Neutrino Physics

Neutrinos offered the greatest surprises in high energy physics during the last decades. In this chapter we review the main milestones of this passionate history: the first neutrino beams which established the separate neutrino identities, the Gargamelle discovery of the weak neutral currents, the LEP determination of three light neutrino species and the discovery of the intriguing phenomenon of neutrino oscillations. The experimental determination of the neutrino mass matrix elements is still in progress with several experiments either taking data or planned for the near future. We end with the present theoretical puzzles and the experiments which may help to solve them.

Leptogenesis in supersymmetry with one L violating coupling

We have shown a new scenario of successful leptogenesis with one L violating coupling and a relative Majorana phase playing the role of CP violation. This is in contrast to the usual consideration of Feynman diagram with at least two L violating couplings. We have considered R-parity violating minimal supersymmetric standard model (MSSM) for leptogenesis at TeV scale. This scenario is also consistent with generating light neutrino mass if asymmetry is generated through semileptonic $$\lambda ^{\prime }$$ λ ′ coupling.

Quasi-Dirac neutrinos in the linear seesaw model

We implement a minimal linear seesaw model (LSM) for addressing the Quasi-Dirac (QD) behaviour of heavy neutrinos, focusing on the mass regime of MN ≲ MW. Here we show that for relatively low neutrino masses, covering the few GeV range, the same-sign to opposite-sign dilepton ratio, Rℓℓ, can be anywhere between 0 and 1, thus signaling a Quasi-Dirac regime. Particular values of Rℓℓ are controlled by the width of the QD neutrino and its mass splitting, the latter being equal to the light-neutrino mass mν in the LSM scenario. The current upper bound on mν1 together with the projected sensitivities of current and future |UN ℓ|2 experimental measurements, set stringent constraints on our low-scale QD mass regime. Some experimental prospects of testing the model by LHC displaced vertex searches are also discussed.

The two-loop contributions to muon MDM in $$U(1)_X$$ SSM

The MSSM is extended to the $$U(1)_X$$ U ( 1 ) X SSM, whose local gauge group is $$SU(3)_C \times SU(2)_L \times U(1)_Y \times U(1)_X$$ S U ( 3 ) C × S U ( 2 ) L × U ( 1 ) Y × U ( 1 ) X . To obtain the $$U(1)_X$$ U ( 1 ) X SSM, we add the new superfields to the MSSM, namely: three Higgs singlets $${\hat{\eta }},~\hat{{\bar{\eta }}},~{\hat{S}}$$ η ^ , η ¯ ^ , S ^ and right-handed neutrinos $${\hat{\nu }}_i$$ ν ^ i . It can give light neutrino tiny mass at the tree level through the seesaw mechanism. The study of the contribution of the two-loop diagrams to the MDM of muon under $$U(1)_X$$ U ( 1 ) X SSM provides the possibility for us to search for new physics. In the analytical calculation of the loop diagrams (one-loop and two-loop diagrams), the effective Lagrangian method is used to derive muon MDM. Here, the considered two-loop diagrams include Barr-Zee type diagrams and rainbow type two-loop diagrams, especially Z–Z rainbow two-loop diagram is taken into account. The obtained numerical results can reach $$7.4\times 10^{-10}$$ 7.4 × 10 - 10 , which can remedy the deviation between SM prediction and experimental data to some extent.

CP violation in rare lepton-number-violating W decays at the LHC

Some models of leptogenesis involve a quasi-degenerate pair of heavy neutrinos N1,2 whose masses can be small, O(GeV). Such neutrinos can contribute to the rare lepton-number-violating (LNV) decay $$ {W}^{\pm}\to {\mathrm{\ell}}_1^{\pm }{\mathrm{\ell}}_2^{\pm }{\left({q}^{\prime}\overline{q}\right)}^{\mp } $$ W ± → ℓ 1 ± ℓ 2 ± q ′ q ¯ ∓ . If both N1 and N2 contribute, there can be a CP-violating rate difference between the LNV decay of a W− and its CP-conjugate decay. In this paper, we examine the prospects for measuring such a CP asymmetry ACP at the LHC. We assume a value for the heavy-light neutrino mixing parameter |BℓN|2 = 10−5, which is allowed by the present experimental constraints, and consider 5 GeV ≤ MN≤ 80 GeV. We consider three versions of the LHC — HL-LHC, HE-LHC, FCC-hh — and show that small values of the CP asymmetry can be measured at 3σ, in the range 1% ≲ ACP ≲ 15%.

Significance of Broken μ − τ Symmetry in Correlating δ CP , θ 13 , Lightest Neutrino Mass, and Neutrinoless Double Beta Decay 0 ν β β

Leptonic CP violating phase δ CP in the light neutrino sector and leptogenesis via present matter-antimatter asymmetry of the Universe entails each other. Probing CP violation in light neutrino oscillation is one of the challenging tasks today. The reactor mixing angle θ 13 measured in reactor experiments, LBL, and DUNE with high precision in neutrino experiments indicates towards the vast dimensions of scope to detect δ CP . The correlation between leptonic Dirac CPV phase δ CP , reactor mixing angle θ 13 , lightest neutrino mass m 1 , and matter-antimatter asymmetry of the Universe within the framework of μ − τ symmetry breaking assuming the type I seesaw dominance is extensively studied here. Here, a SO(10) GUT model with flavor μ − τ symmetry is considered. In this work, the idea is to link baryogenesis through leptogenesis and the hint of CP violation in the neutrino oscillation data to a breaking of the mu-tau symmetry. Small tiny breaking of the μ − τ symmetry allows a large Dirac CP violating phase in neutrino oscillation which in turn is characterized by awareness of measured value of θ 13 and to provide a hint towards a better understanding of the experimentally observed near-maximal value of ν μ − ν τ mixing angle θ 23 ≃ π / 4 . Precise breaking of the μ − τ symmetry is achieved by adding a 120-plet Higgs to the 10 + 1 2 ¯ 6 -dimensional representation of Higgs. The estimated three-dimensional density parameter space of the lightest neutrino mass m 1 , δ CP , and reactor mixing angle θ 13 is constrained here for the requirement of producing the observed value of baryon asymmetry of the Universe through the mechanism of leptogenesis. Carrying out numerical analysis, the allowed parameter space of m 1 , δ CP , and θ 13 is found out which can produce the observed baryon to photon density ratio of the Universe.

Radiative seesaw corrections and charged-lepton decays in a model with soft flavour violation

We consider the one-loop radiative corrections to the light-neutrino mass matrix and their consequences for the predicted branching ratios of the five lepton-flavour-violating decays $$ {\mathrm{\ell}}_1^{-}\to {\mathrm{\ell}}_2^{-}{\mathrm{\ell}}_3^{+}{\mathrm{\ell}}_3^{-} $$ ℓ 1 − → ℓ 2 − ℓ 3 + ℓ 3 − in a two-Higgs-doublet model furnished with the type-I seesaw mechanism and soft lepton-flavour violation. We find that the radiative corrections are very significant; they may alter the predicted branching ratios by several orders of magnitude and, in particular, they may help explain why BR (μ− → e−e+e−) is strongly suppressed relative to the branching ratios of the decays of the τ−. We conclude that, in any serious numerical assessment of the predictions of this model, it is absolutely necessary to take into account the one-loop radiative corrections to the light-neutrino mass matrix.