Heavy Sterile Neutrinos Research Articles

Sterile neutrino dark matter and low scale leptogenesis from a charged scalar.

We show that novel paths to dark matter generation and baryogenesis are open when the standard model is extended with three sterile neutrinos N_i and a charged scalar delta ^+. Specifically, we propose a new production mechanism for the dark matter particle—a multi-keV sterile neutrino, N_1—that does not depend on the active-sterile mixing angle and does not rely on a large primordial lepton asymmetry. Instead, N_1 is produced, via freeze-in, by the decays of delta ^+ while it is in equilibrium in the early Universe. In addition, we demonstrate that, thanks to the couplings between the heavier sterile neutrinos N_{2,3} and delta ^+, baryogenesis via leptogenesis can be realized close to the electroweak scale. The lepton asymmetry is generated either by N_{2,3}-decays for masses M_{2,3}gtrsim TeV, or by N_{2,3}-oscillations for M_{2,3}sim GeV. Experimental signatures of this scenario include an X-ray line from dark matter decays, and the direct production of delta ^+ at the LHC. This model thus describes a minimal, testable scenario for neutrino masses, the baryon asymmetry, and dark matter.

Proton decay and new contribution to 0ν2β decay in SO(10) with low-mass Z′ boson, observable n − n ¯ $$ n-\overline{n} $$ oscillation, lepton flavor violation, and rare kaon decay

In the conventional approach to observable $$ n-\overline{n} $$ oscillation through Pati-Salam intermediate gauge symmetry in SO(10), the canonical seesaw mechanism is also constrained by the symmetry breaking scale M R ∼ M C ≤ 106 GeV which yields light neutrino masses several orders larger than the neutrino oscillation data. A method to evade this difficulty is through TeV scale gauged inverse seesaw mechanism which has been recently exploited while predicting experimentally verifiable W ± , Z R bosons with a new dominant contribution to neutrinoless double beta decay in the W L −W L channel and other observable phenomena, but with proton lifetime far beyond the accessible limits. In the present work, adopting the view that W ± may be heavy and currently inaccessible to accelerator tests, we show how a class of non-supersymmetric SO(10) models allows a TeV scale Z′ boson, experimentally testable proton decay along with observable $$ n-\overline{n} $$ oscillation, and lepto-quark gauge boson mediated rare kaon decays without resorting to additional fine-tuning of parameters. The occurrence of Pati-Salam gauge symmetry with unbroken D-parity and two gauge couplings at the highest intermediate scale guarantees precision unification with vanishing GUT-threshold or gravitational corrections on sin2 θ W (M Z ) prediction in this model. Predictions for neutrinoless double beta decay in the W L − W L channel is analysed in detail including light and heavy sterile neutrino exchange contributions by means of normal and band plots and also by scattered plots while a new formula for half-life is derived. Comparison with available data from various groups by normal and scattered plots reveals how the existing experimental bounds are satisfied irrespective of the mass hierarchy in the light neutrino sector leading to the lower bound on the lightest sterile neutrino mass, $$ {\widehat{M}}_{S_1}\ge 18\pm 2.9 $$ GeV. The model also predicts branching ratios for charged lepton flavor violation verifiable by ongoing search experiments. We also derive new renormalisation group equations constraining the lepto-quark gauge boson mass in the presence of SU(2) L × U(1) R × U(1) B−L × SU(3) C symmetry, specific to the occurrence of extra Z′ boson, leading to a new lower bound on the lepto-quark gauge boson mass mediating rare kaon decay, M LQ ≥ (1.54±0.06) × 106 GeV. We also discuss the symmetry breaking of non-SUSY SO(10) through the well known flipped SU(5) × Ũ(1) path and show, for the first time, how TeV scale Z′ is predicted with gauged inverse seesaw ansatz for neutrino masses and substantial lepton flavor and lepton number violations. As a significant new result along this path, we report a successful unification of the two gauge couplings of SU(5) × Ũ(1) into the single GUT coupling of SO(10).

