arXiv subjects

Manfred Lindner

Publications and source records attributed to Manfred Lindner.

At least 19 recordsLinked to original sources

arXiv2026

Dark matter in scale-invariant gravity with hidden-sector condensation

The origin of the electroweak scale, cosmic inflation, and dark matter are often treated as independent problems beyond the Standard Models of particle physics and cosmology. In this work, we explore the possibility that they instead arise from a common underlying framework based on classically scale-invariant quadratic gravity coupled to a strongly interacting hidden sector. The $R^2$ term naturally realizes Starobinsky inflation, while confinement in the hidden sector dynamically generates the Planck scale and triggers electroweak symmetry breaking through a gravitationally induced Higgs mass generation mechanism. The scalar degree of freedom associated with the $R^2$ term subsequently reheats both the visible and hidden sectors through universal couplings to the energy-momentum tensor, leading to the gravitational freeze-in production of hidden-sector states. We investigate three representative realizations of the hidden sector in which the dark matter candidate is either a hidden $\eta'$ meson, a hidden vector boson, or charged hidden pions, and derive the corresponding dark matter and dark radiation relic abundances.

hep-ph↗

arXiv2026

Search for Axions and Dark Photons Using Single Molecule Magnets

Molecular magnets, although analogous to familiar macroscopic magnets, offer a platform for next generation magnetic storage technologies with far higher data densities and prospective applications in quantum information science. When exposed to an external magnetic field, single molecule magnets enter a frustrated magnetic configuration that is exceptionally sensitive to low energy excitations. Energy deposited by a dark matter particle can trigger the relaxation of a metastable molecule, releasing Zeeman energy that subsequently propagates through neighboring molecules. This magnetic avalanche encodes the energy deposited in the initial excitation. By combining concepts from chemistry, condensed matter physics, and particle physics, we show that dysprosium and manganese molecules can achieve more than an order of magnitude improvement in sensitivity to dark photon and QCD axion models, respectively, compared with existing detection methods.

hep-ph↗

arXiv2025

Testing lepton non-unitarity with the next generation of Germanium-based CE$\nu$NS reactor experiments

Coherent elastic neutrino-nucleus scattering (CE$\nu$NS) has been experimentally confirmed using neutrinos from pion decay at rest, solar neutrinos and reactor antineutrinos. Future CE$\nu$NS experiments will foreseeable lead to precision measurements which will be a powerful tool to search for new physics beyond the Standard Model. In this work, we investigate possible deviations from unitarity in the $3\times3$ leptonic mixing matrix that controls the propagation of active neutrinos. Such deviations may originate from the mixing with additional gauge singlet fermions and depending on their mass scale and mixing, the resulting phenomenology can differ substantially. We explore two well-motivated regimes: the \textit{seesaw limit}, where the new fermions are heavy and kinematically inaccessible, leading to effective deviations from unitarity in the active sector; and the \textit{light sterile limit}, where they are light enough to be produced and participate in neutrino propagation and scattering processes. We show how these scenarios modify both CE$\nu$NS and elastic neutrino--electron scattering (E$\nu e$S), and we present the corresponding sensitivity projections for a future CE$\nu$NS reactor experiment obtained by upscaling the CONUS+ experiment, which reported the first observation of reactor CE$\nu$NS. We identify the leading experimental systematics relevant for such an upscaling and demonstrate the resulting capability to probe TeV-scale new physics. Our results highlight the strong potential of CE$\nu$NS to test the structure of the lepton sector and to search for physics beyond the Standard Model.

hep-ph↗

arXiv2025

Neutrinoless Double Beta Decay in Light of JUNO First Data

The first results from the JUNO reactor neutrino oscillation experiment improve our knowledge of neutrino masses and mixing parameters, especially the solar angle $\theta_s \equiv \theta_{12}$ and the solar mass squared difference $\Delta m^2_s \equiv \Delta m^2_{21}$. We discuss the implications of these results on neutrinoless double beta decay by itself and in combination with the global fit of neutrino oscillation experiments, the JUNO first data, and cosmological constraints on the neutrino mass sum. For the effective mass $\langle m_{ee} \rangle$, the uncertainties in its lower limits for both mass orderings and upper limits for the normal ordering are largely reduced. Since the cosmological CMB and DESI BAO data put a stringent constraint on the neutrino mass scale, we also show how the probability distribution of both the real and imaginary parts of the effective mass $\langle m_{ee} \rangle$ on the complex plane is affected. Especially, the funnel region with $|\langle m_{ee} \rangle| \lesssim 1$\,meV receives larger chance to happen. Correspondingly, the chance of determining the two Majorana CP phases simultaneously in this region also increases with reduced uncertainty.

