Signals of universality violation in e<Superscript>+</Superscript>e<Superscript>-</Superscript> collisions

We examine distributions of leptons produced in e⁺e^- collisions, by a family nonuniversal extra gauge boson Z^′, suggested by the model, and by other neutral gauge bosons occurring in left–right symmetric models and in superstring-inspired E₆ models. We discuss how to distinguish the models by examining the couplings to fermions of the extra Z-boson through its leptonic production cross sections and asymmetries. We show how the universality violation inherent in the model enhances the discovery potential of Z^′ at future planned and proposed e⁺e^- colliders.     

The correction of the littlest Higgs model to the Higgs production process e<Superscript>+</Superscript>e<Superscript>-</Superscript>→e<Superscript>+</Superscript>e<Superscript>-</Superscript>H at the ILC

The littlest Higgs model is the most economical one among various little Higgs models. In the context of the littlest Higgs model, we study the process e⁺e^-→e⁺e^-H at the ILC and calculate the correction of the littlest Higgs model to the cross section of this process. The results show that, in the favorable parameter spaces preferred by the electroweak precision data, the value of the relative correction is in the range from a few percent to tens percent. In most cases, the correction is large enough to reach the measurement precision of the ILC. Therefore, the correction of the littlest Higgs model to the process e⁺e^-→e⁺e^-H might be detected at the ILC, which will give an ideal way to test the model. PACS 12.60.Nz; 14.80.Mz; 12.15.Lk; 14.65.Ha     

Spin correlations in elastic e<Superscript>+</Superscript>e<Superscript>-</Superscript> scattering in QED

Spin correlations are carefully investigated in elastic e⁺e^- scattering in QED, for initially polarized as well as unpolarized particles, with emphasis placed on energy or speed of the underlying particles involved in the process. An explicit expression is derived for the corresponding transition probabilities in closed form to the leading order. These expressions differ from those obtained from simply combining the spins of the relevant particles, which are of kinematic nature. It is remarkable that these explicit results obtained from quantum field theory show a clear violation of Bell's inequality at all energies, in support of quantum theory in the relativistic regime. We hope that our explicit expression will lead to experiments, of the type described in the bulk of this paper, that monitor speed.     

QED radiative corrections to the decay π<Superscript>0</Superscript>→e<Superscript>+</Superscript>e<Superscript>-</Superscript>

We reconsider QED radiative corrections (RC) to the π⁰→e⁺e^- decay width. One kind of RC investigated earlier has a renormalization group origin and can be associated with the final state interaction of electron and positron. It determines the distribution of lepton pair invariant masses in the whole kinematic region. The other type of RC has a double-logarithmic character and is related to almost on-mass-shell behavior of the lepton form factors. The total effect of RC for π⁰→e⁺e^- decay is estimated to be 3.2%, and for η→e⁺e^- decay it is 4.3%. PACS  13.25.Cq; 12.38.Lg; 12.38.-t     

On kaon production in e<Superscript>+</Superscript>e<Superscript>-</Superscript> and semi-inclusive DIS reactions

We consider semi-inclusive unpolarized DIS for the production of charged kaons and the different possibilities to test the conventionally used assumptions s-s̄=0 and D_d ^K+-K-=0. The tests considered have the advantage that they do not require any knowledge of the fragmentation functions. We also show that measurements of both charged and neutral kaons would allow for the determination of the kaon fragmentation functions D_q ^K++K- solely from SIDIS measurements, and we discuss the comparison of (D_u-D_d)^K+-K- obtained independently in SIDIS and e⁺e^- reactions. All analyses are performed in LO and NLO in QCD. The feasibility of the tests to HERMES SIDIS data is considered. PACS 12.38.Bx; 13.85.Ni     

Hadron multiplicities in e<Superscript>+</Superscript>e<Superscript>-</Superscript> annihilation with heavy primary quarks

The multiple hadron production in the events induced by the heavy primary quarks in e⁺e^- annihilation is reconsidered with account of corrected experimental data. A new value for the multiplicity in bb̄ events is presented on the basis of pQCD estimates.     

Measurement of the top-Higgs Yukawa coupling at a linear e<Superscript>+</Superscript>e<Superscript>-</Superscript> collider

Understanding the mechanism of electroweak symmetry breaking and the origin of boson and fermion masses is among the most pressing questions raised in contemporary particle physics. If these issues involve one (several) Higgs boson(s), a precise measurement of all its (their) properties will be of prime importance. Among those, the Higgs coupling to matter fermions (the Yukawa coupling). At a linear collider, the process e⁺e^-→tt̄H will allow a direct measurement of the top-Higgs Yukawa coupling. We present a realistic feasibility study of the measurement in the context of the TESLA collider. Four channels are studied and the analysis is repeated for several Higgs mass values within the range 120–200 GeV/c ². PACS 13.66.Jn; 14.65.Ha; 14.80.Bn     

e<Superscript> + </Superscript>e<Superscript>-</Superscript>-pair production in Pb-Au collisions at 158 GeV per nucleon