Searches for New Physics with the TREK Detector

The KEK Experiment E246 has provided the best upper limit of the transverse muon polarization in Kμ3 decays to date. The E246 detector is now being upgraded by the TREK collaboration and upcoming experiments with this detector at the J-PARC facility will allow new high-precision measurements in search for physics beyond the Standard Model. The first round of planned experiments include the search for lepton universality violation in a measurement of the ratio of the Ke2 and Kμ2 decay widths, as well as the search for heavy sterile neutrinos and possible massive gauge boson A' in the 10 to 100 MeV mass range. The search for T violation in kaon decays requires a higher intensity hadron beam and will be carried out later.

Nearly degenerate heavy sterile neutrinos in cascade decay: Mixing and oscillations

Some extensions beyond the Standard Model propose the existence of nearly degenerate heavy sterile neutrinos. If kinematically allowed these can be resonantly produced and decay in a cascade to common final states. The common decay channels lead to mixing of the heavy sterile neutrino states and interference effects. We implement non-perturbative methods to study the dynamics of the cascade decay to common final states, which features similarities but also noteworthy differences with the case of neutral meson mixing. We show that mixing and oscillations among the nearly degenerate sterile neutrinos can be detected as \emph{quantum beats} in the distribution of final states produced from their decay. These oscillations would be a telltale signal of mixing between heavy sterile neutrinos. We study in detail the case of two nearly degenerate sterile neutrinos produced in the decay of pseudoscalar mesons and decaying into a purely leptonic "visible" channel: $\nu_h \rightarrow e^+ e^- \nu_a$. Possible cosmological implications for the effective number of neutrinos $N_{eff}$ are discussed.

Effect of cancellation in neutrinoless double beta decay

In light of recent experimental results, we carefully analyze the effects of interference in neutrinoless double beta decay, when more than one mechanism is operative. If a complete cancellation is at work, the half-life of the corresponding isotope is infinite, and any constraint on it will automatically be satisfied. We analyze this possibility in detail assuming a cancellation in $^{136}\mathrm{Xe}$, and find its implications on the half-life of other isotopes, such as $^{76}\mathrm{Ge}$. For definiteness, we consider the role of light and heavy sterile neutrinos. In this case, the effective Majorana mass parameter can be redefined to take into account all contributions, and its value gets suppressed. Hence, larger values of neutrino masses are required for the same half-life. The canonical light neutrino contribution cannot saturate the present limits of half-lives or the positive claim of observation of neutrinoless double beta decay, once the stringent bounds from cosmology are taken into account. For the case of cancellation, where all the sterile neutrinos are heavy, the tension between the results from neutrinoless double beta decay and cosmology becomes more severe. We show that the inclusion of light sterile neutrinos in this setup can resolve this issue. Using the recent results from GERDA, we derive upper limits on the active-sterile mixing angles and compare them with the case of no cancellation. The required values of the mixing angles become larger, if a cancellation is at work. A direct test of destructive interference in $^{136}\mathrm{Xe}$ is provided by the observation of this process in other isotopes, and we study in detail the correlation between their half-lives. Finally, we discuss the model realizations which can accommodate light and heavy sterile neutrinos and the cancellation. We show that sterile neutrinos of few hundred MeV or GeV mass range, coming from an Extended seesaw framework or a further extension, can satisfy the required cancellation.

Helicitogenesis: WIMPy baryogenesis with sterile neutrinos and other realizations

We propose a mechanism for baryogenesis from particle decays or annihilations that can work at the TeV scale. Some heavy particles annihilate or decay into a heavy sterile neutrino N (with M > 0.5 TeV) and a "light" one \nu (with m << 100 GeV), generating an asymmetry among the two helicity degrees of freedom of \nu. This asymmetry is partially transferred to Standard Model leptons via fast Yukawa interactions and reprocessed into a baryon asymmetry by the electroweak sphalerons. We illustrate this mechanism in a WIMPy baryogenesis model where the helicity asymmetry is generated in the annihilation of dark matter. This model connects the baryon asymmetry, dark matter, and neutrino masses. Moreover it also complements previous studies on general requirements for baryogenesis from dark matter annihilation. Finally we discuss other possible realizations of this helicitogenesis mechanism.

