arXiv subjects

Frank F. Deppisch

Publications and source records attributed to Frank F. Deppisch.

At least 19 recordsLinked to original sources

arXiv2026

Displaced Signals from Long-lived Particles in Neutrinoless Double Beta Decay

Much of the literature on neutrinoless double beta ($0νββ$) decay with light new-physics mediators focuses on invisible missing energy. We investigate the scenario in which a massive Majoron-like particle $ϕ$ is produced on-shell during $0νββ$ decay and subsequently decays into visible final states after travelling a macroscopic distance. Specifically, we analyze the displaced energy deposition from $ϕ$ decays into a photon pair ($γγ$), a photon and a dark photon ($γγ_D$) and an electron-positron pair ($e^+e^-$). We demonstrate that the displaced decays modify the expected visible energy spectra and provide novel, distinct experimental signatures at current and upcoming $0νββ$ experiments, with the promise of improving the sensitivity of the standard invisible Majoron searches in $0νββ$ decay by more than two orders of magnitude. The relevant effective couplings can naturally arise in well-motivated ultraviolet-complete scenarios that conventional $0νββ$ decay searches cannot probe.

hep-ph↗

arXiv2026

Photon--Dark Matter Elastic Scattering: An Effective-Operator Scan and First Operator-Resolved Sensitivity Estimates from the Galactic Halo

Elastic photon--dark matter scattering attenuates gamma-ray spectra along a line of sight, probing the same operators as dark matter annihilation to photons but at a rate linear, rather than quadratic, in dark matter density. We consider Standard Model gauge-invariant effective operators of mass-dimension 5 to 7, suppressed by a cutoff scale $Λ$, coupling scalar, Majorana or Dirac dark matter to the photon. The leading operators with non-vanishing real-photon amplitudes enter at dimension-5 for Dirac dark matter and dimension-7 for Majorana dark matter. In the electroweak-doublet dipole portal, the inelastic splitting invoked to evade direct detection also closes the CMB annihilation bound, leaving attenuation the only one of the three photon-sector probes that survives. Applying this to a pixel-level reanalysis of 17 years of \textit{Fermi}--LAT Pass~8 data toward the Galactic centre, we derive the first operator-resolved sensitivity estimates for photon--dark matter scattering from the Galactic halo: $Λ\simeq 0.32~\mathrm{GeV}$ for the dimension-5 Dirac dipoles, $0.21~\mathrm{GeV}$ for the dimension-6 scalar Rayleigh operator and $0.79$--$1.06~\mathrm{GeV}$ for the dimension-7 Rayleigh family. The reach is weak: it lies below the EFT-validity threshold across the cold dark matter mass range, and is superseded on the dipole plane by CMB and direct-detection constraints. The framework is calibrated against pseudo-experiments, and recomputes the sensitivity for any instrument that provides a per-bin spectrum with uncertainties and a line-of-sight column density.

astro-ph.HE↗

arXiv2026

The 20 GeV Galactic Halo Excess: Pixel-Level Confirmation and Consistency with Sub-TeV WIMP Annihilation

A recent analysis of 15 years of Fermi-LAT data reported a spherically symmetric, halo-like component of the Galactic diffuse emission that peaks near 20GeV. We independently reproduce this cell-aggregated analysis, then extend it to a pixel-level likelihood on the native $0.125^\circ$ maps, adding energy-dependent point-spread-function forward folding and masking bright sources. Both methods replicate the 20GeV halo spectrum, with the pixel-level normalisation ${\sim}20\%$ above the cellwise fit across NFW emissivity scalings $ρ^p$, $p \in 1,2,2.5$. This 20GeV halo is a high-latitude feature, distinct from the inner-Galaxy excess, and consistent with sub-TeV dark matter (WIMP) annihilation. It is centrally concentrated, strongly disfavouring extragalactic emission. Fitting prompt $s$-wave annihilation spectra, best-fit masses are $m_χ\simeq 0.55$TeV ($W^+W^-$) and $0.72$TeV ($b\bar{b}$) with $\langleσv\rangle \simeq 1\times10^{-24}~\mathrm{cm^3\,s^{-1}}$, in $\sim\!4$-$5\times$ tension with dwarf spheroidal galaxy limits. However, accounting for foreground modelling and $J$-factor systematic uncertainties widens the tension window to $R\simeq1.6$-$9.3$, leaving the $s$-wave interpretation viable. To close the tension, we consider alternative particle dark matter models. $p$-wave annihilation misses relic abundance constraints by $\sim\!7$ orders of magnitude. A decay interpretation evades dwarf limits but is disfavoured by the isotropic gamma-ray background. The only viable velocity structure consistent with dwarf limits, present-day halo rates, and relic density is low-velocity-enhanced annihilation (resonant Sommerfeld or Breit-Wigner). This supplies the required $\approx\!45\times$ boost from a thermal relic. Fully resolving the dwarf tension requires a fine-tuned resonance peaking at the halo velocity and falling for colder systems.

