Measurement of the tau lepton mass and an upper limit on the mass difference between tau+ and tau-

The mass of the tau lepton has been measured in the decay mode tau-->3pinutau using a pseudomass technique. The result obtained from 414 fb-1 of data collected with the Belle detector is Mtau=[1776.61+/-0.13(stat)+/-0.35(sys)] MeV/c2. The upper limit on the relative mass difference between positive and negative tau leptons is |Mtau+-Mtau-|/Mtau<2.8 x 10-4 at 90% confidence level.     

A mass reconstruction technique for a heavy resonance decaying to τ+τ-

For a resonance decaying to τ⁺τ^-, it is difficult to reconstruct its mass accurately because of the presence of neutrinos in the decay products of the τ leptons. If the resonance is heavy enough, we show that its mass can be well determined by the momentum component of the τ decay products perpendicular to the velocity of the τ lepton, p_⊥, and the mass of the visible/invisible decay products, m_vis/inv, for τ decaying to hadrons/leptons. By sampling all kinematically allowed values of p_⊥ and m_vis/inv according to their joint probability distributions determined by the MC simulations, the mass of the mother resonance is assumed to lie at the position with the maximal probability. Since p_⊥ and m_vis/inv are invariant under the boost in the τ lepton direction, the joint probability distributions are independent upon the τ's origin. Thus this technique is able to determine the mass of an unknown resonance with no efficiency loss. It is tested using MC simulations of the physics processes m pp→Z/h(125)/h(750)+X→ ττ+X at 13 TeV. The ratio of the full width at half maximum and the peak value of the reconstructed mass distribution is found to be 20%-40% using the information of missing transverse energy.     

Improved limits on the lepton-flavor violating decays tau{-}-->l{-}l{+}l{-}

A search for the neutrinoless, lepton-flavor violating decay of the tau lepton into three charged leptons has been performed using 376 fb{-1} of data collected at an e{+}e{-} center-of-mass energy around 10.58 GeV with the BABAR detector at the SLAC PEP-II storage rings. In all six decay modes considered, the numbers of events found in data are compatible with the background expectations. Upper limits on the branching fractions are set in the range (4-8)x10{-8} at 90% confidence level.     

Measurement of the {\rm\bf B_{\bf d}^{\bf 0}- {\bf \bar{B}}_{\bf d}^{\bf 0}} oscillation frequency

Time-dependent mixing is studied using about two million hadronic Z decays registered by L3 in 1994 and 1995. For this study three techniques are used. Tagging of the b-quark charge at decay time is performed by identifying leptons from semileptonic B decays. The flavour of the b quark at production time is determined from the charge of the lepton in the opposite hemisphere or by using a jet-charge technique. The proper time of the B-particle decay is obtained by reconstructing the production and decay vertices or by a measurement of the lepton impact parameter. The combined result for the frequency of meson oscillations is Received: 20 February 1998 / Revised version: 23 March 1998 / Published online: 12 August 1998     

Determining the CP parity of Higgs bosons via their tau decay channels at the Large Hadron Collider

If neutral Higgs bosons are discovered at the CERN Large Hadron Collider (LHC), then an important subsequent issue will be the investigation of their CP nature. Higgs boson decays into tau lepton pairs are particularly suited in this respect. By analyzing the three charged pion decay modes of the tau leptons and taking expected measurement uncertainties at the LHC into account, we show that the CP properties of a Higgs boson can be pinned down with appropriately chosen observables, provided that sufficiently large event numbers will eventually be available.     

The Discovery of the Tau Lepton and the Changes in Elementary-Particle Physics in Forty Years

I describe the discovery of the tau lepton in the 1970s using the SPEAR electron-positron collider and the SLAC-LBL detector of the Stanford Linear Accelerator Center. I also describe the subsequent verification of the existence of the tau lepton and its leptonic nature by experiments at SPEAR and at the DORIS electron-positron collider at DESY. As a preliminary to the tau discovery I discuss how I became a physicist and became interested in leptons. This history of the discovery of the tau allows me to give a general picture of the high-energy physics world of forty years ago and to discuss the changes that have occurred in the practice of high-energy physics over these forty years.Martin L. Perl is a professor, experimenter, and group leader at the Stanford Linear Accelerator Center of Stanford University. In 1995 he was awarded the Nobel Prize in Physics for his discovery of the tau lepton.     