Spherical collapse inνΛCDM

The abundance of massive dark matter halos hosting galaxy clusters provides an important test of the masses of relic neutrino species. The dominant effect of neutrino mass is to lower the typical amplitude of density perturbations that eventually form halos, but for neutrino masses $> 0.4eV$ the threshold for halo formation can be changed significantly as well. We study the spherical collapse model for halo formation in cosmologies with neutrino masses in the range $m_{\nu i} =0.05eV$- $1eV$ and find that halo formation is differently sensitive to $\Omega_\nu$ and $m_\nu$. That is, different neutrino hierarchies with common $\Omega_\nu$ are in principle distinguishable. The added sensitivity to $m_{\nu}$ is small but potentially important for scenarios with heavier sterile neutrinos. Massive neutrinos cause the evolution of density perturbations to be scale-dependent at high redshift which complicates the usual mapping between the collapse threshold and halo abundance. We propose one way of handling this and compute the correction to the halo mass function within this framework. For $\sum m_{\nu i} < 0.3eV$, our prescription for the halo abundance is only $\sim< 15\%$ different than the standard calculation. However for larger neutrino masses the differences approach $50-10\%$ which, if verified by simulations, could alter neutrino mass constraints from cluster abundance.

Search for direct and indirect unitarity violation in neutrino oscillation experiments

If three standard neutrinos mix with other degree of freedoms like sterile neutrinos, no matter how heavy the sterile neutrino masses are, it could result in the unitarity violation in the MNSP matrix. Nevertheless, the unitarity violation induced by the existence of light or heavy sterile neutrinos can have very different effects on neutrino oscillations, we call the former case direct unitarity violation and the later case the indirect unitarity violation. We will explain in this paper the difference of these two kinds of unitarity violations, then focus on the possibilities of searching the unitarity violation in neutrino oscillation experiments, of which the precision reactor experiments with multiple baselines are discussed in detail.

Dark neutrinos as Cold Dark Matter

It is shown that heavy sterile neutrinos can be a consequence of a new broken U(1)X symmetry. This sterile symmetry is further speculated to be related to the conservation of the dark number associated with Dark Matter particles, corresponding to a dark neutrino mass mND⩾1keV with a high scale ⩾∼102GeV dark symmetry U(1)D breaking.

On the keV sterile neutrino search in electron capture

A joint effort of cryogenic microcalorimetry (CM) and high-precision Penning-trap mass spectrometry (PT-MS) in investigating atomic orbital electron capture (EC) can shed light on the possible existence of heavy sterile neutrinos with masses from 0.5 to 100 keV. Sterile neutrinos are expected to perturb the shape of the atomic de-excitation spectrum measured by CM after a capture of the atomic orbital electrons by a nucleus. This effect should be observable in the ratios of the capture probabilities from different orbits. The sensitivity of the ratio values to the contribution of sterile neutrinos strongly depends on how accurately the mass difference between the parent and the daughter nuclides of EC transitions can be measured by, for example, PT-MS. A comparison of such probability ratios in different isotopes of a certain chemical element allows one to exclude many systematic uncertainties, and thus could make feasible a determination of the contribution of sterile neutrinos on a level below 1%. Several electron capture transitions suitable for such measurements are discussed.

Neutrino Production via e−e+ Collision at Z-Boson Peak

The production of the three normal neutrinos via e−e+ collision at Z-boson peak (neutrino production in a Z-factory) is investigated thoroughly. The differences of νe-pair production from νμ-pair and ντ-pair production are presented in various aspects. Namely the total cross sections, relevant differential cross sections and the forward-backward asymmetry etc. for these neutrinos are presented in terms of figures as well as numerical tables. The restriction on the room for the mixing of the three species of light neutrinos with possible externals (heavy neutral leptons and/or sterile neutrinos) from refined measurements of the invisible width of Z-boson is discussed.

