Differences in lepton number (i.e., $ L _e - L _μ$, $ L _e - L _τ$, $ L_μ- L _τ$, or combinations thereof) are not conserved charges in the Standard Model due to the observation of neutrino oscillations. We compute the divergence of the corresponding currents in the case of Majorana or Dirac-type neutrinos and show that, in the high energy limit, the vector interactions map onto those of a light scalar coupled to neutrinos with its coupling fixed by the observed neutrino masses and mixing. This leads to amplitudes with external light vectors that scale inversely with the vector mass. By studying these processes, we set new constraints on $ L _i - L _j $ through a combination of semi-leptonic meson decays, invisible neutrino decays, neutrinoless double beta decays, and observations of Big Bang Nucleosynthesis/supernova, which can be much stronger than previous limits for vector masses below an eV. These bounds have important implications on the experimental prospects of detecting $ L _i - L _j $ long-range forces.
One of the major open questions in the neutrino sector is the issue of leptonic CP violation. Current global oscillation data shows a mild preference for a large, potentially maximal value for the Dirac CP phase in the neutrino mixing matrix. In this letter, we point out that New Physics in the form of neutral-current like non-standard interactions with real couplings would likely yield a similar conclusion even if CP in the neutrino sector were conserved. Therefore, the claim for a discovery of leptonic CP violation will require a robust ability to test New Physics scenarios.
I discuss some theoretical aspects of how to observe leptonic CP violation. It is divided into two parts, one for CP violation due to Majorana, and the other more conventional leptonic Kobayashi-Maskawa (KM) phases. In the first part, I estimate the effect of Majorana phase to observable of neutrinoless double beta decay experiments by paying a careful attention to the definition of the atmospheric scale Delta m^2. In the second part, I discuss Tokai-to-Kamioka-Korea two detector complex which receives neutrino superbeam from J-PARC as a concrete setting for discovering CP violation due to the…
We explore the prospects to discover the $h \to Z γ$ decay in $t\bar t$-associated production, featuring a signal-to-background ratio of ${\cal O}(1)$. Performing a detailed analysis of the semi-leptonic $t \bar t $-decay channel, we demonstrate that the production mode could lead to a $\sim5\,σ$ discovery at the high-luminosity LHC, while the effective $h Z γ$ coupling could be extracted with a $\sim15\,\%$ accuracy. Extending the analysis to potential future $pp$ colliders with 27 TeV and 100 TeV center-of-mass energies, we also show that the latter would allow precision measurements at the …
The CP conserved non leptonic $K\to πππ$ decays are discussed within the chiral quark model, including chiral perturbation theory corrections. All amplitudes are parameterized in terms of quark and gluon condensate and the constituent quark mass. The same values for these parameters that can be obtained by a fit of the $ΔI = 1/2$ rule in $K\to 2 π$ give a reasonably good fit in the $K\to 3 π$ case. We compare with the work of other groups.
We study the purely leptonic $W$ decays $W^+ \to e^+ μ^- e^+ ν_e$ and $W^+ \to e^+ e^+ μ^- \bar ν_μ$ (or their charge conjugates) produced at the LHC, induced by sterile neutrinos with mass below $M_W$ in the intermediate state. While the first mode is induced by both Dirac or Majorana neutrinos, the second mode is induced only by Majorana neutrinos, as it violates lepton number. We find that, even when the final (anti-)neutrino goes undetected, one could distinguish between these two processes, thus distinguishing the Dirac or Majorana character of the sterile neutrinos, by studying the muon …
Considering the recently obtained value of $θ_{13}$ from Daya Bay and other reactor experiments we have studied the prospects of considering mono-energetic neutrino beam in studying $CP$ violation in the leptonic sector. Using a neutrino beam from electron capture process for nuclei $^{110}_{50}$Sn and $^{152}$Yb and considering two baselines - 130 Km and 250 Km with Water Cherenkov detector, we have shown the discovery reach of $CP$ violation in neutrino oscillation experiment. Particularly for $^{110}_{50}$Sn nuclei $CP$ violation could be found for about 80% of the possible $δ$ values for a…
It is possible that measurements of vector boson scattering (VBS) at the LHC will reveal disagreement with Standard Model predictions, but no new particles will be observed directly. The task is then to learn as much as possible about the new physics from a VBS analysis carried within the framework of the Effective Field Theory (EFT). In this paper we discuss issues related to the correct usage of the EFT when the WW invariant mass is not directly accessible experimentally, as in purely leptonic W decay channels. The strategies for future data analyses in case such scenario indeed occurs are p…
