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Register a new userLepton Flavor Violating Dilepton Dijet Signatures from Sterile Neutrinos at Proton Colliders
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In this article we investigate the prospects of searching for sterile neutrinos in lowscale seesaw scenarios via the lepton flavour violating (but lepton number conserving) dilepton dijet signature. In our study, we focus on the final state $e^pm mu^mp jj$ at the HL-LHC and the FCC-hh (or the SppC). We perform a multivariate analysis at the detector level including the dominant SM backgrounds from di-top, di-boson, and tri-boson. Under the assumption of the active-sterile neutrino mixings $|V_{ l N}|^2=|theta_e|^2=|theta_mu|^2$ and $|V_{ tau N}|^2 = |theta_tau|^2=0$, the sensitivities on the signal production cross section times branching ratio $sigma(p p to l^pm N)times {rm BR} (N to l^{ mp} jj)$ and on $|V_{ l N}|^2$ for sterile neutrino mass $M_N$ between 200 and 1000 GeV are derived. For the benchmark $M_N=500$ GeV, when ignoring systematic uncertainties at the HL-LHC (FCC-hh/SppC) with 3 (20) ${rm ab}^{-1}$ luminosity, the resulting 2-$sigma$ limits on $|V_{ l N}|^2$ are $4.9times 10^{-3}$ ($7.0times 10^{-5}$), while the 2 -$sigma$ limit on $sigma times {rm BR}$ are $4.4times10^{-2}$ ($1.6times10^{-2}$) fb, respectively. The effect of the systematic uncertainty is also studied and found to be important for sterile neutrinos with smaller masses. We also comment on searches with $tau^pm mu^mp jj$ and $tau^pm e^mp jj$ final states.
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We develop the consequences of introducing a purely leptonic, lepton number violating non-standard interaction (NSI) and standard model neutrino mixing with a fourth, sterile neutrino in the analysis of short-baseline, neutrino experiments. We focus on the muon decay at rest (DAR) result from the Liquid Scintillation Neutrino Experiment (LSND) and the Karlsruhe and Rutherford Medium Energy Neutrino Experiment (KARMEN). We make a comprehensive analysis of lepton number violating, NSI effective operators and find nine that affect muon decay relevant to LSND results. Two of these preserve the standard model (SM) value 3/4 for the Michel rho and delta parameters and, overall, show favorable agreement with precision data and the electron anti-neutrino signal from LSND data. We display theoretical models that lead to these two effective operators. In the model we choose to apply to DAR data, both electron anti-neutrino appearance from muon anti-neutrino oscillation and electron anti-neutrino survival after production from NSI decay of the positive muon contribute to the expected signal. This is a unique feature of our scheme. We find a range of parameters where both experiments can be accommodated consistently with recent global, sterile neutrino fits to short baseline data. We comment on implications of the models for new physics searches at colliders and comment on further implications of the lepton number violating interactions plus sterile neutrino-standard model neutrino mixing.
We consider the minimal 3-3-1 model with three sterile neutrinos transforming as singlet under the $SU(3)_Lotimes U(1)_X$ symmetry. This model, with or without sterile neutrinos, predicts flavor violating interactions in both quark and lepton sectors, since all the charged fermions mass matrices can not be assumed diagonal in any case. Here we accommodate the lepton masses and the Pontecorvo-Maki-Nakawaga-Sakata matrix at the same time, and as consequence the Yukawa couplings and the unitary matrices which diagonalize the mass matrices are not free parameters anymore. We study some phenomenological consequences, i.e., $l_ito l_jl_k bar{l}_k$ and $l_ito l_jgamma$ which are induced by neutral and doubly charged particles present in the model. In particular we find that if the decay $muto eebar{e}$ is observed in the future, the only particle in the model that could explain this decay is the doubly charged vector bilepton.
We assess the status of past and future experiments on lepton flavor violating (LFV) muon and tau decays into a light, invisible, axion-like particle (ALP), $a$. We propose a new experimental setup for MEG II, the MEGII-fwd, with a forward calorimeter placed downstream from the muon stopping target. Searching for $mu to e a$ decays MEGII-fwd is maximally sensitive to LFV ALPs, if these have nonzero couplings to right-handed leptons. The experimental set-up suppresses the (left-handed) Standard Model background in the forward direction by controlling the polarization purity of the muon beam. The reach of MEGII-fwd is compared with the present constraints, the reach of Mu3e and the Belle-II reach from $tau to ell a$ decays. We show that a dedicated experimental campaign for LFV muon decays into ALPs at MEG II and Mu3e will be able to probe the ALP parameter space in an unexplored region well beyond the existing astrophysical constraints. We study the implications of these searches for representative LFV ALP models, where the presence of a light ALP is motivated by neutrino masses, the strong CP problem and/or the SM flavor puzzle. To this extent we discuss the majoron in low-scale seesaw setups and introduce the LFV QCD axion, the LFV axiflavon and the leptonic familon, paying particular attention to the cases where the LFV ALPs constitute cold dark matter.
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.
We study lepton flavor violating (LFV) tau and B decays in models with heavy neutrinos to constrain the mixing matrix parameters U_{tau N}. We find that the best current constraints when the heavy neutrinos are purely left-handed come from LFV radiative tau decay modes. To obtain competitive constraints in LFV B decay it is necessary to probe b -> X_{s} tau^{pm} e^{mp} at the 10^{-7} level. When the heavy neutrinos have both left and right-handed couplings, the mixing parameters can be constrained by studying LFV B decay modes and LFV tau decay into three charged leptons. We find that the branching ratios B(tau^{pm} -> l_1^{pm} l_2^{pm} l_3^{mp}), B(B_{s} -> tau^{pm} e^{mp}) and B(b -> X_{s} l_1^{pm} l_2^{mp}) need to be probed at the 10^{-8} level in order to constrain the mixing parameters beyond what is known from unitarity.
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