTAN to LFV ALP couplings and compare it with multiple low-energy leptonic constraints and the future improvements thereof. We find that $μ$TRISTAN can be generally complementary to searches for low-energy LFV processes and measurements of the leptonic magnetic dipole moments and has the capability to explore unconstrained parameter space for ALP masses in the $\mathcal{O}(1)$ to $\mathcal{O}(100)$~GeV range. In the light ALP regime, however, the parameter space that $μ$TRISTAN is sensitive to, has been already excluded by low-energy searches for LFV decays.

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arXiv2024

Gravitational waves from cosmic strings in Froggatt-Nielsen flavour models

Gravitational waves (GW) are a powerful probe of the earliest moments in the Universe, enabling us to test fundamental interactions at energy scales beyond the reach of laboratory experiments. In this work, we assess the GW capability to probe the origin of the flavour sector of the Standard Model (SM). Within the context of Froggatt-Nielsen models of fermion masses and mixing based on a gauged $U(1)$ flavour symmetry, we investigate the formation of cosmic strings and the resulting stochastic GW background (GWB), estimating the sensitivity to the model's parameter space of future GW experiments. Comparing these results with the bounds from low-energy flavour observables, we find that these two types of experimental probes of the model are nicely complementary. Flavour physics observables can probe low to intermediate symmetry-breaking scales $v_\phi$, while future GW experiments are sensitive to the opposite regime, for which the string tension is large enough to yield sizeable GW signals, and in the long run can set an upper limit on the scale as stringent as $v_\phi \lesssim 10^9$ GeV. In certain scenarios, the combination of flavour constraints and future GW bounds can bring about a complete closure of the available parameter space, which illustrates how GWB searches can play an important role in testing the origin of the SM flavour sector even if that occurs at ultra-high energies.

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arXiv2023

Type-II Majoron Dark Matter

We discuss in detail the possibility that the ``type-II majoron'' -- that is, the pseudo Nambu-Goldstone boson that arises in the context of the type-II seesaw mechanism if the lepton number is spontaneously broken by an additional singlet scalar -- account for the dark matter (DM) observed in the universe. We study the requirements the model's parameters have to fulfill in order to reproduce the measured DM relic abundance through two possible production mechanisms in the early universe, freeze-in and misalignment, both during a standard radiation-dominated era and early matter domination. We then study possible signals of type-II majoron DM and the present and expected constraints on the parameter space that can be obtained from cosmological observations, direct detection experiments, and present and future searches for decaying DM at neutrino telescopes and cosmic-ray experiments. We find that -- depending on the majoron mass, the production mechanism, and the vacuum expectation value of the type-II triplet -- all of the three decay modes (photons, electrons, neutrinos) of majoron DM particles can yield observable signals at future indirect searches for DM. Furthermore, in a corner of the parameter space, detection of majoron DM is possible through electron recoil at running and future direct detection experiments.

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arXiv2023

Testing axion couplings to leptons in $Z$ decays at future $e^+e^-$ colliders

We study the possibility of probing the existence of a light, invisible, axion-like particle (ALP) $a$ in leptonic decays of the $Z$ boson at the proposed high-energy $e^+e^-$ colliders, CEPC and FCC-ee. Both projects plan to run at the $Z$ pole, collecting $10^{12}-10^{13}$ visible $Z$ decays. We show that, searching for the emission of an invisible ALP from leptons in leptonic $Z$ decays, this enormous statistics could allow to constrain the ALP couplings to leptons at an unprecedented level for laboratory experiments. In particular, within a Monte Carlo simulation framework, we estimate that CEPC/FCC-ee can be sensitive to the coupling of an invisible ALP to muons up to $f_a/C^A_{μμ} \approx 1$ TeV - where $f_a$ is the ALP decay constant - corresponding to ${\rm BR}(Z \to μ^+μ^-\,a) \approx 3\times 10^{-11}$.

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arXiv2022

Indirect constraints on lepton-flavour-violating quarkonium decays

Within an effective-field-theory framework, we present a model-independent analysis of the potential of discovering new physics by searching for lepton flavour violation in heavy quarkonium decays and, more in general, we study the phenomenology of lepton-flavour-violating (LFV) 2 quark - 2 lepton ($2q2\ell$) operators with two charm or bottom fields. We compute the constraints from LFV muon and tau decays on the new-physics operators that can induce LFV processes involving $c\bar c$ and $b\bar b$ systems, thus providing a comprehensive list of indirect upper limits on processes such as $J/ψ\to \ell\ell^\prime$, $Υ(nS) \to \ell\ell^\prime$, $Υ(nS) \to \ell\ell^\prime γ$ etc., which can be sought at BESIII, Belle II, and the proposed super tau-charm factory. We show that such indirect constraints are so stringent that they prevent the detection of quarkonium decays into $eμ$. In the case of decays of quarkonia into $\ellτ$ ($\ell=e,μ$), we find that an improvement by 2-3 orders of magnitude on the current sensitivities is in general required in order to discover or further constrain new physics. However, we show that cancellations among different contributions to the LFV tau decay rates are possible, such that $Υ(nS)\to \ellτ$ can saturate the present experimental bounds. We also find that, interestingly, searches for LFV $Z$ decays, $Z\to\ellτ$, at future $e^+e^-$ colliders are complementary probes of $2q2\ell$ operators with third generation quarks.

