Spin physics at e<Superscript>+</Superscript>e<Superscript>−</Superscript> colliders

A large number of measurement related to spin was performed during the last decade at the e ⁺ e ^? colliders LEP and SLC providing important information on the dynamics of high energy interactions. In this paper three main topics will be covered: the measurements of the electroweak couplings, the study of fragmentation dynamics and the search for physics beyond the Standard Model.     

Statistical hadronization with exclusive channels in e<Superscript>+</Superscript>e<Superscript>−</Superscript> annihilation

We perform a systematic analysis of exclusive hadronic channels in e⁺e⁻ collisions at center-of-mass energies between 2.1 and 2.6 GeV within the statistical hadronization model. Because of the low multiplicities involved, calculations have been carried out in the full microcanonical ensemble, including conservation of energy-momentum, angular momentum, parity, isospin, and all relevant charges. We show that the data are in an overall good agreement with the model for an energy density of about 0.5 GeV/fm³ and an extra-strangeness suppression parameter γ _S ∼0.7, essentially the same values found with fits to inclusive multiplicities at higher energy.     

A study of the dynamical-fluctuation property inside jets in e<Superscript>+</Superscript>e<Superscript>−</Superscript> collisions

A study of the dynamical-fluctuation property of 2-jet events is carried out. It is found that the dynamical fluctuations of the hadronic system inside 2-jet events change with the variation of the cut parameter Y_cut. There is a transition point, where the dynamical fluctuations in these systems are circular in the transverse plane; and are elliptical in the longitudinal-transverse planes. It is shown that this transition point corresponds to the scale of visible jets. Meanwhile, the dynamical fluctuation properties inside a single-jet in 2-jet and 3-jet events are compared. The dynamical fluctuations inside quark- and gluon-jets are found to be qualitatively different. A scale for the “visible gluon jet” production is thus obtained.     

Measurement of cross sections for processes involving <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> annihilation to hadrons at the VEPP-2M collider

Results obtained by measuring, in experiments at the VEPP-2M e ⁺ e ^? collider with the CMD-2 and SND detectors at the Novosibirsk Institute of Nuclear Physics, cross sections for e ⁺ e ^? → hadrons annihilation processes at energies in the range 360 ≤ √s ≤ 1380 MeV are surveyed. The measured cross sections and the vector-meson parameters deduced from an analysis of these cross sections are presented. The cross section for the process e ⁺ e ^? → π ⁺ π ^? is compared with the spectral function for the decay τ ^? → π ^? π ⁰ ν _τ.     

Probing little Higgs model in e<Superscript>+</Superscript>e<Superscript>-</Superscript>→νν̄γ process

We study the process e⁺e^-→νν̄γ to search for its sensitivity to the extra gauge bosons Z₂, Z₃ and W₂ ^±, which are suggested by the little Higgs models. We find that the ILC with and CLIC with cover different regions of the LHM parameters. We show that this channel can provide a determination of the parameters, complementary to measurements of the extra gauge bosons obtainable at the upcoming LHC experiments. PACS 14.80.Cp; 12.60.Fr; 12.60.Cn     

Processes e<Superscript>+</Superscript>e<Superscript>−</Superscript> → π<Superscript>0</Superscript>(π<Superscript>0</Superscript>′) γ in extended NJL model

The processes of electron-positron annihilation into π⁰γ and into π′(1300)γ are considered within an extended NJL model. The intermediate vector mesons ρ ⁰, ω ρ′(1450), and ω(1420) are taken into account. The latter two mesons are treated as the first radial excited states. They are incorporated into the NJL model by means of a polynomial form factor. Numerical predictions for the cross-sections of these processes are received for the center-of-mass energies below 2 GeV. Our results for the π⁰γ production are in agreement with experimental data obtained in the energy region 600–1020 MeV.     

The evaluation of the branching ratios of the decays φ(1020)→2π<Superscript>+</Superscript>2π<Superscript>−</Superscript>π<Superscript>0</Superscript>, π<Superscript>+</Superscript>π<Superscript>−</Superscript>3π<Superscript>0</Superscript>

Combining the Okubo-Zweig-Iizuka rule in the decay φ→ρπ→π⁺π^?π⁰ with the ρ→4π decay amplitudes, we calculate the φ→2π⁺2π^?π⁰ and φ→π⁺π^?3π⁰ ones. The partial widths of the above φ decays are evaluated, and the excitation curves in e⁺e^? annihilation are obtained, assuming reasonable particular relations among the parameters characterizing the anomalous terms of the HLS Lagrangian. The evaluated branching ratios B_φ→π⁺_π^?_3π⁰ ≈ 2 × 10^?7 and B_φ→2π⁺_2π^?_π⁰ ≈ 7 × 10^?7 are such that, with the luminosity L=500 pb^?1 attained at DAΦNE φ factory, one may already possess about 1685 events of the decays φ→5π.     

