Narrow Results

The high precision measurements of the cosmic microwave background by the Planck survey yielded tight constraints on cosmological parameters and the statistics of the density fluctuations at the time of recombination. This provides the means for a critical study of structure formation in the Universe by comparing the microwave background results with present epoch measurements of the cosmic large-scale structure. It can reveal subtle effects such as how different forms of Dark Matter may modify structure growth. Currently most interesting is the damping effect of structure growth by massive neutrinos. Different observations of low redshift matter density fluctuations provided evidence for a signature of massive neutrinos. Here we discuss the study of the cosmic large-scale structure with a complete sample of nearby, X-ray luminous clusters from our REFLEX cluster survey. From the observed X-ray luminosity function and its reproduction for different cosmological models, we obtain tight constraints on the cosmological parameters describing the matter density, ${\mathrm{\Omega }}_m$, and the density fluctuation amplitude, ${\sigma }_8$. A comparison of these constraints with the Planck results shows a discrepancy in the framework of a pure $\mathrm{\Lambda }$CDM model, but the results can be reconciled, if we allow for a neutrino mass in the range of 0.17 eV to 0.7 eV. Also some others, but not all of the observations of the nearby large-scale structure provide evidence or trends for signatures of massive neutrinos. With further improvement in the systematics and future survey projects, these indications will develop into a definitive measurement of neutrino masses.

Numerous observations point towards the existence of an unknown elementary particle with no electromagnetic interactions, a large population of which was presumably produced in the early stages of the history of the Universe. This so-called dark matter has survived until the present day, accounting for the 26% of the present energy budget of the Universe. It remains an open question whether the particles comprising the dark matter are absolutely stable or whether they have a finite but very long lifetime, which is a possibility since there is no known general principle guaranteeing perfect stability. In this paper, we review the observational limits on the lifetime of dark matter particles with mass in the GeV–TeV range using observations of the cosmic fluxes of antimatter, gamma-rays and neutrinos. We also examine some theoretically motivated scenarios that provide decaying dark matter candidates.

We review the model building aspects for keV sterile neutrinos as Dark Matter (DM) candidates. After giving a brief discussion of some cosmological, astrophysical and experimental aspects, we first discuss the currently known neutrino data and observables. We then explain the purpose and goal of neutrino model building, and review some generic methods used. Afterwards certain aspects specific for keV neutrino model building are discussed, before reviewing the bulk of models in the literature. We try to keep the discussion on a pedagogical level, while nevertheless pointing out some finer details where necessary and useful. Ideally, this review should enable a grad student or an interested colleague from cosmology or astrophysics with some prior experience to start working on the field.

It has been hypothesized recently that core collapse supernovae are triggered by mildly relativistic jets following observations of radio properties of these explosions. Association of a jet, similar to a gamma-ray burst jet but only slower, allows shock acceleration of particles to high energy and non-thermal neutrino emission from a supernova. Detection of these high energy neutrinos in upcoming kilometer scale Cherenkov detectors may be the only direct way to probe inside these astrophysical phenomena as electromagnetic radiation is thermal and contains little information. Calculation of high energy neutrino signal from a simple and slow jet model buried inside the pre-supernova star is reviewed here. The detection prospect of these neutrinos in water or ice detector is also discussed in this brief review. Jetted core collapse supernovae in nearby galaxies may provide the strongest high energy neutrino signal from point sources.

Neutrino experiments study the least understood of the Standard Model particles by observing their direct interactions with matter or searching for ultra-rare signals. The study of neutrinos typically requires overcoming large backgrounds, elusive signals, and small statistics. The introduction of state-of-the-art machine learning tools to solve analysis tasks has made major impacts to these challenges in neutrino experiments across the board. Machine learning algorithms have become an integral tool of neutrino physics, and their development is of great importance to the capabilities of next generation experiments. An understanding of the roadblocks, both human and computational, and the challenges that still exist in the application of these techniques is critical to their proper and beneficial utilization for physics applications. This review presents the current status of machine learning applications for neutrino physics in terms of the challenges and opportunities that are at the intersection between these two fields.

We review the computation of and associated uncertainties in the current understanding of the relic neutrino background due to core-collapse supernovae, black hole formation and neutron star merger events. We consider the current status of uncertainties due to the nuclear equation of state (EoS), the progenitor masses, the source supernova neutrino spectrum, the cosmological star formation rate, the stellar initial mass function, neutrino oscillations, and neutrino self-interactions. We summarize the current viability of future neutrino detectors to distinguish the nuclear EoS and the temperature of supernova neutrinos via the detected relic supernova neutrino spectrum.

