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Register a new userLinking lepton number violation with $B$ anomalies
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Hints of violation of lepton flavor universality in semileptonic $B$ decays have prompted a renewed interest in leptoquarks at the low TeV scale. Among the different scenarios suggested, some happen to violate also lepton number, yet not much attention has been paid to the expected size of the associated lepton number violating processes. In this note we examine this issue. We find that there is a single leptoquark scenario compatible with the current size of the anomalies which also violates lepton number. In this scenario (Majorana) neutrino masses are radiatively generated. With the leptoquark parameters extracted from fitting the flavor anomalies, one actually gets the right order of magnitude for neutrino masses. We examine the associated effective field theories both at the electroweak scale and at the hadronic scale and estimate the size of the most relevant lepton number violating processes.
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Several experiments observed deviations from the Standard Model (SM) in the flavour sector: LHCb found a $4-5,sigma$ discrepancy compared to the SM in $bto smu^+mu^-$ transitions (recently supported by an Belle analysis) and CMS reported a non-zero measurement of $htomutau$ with a significance of $2.4,sigma$. Furthermore, BELLE, BABAR and LHCb founds hints for the violation of flavour universality in $Bto D^{(*)}tau u$. In addition, there is the long-standing discrepancy in the anomalous magnetic moment of the muon. Interestingly, all these anomalies are related to muons and taus, while the corresponding electron channels seem to be SM like. This suggests that these deviations from the SM might be correlated and we briefly review some selected models providing simultaneous explanations.
Indirect searches have the potential to probe scales beyond the realm of direct searches. In this letter we consider the implications of two parity violating experiments: weak charge of proton $Q_W^p$ and the Caesium atom $Q_W^{Cs}$ on the solutions to lepton flavour non-universality violations (LFUV) in the decay of $B$ mesons. Working in a generic implementation of a minimal $Z^prime$ model, we assume the primary contribution being due to the electron to facilitate comparison with the low $q^2$ parity violating experiments. We demonstrate that the conclusion is characterized by different limiting behavior depending on the chirality of the lepton current. The correlation developed in this study demonstrates the effectiveness in studying the synergy between different experiments leading to a deeper understanding of the interpretation of the existing data. It is shown that a possible future improvement in the parity violating experiments can have far reaching implications in the context of direct searches. We also comment on the prospect of addition of the muon to the fits and the role it plays in ameliorating the constraints on models of $Z$. This offers a complimentary understanding of the pattern of the coupling of the NP to the leptons, strongly suggesting either a muon only or a combination of solutions to the anomalies.
It is shown how pure Dirac neutrino masses can naturally occur at low energies even in the presence of Planck scale lepton number violation. The geometrical picture in five dimensions assumes that the lepton number symmetry is explicitly broken on the Planck brane while the right-handed neutrino is localised on the TeV brane. This physical separation in the bulk causes the global lepton number to be preserved at low energies. A small wavefunction overlap between the left-handed and right-handed neutrinos then naturally leads to a small Dirac Yukawa coupling. By the AdS/CFT correspondence there exists a purely four-dimensional dual description in which the right-handed neutrino is a composite CFT bound state. The global lepton number is violated at the Planck scale in a fundamental sector whose mixing into the composite sector is highly suppressed by CFT operators with large anomalous dimensions. A similar small mixing is then also responsible for generating a naturally small Dirac Yukawa coupling between the fundamental left-handed neutrino and the composite right-handed neutrino.
We did a model independent phenomenological study of baryogenesis via leptogenesis, neutrinoless double beta decay (NDBD) and charged lepton flavour violation (CLFV) in a generic left-right symmetric model (LRSM) where neutrino mass originates from the type I + type II seesaw mechanism. We studied the new physics contributions to NDBD coming from the left-right gauge boson mixing and the heavy neutrino contribution within the framework of LRSM. We have considered the mass of the RH gauge boson to be specifically 5 TeV, 10 TeV and 18 TeV and studied the effects of the new physics contributions on the effective mass and baryogenesis and compared with the current experimental limit. We tried to correlate the cosmological BAU from resonant leptogenesis with the low energy observables, notably, NDBD and LFV with a view to finding a common parameter space where they coexists.
In this note we examine the constraints imposed by muon anomalous magnetic moment ($(g-2)_mu$) and $mu^- to e^+ e^- e^-$ on lepton number violating (LNV) couplings of the triplet Higgs in Little Higgs (LH) model.
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