We present the combined results on electron-pair production in 158 GeV/n Pb-Au ( = 17.2 GeV) collisions taken at the CERN SPS in 1995 and 1996, and give a detailed account of the data analysis. The enhancement over the reference of neutral meson decays amounts to a factor of 2.31 for semi-central collisions (28 ) when yields are integrated over m > 200 MeV/c² in invariant mass. The measured yield, its stronger-than-linear scaling with , and the dominance of low pair p_t strongly suggest an interpretation as thermal radiation from pion annihilation in the hadronic fireball. The shape of the excess centring at 500 MeV/c², however, cannot be described without strong medium modifications of the meson. The results are put into perspective by comparison to predictions from Brown-Rho scaling governed by chiral symmetry restoration, and from the spectral-function many-body treatment in which the approach to the phase boundary is less explicit.Received: 2 March 2005, Published online: 8 June 2005     

Pair production of the heavy leptons in future high energy linear e<Superscript>+</Superscript>e<Superscript>-</Superscript> colliders

The littlest Higgs model with T-parity predicts the existence of the T-odd particles, which can only be produced in pairs. We consider pair production of the T-odd leptons in a future high energy linear e⁺e^- collider (ILC). Our numerical results show that, as long as the T-odd leptons are not too heavy, they can be copiously produced and their possible signals might be detected via the processes e⁺e^-→L̄_iL_j in future ILC experiments.     

Diffractive production of two ρ<Superscript>0</Superscript><Subscript>L</Subscript> mesons in e<Superscript>+</Superscript>e<Superscript>-</Superscript> collisions

We present an estimate of the cross-section for the exclusive production of a ρ_L ⁰-meson pair in e⁺e^- scattering, which will be studied in the future high-energy International Linear Collider. For this aim, we complete calculations of the Born order approximation of the amplitudes γ^* _L,T(Q₁ ²)γ^* _L,T(Q₂ ²)→ρ_L ⁰ρ_L ⁰, for arbitrary polarization of virtual photons and longitudinally polarized mesons, in the kinematical region s≫-t,Q₁ ²,Q₂ ². These processes are completely calculable in the hard region Q₁ ²,Q₂ ²≫Λ² _QCD, and we perform most of the calculations in an analytical way. The resulting cross-section turns out to be large enough for this process to be measurable with foreseen luminosity and energy, for Q₁ ² and Q₂ ² in the range of a few GeV².     

Graviton resonances in e<Superscript>+</Superscript>e<Superscript>-</Superscript>→μ<Superscript>+</Superscript>μ<Superscript>-</Superscript> at linear colliders with beamstrahlung and ISR effects

The electromagnetic radiation emitted by colliding beams is expected to play an important role at the next generation of high energy e⁺e^- linear colliders. Focussing on the simplest process, e⁺e^-→μ⁺μ^-, we show that, for suitable machine parameters and luminosity, radiative effects like initial state radiation (ISR) and beamstrahlung can be used to search for resonant graviton modes of the Randall–Sundrum model in an efficient manner.     

Lepton flavor violating signals of a little Higgs model at high energy linear e<Superscript>+</Superscript>e<Superscript>-</Superscript> colliders

The littlest Higgs (LH) model predicts the existence of the doubly charged scalars , which generally have large flavor changing couplings to leptons. We calculate the contributions of Φ^±± to the lepton flavor violating (LFV) processes l_i→l_jγ and l_i→l_jl_kl_k, and we compare our numerical results with the current experimental upper limits on these processes. We find that some of these processes can give severe constraints on the coupling constant Y_ij and the mass parameter M_Φ. Taking into account the constraints on these free parameters, we further discuss the possible lepton flavor violating signals of at high energy linear e⁺e^- collider (ILC) experiments. Our numerical results show that the possible signals of might be detected via the subprocesses e^±e^±→l^±l^± in future ILC experiments.     

Measurement of W-pair production in e<Superscript> + </Superscript>e<Superscript>-</Superscript> collisions at centre-of-mass energies from 183 to 209 GeV

The W ⁺ W^- production cross section is measured from a data sample corresponding to a total integrated luminosity of 683 pb^-1, collected by the ALEPH experiment at LEP at centre-of-mass energies from 183 to 209 GeV. Individual cross sections for the different topologies arising from W decays into leptons or hadrons, as well as the total W-pair cross section are given at eight centre-of-mass energies. The results are found to be in agreement with recently developed Standard Model calculations at the one percent level. The hadronic branching fraction of the W boson is measured to be = 67.13 0.37(stat.) 0.15(syst.)%, from which the CKM matrix element is determined to be 0.958 0.017(stat.) 0.008(syst.)Received: 1 October 2004, Published online: 15 November 2004     

Lowest lying <Superscript>1</Superscript>D<Superscript>e</Superscript> resonance of H<Superscript>-</Superscript> in Debye plasma

Electron-hydrogen scattering has been investigated in Debye plasma environment employing the close-coupling approximation. Two models, viz., 6-state CCA and 9-state CCA have been used. Plasma screening has been taken into account via Debye-Hückel model potential. The lowest lying ¹D^e^1{\rm D}^{{\rm e}} auto-detaching resonance of the hydrogen negative ion has been successfully predicted for various plasma conditions. The resonant state changes to shape resonance for Debye scattering lengths less than a critical value.     

