arXiv subjects

Robert Ziegler

Publications and source records attributed to Robert Ziegler.

At least 19 recordsLinked to original sources

arXiv2026

Flavor-violating dark matter at MEG-II and Mu3e

We investigate the potential of high-intensity muon experiments such as MEG~II and Mu3e to uncover dark matter (DM) through the lepton-flavor-violating decay $\mu^+\to e^+\chi\bar\chi$. We describe the underlying interactions in terms of dimension-six four-fermion operators and systematically explore all allowed Lorentz structures. We show that precision measurements of the Michel spectrum can probe new-physics scales of ${\cal O}({\rm TeV})$ for DM masses above approximately $1~{\rm MeV}$. For lighter DM, whose signal is confined close to the Michel endpoint, the radiative decay $\mu^+\to e^+\chi\bar\chi\gamma$ at MEG~II opens a complementary window, with sensitivity comparable to and, in some regions, stronger than that of the non-radiative channel. Remarkably, for reheating temperatures below the tens-of-MeV scale but above the BBN bound, MEG~II and Mu3e can probe regions in which freeze-in through the very same LFV interactions accounts for the observed DM abundance.

hep-ph↗

arXiv2026

Flavor phenomenology of light dark particles

We review the flavor phenomenology of light dark particles, focusing on axion-like particles with sub-GeV masses and generic flavor-violating couplings. Such states can naturally emerge from the spontaneous breaking of generic flavored symmetries, and are motivated by dark matter or the Strong CP Problem, with the QCD axion serving as a paradigmatic example. Light dark particles can be produced in two-body decays of Standard Model particles, giving rise to missing energy signals that can not only be observed in high-precision flavor experiments, but also be probed in core-collapse supernovae and the cosmic microwave background. These decays are controlled by dimension-five operators, which makes dedicated laboratory searches sensitive to very large UV scales up to $10^{12} {\rm GeV}$ and thus highly complementary to astrophysical and cosmological probes. We provide a comprehensive survey of the resulting limits and prospects across all relevant channels, highlighting the central role of flavor physics in exploring the landscape of light dark matter.

hep-ph↗

arXiv2025

On the decay of a light spinless particle into two photons

We analyze the effective couplings of a light, spinless, gauge-singlet particle $\phi$ to on-shell photons. Starting from the most general theory at the electroweak scale, which allows for CP-violating interactions suppressed by inverse powers of an ultraviolet scale $\Lambda$, we derive the corresponding low-energy effective theory valid below the GeV scale. Within this framework, we systematically expand the effective couplings of $\phi$ to on-shell photons in powers of small parameters. Working at the one-loop level, we retain terms at first order in $1/\Lambda$. We incorporate both isospin-breaking effects and $\eta$-$\eta'$ mixing, and provide explicit expressions for the couplings up to first order in $m_\phi^2/m_\eta^2$ and $m_\pi^2/m_\eta^2$. As applications, we compute the decay rate of $\phi$ into two photons and illustrate our results in several physically motivated scenarios.

hep-ph↗

arXiv2025

The SN 1987A Cooling Bound on Dark Matter Absorption in Electron Targets

We present new supernova (SN 1987A) cooling bounds on sub-MeV fermionic dark matter with effective couplings to electrons. These bounds probe the parameter space relevant for direct detection experiments in which dark matter can be absorbed by the target material, showing strong complementarity with indirect searches and constraints from dark matter overproduction. Crucially, our limits exclude the projected sensitivity regions of current and upcoming direct detection experiments. Since these conclusions are a priori not valid for light mediators, we extend our analysis to this case. We show that sub-GeV mediators can be produced resonantly both in supernova cores and in the early Universe, altering the SN 1987A analysis for effective couplings. Still, a combination of supernova cooling constraints and limits from dark matter overproduction excludes the entire parameter space relevant for direct detection in this case.

hep-ph↗

arXiv2025

Leptophilic ALPs in Laboratory Experiments

We study the collider phenomenology of leptophilic axion-like particles (ALPs), i.e. pseudoscalar particles that couple only to charged leptons. Loops of charged leptons induce effective interactions of the ALPs with photons, which depend on the momenta of the interacting particles and differ between pseudoscalar and derivative lepton couplings. We systematically discuss the form of the interaction with photons for general external momenta and identify the regimes when it can be safely approximated by an effective coupling constant. We use these results to derive novel constraints from LEP and calculate state-of-the-art limits from E137 and NA64 for four different scenarios, in which the ALPs couple either to a single lepton generation or universally to all, for both pseudoscalar and derivative lepton couplings. We collect complementary bounds from astrophysics, flavour, and other laboratory experiments to chart the allowed parameter space of leptophilic ALPs in the MeV-GeV mass range.