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arXiv2022

Lepton Flavour Violation in minimal grand-unified type II seesaw models

We revisit minimal non-supersymmetric models of SU(5) Grand Unification with the type II seesaw mechanism as the origin of neutrino masses. Imposing the requirement of gauge coupling unification and the proton lifetime bounds, we perform a Bayesian fit and obtain robust quantitative information on the mass scales of the beyond the Standard Model particles. We then study lepton-flavour-violating (LFV) processes induced by the type II scalar triplet and its SU(5) partners, showing that the interplay of upcoming searches for different LFV observables can provide additional information on the masses of the new particles, as well as non-trivial constraints on neutrino parameters.

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arXiv2022

Looking forward to Lepton-flavor-violating ALPs

We assess the status of past and future experiments on lepton flavor violating (LFV) muon and tau decays into a light, invisible, axion-like particle (ALP), $a$. We propose a new experimental setup for MEG II, the MEGII-fwd, with a forward calorimeter placed downstream from the muon stopping target. Searching for $μ\to e a$ decays MEGII-fwd is maximally sensitive to LFV ALPs, if these have nonzero couplings to right-handed leptons. The experimental set-up suppresses the (left-handed) Standard Model background in the forward direction by controlling the polarization purity of the muon beam. The reach of MEGII-fwd is compared with the present constraints, the reach of Mu3e and the Belle-II reach from $τ\to \ell a$ decays. We show that a dedicated experimental campaign for LFV muon decays into ALPs at MEG II and Mu3e will be able to probe the ALP parameter space in an unexplored region well beyond the existing astrophysical constraints. We study the implications of these searches for representative LFV ALP models, where the presence of a light ALP is motivated by neutrino masses, the strong CP problem and/or the SM flavor puzzle. To this extent we discuss the majoron in low-scale seesaw setups and introduce the LFV QCD axion, the LFV axiflavon and the leptonic familon, paying particular attention to the cases where the LFV ALPs constitute cold dark matter.

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arXiv2021

Z lepton flavour violation as a probe for new physics at future $e^+e^-$ colliders

In this work we assess the potential of discovering new physics by searching for lepton-flavour-violating (LFV) decays of the $Z$ boson, $Z\to \ell_i \ell_j$, at the proposed circular $e^+e^-$ colliders CEPC and FCC-ee. Both projects plan to run at the $Z$-pole as a Tera Z factory, i.e., collecting $\mathcal O\left(10^{12}\right)$ $Z$ decays. In order to discuss the discovery potential in a model-independent way, we revisit the LFV $Z$ decays in the context of the Standard Model effective field theory and study the indirect constraints from LFV $μ$ and $τ$ decays on the operators that can induce $Z\to \ell_i \ell_j$. We find that, while the $Z\to μe$ rates are beyond the expected sensitivities, a Tera Z factory is promising for $Z\to τ\ell$ decays, probing New Physics at the same level of future low-energy LFV observables.

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arXiv2021

$B$ anomalies and muon $g-2$ from Dark Matter

Motivated by the result of the Muon g-2 experiment and the long-standing anomalies in semileptonic $B$ meson decays, we systematically build a class of minimal models that can address both experimental results thanks to the contributions of a set of new fields that include a thermal Dark Matter candidate.

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arXiv2021

Implications of the Muon g-2 result on the flavour structure of the lepton mass matrix

The confirmation of the discrepancy with the Standard Model predictions in the anomalous magnetic moment by the Muon g-2 experiment at Fermilab points to a low scale of new physics. Flavour symmetries broken at low energies can account for this discrepancy but these models are much more restricted, as they would also generate off-diagonal entries in the dipole moment matrix. Therefore, if we assume that the observed discrepancy in the muon $g-2$ is explained by the contributions of a low-energy flavor symmetry, lepton flavour violating processes can constrain the structure of the lepton mass matrices and therefore the flavour symmetries themselves predicting these structures. We apply these ideas to several discrete flavour symmetries popular in the leptonic sector, such as $Δ(27)$, $A_4$, and $A_5 \ltimes {\rm CP}$.

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