arXiv subjects

Thomas Schwetz

Publications and source records attributed to Thomas Schwetz.

At least 19 recordsLinked to original sources

arXiv2026

Probing T and CP Violation at DUNE and T2HK

We study the sensitivity of the DUNE and T2HK long-baseline experiments to time reversal (T) violation in neutrino oscillations. Rather than the conventional approach of exchanging initial and final neutrino flavors, we search for T violation through the $L$-dependence of the $\nu_\mu \to \nu_e$ transition probability at fixed neutrino energy using neutrino data only. Within the standard three-flavour framework, we show that the DUNE and T2HK together can establish the presence of an $L$-odd component in the oscillation probability at up to $\sim 4\sigma$ significance, with the optimal sensitivity in the energy range $E_\nu \in [0.68, 0.92]$ GeV. The second oscillation maximum of DUNE plays a crucial role in this analysis. We further show that DUNE is more sensitive to T violation that is running in neutrino-only mode, whereas T2HK provides better sensitivity in the conventional neutrino versus anti-neutrino comparison, making the two experiments complementary to each other in search of the CP phase $\delta_{\rm CP}$.

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arXiv2026

Oscillating Neutrinos vs. Oscillating Scalars: Constraining Scalar Dark Matter-Induced Neutrino Mass

We consider the hypothesis that neutrino masses are generated by a coupling to an ultra-light (pseudo-)scalar field, which provides the dark matter in the universe. This leads to time-varying neutrino masses with a frequency set by the dark matter mass, with implications for neutrino oscillation data. We use that dark matter is in a virialised state in the galaxy and provide a detailed discussion of the relevant time scales. Using data from the T2K, RENO and JUNO experiments, we show that for dark matter masses smaller than about $3\times 10^{-8}$eV down to the smallest allowed dark matter mass of about $10^{-21}$eV only a fraction of between 9\% to 54\% of the total neutrino mass can arise from the coupling to the background scalar, depending on the value of the scalar mass. Future data from JUNO may improve these limits down to 1\% in certain regions of scalar masses. We focus on a real scalar field, but most of our results hold also for a complex scalar.

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arXiv2026

Neutrino mass limits and decaying dark matter: background evolution versus perturbations

We revisit cosmological neutrino mass bounds when a fraction of dark matter is allowed to decay to massless dark radiation. By compensating the late-time increase in the matter density induced by neutrinos becoming non-relativistic, decaying dark matter (DDM) can render datasets solely sensitive to the background density effectively insensitive to neutrino masses. Using data from baryonic acoustic oscillations (BAO) and Type Ia supernovae together with a distance prior from the cosmic microwave background (CMB), we find that neutrino masses as large as ${\cal O}(1\,\mathrm{eV})$ are allowed without degrading the fit. Moreover, the combination of BAO data with the CMB distance prior yields a preference for a non-zero DDM fraction, and alleviates the need for dynamical dark energy with phantom crossing. However, the degeneracy introduced by DDM is decisively broken once perturbation observables are included. Incorporating the full $\textit{Planck}$ CMB likelihood, and in particular CMB lensing, restores strong constraints on the neutrino mass in the DDM scenario, $\sum m_\nu \lesssim 0.079\,\mathrm{eV}$. In contrast, neutrino mass constraints in a smooth dark energy model described by the Chevallier-Polarski-Linder parametrization become merely $\sim 25\%$ stronger compared to background-only analyses. Our results highlight the essential role of structure-growth measurements in assessing extensions of the dark sector and to obtain robust cosmological neutrino mass bounds.

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arXiv2026

Lessons from the first JUNO results

First results from the JUNO reactor neutrino experiment already determine with world-leading precision the small neutrino squared-mass splitting $\Delta m^2_{21}$ and the mixing angle $\theta_{12}$. In this article we perform an exploratory study beyond these, taking advantage of the first JUNO data release to discuss its sensitivity to the large squared-mass splitting, $\Delta m^2_{3\ell}$. When combined with constraints from global oscillation data, this may already contain some information on the neutrino mass ordering. Indeed, we find that the combination of the complementary $\Delta m^2_{3\ell}$-determinations gives a slight preference for Normal Ordering, with a p-value for Inverted Ordering of 2%-2.6% ($2.2\sigma$-$2.3\sigma$). We study the robustness of this result with respect to potential systematic uncertainties and statistical fluctuations. Taken at face value, a full global analysis of oscillation data including the publicly available JUNO information and data leads to a preference for Normal Ordering with $\Delta\chi^2 = 4.6$ and 9.4 without and with Super-K and IceCube-24 atmospheric neutrino data, respectively.