astro-ph.HE↗

arXiv2026

Probing Quasi-Dirac Neutrino Oscillations at Long Baseline Experiments

The Dirac or Majorana nature of neutrinos remains one of the most fundamental open problems in particle physics. A natural intermediate scenario arises when small lepton-number-violating Majorana mass terms break the exact Dirac symmetry. These cause mass eigenstates to form nearly degenerate pairs, each separated by a small mass splitting. We study this quasi-Dirac scenario within a five-neutrino framework, extending the Standard Model by two right-handed neutrinos, which form a nearly degenerate sterile pair alongside the three active states. This extended mixing structure introduces additional phenomenological parameters, including new mixing angles, mass splittings, and CP phases. We use appearance and disappearance data from NO$ν$A and T2K to constrain the active-sterile mixing angles $θ_{sa}$ and the mass splitting within the sterile pair $Δm_{54}^2$ and forecast the sensitivity achievable at DUNE. We analyse how these parameters modify CP-violating observables relative to the standard three-neutrino case, comparing predicted event rates across both mass orderings and as a function of $δ_{CP}$.

hep-ph↗

arXiv2026

Linking the Gauge Hierarchy with Neutrino Masses and Dark Matter via Two-step Cosmological Selection

The hierarchy problem between the electroweak (EW) and Planck scales remains a central puzzle in modern physics. We discuss a promising solution operating through the cosmological selection of the EW vacuum in a multiverse landscape, where the EW scale is dynamically approached as the configuration that maximizes the vacuum energy. By extending the Standard Model with a complex scalar singlet and right-handed neutrinos, charged under a global $U(1)_{B-L}$ symmetry, the model not only explains the smallness of the EW scale. It can also account for neutrino masses via the seesaw mechanism and the matter-antimatter asymmetry via leptogenesis. In addition, it provides a viable dark matter candidate that is testable in future neutrino experiments.

hep-ph↗

arXiv2026

Probing Dark Sector Particles Coupling to Neutrinos with Double Beta Decay

Motivated by the observation of non-zero neutrino masses and the potential for discovering physics beyond the Standard Model, numerous experiments are actively searching for neutrinoless double beta $(0νββ)$ decay. In all of these searches, a substantial amount of data on two-neutrino double beta $(2νββ)$ decay has been collected. In this work, we explore the sensitivity of current and future double beta decay experiments to a massive Majoron-like scalar particle coupled to neutrinos and potentially dark sector fermions, and compare their reach to the relevant cosmological constraints. On- and off-shell production of such a scalar leads to characteristic distortions in the emitted electron spectrum. We investigate how these distortions manifest in current and future double beta decay experiments, deriving the sensitivity to such a scenario. We project the reach of future experiments which can probe scalar-neutrino couplings of $|a_ν| \approx 2\times 10^{-6}$ for sub-MeV scalar particles and remain sensitive to off-shell production above the Q-value of double beta isotopes.

hep-ph↗

arXiv2026

Sterile Neutrinos at MAPP in the B-L Model

The possibility of searching for right-handed neutrinos at the MoEDAL's Apparatus for Penetrating Particles (MAPP) detector is investigated in this work. In particular, pair-production of right-handed (RH) neutrinos $N$ from either a $B-L$ gauge boson $Z'$, as well as Standard Model (SM) $Z$ boson are considered. Under a no-background assumption, we show that the MAPP detector can be sensitive to active-sterile neutrino mixing strengths as low as $V_{μN}^2 \approx 10^{-12}$ for multiple choices of $m_N / m_{Z'}$ values, when taking the $B-L$ gauge coupling $g_{B-L} = 10^{-3}$ near its current limit. The SM $Z$ boson portal can reach a similar sensitivity, when the effective mixing between the $B-L$ and SM gauge boson is $α\approx 0.002$.

