Asymptotic structure of perturbative series for \tau lepton observables

We analyze lepton decay observables, namely the moments of the hadronic spectral density in the finite energy interval , within finite order perturbation theory including corrections. The start of the asymptotic growth of the perturbation theory series is found at this order in a scheme-invariant manner. We establish the ultimate accuracy of finite order perturbation theory predictions and discuss the construction of optimal observables. Received: 6 September 1999 / Published online: 21 December 1999     

Deep inelastic scattering with tagged initial state radiation: Complete \mathcal{O}(\alpha) leptonic QED corrections

In this paper we extend the calculation of the QED corrections to deep inelastic lepton-proton scattering with a tagged photon, taking into account the full corrections on the lepton side. Comparing to previous results that were obtained by considering only large logarithmic terms at leading and next-to-leading accuray, we find that the difference is in general quite small, however, it may be significant in the region of large y and small x. Received: 10 October 2001 / Published online: 14 December 2001     

Precision neutral current asymmetry parameter measurements from the Tau polarization at LEP

Measurements of the lepton polarization and forward-backward polarization asymmetry near the Z resonance using the OPAL detector are described. The measurements are based on analyses of and decays from a sample of 144,810 candidates corresponding to an integrated luminosity of 151 pb. Assuming that the lepton decays according to V–A theory, we measure the average polarization near to be and the polarization forward-backward asymmetry to be , where the first error is statistical and the second systematic. Taking into account the small effects of the photon propagator, photon-Z interference and photonic radiative corrections, these results can be expressed in terms of the lepton neutral current asymmetry parameters: These measurements are consistent with the hypothesis of lepton universality and combine to give . Within the context of the Standard Model this combined result corresponds to . Combing these results with those from the other OPAL neutral current measurements yields a value of . Received: 5 March 2001 / Published online: 6 July 2001     

The \gamma \gamma \to \gamma \gamma process in the standard and SUSY models at high energies

We study the helicity amplitudes of the process at high energy, which in the standard and SUSY models first arise at the one-loop order. In the standard model (SM), the diagrams involve , charged-quark, and lepton loops, while in SUSY, we also have contributions from chargino-, charged-sfermion, and Higgs-loop diagrams. The SUSY contributions are most important in the region above the threshold for producing the supersymmetric partners; there, they interfere most effectively with the primarily imaginary SM amplitudes. Simple expressions for the relevant one-loop functions are given which provide a direct overview of the behaviour of the helicity amplitudes in the whole parameter space at high energies. The various characteristics of a large set of observables are studied in detail. Received: 3 February 1999 / Pulished online: 30 June 1999     

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     

The \gamma \gamma \to \gamma Z process at high energies and the search for virtual SUSY effects

We study the helicity amplitudes and the observables of the process at high energy. As in the case of the process studied before, the relevant diagrams in the standard model (SM) involve W, charged-quark, and lepton loops, while in SUSY we also have contributions from charginos and charged-sfermion or Higgs-loop diagrams. Above 250 GeV, the dominant SM amplitudes are themselves dominated by the W loop, and as for , they are helicity conserving and almost purely imaginary. We discuss the complementary information provided by for the identification of possible nonstandard effects. This process, together with , should provide very useful information on the nature of possible new physics particles, above the threshold of their direct production. Received: 26 April 1999 / Published online: 8 September 1999     

Intrinsic polarized strangeness and \Lambda ^0 polarization in deep-inelastic production

We propose a model for the longitudinal polarization of baryons produced in deep-inelastic lepton scattering at any , based on static SU(6) quark-diquark wave functions and polarized intrinsic strangeness in the nucleon associated with individual valence quarks. The free parameters of the model are fixed by fitting NOMAD data on the longitudinal polarization of hyperons in neutrino collisions. Our model correctly reproduces the observed dependences of the polarization on the kinematic variables. Within the context of our model, the NOMAD data imply that the intrinsic strangeness associated with a valence quark has anticorrelated polarization. We also compare our model predictions with results from the HERMES and E665 experiments using charged leptons. Predictions of our model for the COMPASS experiment are also presented. Received: 25 May 2002 / Revised version: 8 June 2002 / Published online: 7 October 2002     

