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Register a new userUnstable Heavy Majorana Neutrinos and Leptogenesis
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We propose a new mechanism producing a non-vanishing lepton number asymmetry, based on decays of heavy Majorana neutrinos. If they are produced out of equilibrium, as occurs in preheating scenario, and are superpositions of mass eigenstates rapidly decaying, their decay rates contains interference terms provided the mass differences $Delta m$ are small compared to widths $Gamma$. The resulting lepton asymmetry, which is the analogue of the time-integrated CP asymmetry in $B^0-bar{B}^0$ system, is found to be proportional to $Delta m/Gamma$.
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For leptogenesis with heavy sterile neutrinos above the electroweak scale, asymmetries produced at early times (in the relativistic regime) are relevant, if they are protected from washout. This can occur for weak washout or when the asymmetry is partly protected by being transferred to spectator fields. We thus study the relevance of relativistic effects for leptogenesis in a minimal seesaw model with two sterile neutrinos in the strongly hierarchical limit. Starting from first principles, we derive a set of momentum-averaged fluid equations to calculate the final $B-L$ asymmetry as a function of the washout strength and for different initial conditions at order one accuracy. For this, we take the leading fluid approximation for the relativistic $CP$-even and odd rates. Assuming that spectator fields remain in chemical equilibrium, we find that for weak washout, relativistic corrections lead to a sign flip and an enhancement of the asymmetry for a vanishing initial abundance of sterile neutrinos. As an example for the effect of partially equilibrated spectators, we consider bottom-Yukawa and weak-sphaleron interactions in leptogenesis driven by sterile neutrinos with masses $gtrsim 5times10^{12}$ GeV. For a vanishing initial abundance of sterile neutrinos, this can give rise to another flip and an absolute enhancement of the final asymmetry in the strong washout regime by up to two orders of magnitude relative to the cases either without spectators or with fully equilibrated ones. These effects are less pronounced for thermal initial conditions for the sterile neutrinos. The $CP$-violating source in the relativistic regime at early times is important as it is proportional to the product of lepton-number violating and lepton-number conserving rates, and therefore less suppressed than an extrapolation of the nonrelativistic approximations may suggest.
We investigate the search for heavy Majorana neutrinos stemming from a composite model scenario at the upcoming LHC Run II at a center of mass energy of 13 TeV. While previous studies of the composite Majorana neutrino were focussed on gauge interactions via magnetic type transition coupling between ordinary and heavy fermions (with mass $m^*$) here we complement the composite model with contact interactions at the energy scale $Lambda$ and we find that the production cross sections are dominated by such contact interactions by roughly two/three orders of magnitude. This mechanism provides therefore very interesting rates at the prospected luminosities. We study the same sign di-lepton and di-jet signature ($pp to ellell jj$) and perform a fast detector simulation based on Delphes. We compute 3$sigma$ and 5$sigma$ contour plots of the statistical significance in the parameter space ($Lambda,m^*$). We find that the potentially excluded regions at $sqrt{s} =13$ TeV are quite larger than those excluded so far at Run I considering searches with other signatures.
The neutrinophilic two-Higgs-doublet model ($ u$2HDM) provides a natural way to generate tiny neutrino mass from interactions with the new doublet scalar $Phi_ u$ ($H^pm,~H,~A$) and singlet neutrinos $N_R$ of TeV scale. In this paper, we perform detailed simulations for the lepton number violating (LNV) signatures at LHC arising from cascade decays of the new scalars and neutrinos with the mass order $m_{N_R}<m_{Phi_ u}$. Under constraints from lepton flavor violating processes and direct collider searches, their decay properties are explored and lead to three types of LNV signatures: $2ell^pm 4j+cancel{E}_T$, $3ell^pm 4j+cancel{E}_T$, and $3ell^pmell^mp 4j$. We find that the same-sign trilepton signature $3ell^pm4j+cancel{E}_T$ is quite unique and is the most promising discovery channel at the high-luminosity LHC. Our analysis also yields the $95%$ C.L. exclusion limits in the plane of the $Phi_ u$ and $N_R$ masses at 13 (14) TeV LHC with an integrated luminosity of 100~(3000)/fb.
The texture zero mass matrices for the leptons and the seesaw mechanism are used to derive relations between the matrix elements of the lepton mixing matrix and the ratios of the neutrino masses.
We derive constraints on the relic abundance of a generic particle of mass $sim~1-10^{14}$ TeV which decays into neutrinos at cosmological epochs, using data from the Frejus and IMB nucleon decay detectors and the Flys Eye air shower array. The lifetime of such unstable particles which may constitute the dark matter today is bounded to be greater than $sim~10^{14}-10^{18}$ yr, depending on the mass. For lifetimes shorter than the age of the universe, neutrino energy losses due to scattering and the expansion redshift become important and set limits to the ability of neutrino observatories to probe the early universe.
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