Shintaro Eijima

A facility to search for hidden particles at the CERN SPS: the SHiP physics case.

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (search for hidden particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, [Formula: see text] and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the standard model and describe interactions between new particles and four different portals-scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the standard model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation.

Kinetic equations for baryogenesis via sterile neutrino oscillation

We investigate baryogenesis in the νMSM (neutrino Minimal Standard Model), which is the SM extended by three right-handed neutrinos with masses below the electroweak scale. The baryon asymmetry of the universe can be generated by the mechanism via flavor oscillation of right-handed (sterile) neutrinos which are responsible to masses of active neutrinos confirmed by various experiments. We present the kinetic equations for the matrix of densities of leptons which describe the generation of asymmetries. Especially, the momentum dependence of the matrix of densities is taken into account. By solving these equations numerically, it is found that the momentum distribution is significantly distorted from the equilibrium one, since the production for the modes with lower momenta k << T (T is the temperature of the universe) is enhanced, while suppressed for higher modes. As a result, the most important mode for the yields of sterile neutrinos as well as the baryon asymmetry is k 2T, which is smaller than k inferred from the thermal average. The comparison with the previous works is also discussed.

Mixing of active and sterile neutrinos

We investigate mixing of neutrinos in the νMSM (neutrino Minimal Standard Model), which is the MSM extended by three right-handed neutrinos. Especially, we study elements of the mixing matrix ΘαI between three left-handed neutrinos νLα(α = e, μ, τ) and two sterile neutrinos NI(I =2, 3) which are responsible to the seesaw mechanism generating the suppressed masses of active neutrinos as well as the generation of the baryon asymmetry of the universe. It is shown that ΘeI can be suppressed by many orders of magnitude compared with ΘμI and ΘτI, when the Chooz angle θ13 is large in the normal hierarchy of active neutrino masses. We then discuss the neutrinoless double beta decay in this framework by taking into account the contributions not only from active neutrinos but also from all the three sterile neutrinos. It is shown that N2 and N3 give substantial, destructive contributions when their masses are smaller than a few 100 MeV, and as a results ΘeI receive no stringent constraint from the bounds on such decays. Finally, we discuss the impacts of the obtained results on the direct searches of N2,3 in meson decays for the case when N2,3 are lighter than pion mass. We show that there exists the allowed region for N2,3 with such small masses in the normal hierarchy case even if the current bound on the lifetimes of N2,3 from the big bang nucleosynthesis is imposed. It is also pointed out that the direct search by using π+ → e+ + N2,3 and K+ → e+ + N2,3 might miss such N2,3 since the branching ratios can be extremely small due to the cancellation in ΘeI, but the search by K+ → μ+ + N2,3 can cover the whole allowed region by improving the measurement of the branching ratio by a factor of 5.

Direct Search for Right-handed Neutrinos and Neutrinoless Double Beta Decay

We consider an extension of the Standard Model by two right-handed neutrinos, especially with masses lighter than charged K meson. This simple model can realize the seesaw mechanism for neutrino masses and also the baryogenesis by flavor oscillations of right-handed neutrinos. We summarize the constraints on right-handed neutrinos from direct searches as well as the big bang nucleosynthesis. It is then found that the possible range for the quasi-degenerate mass of right-handed neutrinos is MN ≥ 163 MeV for normal hierarchy of neutrino masses, while MN = 188–269 MeV and MN ≥ 285 MeV for inverted hierarchy case. Furthermore, we find in the latter case that the possible value of the Majorana phase is restricted for MN = 188–350 MeV, which leads to the fact that the rate of neutrinoless double beta decay is also limited.

Heavy neutrino search in accelerator-based experiments

A bstractWe explore the feasibility of detecting heavy neutrinos by the existing facilities of neutrino experiments. A heavy neutrino in the mass range 1 MeV ≲ MN ≲ 500 MeV is produced by pion or kaon decay, and decays to charged particles which leave signals in neutrino detectors. Taking the T2K experiment as a typical example, we estimate the heavy neutrino flux produced in the neutrino beam line. Due to massive nature of the heavy neutrino, the spectrum of the heavy neutrino is significantly different from that of the ordinary neutrinos. While the ordinary neutrinos are emitted to various directions in the laboratory frame due to their tiny masses, the heavy neutrinos tend to be emitted to the forward directions and frequently hit the detector. The sensitivity for the mixing parameters is studied by evaluating the number of signal events in the near detector ND280. For the electron-type mixing, the sensitivity of T2K at 1021 POT is found to be better than that of the previous experiment PS191, which has placed the most stringent bounds on the mixing parameters of the heavy neutrinos for 140 MeV ≲ MN ≲ 500 MeV.

