arXiv subjects

Thomas Schwetz

Publications and source records attributed to Thomas Schwetz.

At least 37 records · Page 2Linked to original sources

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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arXiv2023

Decoherence effects in reactor and Gallium neutrino oscillation experiments -- a QFT approach

We adopt the quantum field theoretical method to calculate the amplitude and event rate for a neutrino oscillation experiment, considering neutrino production, propagation and detection as a single process. This method allows to take into account decoherence effects in the transition amplitude induced by the quantum mechanical uncertainties of all particles involved in the process. We extend the method to include coherence loss due to interactions with the environment, similar to collisional line broadening. In addition to generic decoherence induced at the amplitude level, the formalism allows to include, in a straightforward way, additional damping effects related to phase-space integrals over momenta of unobserved particles as well as other classical averaging effects. We apply this method to neutrino oscillation searches at reactor and Gallium experiments and confirm that quantum decoherence is many orders of magnitudes smaller than classical averaging effects and therefore unobservable. The method used here can be applied with minimal modifications also to other types of oscillation experiments, e.g., accelerator based beam experiments.

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arXiv2023

Dipole portal and neutrinophilic scalars at DUNE revisited: the importance of the high-energy neutrino tail

We estimate the sensitivity of the DUNE experiment to new physics particles interacting with neutrinos, considering the dipole portal to heavy neutral leptons and a neutrinophilic scalar with lepton-number $2$ as examples. We demonstrate that neutrinos from the high-energy tail of the DUNE flux, with energies $E_ν\gtrsim 5-10\text{ GeV}$, may significantly improve the sensitivity to these models, allowing to search for particles as heavy as $\simeq 10\text{ GeV}$. We also study the impact of the so-called tau-optimized neutrino beam configuration, which slightly improves sensitivity to the new physics models considered here. For both models, we consider new production channels (such as deep-inelastic scattering) and provide a detailed comparison of different signatures in the detector.

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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.

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arXiv2022

A self-consistent wave description of axion miniclusters and their survival in the galaxy

We present a solution of the Schrödinger-Poisson system based on the WKB ansatz for the wave function. In this way we obtain a description of a gravitationally bound clump of axion dark matter by a superposition of energy eigenstates with random phases. It can be applied to any self-consistent pair of radial density distribution and phase space density $f(E)$ related by Eddington's formula. We adopt this as a model for axion miniclusters in our galaxy and use it to study the mass loss due to a star encounter by using standard perturbation theory methods known from quantum mechanics. Finally, we perform a Monte Carlo study to estimate the surviving fraction of axion miniclusters in the dark matter halo of our galaxy. We find that the reaction to perturbations and the survival probability depend crucially on the density profile. Weakly bound clusters are heated up and eventually destroyed, whereas more strongly bound systems get even more compact as a result of perturbations and are driven towards an axion star configuration.

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arXiv2022

Constraining active-sterile neutrino transition magnetic moments at DUNE near and far detectors

We consider the sensitivity of the DUNE experiment to a heavy neutral lepton, HNL (also known as sterile neutrino) in the mass range from a few MeV to a few GeV, interacting with the Standard Model via a transition magnetic moment to the active neutrinos, the so-called dipole portal. The HNL is produced via the upscattering of active neutrinos, and the subsequent decay inside the detector provides a single-photon signal. We show that the tau-neutrino dipole portal can be efficiently probed at the DUNE far detector, using the tau-neutrino flux generated by neutrino oscillations, while the near detector provides better sensitivity to the electron- and muon-neutrino dipole portal. DUNE will be able to explore large regions of currently unconstrained parameter space and has comparable sensitivity to other planned dedicated experiments, such as SHiP. We also comment briefly on the sensitivity to pure HNL mixing with the tau neutrino at the DUNE far detector.