hep-ph↗

arXiv2025

Flavor phenomenology of light dark sectors

The dark sector offers a compelling theoretical framework for addressing the nature of dark matter while potentially solving other fundamental problems in physics. This review focuses on light dark flavored sector models, which are those where the flavor structure of the interactions with the standard model is non-trivial and distinguish among different fermion families. Such scenarios feature flavor violation leading to unique experimental signatures, such as flavor-changing neutral current decays of heavy hadrons (kaons, $D$ and $B$ mesons, baryons) and leptons (muons and taus) with missing energy carried away by light dark-sector particles. In this article, we review their motivation, summarize current constraints, highlight discovery opportunities in ongoing and future flavor experiments, and discuss implications for astrophysics and cosmology.

hep-ph↗

arXiv2025

Fully-Decentralized MADDPG with Networked Agents

In this paper, we devise three actor-critic algorithms with decentralized training for multi-agent reinforcement learning in cooperative, adversarial, and mixed settings with continuous action spaces. To this goal, we adapt the MADDPG algorithm by applying a networked communication approach between agents. We introduce surrogate policies in order to decentralize the training while allowing for local communication during training. The decentralized algorithms achieve comparable results to the original MADDPG in empirical tests, while reducing computational cost. This is more pronounced with larger numbers of agents.

cs.LG↗

arXiv2024

CPon Dark Matter

We study a class of supersymmetric models where the strong CP problem is solved through spontaneous CP violation, carried out by a complex scalar field that determines the Yukawa couplings of the theory. Assuming that one real component of this field - the CPon - is light, we examine the conditions under which it provides a viable Dark Matter candidate. The CPon couplings to fermions are largely determined by the field-dependent Yukawa interactions, and induce couplings to gauge bosons at 1-loop. All couplings are suppressed by an undetermined UV scale, which needs to exceed $10^{12}$ GeV in order to satisfy constraints on excessive stellar cooling and rare kaon decays. The CPon mass is limited from below by 5th force experiments and from above by X-ray telescopes looking for CPon decays to photons, leaving a range roughly between 10 meV and 1 MeV. Everywhere in the allowed parameter space the CPon can saturate the observed Dark Matter abundance through an appropriate balance of misalignment and freeze-in production from heavy SM fermions.

hep-ph↗

arXiv2024

Flavor Phenomenology of Light Dark Vectors

Light dark matter with flavor-violating couplings to fermions may be copiously produced in the laboratory as missing energy from decays of SM particles. Here we study the effective Lagrangian of a light dark vector with generic dipole or vector couplings. We calculate the resulting two-body decay rates of mesons, baryons and leptons as a function of the dark vector mass and show that existing experimental limits probe UV scales as large as $10^{12} \,\mathrm{GeV}$. We also derive the general RGEs in order to constrain the flavor-universal UV scenario, where all flavor violation arises radiatively proportional to the CKM matrix.

hep-ph↗

arXiv2024

Axion Dark Matter from Heavy Quarks

We propose simple scenarios where the observed dark matter abundance arises from decays and scatterings of heavy quarks through freeze-in of an axion-like particle with mass in the $10 {\rm \, keV} - 1 {\rm \, MeV}$ range. These models can be tested by future X-ray telescopes, and in some cases will be almost entirely probed by searches for two-body decays $K \to π+ {\rm invis.}$ at NA62. As a byproduct, we discuss the cancellation of IR divergencies in flavor-violating scattering processes relevant for thermal axion production, and derive the general contribution to axion-photon couplings from all three light quarks.

hep-ph↗

arXiv2024

A Leptonic ALP Portal to the Dark Sector

We discuss the leptonic ALP portal as a simple scenario that connects observed discrepancies in anomalous magnetic moments to the Dark Matter relic abundance. In this framework an axion-like particle in the multi-MeV range couples to SM leptons and a DM fermion, with mass above the ALP mass but below a GeV. The ALP contributes to $(g-2)_μ$ and $(g-2)_e$ dominantly through 2-loop Barr-Zee diagrams, while the DM abundance is generated by $p$-wave annihilation to ALP pairs. Constraints from beam-dump experiments, colliders and CMB probes are very stringent, and restrict the viable parameter space to a rather narrow region that will be tested in the near future.

