LHC signatures of sterile neutrinos in a minimal radiative extended seesaw framework

The presence of small neutrino masses and flavour mixings can be accounted for naturally in various models about extensions of the standard model, particularly in the seesaw mechanism models. In this work, we present a minimally extended seesaw framework with two right-handed neutrinos, where the active neutrino masses are derived in the radiative regime. Using the framework it can be shown that within certain mass limits, the light neutrino mass term can approach a form that is similar to its form under type-I seesaw mechanism. Apart from this, we show that the decay width of right-handed neutrinos (produced through the decay of [Formula: see text] boson in a particle collider) is short enough to cause a sufficiently long lifetime for the particles, thus ensuring an observable displacement in the LHC between the production and decay vertices. We comment on the fact that these displaced vertex signatures thus can serve as a means to verify the existence of these right-handed neutrinos in future experiments. Lastly, we line up the possibility of our future work where the vertex signatures of particles greater than the mass of [Formula: see text] boson can be worked upon.

A 4π time-of-flight detector for the ND280/T2K upgrade

ND280 is a near detector of the T2K experiment which is located in the J-PARC accelerator complex in Japan. After a decade of fruitful data-taking, ND280 is scheduled for upgrade. The time-of-flight (ToF) detector, which is described in this article, is one of three new detectors that will be installed in the basket of ND280. The ToF detector has a modular structure. Each module represents an array of 20 plastic scintillator bars which are stacked in a plane of 2.4 × 2.2 m2 area. Six modules of similar construction will be assembled in a cube, thus providing an almost 4π enclosure for an active neutrino target and two TPCs. The light emitted by scintillator is absorbed by arrays of large-area silicon photo-multipliers (SiPMs) which are attached to both ends of every bar. The readout of SiPMs, shaping and analog sum of individual SiPM signals within the array are performed by a discrete circuit amplifier. An average time resolution of about 0.14 ns is achieved for a single bar when measured with cosmic muons. The detector will be installed in the basket of ND280, where it will be used to veto particle originating outside the neutrino target, improve the particle identification and provide a cosmic trigger for calibration of detectors which are enclosed inside it.

Heavy neutral leptons in effective field theory and the high-luminosity LHC

Heavy neutral leptons (HNLs) with masses around the electroweak scale are expected to be rather long-lived particles, as a result of the observed smallness of the active neutrino masses. In this work, we study long-lived HNLs in NRSMEFT, a Standard Model (SM) extension with singlet fermions to which we add non-renormalizable operators up to dimension-6. Operators which contain two HNLs can lead to a sizable enhancement of the production cross sections, compared to the minimal case where HNLs are produced only via their mixing with the SM neutrinos. We calculate the expected sensitivities for the ATLAS detector and the future far-detector experiments: AL3X, ANUBIS, CODEX-b, FASER, MATHUSLA, and MoEDAL-MAPP in this setup. The sensitive ranges of the HNL mass and of the active-heavy mixing angle are much larger than those in the minimal case. We study both, Dirac and Majorana, HNLs and discuss how the two cases actually differ phenomenologically, for HNL masses above roughly 100 GeV.

Searching for non-unitary neutrino oscillations in the present T2K and NO$$\nu $$A data

The mixing of three active neutrino flavors is parameterized by the unitary PMNS matrix. If there are more than three neutrino flavors and if the extra generations are heavy iso-singlets, the effective $$3\times 3$$ 3 × 3 mixing matrix for the three active neutrinos will be non-unitary. We have analyzed the latest T2K and NO$$\nu $$ ν A data with the hypothesis of non-unitary mixing of the active neutrinos. We found that the 2019 NO$$\nu $$ ν A data slightly (at $$\sim 1\, \sigma $$ ∼ 1 σ CL) prefer the non-unitary mixing over unitary mixing. In fact, allowing the non-unitary mixing brings the NO$$\nu $$ ν A best-fit point in the $$\sin ^2{\theta _{23}}-\delta _{\mathrm {CP}}$$ sin 2 θ 23 - δ CP plane closer to the T2K best-fit point. The 2019 T2K data, on the other hand, cannot rule out any of the two mixing schemes. A combined analysis of the NO$$\nu $$ ν A and T2K 2019 data prefers the non-unitary mixing at $$1\, \sigma $$ 1 σ CL. We derive constraints on the non-unitary mixing parameters using the best-fit to the combined NO$$\nu $$ ν A and T2K data. These constraints are weaker than previously found. The latest 2020 data from both the experiments prefer non-unitarity over unitary mixing at $$1\, \sigma $$ 1 σ CL. The combined analysis prefers non-unitarity at $$2\, \sigma $$ 2 σ CL. The stronger tension, which exists between the latest 2020 data of the two experiments, also gets reduced with non-unitary analysis.