hep-ph↗

arXiv2025

Constraining the SMEFT Extended with Sterile Neutrinos at FCC-ee

We investigate how extensions of the Standard Model (SM) involving heavy neutral leptons (HNLs) can be probed at FCC-ee, the proposed high-energy circular $e^+e^-$ collider. Using the effective field theory (EFT) approach, we determine the impact of new interactions on the production and decay of HNLs at FCC-ee. In particular, we consider $d\leq 7$ $ν$SMEFT operators which induce vector, scalar and tensor four-fermion and effective charged- and neutral-current interactions of HNLs, that may also mix with the active neutrinos of the SM. We consider sensitivities to the active-sterile mixing and EFT Wilson coefficients from monophoton searches and displaced vertex decay signatures. In both analyses, we consider the scenarios where HNLs are Majorana or Dirac fermions. We translate the upper bounds on the Wilson coefficients to lower limits on the scale of new physics.

hep-ph↗

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.

hep-ex↗

arXiv2025

Inflationary Gravitational Waves and Laboratory Searches as Complementary Probes of Right-handed Neutrinos

We analyze the damping of inflationary gravitational waves (GW) that re-enter the Hubble horizon before or during a post-inflationary era dominated by a meta-stable, right-handed neutrino (RHN), whose out-of-equilibrium decay releases entropy. Within a minimal type-I seesaw extension of the Standard Model (SM), we explore the conditions under which the population of thermally produced RHNs remain long-lived and cause a period of matter-domination. We find that the suppression of the GW spectrum occurs above a characteristic frequency determined by the RHN mass and active-sterile mixing. For RHN masses in the range $0.1$-$10$ GeV and mixing $10^{-12} \lesssim |V_{eN}|^2 \lesssim 10^{-5}$, we estimate such characteristic frequencies and the signal-to-noise ratio to assess the detection prospects in GW observatories such as THEIA, $μ$-ARES, LISA, BBO and ET. Additionally we use LIGO data to put upper bounds on the reheating temperature after inflation, for a given blue-tilted GW spectrum. We find complementarity between GW signals and laboratory searches in SHiP, DUNE and LEGEND-1000. Notably, RHN masses of $0.2$-$2$ GeV and mixing $10^{-10} \lesssim |V_{eN}|^2 \lesssim 10^{-7}$ are testable in both laboratory experiments and GW observations. Additionally, GW experiments can probe the canonical seesaw regime of light neutrino mass generation, a region largely inaccessible to laboratory searches.

hep-ph↗

arXiv2025

Revealing the Origin of Neutrino Masses through Displaced Shower Searches in the CMS Muon System

We study the potential to probe the origin of neutrino masses, by searching for long-lived right-handed neutrinos (RHNs) $N$ in the $B-L$ model and in the RHN-extended Standard Model (SM) Effective Field Theory (EFT). Despite the small active-sterile mixing $|V_{\ell N}|^2$, RHNs are produced abundantly via SM and exotic Higgs production, as long as the Higgs mixing or EFT operator coefficient is sufficiently large. We reinterpret a search for displaced showers in the CMS muon system and we find that it is sensitive to parameter space at and below the seesaw floor, $|V_{\ell N}|^2 \approx 10^{-12}$ ($\ell = e$, $τ$) for $m_N \approx 40$ GeV. With existing data constraining such well-motivated scenarios of neutrino mass generation, we determine the projected sensitivity at the HL-LHC, motivating dedicated searches for long-lived RHNs with decay lengths $\approx 10$ m.

hep-ph↗

arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

hep-ph↗

arXiv2025

Relaxing Limits from Big Bang Nucleosynthesis on Heavy Neutral Leptons with Axion-like Particles

Heavy neutral leptons (HNLs) are constrained by requirements of Big Bang Nucleosynthesis (BBN) as their decays significantly impact the formation of the primordial elements. We propose here a model where the primary decay channel for the HNLs is to an axion-like particle (ALP) and a neutrino. Consequently, HNLs can decay earlier and evade the BBN bound for lower masses, provided the ALPs themselves decay considerably later. Further cosmological and astrophysical constraints limit severely the range of validity of the ALP properties. We find that a new parameter region opens up for HNLs with masses between 1 MeV and 1 GeV, and active-sterile neutrino mixing strengths between $10^{-9}$ and $10^{-6}$ that is consistent with constraints and can be probed in future searches. In such a scenario, current bounds as well as sensitivities of future direct HNL searches such as at NA62 and DUNE will be affected.

hep-ph↗

arXiv2025

Probing the Nature of Heavy Neutral Leptons in Direct Searches and Neutrinoless Double Beta Decay