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arXiv2022

Neutrino mass and mass ordering: No conclusive evidence for normal ordering

The extraction of the neutrino mass ordering is one of the major challenges in particle physics and cosmology, not only for its implications for a fundamental theory of mass generation in nature, but also for its decisive role in the scale of future neutrinoless double beta decay experimental searches. It has been recently claimed that current oscillation, beta decay and cosmological limits on the different observables describing the neutrino mass parameter space provide robust decisive Bayesian evidence in favor of the normal ordering of the neutrino mass spectrum [arXiv:2203.14247]. We further investigate these strong claims using a rich and wide phenomenology, with different sampling techniques of the neutrino parameter space. Contrary to the findings of Jimenez et al [arXiv:2203.14247], no decisive evidence for the normal mass ordering is found. Neutrino mass ordering analyses must rely on priors and parameterizations that are ordering-agnostic: robust results should be regarded as those in which the preference for the normal neutrino mass ordering is driven exclusively by the data, while we find a difference of up to a factor of 33 in the Bayes factors among the different priors and parameterizations exploited here. An ordering-agnostic prior would be represented by the case of parameterizations sampling over the two mass splittings and a mass scale, or those sampling over the individual neutrino masses via normal prior distributions only. In this regard, we show that the current significance in favor of the normal mass ordering should be taken as $2.7σ$ (i.e. moderate evidence), mostly driven by neutrino oscillation data.

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arXiv2022

Cosmic Neutrino Background Detection in Large-Neutrino-Mass Cosmologies

The Cosmic Neutrino Background (CNB) is a definite prediction of the standard cosmological model and its direct discovery would represent a milestone in cosmology and neutrino physics. In this work, we consider the capture of relic neutrinos on a tritium target as a possible way to detect the CNB, as aimed for by the PTOLEMY project. Crucial parameters for this measurement are the absolute neutrino mass $m_ν$ and the local neutrino number density $n_ν^{\rm loc}$. Within the $Λ$CDM model, cosmology provides a stringent upper limit on the sum of neutrino masses of $\sum m_ν< 0.12\,{\rm eV}$, with further improvements expected soon from galaxy surveys by DESI and EUCLID. This makes the prospects for a CNB detection and a neutrino mass measurement in the laboratory very difficult. In this context, we consider a set of non-standard cosmological models that allow for large neutrino masses ($m_ν\sim 1\,{\rm eV}$), potentially in reach of the KATRIN neutrino mass experiment or upcoming neutrinoless double-beta decay searches. We show that the CNB detection prospects could be much higher in some of these models compared to those in $Λ$CDM, and discuss the potential for such a detection to discriminate between cosmological scenarios. Moreover, we provide a simple rule to estimate the required values of energy resolution, exposure, and background rate for a PTOLEMY-like experiment to cover a certain region in the $(m_ν,\, n_ν^{\rm loc})$ parameter space. Alongside this paper, we publicly release a code to calculate the CNB sensitivity in a given cosmological model.

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arXiv2022

Statistical significance of the sterile-neutrino hypothesis in the context of reactor and gallium data

We evaluate the statistical significance of the 3+1 sterile-neutrino hypothesis using $ν_e$ and $\barν_e$ disappearance data from reactor, solar and gallium radioactive source experiments. Concerning the latter, we investigate the implications of the recent BEST results. For reactor data we focus on relative measurements independent of flux predictions. For the problem at hand, the usual $χ^2$-approximation to hypothesis testing based on Wilks' theorem has been shown in the literature to be inaccurate. We therefore present results based on Monte Carlo simulations, and find that this typically reduces the significance by roughly $1\,σ$ with respect to the naïve expectation. We find no significant indication in favor of sterile-neutrino oscillations from reactor data. On the other hand, gallium data (dominated by the BEST result) show more than $5\,σ$ of evidence supporting the sterile-neutrino hypothesis, favoring oscillation parameters in agreement with constraints from reactor data. This explanation is, however, in significant tension ($\sim 3\,σ$) with solar neutrino experiments. In order to assess the robustness of the signal for gallium experiments we present a discussion of the impact of cross-section uncertainties on the results.

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arXiv2022

Model-independent test of T violation in neutrino oscillations

We propose a method to establish time reversal symmetry violation at future neutrino oscillation experiments in a largely model-independent way. We introduce a general parametrization of flavour transition probabilities which holds under weak assumptions and covers a large class of new-physics scenarios. This can be used to search for the presence of T-odd components in the transition probabilities by comparing data at different baselines but at the same neutrino energies. We show that this test can be performed already with experiments at three different baselines and might be feasible with experiments under preparation/consideration.