hep-ph↗

arXiv2024

Thermal production of astrophobic axions

Hot axions are produced in the early Universe via their interactions with Standard Model particles, contributing to dark radiation commonly parameterized as $ΔN_{\text{eff}}$. In standard QCD axion benchmark models, this contribution to $ΔN_{\text{eff}}$ is negligible after taking into account astrophysical limits such as the SN1987A bound. We therefore compute the axion contribution to $ΔN_{\text{eff}}$ in so-called astrophobic axion models characterized by strongly suppressed axion couplings to nucleons and electrons, in which astrophysical constraints are relaxed and $ΔN_{\text{eff}}$ may be sizable. We also construct new astrophobic models in which axion couplings to photons and/or muons are suppressed as well, allowing for axion masses as large as few eV. Most astrophobic models are within the reach of CMB-S4, while some allow for $ΔN_{\text{eff}}$ as large as the current upper bound from Planck and thus will be probed by the Simons Observatory. The majority of astrophobic axion models predicting large $ΔN_{\text{eff}}$ is also within the reach of IAXO or even BabyIAXO.

hep-ph↗

arXiv2023

Supernova Limits on Muonic Dark Forces

Proto-neutron stars formed during core-collapse supernovae are hot and dense environments that contain a sizable population of muons. If these interact with new long-lived particles with masses up to roughly 100 MeV, the latter can be produced and escape from the stellar plasma, causing an excessive energy loss constrained by observations of SN 1987A. In this article we calculate the emission of light dark fermions that are coupled to leptons via a new massive vector boson, and determine the resulting constraints on the general parameter space. We apply these limits to the gauged $L_\mu-L_\tau$ model with dark fermions, and show that the SN 1987A constraints exclude a significant portion of the parameter space targeted by future experiments. We also extend our analysis to generic effective four-fermion operators that couple dark fermions to muons, electrons, or neutrinos. We find that SN 1987A cooling probes a new-physics scale up to $\sim7$ TeV, which is an order of magnitude larger than current bounds from laboratory experiments.

hep-ph↗

arXiv2023

New Physics Searches at Kaon and Hyperon Factories

Rare meson decays are among the most sensitive probes of both heavy and light new physics. Among them, new physics searches using kaons benefit from their small total decay widths and the availability of very large datasets. On the other hand, useful complementary information is provided by hyperon decay measurements. We summarize the relevant phenomenological models and the status of the searches in a comprehensive list of kaon and hyperon decay channels. We identify new search strategies for under-explored signatures, and demonstrate that the improved sensitivities from current and next-generation experiments could lead to a qualitative leap in the exploration of light dark sectors.

hep-ph↗

arXiv2023

Flavor Probes of Axion Dark Matter

Standard Model extensions with light axions are well-motivated by the observed Dark Matter abundance and the Peccei-Quinn solution to the Strong CP Problem. In general such axions can have large flavor-violating couplings to SM fermions, which naturally arise in scenarios where the Peccei-Quinn symmetry also explains the hierarchical pattern of fermion masses and mixings. I will discuss how these couplings allow for efficient axion production from the decays of SM particles, giving the opportunity to probe flavored axion Dark Matter with precision flavor experiments, astrophysics and cosmology.

hep-ph↗

arXiv2023

Axion Dark Matter from Lepton flavor-violating Decays

We propose simple scenarios where lepton flavor-violating couplings generate the observed dark matter abundance through freeze-in of an axion-like particle with mass in the few keV range. Compared to flavor-diagonal freeze-in, this mechanism enhances dark matter stability, softens stellar cooling constraints and improves the experimental sensitivity of accelerator-based searches. These scenarios can be tested by future X-ray telescopes, and in some cases will be almost entirely probed by new searches for lepton flavor violation at high-intensity experiments such as Mu3e and MEG II.

hep-ph↗

arXiv2022

The Charged Higgs from the Bottom-Up: Probing Flavor at the LHC

We systematically study model-independent constraints on the three generic charged Higgs couplings to $b$-quarks and up-type quarks. While existing LHC searches have focussed on the $tb$ coupling, we emphasize that the LHC plays a crucial role in probing also $ub$ and $cb$ couplings, since constraints from flavor physics are weak. In particular we propose various new searches that can significantly extend the present reach on the parameter space by: i) looking for light charged Higgses that decay into $ub$-quarks, ii) probing charged Higgs couplings to light and top quarks using multi-$b$-jet signatures, iii) looking for single $b$-quarks in low-mass dijet searches, iv) searching for charge asymmetries induced by charged Higgs production via $ub$ couplings.

hep-ph↗

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.

hep-ph↗