Heavy Neutral Leptons (HNLs) are a popular extension of the Standard Model to explain the lightness of neutrino masses and the matter-antimatter asymmetry through leptogenesis. Future direct searches, such as fixed target setups like DUNE, and neutrinoless double beta decay are both expected to probe the regime of active-sterile neutrino mixing in a standard Seesaw scenario of neutrino mass generation for HNL masses around m_N <~ 1 GeV. Motivated by this, we analyse the complementarity between future direct searches and neutrinoless double beta decay to probe the nature of HNLs, i.e., whether they are Majorana or quasi-Dirac states, and CP-violating phases in the sterile neutrino sector. Following an analytic discussion of the complementarity, we implement a generic fixed target experiment modelling DUNE. We perform a statistical study in how a combined search for HNLs in direct searches and neutrinoless double beta decay, using DUNE and LEGEND-1000 as representative examples, can probe the nature of sterile neutrinos.

hep-ph↗

arXiv2024

Probing Heavy Neutrino Magnetic Moments at the LHC using Long-Lived Particle Searches

We explore long-lived particle (LLP) searches using non-pointing photons at the LHC as a probe for sterile-to-sterile and active-to-sterile transition magnetic dipole moments of sterile neutrinos. We consider heavy sterile neutrinos with masses ranging from a few~GeV to several hundreds of GeV. We discuss transition magnetic dipole moments using the Standard Model effective field theory and low-energy effective field theory extended by sterile neutrinos ($N_R$SMEFT and $N_R$LEFT) and also provide a simplified UV-complete model example. LLP searches at the LHC using non-pointing photons will probe sterile-to-sterile dipole moments two orders of magnitude below the current best constraints from LEP, while an unprecedented sensitivity to sterile neutrino mass of about 700 GeV is expected for active-to-sterile dipole moments. For the UV model example with one-loop transition magnetic moments, the searches for charged lepton flavour violating processes in synergy with LLP searches at the LHC can probe new physics at several TeV mass scales and provide valuable insights into the lepton flavour structure of new physics couplings.

hep-ph↗

arXiv2024

Testing Leptogenesis and Seesaw using Long-lived Particle Searches in the $B-L$ Model

We discuss the potential of using long-lived particle (LLP) searches for right-handed neutrinos (RHNs) to test resonant leptogenesis and the seesaw mechanism. This is challenging if only RHNs are added to the Standard Model (SM), as naturally the active-sterile mixing strengths $|V_{\ell N}|^2$ are small, for 1 GeV $\lesssim M_N \lesssim 1000$ GeV. Instead, we consider the minimal $B-L$ gauge model, where a $Z^\prime$ gauge boson couples to fermions including the RHNs. During leptogenesis, this gauge coupling introduces scattering processes that washout the $B-L$ asymmetry. At colliders, it can lead to abundant production of RHNs which allows probing the associated seesaw mechanism using LLP searches. We find that LLP searches at the FCC-hh can test leptogenesis and the seesaw mechanism simultaneously, and probe the active-sterile mixing at or below the seesaw floor.

hep-ph↗

arXiv2024

Dark Matter and Exotic Neutrino Interactions in Direct Detection Searches

We investigate the effect of new physics interacting with both Dark Matter (DM) and neutrinos at DM direct detection experiments. Working within a simplified model formalism, we consider vector and scalar mediators to determine the scattering of DM as well as the modified scattering of solar neutrinos off nuclei. Using existing data from LUX as well as the expected sensitivity of LUX-ZEPLIN and DARWIN, we set limits on the couplings of the mediators to quarks, neutrinos and DM. Given the current limits, we also assess the true DM discovery potential of direct detection experiments under the presence of exotic neutrino interactions. In the case of a vector mediator, we show that the DM discovery reach of future experiments is affected for DM masses $m_χ\lesssim 10$ GeV or DM scattering cross sections $σ_χ\lesssim 10^{-47}$ cm$^2$. On the other hand, a scalar mediator will not affect the discovery reach appreciably.

hep-ph↗

arXiv2023

Sensitivity of Future Tritium Decay Experiments to New Physics

Tritium beta-decay is the most promising approach to measure the absolute masses of active light neutrinos in the laboratory and in a model-independent fashion. The development of Cyclotron Radiation Emission Spectroscopy techniques and the use of atomic tritium has the potential to improve the current limits by an order of magnitude in future experiments. In this paper, we analyse the potential sensitivity of such future searches to keV-mass sterile neutrinos and exotic interactions of either the active or sterile neutrinos. We calculate the relevant decay distributions in both energy and angle of the emitted electron with respect to a potential polarisation of the tritium, including the interference with the Standard Model case as well as incorporating relevant final state corrections for atomic tritium. We present projected sensitivities on the active-sterile neutrino mixing and effective coupling constants of exotic currents, demonstrating the potential to probe New Physics in tritium experiments.

hep-ph↗