Exploring invisible neutrino decay at ESSnuSB

We explore invisible neutrino decay in which a heavy active neutrino state decays into a light sterile neutrino state and present a comparative analysis of two baseline options, 540 km and 360 km, for the ESSnuSB experimental setup. Our analysis shows that ESSnuSB can put a bound on the decay parameter τ3/m3 = 2.64 (1.68) × 10−11 s/eV for the baseline option of 360 (540) km at 3σ. The expected bound obtained for 360 km is slightly better than the corresponding one of DUNE for a charged current (CC) analysis. Furthermore, we show that the capability of ESSnuSB to discover decay, and to measure the decay parameter precisely, is better for the baseline option of 540 km than that of 360 km. Regarding effects of decay in δCP measurements, we find that in general the CP violation discovery potential is better in the presence of decay. The change in CP precision is significant if one assumes decay in data but no decay in theory.

Light dark matter from inflaton decay

We propose a simple mechanism of light dark matter (DM) production from the decay of the oscillating inflaton condensation. If the reheating temperature after inflation is higher than the inflaton mass, which is of the same order of the momentum of the DM at the time of the production, the DM momentum can be suppressed compared to the temperature of the thermal plasma if the interaction of the DM is weak enough. Consequently, the DM can be cold enough to avoid the observational constraints on the warm DM, like the Lyman-α bound even if the DM mass is small. We study the bosonic and fermionic DM production from the inflaton decay, taking into account the effect of the stimulated emission and Pauli blocking, respectively. In both cases, the DM can be cold and abundant enough to be a viable candidate of the DM. We also apply our mechanism to the production of isocurvature-problem-free axion DM and Dirac sea DM of right-handed neutrino consistent the seesaw relation for the active neutrino masses.

Renormalizable SO(10) GUT with Suppressed Dimension-5 Proton Decays

We study a renormalizable SUSY SO(10) GUT model where the Yukawa couplings of single 10, single $${\bf \overline{126}}$$ and single 120 fields, Y10, Y126, Y120, account for the quark and lepton Yukawa couplings and the neutrino mass. We pursue the possibility that Y10, Y126, Y120 reproduce the correct quark and lepton masses, CKM and PMNS matrices and neutrino mass differences, and at the same time suppress dimension-5 proton decays (proton decays via colored Higgsino exchange) through their texture, so that the soft SUSY breaking scale can be reduced as much as possible without conflicting the current experimental bound on proton decays. We perform a numerical search for such a texture, and investigate implications of that texture on unknown neutrino parameters, the Dirac CP phase of PMNS matrix, the lightest neutrino mass and the (1, 1)-component of the neutrino mass matrix in the charged lepton basis. Here we concentrate on the case when the active neutrino mass is generated mostly by the Type-2 seesaw mechanism, in which case we can obtain predictions for the neutrino parameters from the condition that dimension-5 proton decays be suppressed as much as possible.

Current and future neutrino oscillation constraints on leptonic unitarity

The unitarity of the lepton mixing matrix is a critical assumption underlying the standard neutrino-mixing paradigm. However, many models seeking to explain the as-yet-unknown origin of neutrino masses predict deviations from unitarity in the mixing of the active neutrino states. Motivated by the prospect that future experiments may provide a precise measurement of the lepton mixing matrix, we revisit current constraints on unitarity violation from oscillation measurements and project how next-generation experiments will improve our current knowledge. With the next-generation data, the normalizations of all rows and columns of the lepton mixing matrix will be constrained to ≲10% precision, with the e-row best measured at ≲1% and the τ-row worst measured at ∼10% precision. The measurements of the mixing matrix elements themselves will be improved on average by a factor of 3. We highlight the complementarity of DUNE, T2HK, JUNO, and IceCube Upgrade for these improvements, as well as the importance of ντ appearance measurements and sterile neutrino searches for tests of leptonic unitarity.