Rare decay B\rightarrow X_sl^+l^- in a CP spontaneously broken two-Higgs doublet model

The Higgs boson mass spectrum and couplings of the neutral Higgs bosons to the fermions are worked out in a CP spontaneously broken two-Higgs doublet model in the large case. The differential branching ratio, forward-backward asymmetry, CP asymmetry and lepton polarization for are computed. It is shown that the effects of neutral Higgs bosons are quite significant when is large. Especially, the CP violating normal polarization can be as large as several percents. Received: 15 October 2001 / Revised version: 5 March 2002 / Published online: 26 July 2002     

The \gamma \gamma \to Z Z process and the search for virtual SUSY effects at a \gamma \gamma Collider

We study the helicity amplitudes of the process in the Standard Model at high energy. These amplitudes receive contributions from the W and charged quark and lepton loops, analogous to those encountered in the cases studied before. But also receives contributions from the Higgs s-channel poles involving the effective Higgs- vertex. At energies , the amplitudes in all three processes are mainly helicity-conserving and almost purely imaginary; which renders them a very useful tool in searching for New Physics. As an example, a SUSY case is studied, and the signatures due to the virtual effects induced by a chargino-, charged slepton- or a lightest stop-loop in , are explored. These signatures, combined with the analogous ones in and , should help identifying the nature of possible New Physics particles. Received: 3 September 1999 / Published online: 3 February 2000     

Lepton flavor changing in neutrinoless \tau decays

Neutrino oscillations, as recently reported by the Super-Kamiokande collaboration, imply that lepton numbers could be violated, and are some typical examples. We point out that in these neutrinoless modes, the GIM cancelation is much milder with only a logarithmic behavior where are the neutrino masses. This is in sharp contrast with the vanishingly small amplitude strongly suppressed by the quadratic power . In comparison with the hopelessly small branching ratio B, the B could be larger than . The latter mode, if measurable, could give one more constraint to the lepton mixing angle and the neutrino mass ratio , and therefore is complementary to neutrino oscillation experiments. Received: 29 October 1998 / Published online: 11 March 1999     

A precise measurement of the \tau polarisation at LEP-1

The polarisation has been studied with the data collected by the DELPHI detector at LEP in 1993, 1994 and 1995 around the Z resonance firstly through the exclusive decay channels , , , and and secondly with an inclusive hadronic analysis which benefits from a higher efficiency and a better systematic precision. The results have been combined with those previously published on 1990 to 1992 DELPHI data, to produce results which reflect the full LEP-1 statistics. The fit of the polarisation dependence on the production angle yielded the polarisation parameters and . From these results the ratio of the vector and axial-vector effective couplings and have been derived, compatible with e universality. With the assumption of lepton universality, the ratio of vector to axial-vector effective couplings for leptons was obtained, implying a value of the effective weak mixing angle . Received: 1 December 1999 / Revised version: 2 February 2000 / Published online: 14 April 2000     

Dilepton charge asymmetry inB - \bar B decays

The “partonic” calculation of the charge asymmetry of like-sign lepton pairs coming from \(B_d - \bar B_d \) decays as a result of mixing is reconsidered. Several possible corrections do not change the prediction in an important way. It is unlikely, in the minimal standard model, that the absolute value of the asymmetry is larger than 4×10^?3.     