hep-ph↗

arXiv2025

Primordial Dirac Leptogenesis

We present a novel realization of Dirac leptogenesis based on the post-inflationary reheating phase of the early universe. An asymmetry generated within the scalar sector via CP-violating and out-of-equilibrium inflaton decays is transferred to chiral neutrinos through Yukawa interactions and then to baryons via electroweak sphalerons. We describe in detail a minimal realization of this mechanism that naturally accommodates small neutrino Yukawa couplings and results in contributions to the effective number of relativistic species, $N_{\text{eff}}$, testable in upcoming cosmological observations.

hep-ph↗

arXiv2025

Gravity and the Hierarchy Problem

We propose a mechanism where the dynamical generation of the Planck mass in scale invariant gravity leads to Einstein gravity, successful inflation and an explanation of the hierarchy problem of the Standard Model. We will discuss the scale generation by dynamical symmetry breaking and phenomenological consequences.

hep-ph↗

arXiv2025

Hidden Sector Custodial Naturalness

Custodial Naturalness is a recently introduced idea that combines conformal and scalar-sector custodial symmetry to address the electroweak (EW) scale hierarchy problem of the Standard Model (SM). We introduce a new model that realizes Custodial Naturalness without extension of the SM gauge group. The number of new dynamical degrees of freedom is minimized and the custodial symmetry is reduced to $\mathrm{SO}(5)$. This requires a new scalar singlet field that automatically is a good Dark Matter (DM) candidate, produced via freeze-in with moderate couplings. The most minimal scenario allows the quantum critical generation of the EW scale in a phenomenologically viable way requiring a UV completion at around $10^{11}\,\mathrm{GeV}$. Including ingredients for neutrino mass generation can push this scale to $M_{\mathrm{Pl}}$.

hep-ph↗

arXiv2025

Beyond the Veil: Charting WIMP Territories at the Neutrino Floor

We establish comprehensive theoretical benchmarks for Weakly Interacting Massive Particles (WIMPs) accessible to ultimate direct detection experiments, focusing on the challenging parameter space between current experimental limits and the irreducible neutrino background. We systematically examine both thermal freeze-out and freeze-in production mechanisms across a range of simplified dark matter models, including s-channel scalar and vector portals, t-channel mediator scenarios, and electroweakly interacting multiplets. For thermal relics, we identify parameter regions where suppressed direct detection cross-sections naturally arise through momentum-dependent interactions and blind-spot configurations, while maintaining the correct relic abundance. We extensively investigate freeze-in scenarios, demonstrating how feebly interacting massive particles (FIMPs) in portal models can populate experimentally accessible parameter space despite their ultra-weak couplings. Additionally, we explore how non-standard cosmological histories including early matter domination and fast-expanding Universe scenarios can dramatically alter the relationship between relic density and detection prospects, opening new avenues for discovery. Our analysis provides a roadmap for next-generation experiments approaching the neutrino floor, highlighting complementary detection strategies and identifying the most promising theoretical targets for ultimate sensitivity dark matter searches. These benchmarks establish the theoretical foundation for the final push toward comprehensive coverage of well-motivated WIMP parameter space.

hep-ph↗

arXiv2025

The quantum criticality of the Standard Model and the hierarchy problem

The naturalness principle has long guided efforts to understand physics beyond the Standard Model, with the hierarchy problem as the central issue. We revisit the role of quantum corrections in the fine-tuning of the low-energy effective description and its phase structure. We implement, for the first time in this context, the full Standard Model within the Wilsonian functional renormalization group. Crucially, this method captures conveniently both logarithmic and quadratic scalings, which must both be considered in the tuning, and allows us to provide a new generic and quantitative study of fine-tuning and its interpretation in terms of critical phenomena. We emphasize on the connection between the hierarchy problem and the near-criticality of the Standard Model and extract scheme-independent information on the infrared Higgs phases and the associated quantum phase transition as well as discuss a related enhanced fine-tuning usually not considered in tuning estimates. Finally, we illustrate the framework's versatility by exploring new physics coupled to the Higgs sector that can soften high-scale sensitivity, recovering also the large-anomalous-dimension solution to the hierarchy problem.

hep-ph↗

arXiv2025

Improved cosmological limits on $Z^\prime$ models with light right-handed neutrinos

