Study of Z pair production and anomalous couplingsin e + e- collisions at $\sqrt{s}$ between 190 GeV and 209 GeV

A study of Z-boson pair production in e ⁺ e^- annihilation at center-of-mass energies between 190 GeV and 209 GeV is reported. Final states containing only leptons, ( and ), quark and lepton pairs, ( , ) and only hadrons ( ) are considered. In all states with at least one Z boson decaying hadronically, lifetime, lepton and event-shape tags are used to separate pairs from final states. Limits on anomalous ZZ and ZZZ couplings are derived from the measured cross sections and from event kinematics using an optimal observable method. Limits on low scale gravity with large extra dimensions are derived from the cross sections and their dependence on polar angle.Received: 14 July 2003, Published online: 18 December 2003     

The $\eta_{c}(2980)$ formation in two-photon collisions at LEP energies

production in interactions has been detected via its decays into ,K ⁺ K ^- K ⁺ K ^- and in the data taken with the DELPHI detector at LEP1 and LEP2 energies. The two-photon radiative width averaged over all observed decay channels is = 13.9 2.0 (stat.) 1.4(syst.) 2.7 (BR) keV. No direct decay channel has been observed. An upper limit < 5.5 keV at 95% confidence level has been evaluated for this decay mode.Received: 3 July 2003, Published online: 7 November 2003     

Search for anomalous weak dipole moments of the $\tau$ lepton

The anomalous weak dipole moments of the lepton are measured in a data sample collected by ALEPH from 1990 to 1995 corresponding to an integrated luminosity of 155 pb^-1. Tau leptons produced in the reaction at energies close to the mass are studied using their semileptonic decays to , , or . The real and imaginary components of both the anomalous weak magnetic dipole moment and the CP-violating anomalous weak electric dipole moment, , , and , are measured simultaneously by means of a likelihood fit built from the full differential cross section. No evidence of new physics is found. The following bounds are obtained (95% CL): , , , and .Received: 25 September 2002, Published online: 29 August 2003     

New gauge bosons from the littlest Higgs modeland the process $e^{ + }e^{-}\rightarrow t\bar{t}$

In the context of the littlest Higgs (LH) model, we study the process . We find that the new gauge bosons Z _H and B _H can produce significant correction effects on this process, which can be further enhanced by suitably polarized beams. In most of the parameter space preferred by the electroweak precision data, the absolute value of the relative correction parameter is larger than 5 %. As long as and the absolute value of the relative correction parameter is larger than . With reasonable values of the parameters of the LH model, the possible signals of the new gauge bosons B _H and Z _H can be detected via the process in the future LC experiments with the CM energy . B _H exchange and Z _H exchange can generate significantly corrections to the forward-backward asymmetry only in a small part of the parameter space.Received: 29 October 2004, Revised: 29 November 2004, Published online: 15 February 2005     

Di-jet production in photon-photon collisionsat $\sqrt{s_{\protect\bf\mathrm{ee}}}$ from 189 to 209 GeV

Di-jet production is studied in collisions of quasi-real photons at e ⁺ e^- centre-of-mass energies from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive -clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy of the two leading jets, and as a function of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, , for different regions of . Angular distributions in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of and is presented, where is the jet pseudo-rapidity. Different regions of the - -space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.Received: 20 December 2002, Published online: 17 October 2003     

Leptoproduction of $\varrho$ mesons within Regge approach

A model describing the leptoproduction of mesons is considered. For the amplitude of the photon dissociation into a pair, the light-cone wave function is used. Scattering of the colorless pair off the nucleon is computed within reggeon exchange phenomenology. The transition of the scattered quark and antiquark into the final meson is treated with the aid of the parton-hadron duality concept. Numerical calculations of and r⁰⁴₀₀ describe the world data at Q² < 4 GeV² rather reasonably. The calculations of the meson spin density matrix show that the computed matrix elements except r⁰⁴₀₀ are in good agreement with the available experimental data at Q² up to 8 GeV² and the total mass of the system greater than 4 GeV. The Regge phenomenology predictions at the highest experimentally available energies agree with both the HERA data and calculations performed within the perturbative QCD approach even for very high Q² ( -20 GeV²). The predicted scale of the S-channel helicity non-conservation is in reasonable agreement with the experimental data.Received: 18 September 2003, Revised: 17 December 2003, Published online: 13 February 2004     

Polarisation in sfermion decays: determining $\tan\beta$ and trilinear couplings

The basic parameters of supersymmetric theories can be determined at future e ⁺ e ^- linear colliders with high precision. We investigate in this report how polarisation measurements in and or decays to leptons and t quarks plus neutralinos or charginos can be used to measure (in particular for large values) and to determine the trilinear couplings , A _t and A _b in sfermion pair production.Received: 17 April 2003, Revised: 3 July 2003, Published online: 29 August 2003     

Search for stable hadronizing squarks and gluinosin e + e- collisions up to \sqrt{s} = 209 GeV