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arXiv2025

How light can ALP dark matter be?

We assume axion-like particles (ALPs) to provide the full dark matter abundance and derive various lower bounds on the ALP mass. We contrast the post- and pre-inflationary symmetry breaking cases and present allowed regions in the plane of ALP mass and energy scale of inflation. For the post-inflationary case, we revisit bounds from isocurvature perturbations taking into account that, as suggested by simulations, axion radiation by cosmic strings during the scaling regime provides the dominant production mechanism of dark matter, obtaining significantly weaker limits than previously. Combining isocurvature, with constraints from black hole superradiance and free streaming, we find that the bound $m_a \gtrsim 10^{-17}$ eV applies for most cases considered here. It can be potentially relaxed to $\sim 6\times 10^{-19}$ eV only in the post-inflationary case with a strongly temperature-dependent axion mass, subject to uncertainties on the axion emission spectrum. Significantly stronger bounds are obtained in the post-inflationary scenario from the non-observation of CMB tensor modes, which can be as strong as $m_a > 5\times 10^{-7}$ eV for small reheating efficiencies, $\epsilon \lesssim 5\times 10^{-4}$.

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arXiv2025

T versus CP effects in DUNE and T2HK

Time reversal (T) symmetry violations in neutrino oscillations imply the presence of an $L$-odd component in the transition probability at fixed neutrino energy, with $L$ denoting the distance between neutrino source and detector. Within the standard three-flavour framework, we show that the combination of the transition probabilities determined at the DUNE and T2HK experiments can establish the presence of an $L$-odd component, and therefore provide sensitivity to T violation, up to $4\sigma$ significance. The optimal neutrino energy window is from 0.68 to 0.92 GeV, and therefore a crucial role is played by the low-energy part of the DUNE event spectrum covering the second oscillation maximum. We compare the sensitivity to T violation based on this energy range using neutrino data only with the more traditional search for charge-parity (CP) violation based on the comparison of neutrino versus anti-neutrino beam data. We show that for DUNE it is advantageous to run in neutrino mode only, i.e., searching for T violating effects, whereas T2HK is more sensitive to CP violation, comparing neutrino and anti-neutrino data. Hence, the two experiments offer complementary methods to determine the complex phase in the PMNS mixing matrix.

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arXiv2025

Neutrino oscillations and scattering theory

We derive the neutrino oscillation probability in vacuum using scattering theory methods developed earlier in the context of collider physics. It is computed from Feynman diagrams that combine neutrino production and detection processes into a single quantum amplitude. Initial-state particles in the neutrino source and the detector are treated as wave packets. In contrast to many other approaches, we work with transition probabilities, rather than the amplitude itself, and do not specify the form of the wave packets to arrive at the neutrino oscillation formula. Our approach offers a simple and transparent framework to discuss decoherence effects in neutrino oscillations, as well as the effects of the finite lifetime of the neutrino source. The latter are particularly relevant for oscillation experiments using neutrinos from pion decays in flight.

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arXiv2024

Dynamics of metastable Standard Model particles from long-lived particle decays in the MeV primordial plasma

We investigate the cosmological impact of hypothetical unstable new physics particles that decay in the MeV-scale plasma of the Early Universe. Focusing on scenarios where the decays produce metastable species such as muons, pions, and kaons, we systematically analyze the dynamics of these particles using coupled Boltzmann equations governing their abundances. Our results demonstrate that the metastable species can efficiently annihilate or interact with nucleons, often leading to their disappearance before decay. The suppression of decay significantly alters the properties of cosmic neutrinos, impacting cosmological observables like Big Bang nucleosynthesis and the Cosmic Microwave Background. To support further studies, we provide two public codes: the Mathematica code that traces the evolution of these metastable particles, as well as the python-based unintegrated neutrino Boltzmann solver that uses this evolution as an input and may be applied to a broad range of scenarios. We then utilize them for studying a few particular new physics models.