Electromagnetic corrections in the anomaly sector and the \pi^0 lifetime

The framework for computing radiative corrections in ChPT in the anomaly sector is set up. Application to the lifetime of the and the possibility to extract information on m _d -m _u from precision data is discussed.Received: 30 September 2002, Published online: 22 October 2003PACS: 11.30.Rd Chiral symmetries - 12.15.Ff Quark and lepton masses and mixing - 12.39.Fe Chiral Lagrangians     

Processes with heavy ion collisions

Some Coulomb effects in heavy ion collisions are considered. Among them the process of muon and lepton pair creation, Coulomb and unitary corrections, statistics of multiple pair production. Effects of multiple photon exchange in process of lepton pair production by linearly polarized photon on nuclei are considered. This process is used for measuring the polarization of initial photon. Relativistic muon energy loss due to the light lepton pair production in the Coulomb field are calculated. Also we consider the effects of multiple photon exchange in elastic lepton scattering on unscreened atomic field and discuss the possible experimental testing.     

Scale of leptogenesis

We study the scale at which one can generate the lepton asymmetry of the universe which could then get converted to a baryon asymmetry during the electroweak phase transition. We consider the possibility that the Yukawa couplings could be arbitrarily small but sufficiently large to generate enough lepton asymmetry. This forbids the possibility of the breaking scale to be less than 10 TeV. Received: 14 April 1998 / Revised version: 8 August 1998 / Published online: 11 February 1999     

Measurement of the lifetime of b-baryons

The average lifetime of weakly decaying b-baryons was studied using 3.6 million hadronic decays collected by the DELPHI detector at LEP. The measurement of the proper decay time distribution of secondary vertices was used on three complementary samples. The first sample consisted of events with a fully reconstructed and an opposite charge lepton, or an oppositely charged lepton pair accompanied by a . The other two samples were more inclusive, where b-baryon semileptonic decays were recognized by the presence of either a proton identified by the RICH detector or a and a lepton of charge opposite to that of the proton. The combined result was: Received: 19 February 1999 / Revised version: 6 April 1999 / Published online: 3 August 1999     

Nuclear effects on lepton polarization in charged-current quasielastic neutrino scattering

We use a correlated local Fermi gas (LFG) model, which accounts also for long distance corrections of the RPA type and final-state interactions, to compute the polarization of the final lepton in charged-current quasielastic neutrino scattering. The present model has been successfully used in recent studies of inclusive neutrino nucleus processes and muon capture. We investigate the relevance of nuclear effects in the particular case of τ polarization in tau-neutrino induced reactions for several kinematics of relevance for neutrino oscillation experiments.     

Radiative return at $\Phi$ - and B-factories: FSR for muon pair production at next-to-leading order

Muon pair production through the radiative return is of importance for a measurement of the hadronic production cross section in two ways: it provides an independent calibration and it may give rise to an important background for the measurement of the pion form factor. With this motivation the Monte Carlo event generator PHOKHARA is extended to include next-to-leading order radiative corrections to the reaction . Furthermore, virtual ISR corrections to FSR from pions are introduced, which extends the applicability of the generator into a new kinematical regime. Finally, the effect of photon vacuum polarization is introduced into this new version of the generator.Received: 8 April 2004, Revised: 3 December 2004, Published online: 25 January 2005Work supported in part by BMBF under grant number 05HT9VKB0, EC 5th Framework Programme under contract HPRN-CT-2002-00311 (EURIDICE network), TARI project HPRI-CT-1999-00088, Polish State Committee for Scientific Research (KBN) under contract 2 P03B 017 24, BFM2002-00568, Generalitat Valenciana under grant GRUPOS03/013, and MCyT under grant FPA-2001-3031.     

Uncertainties from fermion masses ine^ + e^ - \to v\bar v\gamma

In the reaction \(e^ + e^ - \to v\bar v\gamma \) , the influence of the unknown masses of heavy fermions for three and four fermion generations is studied, which arises via virtual 1-loop corrections in the framework of the standard model. These masses are the top quark massm _t and in the case of 4 generations the lepton massm t, wherel ^? is theSU(2) partner of the fourth generation neutrino. Increasing the fermion masses from 50 GeV to 250 GeV, the maximum of the total cross section is found to decrease by a few percent.