Searches for supersymmetric particles in e<Superscript> + </Superscript>e<Superscript>-</Superscript> collisions up to 208 GeV and interpretation of the results within the MSSM

DELPHI data collected at centre-of-mass energies up to 208 GeV have been analysed to search for charginos, neutralinos and sfermions in the framework of the Minimal Supersymmetric Standard Model (MSSM) with R-parity conservation. No evidence for a signal was found in any of the channels. The results of each search were used to derive limits on production cross-sections and particle masses. In addition, the combined result of all searches excludes regions in the parameter space of the constrained MSSM, leading to limits on the mass of the Lightest Supersymmetric Particle and other supersymmetric particles.Received: 22 February 2003, Revised: 24 July 2003, Published online: 12 November 2003     

Generation-changing interaction of sneutrinos in <Emphasis Type="Italic">e</Emphasis><Superscript> + </Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>-</Superscript> collisions

The measurability of generation mixing is studied on pair production of sneutrinos in e ⁺ e ^- collisions and their subsequent decays into two different charged leptons e and μ with two lighter charginos. The analyses are made systematically in a general framework of the supersymmetric extension of the standard model. The production and decay process depends on the parameters of the chargino sector as well as those of the sneutrino sector. Although generation-changing interactions are severely constrained by radiative charged-lepton decays, sizable regions in the parameter space could still be explored at e ⁺ e ^- colliders in the near future. Received: 6 October 2004, Revised: 14 November 2004, Published online: 14 January 2005 PACS: 11.30.Hv, 12.15.Ff, 12.60.Jv, 14.80.Ly     

Simulation on the response of new phosphate glass RPC for <Emphasis Type="Italic">γ</Emphasis>-rays and e<Superscript>+</Superscript>/e<Superscript>−</Superscript> particles using GEANT3

The influence of phosphate glass electrode in the configuration of resistive plate chamber has been studied using GEANT3.21 Monte Carlo code. Bakelite electrodes were replaced by phosphate glass electrodes, as these glass materials have low bulk resistivity, are portable and easy to handle. These types of RPCs in their compact form of all materials are suitable for high rate background environment. We find that these new types of RPCs give little higher response to γ-rays and e⁺/e⁻ particles, both for single-and double gap RPCs. The results of simulation are discussed.      

Sneutrino production at <Emphasis Type="Italic">e</Emphasis><Superscript> + </Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>-</Superscript> linear colliders: addendum to slepton production

Complementing the preceding study of charged scalar leptons, the sector of the neutral scalar leptons, sneutrinos, is investigated in a high-precision analysis for future e ⁺ e ^- linear colliders. The theoretical predictions for the cross-sections are calculated at the thresholds for non-zero widths and in the continuum including higher-order corrections at the one-loop level. Methods for measuring the sneutrino masses and the electron-sneutrino-gaugino Yukawa couplings are presented, addressing theoretical problems specific for the sneutrino channels.Received: 10 September 2004, Revised: 11 October 2004, Published online: 3 March 2005     

Associated single photons and doubly-charged scalars at linear <Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> colliders

Doubly-charged scalars, predicted in many models having exotic Higgs rep-resentations, can in general have lepton-number violating (LFV) couplings. We show that by using an associated monoenergetic final state photon seen at a future linear e ⁻ e ⁻ collider, we can have a clear and distinct signature for a doubly-charged resonance. The strength of the ΔL = 2 coupling can also be probed quite effectively as a function of the recoil mass of the doubly-charged scalar.      

Prospects for the measurement of the Higgs boson masswith a linear <Emphasis Type="Italic">e</Emphasis><Superscript> + </Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>-</Superscript> collider

The potential of a linear e ⁺ e^- collider operated at a centre-of-mass energy of 350 GeV is studied for the measurement of the Higgs boson mass. An integrated luminosity of 500 fb^-1 is assumed. For Higgs boson masses of 120, 150 and 180 GeV the uncertainty on the Higgs boson mass measurement is estimated to be 40, 65 and 70 MeV, respectively. The effects of beam related systematics, namely a bias in the beam energy measurement, the beam energy spread and the luminosity spectrum due to beamstrahlung, on the precision of the Higgs boson mass measurement are investigated. In order to keep the systematic uncertainty on the Higgs boson mass well below the level of the statistical error, the beam energy measurement must be controlled with a relative precision better than 10^-4. Received: 30 May 2005, Revised: 6 July 2005, Published online: 6 October 2005