Investigation of the process <Emphasis Type="Italic">J/ψ</Emphasis> → <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> with the KEDR detector

The product of the electron width of the J/ψ meson and the probability for its decay to an electron-positron pair was measured by using data from the KEDR experiment at the VEPP-4M electron-positron collider. The result was Γ _ee × Γ _ee /Γ = 0.3392 ± 0.0068(stat.) ± 0.0063(syst.) keV.     

Energy dependence of Lévy stability and multi-fractal spectrum in e<Superscript>+</Superscript>e<Superscript>−</Superscript> collisions

Various nuclear reactions like quasi-fission, fusion-fission or particle and cluster evaporation from excited compound nuclei were studied in heavy-ion reactions at the velocity filter SHIP of GSI. The velocity filter offers the possibility to detect all reaction products under zero degree relative to the beam direction. Together with the measurement of the product velocity distribution this allows for an identification of the underlying reaction mechanism. This article is focussed on reactions of ²⁵Mg and ⁶⁴Ni beams on ^{206, 207}Pb targets at energies of 5.9×AMeV and 8.7×AMeV . Besides evaporation residues from ²⁵Mg + ²⁰⁶Pb collisions we found evidence for rotation and quasi-fission of nuclear molecules formed in the entrance channel after the capture stage. The break-up of the systems showed a preferred clustering leading to isotopes in the region 84 ⩽ Z ⩽ 88 and 122 ⩽ N ⩽ 127 of the chart of nuclei.     

Tests of analytical hadronisation models using event shape moments in e<Superscript>+</Superscript>e<Superscript>−</Superscript> annihilation

Predictions of analytical models for hadronisation, namely the dispersive model, the shape function and the single dressed gluon approximation, are compared with moments of hadronic event shape distributions measured in e⁺e⁻ annihilation at centre-of-mass energies between 14 and 209 GeV. In contrast to Monte Carlo models for hadronisation, analytical models require to adjust only two universal parameters, the strong coupling and a second quantity parametrising non-perturbative corrections. The extracted values of α _S are consistent with the world average and competitive with previous measurements. The variance of event shape distributions is compared with predictions given by some of these models. Limitations of the models, probably due to unknown higher order corrections, are demonstrated and discussed.     

Polarization and spin correlations of top quarks at a future <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> linear collider

We discuss the polarization and spin correlations of top quarks produced above threshold at a future linear collider, including QCD radiative corrections.     

Two-neutrino positron double-beta decay of <Superscript>106</Superscript>Cd for the 0<Superscript>+</Superscript> → 0<Superscript>+</Superscript> transition

The two-neutrino positron double-beta decay of ¹⁰⁶Cd for the 0 ⁺ 0⁺ transition has been studied in the Hartree-Fock-Bogoliubov model in conjunction with the summation method. In the first step, the reliability of the intrinsic wave functions of ¹⁰⁶Cd and ¹⁰⁶Pd nuclei has been tested by comparing the theoretically calculated results for yrast spectra, reduced B(E2: 0⁺ 2⁺) transition probabilities, quadrupole moments Q(2⁺) and gyromagnetic factors g(2⁺) with the available experimental data. In the second step, the nuclear transition matrix element M₂ and the half-life T_1/2² for the 0 ⁺ 0⁺ transition have been calculated with these wave functions. Moreover, we have studied the effect of deformation on the nuclear transition matrix element M₂.     

Rare decay π<Superscript>0</Superscript> → <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> as a test of standard model

Experimental and theoretical progress concerning the rare decay π⁰ → e ⁺ e ⁻ is briefly reviewed. It includes the latest data from KTeV and a new model-independent estimate of the decay branching which show the deviation between experiment and theory at the level of 3.3σ. The predictions for η and η′ decays into lepton pair are presented. We also comment on the impact on the pion rare decay estimate of the data of BABAR collaboration on the pion transition form factor at large momentum transfer.     

Radioactive Production of the Lightest Neutralino in the Process of <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> Annihilation

The production of the lightest stable neutralino in the reaction with general mixing of the higgsino and gaugino is studied. General formulas for differential cross sections are presented. The dependence of the effective differential cross section on the photon energy spectrum and electron mass is illustrated together with the dependence of the effective total cross section on the beam energy for three different scenarios. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 48–57, June, 2005.     

