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Register a new userLooking forward to Lepton-flavor-violating ALPs
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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.
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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.
Exotic Higgs decays are promising channels to discover new physics in the near future. We present a simple model with a new light scalar that couples to the Standard Model through a charged lepton-flavor violating interaction. This can yield exciting new signatures, such as $h to e^+ e^+ mu^-mu^-$, that currently have no dedicated searches at the Large Hadron Collider. We discuss this model in detail, assess sensitivity from flavor constraints, explore current constraints from existing multi-lepton searches, and construct a new search strategy to optimally target these exotic, lepton-flavor violating Higgs decays.
The lepton flavor violating process $J/psito ll (l eq l)$ serves as an ideal place to probe the unparticle theory. Such process can only occur at loop level in the Standard model (SM), so that should be very suppressed, by contrast in unparticle scenario, it happens at tree level and its contribution may be sizable for practical measurement. Moreover, the BESIII will offer the largest database on $J/psi$ which makes more accurate measurements possible. Furthermore, for such purely leptonic decays background is relatively low and signal would be cleaner. Our work carefully investigates the possibility of observing such processes from both theoretical and experimental aspects.
Here I review the status and prospects of experimental investigations into lepton flavor violation (LFV) in charged leptons. Rare LFV processes are naturally expected to occur through loops of TeV scale particles predicted by supersymmetric theories or other models beyond the Standard Model. In contrast to physics of quark flavors that is dominated by the Cabibbo-Kobayashi-Maskawa matrix, LFV in charged leptons is a definitive signal of new physics. Currently active researches are rare tau decay searches at the B factories. The MEG experiment will soon start a sensitive search for the LFV muon decay, mu to e gamma. Prospects for searches at the LHC, a possibility of a fixed target LFV experiment with high energy muons, and a sensitivity of leptonic kaon decays to LFV are also briefly discussed.
Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays $tau^{pm}rightarrowell^{pm}gamma$ ($ell=e,mu$) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via $tau^{pm}rightarrowell^{pm}gamma$ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb$^{-1}$, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, $mathcal{B}(tau^{pm}rightarrow mu^{pm}gamma)$ $leq$ $4.2times10^{-8}$ and $mathcal{B}(tau^{pm}rightarrow e^{pm}gamma)$ $leq$ $5.6times10^{-8}$, are set at 90% confidence level.
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