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arXiv2024

Large neutrino mass in cosmology and keV sterile neutrino dark matter from a dark sector

We consider an extended seesaw model which generates active neutrino masses via the usual type-I seesaw and leads to a large number of massless fermions as well as a sterile neutrino dark matter (DM) candidate in the $\mathcal{O}(10-100) {\rm~keV}$ mass range. The dark sector comes into thermal equilibrium with Standard Model neutrinos after neutrino decoupling and before recombination via a U(1) gauge interaction in the dark sector. This suppresses the abundance of active neutrinos and therefore reconciles sizeable neutrino masses with cosmology. The DM abundance is determined by freeze-out in the dark sector, which allows avoiding bounds from X-ray searches. Our scenario predicts a slight increase in the effective number of neutrino species $N_{\rm eff}$ at recombination, potentially detectable by future CMB missions.

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arXiv2024

New physics decaying into metastable particles: impact on cosmic neutrinos

We investigate decays of hypothetical unstable new physics particles into metastable species such as muons, pions, or kaons in the Early Universe, when temperatures are in the MeV range, and study how they affect cosmic neutrinos. We demonstrate that the non-trivial dynamics of metastables in the plasma alters the impact of the new physics particles on the neutrino population, including the effective number of neutrino degrees of freedom, $N_{\rm eff}$, modifies neutrino spectral distortions, and may induce asymmetries in neutrino and antineutrino energy distributions. These modifications have important implications for observables such as Big Bang Nucleosynthesis and the Cosmic Microwave Background, especially in light of upcoming CMB observations aiming to reach percent-level precision on $N_{\rm eff}$. We illustrate our findings with a few examples of new physics particles and provide a computational tool available for further exploration.

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arXiv2024

NuFit-6.0: Updated global analysis of three-flavor neutrino oscillations

We present an updated global analysis of neutrino oscillation data as of September 2024. The parameters $\theta_{12}$, $\theta_{13}$, $\Delta m^2_{21}$, and $|\Delta m^2_{3\ell}|$ ($\ell = 1,2$) are well-determined with relative precision at $3\sigma$ of about 13\%, 8\%, 15\%, and 6\%, respectively. The third mixing angle $\theta_{23}$ still suffers from the octant ambiguity, with no clear indication of whether it is larger or smaller than $45^\circ$. The determination of the leptonic CP phase $\delta_{CP}$ depends on the neutrino mass ordering: for normal ordering the global fit is consistent with CP conservation within $1\sigma$, whereas for inverted ordering CP-violating values of $\delta_{CP}$ around $270^\circ$ are favored against CP conservation at more than $3.6\sigma$. While the present data has in principle $2.5$--$3\sigma$ sensitivity to the neutrino mass ordering, there are different tendencies in the global data that reduce the discrimination power: T2K and NOvA appearance data individually favor normal ordering, but they are more consistent with each other for inverted ordering. Conversely, the joint determination of $|\Delta m^2_{3\ell}|$ from global disappearance data prefers normal ordering. Altogether, the global fit including long-baseline, reactor and IceCube atmospheric data results into an almost equally good fit for both orderings. Only when the $\chi^2$ table for atmospheric neutrino data from Super-Kamiokande is added to our $\chi^2$, the global fit prefers normal ordering with $\Delta\chi^2 = 6.1$. We provide also updated ranges and correlations for the effective parameters sensitive to the absolute neutrino mass from $\beta$-decay, neutrinoless double-beta decay, and cosmology.

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arXiv2024

Model-independent search for T violation with T2HK and DUNE

We consider the time reversal (T) transformation in neutrino oscillations in a model-independent way by comparing the observed transition probabilities at two different baselines at the same neutrino energy. We show that, under modest model assumptions, if the transition probability $P_{\nu_\mu\to\nu_e}$ around $E_\nu \simeq 0.86$ GeV measured at DUNE is smaller than the one at T2HK the T symmetry has to be violated. Experimental requirements needed to achieve good sensitivity to this test for T violation are to obtain enough statistics at DUNE for $E_\nu \lesssim 1$ GeV (around the 2nd oscillation maximum), good energy resolution (better than 10%), and near-detector measurements with a precision of order 1% or better.

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arXiv2024

Not-so-inelastic Dark Matter

Models of inelastic (or pseudo-Dirac) dark matter commonly assume an accidental symmetry between the left-handed and right-handed mass terms in order to suppress diagonal couplings. We point out that this symmetry is unnecessary, because for Majorana fermions the diagonal couplings are not strongly constrained. Removing the requirement of such an ad-hoc symmetry instead relaxes the relic density constraint due to additional annihilation modes. We consider a simple UV-complete model realising this setup and study constraints from (in)direct detection, beam dump experiments and colliders. We identify two viable mass regions for the dark matter mass, around a few hundred MeV and around a few GeV, respectively. The former region will be fully tested by near-future analyses of NA64 and Belle II data, while the latter turns out to be challenging to explore even with future experiments.