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arXiv2022

Testing sterile neutrino mixing with present and future solar neutrino data

We investigate the sensitivity of solar neutrino data to mixing of sterile neutrinos with masses $\gtrsim$ eV. For current data, we perform a Feldman-Cousins analysis to derive a robust limit on the sterile neutrino mixing. The solar neutrino limit excludes significant regions of the parameter space relevant to hints from reactor and radioactive gallium source experiments. We then study the sensitivity of upcoming solar neutrino data, most notably elastic neutrino-electron scattering in the DARWIN and DUNE experiments as well as coherent neutrino-nucleus scattering in DARWIN. These high precision measurements will increase the sensitivity to sterile neutrino mixing by about a factor of 4.5 compared to present limits. As a by-product, we introduce a simplified solar neutrino analysis using only four data points: the low- and high-energy $ν_e$ survival and transition probabilities. We show that this simplified analysis is in excellent agreement with a full solar neutrino analysis; it is very easy to handle numerically and can be applied to any new physics model in which the energy dependence of the $ν_e$ transition probabilities is not significantly modified.

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arXiv2021

On T violation in non-standard neutrino oscillation scenarios

We discuss time reversal (T) violation in neutrino oscillations in generic new physics scenarios. A general parameterization is adopted to describe flavour evolution, which captures a wide range of new physics effects, including non-standard neutrino interactions, non-unitarity, and sterile neutrinos in a model-independent way. In this framework, we discuss general properties of time reversal in the context of long-baseline neutrino experiments. Special attention is given to fundamental versus environmental T violation in the presence of generic new physics. We point out that T violation in the disappearance channel requires new physics which modifies flavour mixing at neutrino production and detection. We use time-dependent perturbation theory to study the effect of non-constant matter density along the neutrino path, and quantify the effects for the well studied baselines of the DUNE, T2HK, and T2HKK projects. The material presented here provides the phenomenological background for the model-independent test of T violation proposed by us in Ref. [1].

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arXiv2021

NuFIT: Three-Flavour Global Analyses of Neutrino Oscillation Experiments

In this contribution, we summarise the determination of neutrino masses and mixing arising from global analysis of data from atmospheric, solar, reactor, and accelerator neutrino experiments performed in the framework of three-neutrino mixing and obtained in the context of the NuFIT collaboration. Apart from presenting the latest status as of autumn 2021, we discuss the evolution of global-fit results over the last 10 years, and mention various pending issues (and their resolution) that occurred during that period in the global analyses.

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arXiv2021

Resolving the LMA-dark NSI degeneracy with coherent neutrino-nucleus scattering

In the presence of non-standard neutrino interactions (NSI), a degeneracy exists in neutrino oscillation data, which involves the flipping of the octant of the mixing angle ${θ_{12}}$ and the type of the neutrino mass ordering. In this article, we revisit the status of this degeneracy in the light of recent data on coherent elastic neutrino-nucleus scattering (CE$ν$NS) from the COHERENT experiment. For general relative couplings to up and down quarks, the degeneracy is disfavoured at the $2σ$ level by the latest data but remains at a higher confidence level. We investigate the requirements of future CE$ν$NS measurements to resolve the degeneracy with high significance. We find that a measurement involving both, electron and muon neutrino flavours and a target with a neutron-to-proton ratio close to 1 is required. For example, an experiment with a silicon target at the European Spallation Source can resolve the degeneracy at more than $4σ$ for arbitrary relative couplings to up and down quarks.