Study of multihadron events with isolated leptons in e+e− annihilation at 50⩽sles;55 GeV

We have studied inclusive lepton production in e⁺e⁻ annihilations into multihadrons in the energy range between √s=50 and 55 GeV using the VENUS detector at TRISTAN, KEK. Though MARK-J and JADE groups at PETRA found an excess of isolated muon events at √s⩾46.3 GeV, we have not observed such an anomaly in muon nor in electron production. The observed rates of isolated leptons are consistent with the production and decay of five known quarks.     

Charm,bottom and top production in high energy electron proton collisions

Differential cross sections for electroproduction of charm, bottom and top quarks are calculated via all contributing 2 → 2 and 2 → 3 QCD subprocesses. Fragmentation of heavy quarks to heavy hadrons and effects due to weak chain decays of the heavy quarks are taken into account. We also calculated background contributions given by the production of two or three light-quark jets both via the neutral and charged current processes. We point out that, similarly to the case of hadron colliders, it will be necessary to require final state lepton(s) to suppress the jet background. To separate charm and bottom production we have to require at least one hard muon or two isolated leptons and one jet in the final state. We show that two or more jet production via the charged current mechanism and bottom production with hard gluon bremssrahlung are the most important background contributions in top search. However, we also show that they become negligible by requiring one lepton, two or three jets and large missing energy in the final state. We estimate that the discovery limit on the top quark mass value is about 70 GeV at HERA.     

Azimuthal angular dependence of decay lepton in e^+ e^- \to t \bar{t}

We discuss top quark production and its subsequent decay as used for searching new physics at lepton colliders. The angular dependence of the decay leptons is calculated including both QCD corrections and anomalous couplings. The off-diagonal spin basis for the top and anti-top quarks is shown to be useful to probe the anomalous couplings. Received: 11 August 2000 / Published online: 27 November 2000     

Formation and decay of scalar Leptoquarks/Squarks in ep collisions

The cross sections for the formation of scalar resonances, leptoquarks or squarks, in electron/positron-proton collisions at HERA are presented, including next-to-leading order QCD corrections. Depending mildly on the mass of the resonances, the K-factors increase the production cross sections by up to 30% if the target quarks are valence quarks. The QCD corrections to the partial decay widths of leptoquarks/ squarks to leptons and quarks are small. The electron spectrum in the decays is nevertheless softened by perturbative gluon radiation at a level of 2 GeV for a leptoquark/ squark mass of 200 GeV.     

Signatures of MSSM charged Higgs bosons via chargino–neutralino decay channels at the LHC

We assess the potential of detecting a charged Higgs boson of the MSSM at the LHC via its decays into a chargino and a neutralino. We focus our attention on the region of parameter space with and , where identification of the via other decay modes has proven to be ineffective. Searching for means to plug this hole, we simulate the decays and – the former can yield a single hard lepton (from the chargino decay) while the latter can yield three leptons (from the chargino and neutralino decays). Coupled with the dominant top quark + charged Higgs boson production mode, the resulting signature is one or three hard, isolated leptons, substantial missing transverse momentum and a reconstructed (via a 3-jet invariant mass) top quark. The single lepton channel is swamped by background processes; however, with suitable cuts, a trilepton signal emerges. While such a signal suffers from a low number of surviving events (after cuts) and is dependent on several MSSM input parameters (notably , and slepton masses), it does fill at least some of the void left by previous investigations. Received: 21 October 2000 / Published online: 23 February 2001     

Bounds on radii and anomalous magnetic dipole moments of quarks and leptons from LEP,SLC and HERA

Leptons, quarks and gauge bosons are assumed to be pointlike particles in the Standard Model. Stringent bounds on the radii of quarks and leptons and their weak anomalous magnetic moments can be derived from the high-precision measurements at LEP and SLC. We find a model-independent bound ofR?10^?17 cm for quark and lepton radii. HERA will provide complementary information on the electromagnetic static properties of the quarks and the parameters of the charged quark currents.     

Lepton number violation at the LHC with leptoquark and diquark

We investigate a model in which tiny neutrino masses are generated at the two-loop level by using scalar leptoquark and diquark multiplets. The diquark can be singly produced at the LHC, and it can decay into a pair of leptoquarks through the lepton number violating interaction. Subsequent decays of the two leptoquarks can provide a clear signature of the lepton number violation, namely two QCD jets and a pair of same-signed charged leptons without missing energy. We show that the signal process is not suppressed while neutrino masses are appropriately suppressed.     