Following earlier studies that provided a consistent theory of kinematics for tachyons (faster-than-light particles), we here embark on a study of tachyon dynamics, both in classical physics and in the quantum theory. Examining a general scattering process, we come to recognize that the labels given to “in” and “out” states are not Lorentz invariant for tachyons; and this lets us find a sensible interpretation of negative energy states. For statistical mechanics, as well as for scattering problems, we study what should be the proper expression for density of states for tachyons. We review the previous work on quantization of a Dirac field for tachyons and go on to expand earlier considerations of neutrinos as tachyons in the context of cosmology. We stumble into the realization that tachyon neutrinos would contribute to gravitation with the opposite sign compared to tachyon antineutrinos. This leads to the gobsmacking prediction that the Cosmic Neutrino Background, if they are indeed tachyons, might explain both phenomena of Dark Matter and Dark Energy. This theoretical study also makes contact with the anticipated results from the experiments KATRIN and PTOLEMY, which focus on beta decay and neutrino absorption by Tritium.

We show that a very simple solution to the strong CP problem naturally leads to Dirac neutrinos. Small effective neutrino masses emerge from a type-I Dirac seesaw mechanism. Neutrino mass limits probe the axion parameters in regions currently inaccessible to conventional searches.

The electron and muon number violating muonium–antimuonium oscillation process can proceed provided that neutrinos have nonzero masses and mix among the various generations. Modifying the Standard Model only by the inclusion of singlet right-handed neutrino fields and allowing for general neutrino masses and mixings, the leading order matrix element contributing to this process is computed. For the particularly interesting case where the neutrino masses are generated by a seesaw mechanism with a very large Majorana mass M_R≫M_W, it is found that both the very light and very heavy Majorana neutrinos each give comparable contributions to the oscillation time scale proportional to . Present experimental limits set by the non-observation of the oscillation process sets a lower limit on M_R of roughly of order 10⁴ GeV.

The Cosmic Microwave Background (CMB) yields information about our Universe at around 380,000 years after the Big Bang (BB). Due to the weak interaction of the neutrinos with matter, the Cosmic Neutrino Background (CNB) should give information about a much earlier time of our Universe, around one second after the BB. Probably, the most promising method to “see” the CNB is the capture of the electron neutrinos from the Background by Tritium, which then decays into ³He and an electron with the energy of the the $Q$-value $=$ 18.562keV plus the electron neutrino rest mass. The “KArlsruhe TRItium Neutrino” (KATRIN) experiment, which is in preparation, seems presently the most sensitive proposed method for measuring the electron antineutrino mass. At the same time, KATRIN can also look by the reaction ${\nu }_e(\sim 1.95\mathrm{\text{K}}){+}^3\mathrm{\text{H}}{\to }^3\mathrm{\text{He}}+e^{-}(Q=18.6\mathrm{\text{keV}}+m_{\nu e}{c}^2)$. The capture of the Cosmic Background Neutrinos (CNB) should show in the electron spectrum as a peak by the electron neutrino rest mass above $Q$. Here, the possibility to see the CNB with KATRIN is studied. A detection of the CNB by KATRIN seems not to be possible at the moment. But KATRIN should be able to determine an upper limit for the local electron neutrino density of the CNB.

Substantial fluxes of protons and leptons with energies below the geomagnetic cutoff have been measured by the AMS experiment at altitudes of 370-390 Km, in the latitude interval ±51.7°. The production mechanisms of the observed trapped fluxes are investigated in detail by means of the FLUKA Monte Carlo simulation code. All known processes involved in the interaction of the cosmic protons with the atmosphere (detailed descriptions of the magnetic field and atmospheric density, as well as the electromagnetic and nuclear interaction processes) are included in the simulation. The results are presented and compared with the experimental data, indicating good agreement with the observed fluxes. The impact of secondary proton flux on particle production in atmosphere is briefly discussed.

The CDF-II collaboration’s recent high-precision measurement of $W$-boson mass indicates new physics contribution(s) beyond the Standard Model. We investigate the possibility of the well-known canonical Scotogenic model to explain the CDF-II measurement. The Scotogenic model is a popular scenario beyond the Standard Model that induces neutrino masses at the one-loop level and includes a viable dark matter candidate, either scalar or fermionic. For both scalar and fermionic dark matter possibilities, we simultaneously examine the constraints coming from (a) neutrino mass, oscillation, neutrinoless double beta decay and lepton flavor violation experiments, (b) from LEP and LHC, (c) from dark matter relic density and direct detection experiments, (d) from the oblique $S,T,U$ parameter values consistent with CDF-II $W$-boson measurement. We demonstrate that the new CDF-II measurement rules out the feasible parameter space of the scalar dark matter in the high mass regions ($m_{{\eta }_R}\gtrsim 500\mathrm{\text{GeV}}$), while still allowing the intermediate mass regions $54\mathrm{\text{GeV}}\lesssim m_{{\eta }_R}\lesssim 76\mathrm{\text{GeV}}$. We also showed that the fermionic dark matter candidate in the canonical Scotogenic model, in the range $M_{N_1}\lesssim 500\mathrm{\text{GeV}}$, can simultaneously explain all the aforementioned issues. Furthermore, we investigated how the recent findings from ATLAS 2023 impact this study.