Time evolution of cascade decay

We study non-perturbatively the time evolution of cascade decay for generic fields and obtain the time dependence of amplitudes and populations for the resonant and final states. We analyze in detail the different time scales and the manifestation of unitary time evolution in the dynamics of production and decay of resonant intermediate and final states. The probability of occupation (population) ‘flows’ as a function of time from the initial to the final states. When the decay width of the parent particle is much larger than that of the intermediate resonant state there is a ‘bottleneck’ in the flow, the population of resonant states builds up to a maximum at nearly saturating unitarity and decays to the final state on the longer time scale . As a consequence of the wide separation of time scales in this case the cascade decay can be interpreted as evolving sequentially . In the opposite limit the population of resonances () does not build up substantially and the cascade decay proceeds almost directly from the initial parent to the final state without resulting in a large amplitude of the resonant state. An alternative but equivalent non-perturbative method useful in cosmology is presented. Possible phenomenological implications for heavy sterile neutrinos as resonant states and consequences of quantum entanglement and correlations in the final state are discussed.

Charged lepton mixing via heavy sterile neutrinos

Pseudoscalar meson decay leads to an entangled state of charged leptons (μ,e) and massive neutrinos. Tracing out the neutrino degrees of freedom leads to a reduced density matrix for the charged leptons whose off-diagonal elements reveal charged lepton oscillations. Although these decohere on unobservably small time scales ≲10−23 s they indicate charged lepton mixing as a result of common intermediate states. The charged lepton self-energy up to one loop features flavor off-diagonal terms responsible for charged lepton mixing: a dominant “short distance” contribution with W bosons and massive neutrinos in the intermediate state, and a subdominant “large distance” contribution with pseudoscalar mesons and massive neutrinos in the intermediate state. Mixing angle(s) are GIM suppressed, and are momentum and chirality dependent. The difference of negative chirality mixing angles near the muon and electron mass shells is θL(Mμ2)−θL(Me2)∝GF∑Uμjmj2Uje⁎ with mj the mass of the neutrino in the intermediate state. Recent results from TRIUMF, suggest an upper bound θL(p2≃Mμ2)−θL(p2≃Me2)<10−14(MS/100 MeV)2 for one generation of a heavy sterile neutrino with mass MS. We obtain the wavefunctions for the propagating modes, and discuss the relation between the lepton flavor violating process μ→eγ and charged lepton mixing, highlighting that a measurement of such process implies a mixed propagator μ, e. Furthermore writing flavor diagonal vertices in terms of mass eigenstates yields novel interactions suggesting further contributions to lepton flavor violating process as a consequence of momentum and chirality dependent mixing angles.

Searching for the Dirac Nature of Neutrinos: Combining Neutrinoless Double Beta Decay and Neutrino Mass Measurements

We studied the neutrinoless double beta decay process to tackle the issue about the nature of neutrino. Establishing the nature of neutrinos, whether they are Dirac or Majorana particles is one of the fundamental questions we need to answer in particle physics, and is related to the conservation of lepton number. Neutrinoless double beta decay ((ββ)0ν) is the tool of choice for testing the Majorana nature of neutrinos. However, up to now, this process has not been observed, but a wide experimental effort is taking place worldwide and soon new results will become available.Different mechanisms can induce (ββ)0ν-decay and might interfere with each other, potentially leading to suppressed contributions to the decay rate. This possibility would become of great interest if upcoming neutrino mass measurements from KATRIN and cosmological observations found that mν>0.2eV but no positive signal was observed in (ββ)0ν-decay experiments. We focus on the possible interference between light Majorana neutrino exchange with other mechanisms, such as heavy sterile neutrinos and R-parity violating supersymmetric models. We show that in some cases the use of different nuclei would allow to disentangle the different contributions and allow to test the hypothesis of destructive interference. Finally, we present a model in which such interference can emerge and we discuss the range of parameters which would lead to a significant suppression of the decay rate.

MeV dark matter in the3+1+1model

The existence of light sterile neutrinos in the eV mass range with relatively large mixing angles with the active neutrinos has been proposed for a variety of reasons, including to improve the fit to the LSND and MiniBooNE neutrino oscillation experiments, and reactor disappearance experiments. In ref. Phys. Rev. D 84, 053001 (2011), it was shown that neutrino mixing with a heavier sterile neutrino, in the mass range between 33 eV and several GeV, could significantly affect and improve the agreement between neutrino oscillation models with light sterile neutrinos and short baseline experimental results, allowing for a new source of CP violation in appearance experiments and for different apparent mixing angles in appearance and disappearance experiments. However in refs. Phys. Rev. D 86, 033015 (2012) and JHEP 1204, 083 (2012) it was shown that various collider experiment, supernovae, and cosmological constraints can eliminate most of the parameter region where such a heavy sterile neutrino can have a significant effect on neutrino oscillations. In this paper we consider the effects of allowing a new light scalar in the MeV mass region, which is a potential dark matter candidate, to interact with the sterile neutrinos, and show that the resulting model is a consistent theory of neutrino oscillation anomalies and dark matter which can also potentially explain the INTEGRAL excess of 511 keV gamma rays in the central region of the galaxy.