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arXiv2021

Statistical interpretation of sterile neutrino oscillation searches at reactors

A considerable experimental effort is currently under way to test the persistent hints for oscillations due to an eV-scale sterile neutrino in the data of various reactor neutrino experiments. The assessment of the statistical significance of these hints is usually based on Wilks' theorem, whereby the assumption is made that the log-likelihood is $χ^2$-distributed. However, it is well known that the preconditions for the validity of Wilks' theorem are not fulfilled for neutrino oscillation experiments. In this work we derive a simple asymptotic form of the actual distribution of the log-likelihood based on reinterpreting the problem as fitting white Gaussian noise. From this formalism we show that, even in the absence of a sterile neutrino, the expectation value for the maximum likelihood estimate of the mixing angle remains non-zero with attendant large values of the log-likelihood. Our analytical results are then confirmed by numerical simulations of a toy reactor experiment. Finally, we apply this framework to the data of the Neutrino-4 experiment and show that the null hypothesis of no-oscillation is rejected at the 2.6\,$σ$ level, compared to 3.2\,$σ$ obtained under the assumption that Wilks' theorem applies.

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arXiv2020

How to suppress exponential growth -- on the parametric resonance of photons in an axion background

Axion--photon interactions can lead to an enhancement of the electromagnetic field by parametric resonance in the presence of a cold axion background, for modes with a frequency close to half the axion mass. In this paper, we study the role of the axion momentum dispersion as well as the effects of a background gravitational potential, which can detune the resonance due to gravitational redshift. We show, by analytical as well as numerical calculations, that the resonance leads to an exponential growth of the photon field only if (a) the axion momentum spread is smaller than the inverse resonance length, and (b) the gravitational detuning distance is longer than the resonance length. For realistic parameter values, both effects strongly suppress the resonance and prevent the exponential growth of the photon field. In particular, the redshift due to the gravitational potential of our galaxy prevents the resonance from developing for photons in the observable frequency range, even assuming that all the dark matter consists of a perfectly cold axion condensate. For axion clumps with masses below $\sim 10^{-13}\, M_\odot$, the momentum spread condition is more restrictive, whereas, for more massive clumps, the redshift condition dominates.

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arXiv2020

Post-inflationary axion isocurvature perturbations facing CMB and large-scale structure

Dark matter comprised of axion-like particles (ALPs) generated by the realignment mechanism in the post-inflationary scenario leads to primordial isocurvature fluctuations. The power spectrum of these fluctuations is flat for small wave numbers, extending to scales accessible with cosmological surveys. We use the latest measurements of Cosmic Microwave Background (CMB) primary anisotropies together with CMB lensing, Baryonic Acoustic Oscillations (BAO) and Sunyaev Zel'dovich (SZ) cluster counts to measure the amplitude and tilt of the isocurvature component. We find preference for a white-noise isocurvature component in the CMB primary anisotropies; this conclusion is, however, weakened by current large-scale structure (LSS) data. Interpreting the result as a conservative upper limit on the isocurvature component, the combined bound on the ALP mass from all probes is $m_{a} \gtrsim 10^{-19}$ eV, with some dependence on how $m_{a}$ evolves with temperature. The expected sensitivity of cosmic shear and galaxy clustering from future LSS experiments and CMB lensing suggests improved bounds of $m_{a} \gtrsim 10^{-18}$-$10^{-13}$ eV, depending on scale cuts used to avoid non-linearities and the ALP mass-temperature dependence.

astro-ph.CO↗

arXiv2020

The fate of hints: updated global analysis of three-flavor neutrino oscillations

Our herein described combined analysis of the latest neutrino oscillation data presented at the Neutrino2020 conference shows that previous hints for the neutrino mass ordering have significantly decreased, and normal ordering (NO) is favored only at the $1.6σ$ level. Combined with the $χ^2$ map provided by Super-Kamiokande for their atmospheric neutrino data analysis the hint for NO is at $2.7σ$. The CP conserving value $δ_\text{CP} = 180^\circ$ is within $0.6σ$ of the global best fit point. Only if we restrict to inverted mass ordering, CP violation is favored at the $\sim 3σ$ level. We discuss the origin of these results - which are driven by the new data from the T2K and NOvA long-baseline experiments -, and the relevance of the LBL-reactor oscillation frequency complementarity. The previous $2.2σ$ tension in $Δm^2_{21}$ preferred by KamLAND and solar experiments is also reduced to the $1.1σ$ level after the inclusion of the latest Super-Kamiokande solar neutrino results. Finally we present updated allowed ranges for the oscillation parameters and for the leptonic Jarlskog determinant from the global analysis.

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