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Register a new userProbing unparticle theory via lepton flavor violating process $J/psito ll$ at BESIII
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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.
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A search for the charged lepton flavor violating decay $J/psito e^{pm}tau^{mp}$ with $tau^{mp} to pi^{mp}pi^0 u_{tau}$ is performed with about $10$ billion $J/ psi$ events collected with the BESIII detector at the BEPCII. No significant signal is observed, and an upper limit is set on the branching fraction $mathcal{B}(J/psito e^{pm}tau^{mp})<7.5times10^{-8}$ at the 90$%$ confidence level. This improves the previously published limit by two orders of magnitude.
We search for the lepton-flavor-violating decay of the $J/psi$ into an electron and a muon using $(225.3pm2.8)times 10^{6}$ $J/psi$ events collected with the BESIII detector at the BEPCII collider. Four candidate events are found in the signal region, consistent with background expectations. An upper limit on the branching fraction of $mathcal{B}(J/psi to emu)< 1.5 times 10^{-7}$ (90% C.L.) is obtained.
Recently, it was pointed out that the electron and muon g-2 discrepancies can be explained simultaneously by a flavor-violating axion-like particle (ALP). We show that the parameter regions favored by the muon g-2 are already excluded by the muonium-antimuonium oscillation bound. In contrast, those for the electron g-2 can be consistent with this bound when the ALP is heavier than 1.5 GeV. We propose to search for a signature of the same-sign and same-flavor lepton pairs and the forward-backward muon asymmetry to test the model at the Belle II experiment.
Using a data sample of $1.31 times 10^{9}$ $J/psi$ events accumulated with the BESIII detector, the decay $J/psito pbar{p}phi$ is studied via two decay modes, $phito K^{0}_{S}K^{0}_{L}$ and $phito K^{+}K^{-}$. The branching fraction of $J/psito pbar{p}phi$ is measured to be $mathcal{B}(J/psito pbar{p}phi)=[5.23pm0.06(mbox{stat})pm0.33(mbox{syst})]times10^{-5}$, which agrees well with a previously published measurement, but with a significantly improved precision. No evident enhancement near the $pbar{p}$ mass threshold, denoted as $X(pbar{p})$, is observed, and the upper limit on the branching fraction of $J/psito X(pbar{p})phito pbar{p}phi$ is determined to be $mathcal{B}(J/psito X(pbar{p})phito pbar{p}phi)<2.1times10^{-7}$ at the 90% confidence level.
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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