We compute supersymmetric contributions to pion leptonic (π_{l2}) decays in the Minimal Supersymmetric Standard Model (MSSM). When R-parity is conserved, the largest contributions to the ratio R_{e/μ} = Γ[ π^+ \to e^+ ν_e(γ)]/Γ[ π^+ \to μ^+ ν_μ(γ)] arise from one-loop (V-A)x(V-A) corrections. These contributions can be potentially as large as the sensitivities of upcoming experiments; if measured, they would imply significant bounds on the chargino and slepton sectors complementary to current collider limits. We also analyze R-parity violating interactions, which may produce a detectable devia…
In this talk I have raised the question of how the future discovery of leptonic CP violation can be made robust even at accepting the rather large current experimental uncertainties in our knowledges of neutrino propagation in matter. To make progress toward answering the difficult question, I listed ways to proceed: (1) Obtain tighter constraints on the MSW theory by testing it by various neutrino experiments. (2) Measure the matter effect in situ, namely within the experiment for discovering CP violation itself. (3) Uncover leptonic CP violation in a matter effect free environment. I also re…
We study the non-standard interactions of neutrinos with light leptonic scalars ($ϕ$) in a global $(B-L)$-conserved ultraviolet (UV)-complete model. The model utilizes Type-II seesaw motivated neutrino interactions with an $SU(2)_L$-triplet scalar, along with an additional singlet in the scalar sector. This UV-completion leads to an enriched spectrum and consequently new observable signatures. We examine the low-energy lepton flavor violation constraints, as well as the perturbativity and unitarity constraints on the model parameters. Then we lay out a search strategy for the unique signature …
Motivated by a number of theoretical considerations, predicting the deeply bound double-heavy tetraquarks $T^{\{bb\}}_{[\bar u \bar d]}$, $T^{\{bb\}}_{[\bar u \bar s]}$ and $T^{\{bb\}}_{[\bar d \bar s]}$, we explore the potential of their discovery at Tera-$Z$ factories. Using the process $Z \to b \bar b b \bar b$, we calculate, employing the Monte Carlo generators MadGraph5$\_$aMC@NLO and Pythia6, the phase space configuration in which the~$b b$ pair is likely to fragment as a diquark. In a jet-cone, defined by an invariant mass interval $m_{bb} < M_{T^{\{bb\}}_{[\bar q \bar q']}} + ΔM$, the …
We reconsider observables for discovering a heavy Higgs boson (with m_h > 2m_W) via its di-leptonic decays h -> WW -> l nu l nu. We show that observables generalizing the transverse mass that take into account the fact that both of the intermediate W bosons are likely to be on-shell give a significant improvement over the variables used in existing searches. We also comment on the application of these observables to other decays which proceed via narrow-width intermediates.
We investigate the leptonic decay constants of the pion and its excitations with a 5-d holographic model for quantum chromodynamics. We prove numerically that the leptonic decay constants of the excited states of the pion vanish in the chiral limit when chiral symmetry is dynamically broken. This nontrivial result is in agreement with a solid prediction of quantum chromodynamics and is based on a generalized Gell-Mann-Oakes-Renner relationship involving the decay constants and masses of the excited states of the pion. We also obtain an extended partially conserved axial-vector current relation…
We discuss a model of dark matter consisting of high energy anti-electron-neutrinos with leptonic force, which is produced by the conserved leptonic charge $g_\ell$ associated with Lee-Yang's $U_1$ gauge symmetry. Based on particle-cosmology for early universe, the high energy neutrino (HEN) model of dark matter assumes that the neutron decay processes, $n\to p^+ + e^- +\ovν_e$, dominate the epoch after the creation, collision and confinement processes of quarks and antiquarks in the beginning. The HEN model implies the following results: There are almost equal numbers of electrons, protons an…
We reconsider observables for discovering and measuring the mass of a Higgs boson via its di-leptonic decays: H --> WW* --> l nu l nu. We define an observable generalizing the transverse mass that takes into account the fact that one of the intermediate W-bosons is likely to be on-shell. We compare this new variable with existing ones and argue that it gives a significant improvement for discovery in the region m_h < 2 m_W.