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arXiv2024

Comment on "Axion Electrodynamics in the Presence of Current Sources"

In this note we refute the critique raised in a preprint by J.~Berger and A.~Bhoonah~\cite{Berger:2023muj} on the method used in our earlier paper, Beutter et al.~\cite{Beutter:2018xfx}, to calculate the electromagnetic fields induced by an axion background in the presence of a magnetic field.

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arXiv2023

Complementarity of $B\to K^{()} μ\bar μ$ and $B\to K^{()} + \mathrm{inv}$ for searches of GeV-scale Higgs-like scalars

The rare decays $B^+\to K^+ μ\bar μ$ and $B^0\to K^{*0} μ\barμ$ provide the strongest constraints on the mixing of a light scalar with the Higgs boson for GeV-scale masses. The constraints sensitively depend on the branching ratio to muons. Additional decay channels like an invisible partial width may substantially weaken the constraints. This scenario will be probed at Belle II in $B\to K^{(*)} + \mathrm{inv}$. We illustrate the complementarity of scalar decays to muons and invisible decays using the currently available results of LHCb and BaBar. We provide two simple model realisations providing a sizeable invisible scalar width, one based on a real scalar and one based on a $U(1)_{B-L}$ gauge symmetry. In both examples the scalar decays into heavy neutral leptons which can be motivated by the seesaw mechanism for neutrino masses.

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arXiv2023

A decoherence explanation of the gallium neutrino anomaly

Gallium radioactive source experiments have reported a neutrino-induced event rate about 20\% lower than expected with a high statistical significance. We present an explanation of this observation assuming quantum decoherence of the neutrinos in the gallium detectors at a scale of 2~m. This explanation is consistent with global data on neutrino oscillations, including solar neutrinos, if decoherence effects decrease quickly with energy, for instance with a power law $E_ν^{-r}$ with $r\simeq 12$. Our proposal does not require the presence of sterile neutrinos but implies a modification of the standard quantum mechanical evolution equations for active neutrinos.

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arXiv2023

Impact of nuclear matrix element calculations for current and future neutrinoless double beta decay searches

Nuclear matrix elements (NME) are a crucial input for the interpretation of neutrinoless double beta decay data. We consider a representative set of recent NME calculations from different methods and investigate the impact on the present bound on the effective Majorana mass $m_{ββ}$ by performing a combined analysis of the available data as well as on the sensitivity reach of future projects. A crucial role is played by the recently discovered short-range contribution to the NME, induced by light Majorana neutrino masses. Depending on the NME model and the relative sign of the long- and short-range contributions, the current $3σ$ bound can change between $m_{ββ} < 40$ meV and 600 meV. The sign-uncertainty may either boost the sensitivity of next-generation experiments beyond the region for $m_{ββ}$ predicted for inverted mass ordering or prevent even advanced setups to reach this region. Furthermore, we study the possibility to distinguish between different NME calculations by assuming a positive signal and by combining measurements from different isotopes. Such a discrimination will be impossible if the relative sign of the long- and short-range contribution remains unknown, but can become feasible if $m_{ββ} \gtrsim 40$ meV and if the relative sign is known to be positive. Sensitivities will be dominated by the advanced $^{76}$Ge and $^{136}$Xe setups assumed here, but NME model-discrimination improves if data from a third isotope is added, e.g., from $^{130}$Te or $^{100}$Mo.

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arXiv2023

Quantifying the tension between cosmological and terrestrial constraints on neutrino masses

The sensitivity of cosmology to the total neutrino mass scale $Σm_ν$ is approaching the minimal values required by oscillation data. We study quantitatively possible tensions between current and forecasted cosmological and terrestrial neutrino mass limits by applying suitable statistical tests such as Bayesian suspiciousness, parameter goodness-of-fit tests, or a parameter difference test. In particular, the tension will depend on whether the normal or the inverted neutrino mass ordering is assumed. We argue, that it makes sense to reject inverted ordering from the cosmology/oscillation comparison only if data are consistent with normal ordering. Our results indicate that, in order to reject inverted ordering with this argument, an accuracy on the sum of neutrino masses $σ({m_ν})$ of better than 0.02~eV would be required from future cosmological observations.

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