We improve limits on $Z^\prime$ extensions of the Standard Model (SM) with light right-handed neutrinos. The presence of shared gauge interactions between the light right-handed neutrinos and other SM fermions allows for production of $\nu_R$ in the early Universe and we use the excess in the effective number of neutrino species $\Delta N_\text{eff}$ to place limits. Our benchmark model is a minimal gauged $U(1)_{B-L}$ that often arises as a building block in other models, and we discuss applicability to more general $U(1)$ extensions. We devise an improved Monte Carlo integration scheme convenient for implementation of generic integrated Boltzmann equations with minimal simplifying assumptions. We sketch our numerical implementation in detail for future reference. Using the new ACT DR6 limit $\Delta N_\text{eff}<0.17$, we improve constraints on the gauge coupling for $1\mathrm{\,GeV} < m_{Z^\prime} < 100\mathrm{\,TeV}$ by orders of magnitude and find the strongest limits thus far, surpassing even current and future colliders, and explore the potential of future CMB experiments to test $U(1)$ extensions up to the GUT scale. We perform a detailed analysis of the robustness of cosmological limits within standard and non-standard thermal histories and find that a strong first order phase transition, early dark energy or early matter domination could dilute $\nu_R$ abundances beyond detection. We investigate the effect of reheating on $\nu_R$-genesis and provide results and prescriptions to apply our bounds to non-standard thermal histories. Limits are generically weakened for reheating $T_\text{reh}\ll m_{Z^\prime}$. Our results suggest that projected limits on $Z^\prime$ with Dirac neutrinos can only be accommodated for in non-standard thermal histories, thus limiting the options to include dark matter candidates or Dirac leptogenesis.

hep-ph↗

arXiv2025

Radiative Symmetry Breaking with a Scale Invariant Seesaw

We study a scale invariant inverse seesaw model that radiatively generates the electroweak scale, the Standard Model (SM) neutrino masses, and stabilizes the electroweak vacuum. Previous studies have noted that the SM Higgs potential and the electroweak scale can be radiatively generated via the minimal seesaw mechanism. This scenario, called the ``Neutrino Option'', was UV-completed by a scale invariant framework. However, these models predict singlet neutrino and scalar masses at $10^7-10^9$ GeV, beyond any experimental reach and leave the electroweak vacuum meta/un-stable at high energies. In this work, we propose modifications to this framework that lower the singlet neutrino masses to experimentally accessible scales through an inverse seesaw mechanism, while fully stabilizing the electroweak vacuum with an additional singlet scalar. The possibility of generating the observed baryon asymmetry of the universe via leptogenesis is also explored.

hep-ph↗

arXiv2025

Custodial Naturalness

Custodial Naturalness is a new symmetry-based idea to explain the large separation between the electroweak (EW) scale and ultraviolet completions of the Standard Model (SM). Classical scale invariance is combined with an enhanced scalar-sector custodial symmetry and both are spontaneously broken by dimensional transmutation at a new intermediate scale. The SM-like Higgs boson is an elementary pseudo-Nambu-Goldstone-Boson (pNGB) of the extended custodial symmetry, which naturally explains the suppression of the EW scale without a little hierarchy problem. We explain details of the general mechanism, its minimal realization and simplest extensions which populate Higgs-, gauge-, and neutrino portals and introduce candidates for particle Dark Matter (DM). We show the stability of the mechanism under inclusion of new sources of explicit custodial symmetry violation, as well as under variations of boundary conditions at the high scale. Custodial Naturalness is experimentally testable - including a specific correlation between the Higgs and top quark masses, as well as by the prediction of a new heavy $Z'$ gauge boson and a new dilaton-like scalar which are well-motivated targets for future colliders and Higgs factories. The cosmological evolution features a strongly supercooled phase transition implying that consequences of Custodial Naturalness may also be tested by gravitational wave observatories.

hep-ph↗

arXiv2024

Radiative Origin of Fermion Mass Hierarchy in Left-Right Symmetric Theory

Despite the remarkable success of the Standard Model, the hierarchy and patterns of fermion masses and mixings remain a profound mystery. To address this, we propose a model employing the rank mechanism, where the originally massless quarks and leptons sequentially get masses. The third-generation masses originate from the seesaw mechanism at the tree level, while those of the second and first generations emerge from one-loop and two-loop radiative corrections, respectively, with a progressive increase in the rank of the mass matrix. This approach does not require new discrete or global symmetries. Unlike other theories of this type that require the introduction of additional scalars, we employ the double seesaw mechanism within a left-right symmetric framework, which allows us to realize this scenario solely through gauge interactions.