Parton structure of the Pomeron in hard single-Pomeron exchange process with μ<Superscript>+</Superscript>μ<Superscript>−</Superscript> pair production on the CMS (LHC)

Process of the μ⁺μ⁻ pair production in hard single-Pomeron exchange is studied on the CMS (LHC) at the energy range $ \sqrt s $ \sqrt s = 7 and 14 TeV. Using H1(DESY) data for quark/antiquark parton distributions in the single-Pomeron, calculations were performed to predict the quark/antiquark component of the parton distribution. The estimations show that the quark/antiquark contribution does not exceed ∼5%. Using CMSSW package the events of the process were reconstructed. The method for estimation of the valent quark/antiquark contribution in the parton structure of the single-Pomeron is developed. This method could be used at the experimental data performance obtained on the CMS detector.     

Laser control of the low-temperature <Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript>+O<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> reaction

The dissociative recombination theory of slow electrons with molecular ions in a strong monochromatic light field is developed. Classifications of all possible transitions and reaction mechanisms are presented. The potential energy curves of the oxygen molecule O₂^** dissociative states and partial cross sections are calculated.     

Parity-Violating in <Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript> Scattering at Finite Temperature

Parity violation implies that physics laws are not invariant under spatial coordinate reversal. Electron-positron scattering is a process that displays parity violation. Using the Thermo Field Dynamics formalism this scattering at finite temperature is analyzed. The transition amplitude is calculated as a function of temperature. The parity violation at very high temperatures tend to go to zero.     

Pseudoscalar Higgs boson in experiments at <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> colliders

The potential of experiments at e ⁺ e ^? colliders to search for a signal of a new pseudoscalar Higgs boson and to study its CP properties is analyzed. A new Higgs boson coupling to the b quark and τ lepton, which is parameterized in a model-independent way as m _b /v(a + iγ₅ b), is chosen to be an indicator of its CP nature. We study the e ⁺ e ^? → τ⁺τ^?vv, e ⁺ e ^? → b \(\bar b\)vv, and e ⁺ e ^? → e ⁺ e ^? b \(\bar b\) processes highly sensitive to anomalous Higgs boson couplings due to the dominant contributions from the vector boson fusion processes W*W* → H and Z*Z* → H in the central region. It is shown that the study of polarization of particles in the final state could help to separate the contributions from the scalar and pseudoscalar Higgs bosons. For instance, the consideration of the cascade decays of τ leptons in the e ⁺ e ^? → τ⁺τ^?vv process allows one to determine reliably the CP state of the Higgs boson as well as the value and the sign of the b parameter. As a result of our analysis, we develop a search strategy for a signal of a new pseudoscalar Higgs boson in future experiments at linear e ⁺ e ^? colliders, which will allow one to understand the CP nature of the Higgs boson and set the bounds on its couplings to fermions.     

On the question of supersymmetric particles production in <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> annihilation

Some aspects of supersymmetric particles pair production in e ⁺ e ^?-annihilation in processes \(e^ + + e^ - \to \tilde l^ + + \tilde l^ -\), \(e^ + + e^ - \to \tilde \chi ^ + + \tilde \chi ^ -\) are studied. Aparatus, in which long-lived charged particles deceleraition and accumulation take place, is considered. Binding energy of a negatively charged particle with nucleus is obtained. Probability of β-decay of a nucleus bounded with a long-lived negatively charged particle is considered.     

A new determination of the γπ → ππ anomalous amplitude via πe<Superscript>-</Superscript> → π<Superscript>-</Superscript>e<Superscript>-</Superscript>π<Superscript>0</Superscript> data

We discuss the reaction π^-e^- → π^-e^-π⁰ with the purpose of obtaining information on the γπ → ππ anomalous amplitude _3π. We compare a full calculation at (p⁶) in chiral perturbation theory and various phenomenological predictions with the existing data of Amendolia et al. By integrating our theory results using Monte Carlo techniques, we obtain σ = 2.05 nb at (p⁶) and σ = 2.17 nb after including the dominant electromagnetic correction. Both results are in good agreement with the experimental cross-section of σ = (2.11±0.47) nb. On the basis of the ChPT results one would extract from the experimental cross-section as amplitudes _3π^(0)extr = (9.9±1.1) GeV^-3 and _3π^(0)extr = (9.6±1.1) GeV^-3, respectively, which have to be compared with the low-energy theorem _3π = e/(4π²F_π³) = 9.72GeV^-3. We emphasize the need for new data to allow for a comparison of experimental and theoretical distributions and to obtain _3π with smaller uncertainty.