Lepton universality in the nu MSM

We consider the nu MSM which is an extension of the Standard Model by three right-handed neutrinos with masses below the electroweak scale, in which the origins of neutrino masses, dark matter, and baryon asymmetry of the universe are simultaneously explained. Among three heavy neutral leptons, N-2 and N-3, which are responsible to the seesaw mechanism of active neutrino masses and the baryogenesis via flavor oscillation, can induce sizable contributions to various lepton universality in decays of charged mesons. Then the possible deviations of the universality in the nu MSM are investigated. We find that the deviation in kaon decay can be as large as O(10(-3)), which will be probed in near future experiments

On neutrinoless double beta decay in the .NU.MSM

Abstract We consider the neutrinoless double beta ( 0 ν β β ) decay in the ν MSM, in which three right-handed neutrinos with masses below the electroweak scale are additionally introduced to the Standard Model. In this model there appear three heavy neutral leptons N 1 , N 2 , and N 3 corresponding to right-handed neutrinos. It has been known that the lightest one N 1 with keV mass, which is a candidate for dark matter, gives a negligible contribution to the 0 ν β β decay. By contrast, the heavier ones N 2 and N 3 , which are responsible to the seesaw mechanism of neutrino masses and baryogenesis, give the destructive contribution (compared with one from active neutrinos). This is because their mass degeneracy at high precision has been assumed, which is expected by analytical studies of baryogenesis. In this analysis, we find that the effective mass of the 0 ν β β decay becomes larger than one from active neutrinos due to the N 2 and N 3 constructive contribution when the mass difference becomes larger and the mass ordering of active neutrinos is inverted. Such a possibility will be explored by the current and near future experiments of the 0 ν β β decay.

Neutrinoless double

The extension of the Standard Model by right handed neutrinos with masses in the GeV range can simultaneously explain the observed neutrino masses via the seesaw mechanism and the baryon asymmetry of the universe via leptogenesis. It has previously been claimed that the requirement for successful baryogenesis implies that the rate of neutrinoless double β decay in this scenario is always smaller than the standard prediction from light neutrino exchange alone. In contrast, we find that the rate for this process can also be enhanced due to a dominant contribution from heavy neutrino exchange. In a small part of the parameter space it even exceeds the current experimental limit, while the properties of the heavy neutrinos are consistent with all other experimental constraints and the observed baryon asymmetry is reproduced. This implies that neutrinoless double β decay experiments have already started to rule out part of the leptogenesis parameter space that is not constrained by any other experiment, and the lepton number violation that is responsible for the origin of baryonic matter in the universe may be observed in the near future.

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Abstract The existence of baryon asymmetry and dark matter in the Universe may be related to CP-violating reactions of three heavy neutral leptons (HNLs) with masses well below the Fermi scale. The dynamical description of the lepton asymmetry generation, which is the key ingredient of baryogenesis and of dark matter production, is quite complicated due to the presence of many different relaxation time scales and the necessity to include quantum-mechanical coherent effects in HNL oscillations. We derive kinetic equations accounting for fermion number violating effects missed so far and identify one of the domains of HNL masses that can potentially lead to large lepton asymmetry generation boosting the sterile neutrino dark matter production.

decay and low scale leptogenesis

We consider a baryogenesis scenario via the oscillation of right-handed neutrinos with Majorana masses of the order of GeV, which are also responsible for neutrino masses by the seesaw mechanism. We study how the initial condition alters the prediction of the present baryon asymmetry by this mechanism. It is usually assumed that the abundance of right-handed neutrinos is zero after the reheating of the inflationary universe and they are produced in scattering processes by the renomalizable Yukawa interaction. However, the higher-dimensional operator with right-handed neutrinos may provide an additional production which is most effective at the reheating epoch. It is shown that such an initial abundance of right-handed neutrinos can significantly modify the prediction when the strong washout of the asymmetry is absent. This leads to the parameter space of the model for the successful baryogenesis being enlarged.