Self-interacting dark matter (SIDM) is a hypothetical form of dark matter (DM), characterized by relatively strong (compared to the weak interaction strength) self-interactions (SIs), which has been proposed to resolve a number of issues concerning tensions between simulations and observations at the galactic or smaller scales. We review here some recent developments discussed at the 14th Marcel Grossmann Meeting (MG14), paying particular attention to restrictions on the SIDM (total) cross-section from using novel observables in merging galactic structures, as well as the rôle of SIDM on the Milky Way halo and its central region. We report on some interesting particle-physics inspired SIDM models that were discussed at MG14, namely the glueball DM, and a right-handed neutrino DM (with mass of a few tens of keV, that may exist in minimal extensions of the standard model (SM)), interacting among themselves via vector bosons mediators in the dark sector. A detailed phenomenology of the latter model on galactic scales, as well as the potential role of the right handed neutrinos in alleviating some of the small-scale cosmology problems, namely the discrepancies between observations and numerical simulations within standard $\mathrm{\Lambda }$CDM and $\mathrm{\Lambda }$WDM cosmologies are reported.

In this work, we study the interaction between a complex neutral scalar boson, the axion and neutrinos, as a possible channel for the generation of the neutrino mass. We calculate one-loop corrections to the neutrino propagator and determine the value of the neutrino mass induced by the interactions with axions. We have obtained a set of relationships between the axion mass, the electron–neutrino mass and the strength of the coupling using current experimental limits on the nonobservation of the neutrinoless double beta decay.

We investigate the interaction between the cosmological relic neutrinos, and primordial gravitational waves entering the horizon before the electroweak phase transition, corresponding to observable frequencies today ν₀≳10^-5Hz. We give an analytic formula for the traceless transverse part of the anisotropic stress tensor, due to weakly interacting neutrinos, and derive an integro-differential equation describing the propagation of cosmological gravitational waves at these conditions. We find that this leads to a decrease of the wave intensity in the frequency region accessible to the LISA space interferometer, that is at the present the most promising way to obtain a direct detection of a cosmological gravitational wave. The absorbed intensity does not depend neither on the perturbation wavelength, nor on the details of neutrino interactions, and is affected only by the neutrino fraction f_ν. The transmitted intensity amounts to 88% for the standard value f_ν=0.40523. An approximate formula for non-standard values of f_ν is given.

A minus sign is inserted, for good reason, into the formula for the energy–momentum tensor for tachyons. This leads to remarkable theoretical consequences and a plausible explanation for the phenomenon called dark energy in the cosmos.

One of the main purposes of SHiP experiment is to shed light on neutrino mass generation mechanisms like the so-called seesaw. We consider a minimal type-I seesaw neutrino mass mechanism model with two heavy neutral leptons (right-handed or sterile neutrinos) with arbitrary masses. Extremely high active-sterile mixing angle requires a correlation between the phases of the Dirac neutrino couplings. Actual experimental limits on the half-life of neutrinoless double beta decay $0\nu \beta \beta$-rate on the active-sterile mixing angle are not significative in constraining the masses or the mixing measurable by SHiP.

This work expands previous efforts, within the classical theories of Special and General Relativity, to include tachyons (faster-than-light particles) along with ordinary (slower-than-light) particles at any energy. The objective here is to construct a Hamiltonian that includes both the particles and the gravitational field that they produce. We do this with a linear approximation for the Einstein field equations; and we also assume a time-independent gravitational metric implied by a static picture of the particles’ motion. The resulting formulas will allow serious modeling to test the idea that cosmic background neutrinos may be tachyons, which can produce the observed gravitational effects now ascribed to some mysterious Dark Matter.

Chiral symmetry is included into the Dirac equation using the irreducible representations of the Poincaré group. The symmetry introduces the chiral angle that specifies the chiral basis. It is shown that the correct identification of these basis allows explaining small masses of neutrinos and predicting a new candidate for Dark Matter massive particle.

Neutrino oscillations in a core-collapse supernova may be responsible for the observed rapid motions of pulsars. Given the present bounds on the neutrino masses, the pulsar kicks require a sterile neutrino with mass 2–20 keV and a small mixing with the active neutrinos. The same particle can be the cosmological dark matter. Its existence can be confirmed the by the X-ray telescopes if they detect a 1–10 keV photon line from the decays of the relic sterile neutrinos. In addition, one may be able to detect gravity waves from a pulsar being accelerated by neutrinos in the event of a nearby supernova.