hep-ph↗

arXiv2024

Ultraheavy Dark Matter and WIMPs Production aided by Primordial Black Holes

The unitary bound restricts thermal relics to be lighter than $100$ TeV. This work investigates the production of ultraheavy dark matter and WIMPs in the presence of primordial black holes. Firstly, we describe how Hawking evaporation can produce ultraheavy dark matter with masses above $10^{12}$ GeV in radiation and matter-domination eras. Later, we assess how primordial black holes that induce a non-standard cosmology impact the predicted relic density of a thermal relic and explore the interplay between them, considering the restrictions arising from entropy injection due to the evaporation of primordial black holes. Considering a concrete B-L model, where the dark matter is a Dirac particle, we obtain the correct relic density for various freeze-out scenarios and show that a dark matter particle can nicely reproduce the correct relic density in agreement with current limits with masses above the $10$ TeV scale. Hence, this work strengthens the continuous search for heavy dark matter particles.

hep-ph↗

arXiv2024

Attenuation of Cosmic Ray Electron Boosted Dark Matter

We consider a model of boosted dark matter (DM), where a fraction of DM is upscattered to relativistic energies by cosmic ray electrons. Such interactions responsible for boosting the DM also attenuate its flux at the Earth. Considering a simple model of constant interaction cross-section, we make analytical estimates of the variation of the attenuation ceiling with the DM mass and confirm it numerically. We then extend our analysis to a $Z'$-mediated leptophilic DM model. We show that the attenuation ceiling remains nearly model-independent for DM and mediator particles heavier than the electron, challenging some previous discussions on this topic. Using the XENONnT direct detection experiment, we illustrate how constraints based on energy-dependent scattering can significantly differ from those based on an assumed constant cross-section. This highlights the importance of re-evaluating these constraints in the context of specific models.

hep-ph↗

arXiv2024

Electroweak hierarchy from conformal and custodial symmetry

We present "Custodial Naturalness" as a new mechanism to explain the separation between the electroweak (EW) scale and the scale of potential ultraviolet completions of the Standard Model (SM). We assume classical scale invariance as well as an extension of the SM scalar sector custodial symmetry to $\mathrm{SO}(6)$. This requires a single new complex scalar field charged under a new $\mathrm{U}(1)_\mathrm{X}$ gauge symmetry which partially overlaps with $B-L$. Classical scale invariance and the high-scale scalar sector $\mathrm{SO}(6)$ custodial symmetry are radiatively broken by quantum effects that generate a new intermediate scale by dimensional transmutation. The little hierarchy problem is solved because the Higgs boson arises as an elementary (i.e. non-composite) pseudo-Nambu-Goldstone boson (pNGB) of the spontaneously broken $\mathrm{SO}(6)$ custodial symmetry. The minimal setting has the same number of parameters as the SM and predicts new physics in the form of a heavy $Z'$ with fixed couplings to the SM and a mass of $m_{Z'}\approx4-100\,\mathrm{TeV}$, as well as a light but close-to invisible dilaton with a mass $m_{h_\Phi}\approx75\,\mathrm{GeV}$.

hep-ph↗

arXiv2024

The Waning of the WIMP: Endgame?

Weakly Interacting Massive Particles (WIMPs) continue to be considered some of the best-motivated Dark Matter (DM) candidates. No conclusive signal, despite an extensive search program that combines, often in a complementary way, direct, indirect, and collider probes, has been however detected so far. This situation might change in the near future with the advent of even larger, multi-ton Direct Detection experiments. We provide here an updated review of the WIMP paradigm, with a focus on selected models that can be probed with upcoming facilities, all relying on the standard freeze-out paradigm for the relic density. We also discuss Collider and Indirect Searches when they provide complementary experimental information.

hep-ph↗

arXiv2024

Light vector bosons and the weak mixing angle in the light of future germanium-based reactor CE$\nu$NS experiments

In this work, the sensitivity of future germanium-based reactor neutrino experiments to the weak mixing angle $\sin^{2}\theta_{W}$, and to the presence of new light vector bosons is investigated. By taking into account key experimental features with their uncertainties and the application of a data-driven and state-of-the-art reactor antineutrino spectrum, the impact of detection threshold and experimental exposure is assessed in detail for an experiment relying on germanium semiconductor detectors. With the established analysis framework, the precision on the Weinberg angle, and capability of probing the parameter space of a universally coupled mediator model, as well as a U(1)$_{\rm B-L}$-symmetric model are quantified. Our investigation finds the next-generation of germanium-based reactor neutrino experiments in good shape to determine the Weinberg angle $\sin^{2}\theta_{W}$ with $<10$ % precision using the low-energetic neutrino channel of CE$\nu$NS. In addition, the current limits on new light vector bosons determined by reactor experiments can be lowered by about an order of magnitude via the combination of both CE$\nu$NS and E$\nu$eS. Consequently, our findings provide strong phenomenological support for future experimental endeavours close to a reactor site.

hep-ph↗