Searches for stable, hadronizing scalar quarks and gluinos are performed using the data collected with the ALEPH detector at LEP. Gluon splitting into a gluino or a squark pair is searched for at centre-of-mass energies around the Z resonance, in the e ⁺ e^- and processes. Stable squark pair production, and stop pair production with subsequent decays into a stable gluino, , are also directly searched for at centre-of-mass energies from 183 to 209 GeV. Altogether, stable hadronizing stop (sbottom) quarks are excluded up to masses of 95 (92) , and stable hadronizing gluinos are excluded up to 26.9 , at 95% confidence level. In the framework of R-parity-conserving supersymmetric models in which the gluino and the stop quark are the two lightest supersymmetric particles, a 95% C.L. lower limit of 80 is set on the stop quark mass.Received: 23 May 2003, Published online: 24 October 2003     

Multi-ring signatures of the oscillation $\nu_\mu \rightarrow \nu_e$ in a water Cherenkov detector

Multi-ring signatures of appearance via the oscillation are formulated for a water Cherenkov detector. These signatures are appropriate for long-baseline neutrino experiments operating at relatively high neutrino energies GeV that emphasize the matter effect. The NC background is less for selected multi-ring events than for 1e-like events, and may be directly estimated from the data. Our results suggest that best sensitivity to and to the sign of can be reached with baselines over some 6000 km.Received: 14 April 2003, Revised: 19 June 2003, Published online: 26 September 2003M.A. Kubantsev: Now at Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA     

Search for charged Higgs bosons at LEP in general two Higgs doublet models

A search for pair-produced charged Higgs bosons was performed in the data collected by the DELPHI detector at LEP II at centre-of-mass energies from 189 GeV to 209 GeV. Five different final states, , , , and were considered, accounting for the major expected decays in type I and type II Two Higgs Doublet Models. No significant excess of data compared to the expected Standard Model processes was observed. The existence of a charged Higgs boson with mass lower than 76.7 GeV/c² (type I) or 74.4 GeV/c² (type II) is excluded at the 95% confidence level, for a wide range of the model parameters. Model independent cross-section limits have also been calculated.Received: 30 September 2003, Revised: 12 February 2004, Published online: 8 April 2004     

The pion form factor within the hidden local symmetry model

We analyze a pion form factor formulation which fulfills the Analyticity requirement within the Hidden Local Symmetry (HLS) Model. This implies an s-dependent dressing of the VMD coupling and an account of several coupled channels. The corresponding function provides nice fits of the pion form factor data from s=-0.25 to s=1 GeV². It is shown that the coupling to has little effect, while improves significantly the fit probability below the mass. No need for additional states like shows up in this invariant-mass range. All parameters, except for the subtraction polynomial coefficients, are fixed from the rest of the HLS phenomenology. The fits show consistency with the expected behaviour of at s=0 up to and with the phase shift data on from threshold to somewhat above the mass. The sector is also examined in relation with recent data from CMD-2. Received: 13 January 2003, Revised: 12 March 2003, Published online: 2 June 2003     

M easur em ent of th e $\mathit{H Z\gamma}$ coupling at th e futur e lin ear e+ e- collid er

We examine the prospects for measuring the coupling of a standard model-like Higgs boson with a mass between 120 and 160 GeV at the future TESLA linear e ⁺ e ^- collider, assuming an integrated luminosity of 1 ab^-1 and a center-of-mass energy of 500 GeV. We consider the Higgs boson produced in association with via the W W fusion reaction , followed by the rare decay into a Z boson and a photon, . Accounting for all main background contributions, a precision of 27% can be achieved in unpolarized e ⁺ e ^- collisions for M _H = 140 GeV. With appropriate initial state polarisations BF( )/BF( ), or the precisions on the partial width, can be improved to 17% and provide valuable information on the coupling. For M _H = 120 and 160 GeV, the small significance of the signals in unpolarized collisions sets upper limits of 79% respectively 72% at 90% confidence level on the branching fraction.Received: 5 March 2003, Revised: 14 July 2003, Published online: 5 September 2003     

$\overline pp$ annihilation into four charged pions at rest and in flight

The spin-parity analysis of the data on the annihilation reaction at rest in liquid and in gaseous hydrogen at 3 bar pressure and in flight at momentum of MeV/c, collected by the Obelix spectrometer at the LEAR complex of CERN, is presented. The relative branching ratios and are obtained. It is also shown that the inclusion of the exotic meson , J^PC = 1^{- +} , mass and width , GeV/c², in its decay to , improves the fit and some implications of these results are briefly discussed. The relative S and P-wave annihilation percentages in four charged pions at two target densities are obtained.Received: 2 December 2003, Published online: 23 April 2004     

<Emphasis Type="Italic">T</Emphasis>-odd correlations in the decay of scalar fermions

We define a CP-sensitive asymmetry in the sfermion decays , based on triple product correlations between the momenta of the outgoing fermions. We study this asymmetry in the MSSM with complex parameters. We show that the asymmetry is sensitive to the phases of the parameters and M₁. The leading contribution stems from the decay chain , for which we obtain analytic formulae for the amplitude squared. The asymmetry can go up to for , and up to for . We also estimate the rates necessary to measure the asymmetry.Received: 2 July 2003, Revised: 7 November 2003, Published online: 13 February 2004     