Search for lepton-flavor and lepton-number violation in the decay tau(-) -->l-(+)h+(-)h'(-)

A search for lepton-flavor and lepton-number violation in the decay of the tau lepton into one charged lepton and two charged hadrons is performed using 221.4 fb(-1) of data collected at an e+e- center-of-mass energy of 10.58 GeV with the BABAR detector at the SLAC PEP-II storage ring. In all 14 decay modes considered, the observed data are compatible with background expectations, and upper limits are set in the range B(tau-->lhh')<(0.7 - 4.8) x 10(-7) at 90% confidence level.     

Production and decay properties of possible spin 32 leptons

A systematic study of production and decay properties of hypothetical charged spin $\text{3}\text{2}$ leptons is presented. These particles are assumed to occur in each lepton family. The simplest V ± A currents constructed with these particle fields and the corresponding spin $\text{1}\text{2}$ neutrinos are assumed to be coupled to the familiar V ? A lepton charged weak currents. Our results are independent from structure models and indicate ways for an experimental search for these particles.     

Heavy majorana neutrinos atep colliders

Heavy Majorana neutrinos (N), predicted in various extensions of the standard model, are examined with respect to the present limits on their masses and mixings with ordinary leptons resulting in explicit examples of allowed values of interest for present and planned accelerator energies. The decayN→Zv is added to the previously available formalism and all dominating branching ratios are calculated. The production of Majorana neutrinos through charged current interactions in theep colliders HERA and LEP ⊕ LHC is investigated using Monte Carlo event simulation. Signals in terms of isolated leptons and jets are found and shown to be effective in suppressing the dominating standard model backgrounds. The discovery limits of such Majorana neutrinos are, for a mixing of 1%, about 160 GeV at HERA and 700 GeV at LEP ⊕ LHC.     

Lepton mixing, μ → e + γ decay and neutrino oscillations

The μ → eγ decay is investigated in a gauge theory with lepton mixing under the assumption that in nature there exist heavy leptons. It is shown that for lepton masses of the order of a few GeV the μ → eγ decay probability may well be close to its experimentally determined upper limit. The relation between such a decay process and neutrino oscillations is briefly considered.     

Observation of events with an isolated high energy lepton and missing transverse momentum at HERA

A search for events with an imbalance in transverse momentum and with isolated high energy leptons has been carried out at the positron-proton collider HERA. One event with an and five events with a are found together with evidence for undetected particles carrying transverse momentum. Within the Standard Model the dominant origin of events with this kind of topology is the production of W bosons with subsequent leptonic decay. Three of the six events are within measurement errors found in a region of phase space likely to be populated by this process, while the remaining events show kinematic properties which are atypical of all Standard Model processes considered. Received: 9 June 1998 / Published online: 26 August 1998     

QCD corrections and the Leptoquark interpretation of the HERA high-Q 2 events

The excess of high-Q ² events found by H1 and ZEUS at HERA in e ⁺ p deep-inelastic scattering above the Standard Model prediction motivates us to calculate the NLO QCD corrections to the HERA scalar leptoquark (or squark) production cross sections. We find that the corrections are significant, of order 50% in the mass range of interest. We also calculate the leptoquark average transverse momentum squared and find it to be rather small. Various leptoquark production cross sections at the Tevatron pp? collider are also considered. We investigate in detail the leptoquark interpretation of the HERA data. First we assume a minimal leptoquark model with a single diagonal Yukawa coupling to first family lepton and quark mass eigenstates only. In this case constraints from atomic parity violating experiments allow only isodoublet scalar leptoquark production at HERA. This interpretation can be confirmed or ruled out in the near future by high luminosity data at the Tevatron. The Tevatron data already appear to rule out the vector leptoquark interpretation of the HERA data. We also consider a more general model which allows for all possible left-handed, right-handed, flavour and lepton number changing couplings. The allowed values of the Yukawa couplings of this general model offer several different interpretations of the data which are radically different from the minimal model solutions. However these somewhat ad hoc tuned solutions can easily be tested by future HERA experiments.     

Excited lepton production at LEP and HERA

General expressions for single and pair production cross sections of excited leptons (e ^*,µ^*,v ^*) are presented. Specific results are shown for a realisticSU(2)×U(1) invariant model. Pair production ine ⁺ e ^? annihilation can measure anomalous magnetic moments of excited leptons. Single production ofe ^* is dominated by thet-channel γ exchange contribution which makes its detection feasible up to masses just below thee ⁺ e ^? c.m. energy. Due to this small |t| enhancement effect, contributions from elastic and resonance scattering inep production ofe ^* are substantial. Realistic estimates of the excited lepton production cross section at HERA are given