Neutrino masses, dominant neutrinoless double beta decay, and observable lepton flavor violation in left-right models and SO(10) grand unification with low mass W R , Z R bosons

While the detection of $W_R$-boson at the Large Hadron Collider is likely to resolve the mystery of parity violation in weak interaction, observation of neutrinoless double beta decay ($0\nu\beta\beta$) is expected to determine whether neutrinos are Majorana fermions. In this work we consider a class of LR models with TeV scale $W_R, Z_R$ bosons but having parity restoration at high scales where they originate from well known Pati-Salam symmetry or $SO(10)$ grand unified theory minimally extended to accommodate inverse seesaw frame work for neutrino masses. Most dominant new contribution to neutrinoless double beta decay is noted to occur via $W_L^{-}W_L^{-}$ mediation involving lighter sterile neutrino exchanges. The next dominant contribution is found to be through $W_L^{-}W_R^{-}$ mediation involving both light and heavy right-handed neutrino or sterile neutrino exchanges. The quark-lepton symmetric origin of the computed value of the Dirac neutrino mass matrix is also found to play a crucial role in determining these and other results on lepton flavor violating branching ratios for $\tau \rightarrow e + \gamma$, $\tau \rightarrow \mu + \gamma$, and $\mu \rightarrow e + \gamma$ accessible to ongoing search experiments. The underlying non-unitarity matrix is found to manifest in substantial CP-violating effects even when the leptonic Dirac phase $\delta_{\rm CP} \simeq 0, \pi, 2 \pi$. Finally we explore a possible origin of the model in non-supersymmetric SO(10) grand unified theory where, in addition to low mass $W_R^\pm$ and $Z_R$ bosons accessible to Large Hadron Collider, the model is found to predict observable neutron-antineutron oscillation and lepto-quark gauge boson mediated rare kaon decay with $\mbox{Br} \left(K_{\rm L} \rightarrow \mu\, \bar{e}\right) \simeq \left(10^{-9}- 10^{-11} \right)$.

Searching for sterile neutrinos fromπandKdecays

The production of heavy sterile neutrinos from ${\ensuremath{\pi}}^{\ensuremath{-}}$, ${K}^{\ensuremath{-}}$ decay at rest yields charged leptons with negative helicity (positive for ${\ensuremath{\pi}}^{+}$, ${K}^{+}$). We obtain the branching ratio for this process and argue that a Stern-Gerlach filter with a magnetic field gradient leads to spatially separated domains of both helicity components with abundances determined by the branching ratio. Complemented with a search of the monochromatic peak, this setup can yield both the mass and mixing angles for sterile neutrinos with masses in the range $3\text{ }\text{ }\mathrm{MeV}\ensuremath{\lesssim}{m}_{s}\ensuremath{\lesssim}414\text{ }\text{ }\mathrm{MeV}$ in next generation high intensity experiments. We also study oscillations of light Dirac and Majorana sterile neutrinos with ${m}_{s}\ensuremath{\simeq}\mathrm{eV}$ produced in meson decays including decoherence aspects arising from lifetime effects of the decaying mesons and the stopping distance of the charged lepton in short baseline experiments. We obtain the transition probability from production to detection via charged current interactions including these decoherence effects for $3+1$ and $3+2$ scenarios, also studying $|\ensuremath{\Delta}L|=2$ transitions from $\overline{\ensuremath{\nu}}\ensuremath{\leftrightarrow}\ensuremath{\nu}$ oscillations for Majorana neutrinos and the impact of these effects on the determination of $CP$-violating amplitudes. We argue that decoherence effects are important in current short baseline accelerator experiments, leading to an underestimate of masses, mixing and $CP$-violating angles. At MiniBooNE/SciBooNE we estimate that these effects lead to an $\ensuremath{\sim}15%$ underestimate for sterile neutrino masses ${m}_{s}\ensuremath{\gtrsim}3\text{ }\text{ }\mathrm{eV}$. We argue that reactor and current short baseline accelerator experiments are fundamentally different and suggest that in future high intensity experiments with neutrinos produced from $\ensuremath{\pi}$, $K$ decay at rest, stopping the charged leptons on distances much smaller than the decay length of the parent meson suppresses considerably these decoherence effects.