Waiting for the discovery of $B^0_d\to K^0\bar K^0$

The CP asymmetries of the decay , which originates from flavor-changing neutral-current processes, and its CP-averaged branching ratio BR offer interesting avenues to explore flavor physics. We show that we may characterize this channel, within the standard model, in a theoretically clean manner through a surface in observable space. In order to extract the relevant information from BR , further information is required, which is provided by the system and the SU(3) flavor symmetry, where we include the leading factorizable SU(3)-breaking corrections and discuss how experimental insights into non-factorizable effects can be obtained. We point out that the standard model implies a lower bound for BR , which is very close to its current experimental upper bound, thereby suggesting that this decay should soon be observed. Moreover, we explore the implications for color suppression in the system, and convert the data for these modes in a peculiar standard-model pattern for the CP-violating observables.Received: 3 August 2004, Published online: 15 November 2004     

Probing signals of the littlest Higgs model via the WW fusion processes at the high energy e + e- collider

In the framework of the littlest Higgs (LH) model, we consider the processes and , and we calculate the contributions of new particles to the cross sections of these processes in the future high energy e ⁺ e^- collider (ILC) with TeV. We find that, with reasonable values of the free parameters, the deviations of the cross sections for the processes from their SM values might be comparable to the future ILC measurement precision. The contributions of the light Higgs boson H⁰ to the process are significantly large in all of the parameter space preferred by the electroweak precision data, which might be detected in the future ILC experiments. However, the contributions of the new gauge bosons B_H and Z_H to this process are very small.Received: 22 February 2005, Revised: 27 April 2005, Published online: 6 July 2005PACS: 12.60.Cn, 14.70.Pw, 14.80.Cp     

The rare annihilation decays $\bar{B}^0_{s,d} \to J/\psi\gamma$

We investigate the physics potential of the annihilation decays in the standard model and beyond. In a naive factorization approach, the branching ratios are estimated to be and . In the framework of QCD factorization, we compute the non-factorizable corrections and get , . Future measurements of these decays would be useful for testing the factorization frameworks. The smallness of these decays in the SM makes them sensitive probes of new physics. As an example, we will consider the possible admixture of the (V + A) charge current to the standard (V-A) current. This admixture will give a significant contribution to the decays.Received: 29 August 2003, Revised: 17 January 2004, Published online: 19 March 2004Corresponding author: Y.D. Yang     

Multiphoton radiation in leptonic <Emphasis Type="Italic">W</Emphasis>-boson decays

We explore CP violation in decay processes in the presence of the anomalous right-handed and couplings. The complex anomalous top coupling can be a source of new CP violation and may lead to a deviation of the observed weak phase in decays, which accounts for the present disagreement of the observed between and decays. Direct CP violation is also predicted. Received: 27 November 2002, Revised: 28 March 2003, Published online: 2 June 2003     

Lepton flavor violating <Emphasis Type="Italic">Z</Emphasis>-decays in supersymmetric seesaw model

In the supersymmetric seesaw model, large flavor mixings of sleptons induce the lepton flavor violating (LFV) interactions ( ), which give rise to various LFV processes. In this work we examine the LFV decays . Subject to the constraints from the existing neutrino oscillation data and the experimental bounds on the decays , these LFV Z-decays are found to be sizable, and among them the largest-rate channel can occur with a branching ratio of 10^-8 and may be accessible at the LHC or GigaZ experiment.Received: 1 September 2003, Published online: 20 November 2003     

Pair-produced heavy particle topologies: MSSM neutralino properties at the LHC from gluino/squark cascade decays

Processes of the form pp → anything → X_iX_j → + + notE are studied via a technique that may be viewed as an adaptation of time-honoured Dalitz plot analyses. X_i and X_j are new heavy states (with i, j =1, . . .,n), which may be identical or distinct; and and are necessarily distinct standard model (SM) fermion pairs whose invariant masses can be measured. A Dalitz-like plot of said invariant masses, versus , exhibits a topology connected to the masses and specific decay chains of X_i and X_j. Aside from relatively minor details, observed patterns consist of a collection of box and wedge shapes. This collection is model-dependent: comparison of the observed pattern to the possibilities for a specific model yields information on which new particle pair combinations are actually being produced, information beyond that extractable from conventional one-dimensional invariant mass distributions. The technique is illustrated via application to the minimal supersymmetric standard model (MSSM) process pp → → e⁺e^- + μ⁺μ^- notE. Here the heavy states are neutralinos (i = 2,3,4) - note that is excluded - which are produced in gluino/squark ( / ) cascade decay chains. Even with fairly modest expectations for the LHC performance during the first few years, this method still provides substantial insight into the neutralino mass spectrum and couplings if gluino/squark masses are relatively low (≃ 400 GeV). Arrival of the final proofs: 29 November 2005