Search for the lepton flavour-violating decay μ→eγ

I introduce the upcoming MEG experiment, which will search for the rare decay μ→eγ down to the branching ratio of 10^-13. In order to suppress the background and achieve this unprecedented sensitivity, a great deal of thought went into designing this experiment. Here, I describe the essential components of this experiment, the beam line, the positron spectrometer, and the liquid xenon γ-ray detector. PACS 29.40.Gx; 29.40.Mc     

Observation of radiative leptonic decay of the tau lepton

Using 4.68 fb(-1) of e(+)e(-) annihilation data collected with the CLEO II detector at the Cornell Electron Storage Ring, we have studied tau radiative decays tau(-)-->nu(tau)&mgr;(-)nu;(&mgr;)gamma and tau(-)-->nu(tau)e(-)nu;(e)gamma. For a 10 MeV minimum photon energy in the tau rest frame, the branching fraction for radiative tau decay to a muon or electron is measured to be (3.61+/-0.16+/-0. 35)x10(-3) or (1.75+/-0.06+/-0.17)x10(-2), respectively. The branching fractions are in agreement with standard model theoretical predictions.     

The decay of theτ lepton to six pions

We present the branching ratio predictions of an isospin statistical model forτ decays to four, five and six pions. Limits on the branching ratios of the three possible six pionτ decay modes using the Conserved Vector Current (CVC) Hypothesis and thee ⁺ e ^?→6π cross section data are also presented. We find that the isospin model prediction is in good agreement with the well-measured four pion decay modes and consistent with the five pion decay modes. However, we find that some of the recent six pionτ measurements do not agree with the isospin model and the CVC prediction.     

The littlest Higgs model with T-parity and some lepton flavor violation decay processes

The new particles predicted by the littlest Higgs model with T-parity (called LHT model) can induce the lepton flavor violation (LFV) couplings at the one-loop level,which can add contributions to some LFV processes.Taking into account the constraints of the experimental data on the relevant free parameters,we calculate the branching ratios of the LFV decay processes Z → l ˉl and τ-→μ-P 1 P 2 with P 1 P 2 = π + π-,K + K-,and K0 ˉK 0 in the context of this new physics model.We find that the LHT model can indeed make significant contributions to some of these LFV decay processes.The Z factory option for the future high energy e + e-collider experiments will give severe constraints on the LHT model.     

On the decay of the Higgs particle into two lepton pairs

The decay of the standard Higgs into two lepton pairs is found to have a very small branching ratio, contrary to a recent result in the literature. The physical reasons for this discrepancy are discussed.     

Suppressing proton decay in the minimal SO(10) model

We show that in a class of minimal supersymmetric SO(10) models which have been found to be quite successful in predicting neutrino mixings, all proton decay modes can be suppressed by a particular choice of Yukawa textures. The required texture not only fits all lepton and quark masses as well as Cabibbo-Kobayashi-Maskawa parameters, but it also predicts neutrino mixing parameter U(e3) and Dirac CP phase sin(/delta(MNS)/ to be 0.07-0.09 and 0.3-0.7, respectively.     

Electron (muon) decay of a heavy lepton

Expressions are derived for the differential probability for the decay e(). The possibly nonzero mass of and the polarizations of both the decaying lepton and the charged lepton which is produced are all taken into account. A quantitative analysis is carried out for various masses of the neutrino and for various energies of the lepton emitted during the decay.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 97–100, June 1981.     

A quantum-relativistic puzzle of the meso-atom decay mode

Dilatation of life-time of a composite particle whose instability is only due to instability of its constituent exhibits an ambiguity of its decay mode which follows from two geometrically different pictures of microstructures. Some measurable characteristics are put forward that should decide which of them is accepted by experiment.     

The calculation of the matrix element for proton decay

Matrix elements and proton decay probabilities are calculated in the SU(5) model of grand unification with the help of the matrix element of the proton transition into three quarks, found previously by the QCD sum rules. The results show that an increase of the existing experimental lower limit for the proton lifetime by a factor of 3–4 would rule out the standard SU(5) model.     

A new decay mode of higher charmonium

We calculate the \(\Lambda _c{\bar{\Lambda }}_c\) partial decay width of the excited vector charmonium states around 4.6 GeV with the quark pair creation model. We find that the partial decay width of the \(\Lambda _c{\bar{\Lambda }}_c\) mode can reach up to several MeV for \(\psi (4S,~5S,~6S)\). In contrast, the partial \(\Lambda _c{\bar{\Lambda }}_c\) decay width of the states \(\psi (3D,~4D,~5D)\) is less than one MeV. If the enhancement Y(4630) reported by the Belle Collaboration in \(\Lambda _c{\bar{\Lambda }}_c\) invariant-mass distribution is the same structure as Y(4660), the Y(4660) resonance is most likely to be a S-wave charmonium state.