Towards testing the unitarity of the [formula omitted] lepton flavor mixing matrix in a precision reactor antineutrino oscillation experiment

The 3×3 Maki–Nakagawa–Sakata–Pontecorvo (MNSP) lepton flavor mixing matrix may be slightly non-unitary if the three active neutrinos are coupled with sterile neutrinos. We show that it is in principle possible to test whether the relation |Ve1|2+|Ve2|2+|Ve3|2=1 holds or not in a precision reactor antineutrino oscillation experiment, such as the recently proposed Daya Bay II experiment. We explore three categories of non-unitary effects on the 3×3 MNSP matrix: 1) the indirect effect in the (3+3) flavor mixing scenario where the three heavy sterile neutrinos do not take part in neutrino oscillations; 2) the direct effect in the (3+1) scenario where the light sterile neutrino can oscillate into the active ones; and 3) the interplay of both of them in the (3+1+2) scenario. We find that both the zero-distance effect and flavor mixing factors of different oscillation modes can be used to determine or constrain the sum of |Ve1|2, |Ve2|2 and |Ve3|2 and its possible deviation from one, and the active neutrino mixing angles θ12 and θ13 can be cleanly extracted even in the presence of light or heavy sterile neutrinos. Some useful analytical results are obtained for each of the three scenarios.

On neutrino flavor states

We review the issues associated with the construction of neutrino flavor states. We then provide a consistent proof that the flavor states are approximately well-defined only if neutrinos are ultra-relativistic or the mass differences are negligible compared to energy. However, we show that weak interactions can be consistently described by only neutrino mass eigenstates. Meanwhile, the second quantization of neutrino flavor fields generally has no physical relevance as their masses are indefinite. Therefore, the flavor states are not physical quantum states and they should simply be interpreted as definitions to denote specific linear combinations of mass eigenstates involved in weak interactions. We also briefly discuss the implication of this work for the mixing between active and heavy sterile neutrinos.

Neutrino phenomenology in a3+1+1framework

Evidence continues to grow in the MiniBooNE (MB) antineutrino mode supporting a low-energy excess compatible with the MB neutrino mode and possibly also confirming the results of the LSND experiment. At least one sterile neutrino is required to explain the anomalies consistent with the observations of other experiments. At the same time, there is a strong tension between the positive signals of LSND and MB and the null results of ${\ensuremath{\nu}}_{e}$ and ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ disappearance experiments. We explore a scenario, first proposed in [A. E. Nelson, Phys. Rev. D 84, 053001 (2011).], where the presence of an additional heavy sterile neutrino (with mass well above an eV) can alleviate tension between LSND, MB and the null results of disappearance experiments. We compare and contrast this $3+1+1$ scenario with the more standard $3+1$ scenario and carry out global fits to all oscillation data including new 2011 MB $\overline{\ensuremath{\nu}}$ data. We find that the tension can be somewhat alleviated and that a phenomenologically viable window for the heavy neutrino, consistent with rare decays and big bang nucleosynthesis constraints, can be found if the fifth neutrino has a mass of order $0.3--10\text{ }\text{ }\mathrm{GeV}$. We also find, however, that the 2011 MB $\overline{\ensuremath{\nu}}$ data exacerbates the tension with null experiments in both the $3+1$ and $3+1+1$ models when the lowest energy bins are included, resulting in little improvement in the global fit. We also discuss the implications of an additional neutrino for the reactor and gallium anomalies, and show that an oscillation explanation of the anomalies is disfavored by cosmological considerations, direct searches, and precision electroweak tests.