The Origin Of Neutrino Masses And Leptonic CP Violation

Neutrino oscillation experiments have firmly established that neutrinos are massive and lepton flavors are mixed,which is the only new physics beyond the standard model that has been discovered with solid experimental evidence.Therefore,studying the origin of neutrino masses and their flavor mixing patterns is an important foreword topic in neutrino physics.In the standard model,CP violation originates from the complex phase in the quark mixing matrix,but the strength of the CP violation is not enough to explain the matter-antimatter asymmetry in the universe.Therefore,searching for a new source of CP violation beyond the standard model is also one of the important issues in neutrino physics.In this thesis,in order to explain neutrino masses and lepton flavor mixing,we present a simple but viable scenario in the framework of canonical Type-Ⅰ Seesaw mech-anism to explicitly break an S3L × S3R flavor symmetry in the leptonic sector.It turns out that the leptonic flavor mixing matrix is completely determined by the mass ra-tios of charged leptons(i.e.,me/mμ and mμ/mτ)and those of light neutrinos(i.e.,m1/m2 and m2/m3).Making use of the latest global-fit results of neutrino oscillation parameters at the 3σ level,the predictions of neutrino mass spectrum,flavor mixing angles and Dirac CP-violating phase can be given as follows:(1)the neutrino mass spectrum shows a hierarchical pattern and a normal ordering,e.g.,m1≈2.2 meV.n2≈8.8 meV and m3≈52.7 meV;(2)the allowed regions of three mixing an-gles are 41.8°(?)θ23(?)43.3°,31.40(?)θ12(?)35.5°,and 8.45°(?)θ13(?)8.90°;(3)the Dirac CP-violating phase θ≈-22°deviates significantly from the maximal value-90°.All these predictions are ready to be tested in the ongoing and forthcoming neu-trino oscillation experiments.The Seesaw mechanisms can not only explain the origin of neutrino masses,but also deduce the Leptogenesis mechanism for the cosmologi-cal matter-antimatter asymmetry.Furthermore,we calculate the CP asymmetries in the decays of heavy Majorana neutrinos in the early universe,and find that the observed matter-antimatter asymmetry can be successfully explained via thermal Leptogenesis.The phenomenological studies in this thesis are helpful in looking for a viable way of flavor symmetry breaking,and instructive for understanding neutrino mass spectra,flavor mixing pattern and CP violation.The most direct way to test the Type-I Seesaw mechanism is to search for the ex-istence of the introduced heavy Majorana neutrino.Since the Majorana neutrino mass term violates the lepton-number by two units,we can probe the production signal of the heavy Majorana neutrino via the lepton-number violating processes at present and forthcoming experiments.In this thesis,we explore the heavy Majorana neutrino pro-duction and decay in the context of W*γ interaction at future ep colliders from the phenomenological point of view.The corresponding cross sections for the processes e-p→e-μ±μ±+X and e-p→veμ-μ±+X at future LHeC,FCC-ep and ILC(?)FCC are predicted.Taking the process e-p→e-μ±μ±+X as an example,we further investigate the transverse momentum distributions and the reconstructed invariant mass distributions of the final state same-sign dileptons and jets.After simulating the detector effects and suppressing the relevant backgrounds in the standard model,the discovery potential of the heavy Majorana neutrinos at future ep colliders are estimated.It shows that,with the integrated luminosity of 300 fb-1,the heavy Majorana neutrino mass can reach 110 GeV(79 GeV)[74 GeV]at LHeC,125 GeV(88 GeV)[75 GeV]at FCC-ep and 665 GeV(425 GeV)[220 GeV]at ILC(?)FCC for 2a(3σ)[5σ]discovery.Combined with the results of the heavy Majorana neutrino production via single W exchange,this work can provide helpful information to search for the heavy Majorana neutrinos in various rapidity region at future ep colliders.