In this study, we work in the framework of the Next-to-Minimal extension of the Standard Model (NMSSM) extended by six singlet leptonic superfields. Through the mixing with the three doublet leptonic superfields, the non-zero tiny neutrino masses can be generated through the inverse seesaw mechanism. While $R$-parity is conserved in this model lepton number is explicitly violated. We quantify the impact of the extended neutrino sector on the NMSSM Higgs sector by computing the complete one-loop corrections with full momentum dependence to the Higgs boson masses in a mixed on-shell-$\overline{\…
We analyze the phenomenological consequences of embedding a flavor symmetry based on the groups $A_5$ and CP in a supersymmetric framework. We concentrate on the leptonic sector, where two different residual symmetries are assumed to be conserved at LO for charged and neutral leptons. All possible realizations to generate neutrino masses at tree level are investigated. Sizable flavor violating effects in the charged lepton sector are unavoidable due to the non-universality of soft-breaking terms determined by the symmetry. We derive testable predictions for the neutrino spectrum, lepton mixing…
If $R$-parity is not conserved, the lightest supersymmetric particle (LSP) could decay via lepton number violating or baryon number violating interactions. The latter case is particularly insidious since it leads to a reduction of the $\eslt$ as well as leptonic signals for supersymmetry. We evaluate cross sections for jets plus $\eslt$, 1$\ell$, $2\ell$ (same-sign and opposite sign) and $3\ell$ event topologies that result from the simultaneous production of all sparticles at the CERN Large Hadron Collider (LHC), assuming that the lightest supersymmetric particle $\tz_1$ decays hadronically i…
We extend the standard model by three types of inert fields including Majorana fermion singlets/triplets, real Higgs singlets/triplets and leptonic Higgs doublets. In the presence of a softly broken lepton number and an exactly conserved Z_2 discrete symmetry, these inert fields together can mediate a one-loop diagram for a Majorana neutrino mass generation. The heavier inert fields can decay to realize a successful leptogenesis while the lightest inert field can provide a stable dark matter candidate. As an example, we demonstrate the leptogenesis by the inert Higgs doublet decays. We also pe…
A classification of leptonic currents with respect to C-operation requires the separation of elementary particles into the two classes of vector C-even and axial-vector C-odd character. Their nature has been created so that to each type of lepton corresponds a kind of neutrino. Such pairs are united in families of a different C-parity. Unlike the neutrino of a vector type, any C-noninvariant Dirac neutrino must have his Majorana neutrino. They constitute the purely neutrino families. We discuss the nature of a corresponding mechanism responsible for the availability in all types of axial-vecto…
Heavy sterile neutrinos with masses below $M_W$ can induce trilepton events at the 14 TeV LHC through purely leptonic $W$ decays of $W^\pm \to e^\pm e^\pm μ^\mp ν$ and $μ^\pm μ^\pm e^\mp ν$ where the heavy neutrino will be in an intermediate state on its mass shell. Discovery and exclusion limits for the heavy neutrinos are found using both Cut-and-Count (CC) and a Multi-Variate Analysis (MVA) methods in this study. We also show that it is possible to discriminate between a Dirac and a Majorana heavy neutrino, even when lepton number conservation cannot be directly tested due to unobservabilit…
We present a non-minimal renormalizable SUSY $SU(5)$~model, with extended Higgs sector and right-handed neutrinos, where the flavour sector exhibits a $Q_{6}$ flavour symmetry. We analysed the simplest version of this model, in which R-parity is conserved and the right-handed neutrino masses in the flavour doublet are considered with and without degeneration. We find the generic form of the mass matrices both in the quark and lepton sectors. We reproduce, according to current data, the mixing in the CKM matrix. In the leptonic sector, in the general case where the right-handed neutrino masses …
We perform a detailed study of a variety of LHC signals in supersymmetric models where lepton number is promoted to an (approximate) U(1)_R symmetry. Such a symmetry has interesting implications for naturalness, as well as flavor- and CP-violation, among others. Interestingly, it makes large sneutrino vacuum expectation values phenomenologically viable, so that a slepton doublet can play the role of the down-type Higgs. As a result, (some of) the leptons and neutrinos are incorporated into the chargino and neutralino sectors. This leads to characteristic decay patterns that can be experimental…
This paper investigates the $e^-e^+\to Zh_1$ sensitivity for Higgs boson's rare decay into heavy neutrinos $h_1 \to NN$ at the proposed electron-positron collider, with the focus on multi-lepton final states that contain same-sign lepton pairs. $h_1 \to NN$ decay can derive from Higgs boson's mixing with new physics scalar(s) that is complementary to the contribution from active-sterile neutrino mixings. We consider the scenario with a singlet scalar which gives the heavy neutrino mass and has a small mixing with the SM Higgs boson. We analyze the semileptonic, fully leptonic and mixed $NN$ de…
