Associated production of light gravitinos in e+e? and e?γ collisions

Light gravitino productions in association with a neutralino (selectron) in e ⁺ e ⁻ (e ⁻ γ) collisions are restudied in a scenario that the lightest supersymmetric particle is a gravitino and the produced neutralino (selectron) promptly decays into a photon (electron) and a gravitino. We explicitly give the helicity amplitudes for the production processes by using the effective goldstino interaction Lagrangian, and present the cross sections with different collision energies and mass spectra. We also examine selection efficiencies by kinematical cuts and beam polarizations for the signal and background processes, and show that the energy and angular distributions of the photon (electron) can explore the mass of the t-channel exchange particle as well as the mass of the decaying particle at a future e ⁺ e ⁻ (e ⁻ γ) collider.     

Double S- and P-wave charmonium production in e+e? annihilation

On the basis of perturbative QCD and the relativistic quark model we calculate relativistic and bound state corrections in the production processes of a pair of S-wave and P-wave charmonium states. Relativistic factors in the production amplitude connected with the relative motion of heavy quarks and the transformation law of the bound state wave function to the reference frame of the moving S- and Pwave mesons are taken into account. For the gluon and quark propagators entering the production vertex function we use a truncated expansion in the ratio of the relative quark momenta to the center-of-mass energy $ \sqrt s $ \sqrt s up to the second order. Relativistic corrections to the quark bound state wave functions in the rest frame are considered by means of the Breit-like potential. It turns out that the examined effects change essentially the nonrelativistic results of the cross section for the considered reactions at the center-of-mass energy $ \sqrt s $ \sqrt s = 10.6 GeV.     

Measurement of the photon structure functionF2γ in the reactione+e?→e+e?+hadrons at LEP

We present measurements of the hadronic photon structure functionF ₂ (x), in twoQ ² ranges with mean values of 5.9 GeV² and 14.7 GeV². The data were taken by the OPAL experiment at LEP, with close to theZ ⁰ mass and correspond to an integratede ⁺ e ^– luminosity of 44.8 pb^–1. In the context of a QCD-based model we find the quark transverse momentum cutoff separating the vector meson dominance (VMD) and perturbative QCD regions to be 0.27±0.10 GeV. We confirm that there is a significant pointlike component of the photon when the probe photon hasQ ²>4 GeV². Our measurements extend to lower values ofx than any previous experiment, and no increase ofF ₂ (x) is observed.     

Search for contact interactions in the reactionse+e?→l+l? ande+e?→γγ

Contact interactions are searched for using the differential cross sections for the reactionse ⁺ e ^–e ⁺ e ^–,e ⁺ e ^–µ ⁺ µ ^–,e ⁺ e ^{– + –} ande ⁺ e ^– measured at 12 energies around theZ peak and corresponding to about 20 pb^–1 of cumulated luminosity. Four-fermion contact term models assuming various chiralities of lepton currents are fitted to the lepton data and lower limits on the energy scale of such terms are set at 95% c.l. The limits vary in the range 0.9–4.7 TeV, depending on the model and on the lepton flavour. Theee contact terms are searched for assuming various chiralities. Limits on the energy scale between 79 and 130 GeV are extracted from the data. The results are compared and combined with those reported at lower energies.Deceased     

Electromagnetic structure of W± bosons in the e–e+ → W+W– reaction

A model-independent analysis of anomalous gauge coupling constants of W ^± bosons is presented and the corresponding restrictions on them and on the electromagnetic characteristics of W ^± bosons following from the experiments on measuring the e⁺ e^- ? W⁺ W^- ? ( e

Mono-γ production of a dark vector at future e+e? colliders

The associated production of a dark particle and photon, represented as a mono-γ event, is a promising channel to probe particle content and dynamics in the dark sector. In this study, we investigate the properties of the mono-γ production of vector dark matter at future \begin{document}$ e^+e^- $\end{document} colliders. Photon-like and Pauli operators as well as triple gauge boson interactions involving dark matter are considered in the framework of effective field theory. We show that, in comparison with the Pauli operator, the triple gauge boson couplings are significantly more interesting in high energy colliders. Beam polarization effects are also analyzed, and we show that the experimental sensitivities cannot be significantly enhanced because of the smaller luminosity.     

Monte-Carlo generator photon jets for the process e+e?→γγ

Monte-Carlo generator with photon jets radiation in collinear regions for the process e ⁺ e ⁻→γγ is described in detail. Radiative corrections in the first order of α are treated exactly. Large leading logarithmic corrections coming from collinear regions are taken into account in all orders of α by applying the Structure Function approach. Theoretical precision of the cross section with radiative corrections is estimated to be 0.2%. This process is considered as an additional tool to measure luminosity in forthcoming experiments with the CMD-3 detector at the e ⁺ e ⁻ collider VEPP-2000.     

K+→π+π0e+e?: a novel short-distance probe

We study the decay K ⁺→π ⁺ π ⁰ e ⁺ e ⁻, currently under analysis by the NA62 Collaboration at CERN. In particular, we provide a detailed analysis of the Dalitz plot for the long-distance, γ ^∗-mediated, contributions (Bremsstrahlung, direct emission and its interference). We also examine a set of asymmetries to isolate genuine short-distance effects. While we show that charge asymmetries are not required to test short distances, they provide the best environment for its detection. This constitutes by itself a strong motivation for NA62 to study K ⁻ decays in the future. We therefore provide a detailed study of different charge asymmetries and the corresponding estimated signals. Whenever possible, we make contact with the related processes K ⁺→π ⁺ π ⁰ γ and K _L →π ⁺ π ⁻ e ⁺ e ⁻ and discuss the advantages of K ⁺→π ⁺ π ⁰ e ⁺ e ⁻ over them.     

Power corrections in e+e?→π+π?,K+K? and B→Kπ,ππ

CLEO-c measurements of the timelike form factors F _π , F _K at ?s=3.671\sqrt{s}=3.671 GeV provide a direct probe of power corrections (PC’s) at energies near m _B . PC’s in F _π,K and B→Kπ,ππ are separated into perturbative and soft parts. In F _π,K the latter are ≥O(10) larger. A PC fit to the B→Kπ,ππ data also yields a ≥O(10) soft-to-perturbative hierarchy for the QCD penguin PC’s. Hence, both can be attributed to dominance of the soft-overlap between energetic (approximately) back-to-back collinear partons, and consistency of the B→Kπ,ππ fit with the Standard Model appears to be naturally realized. The CP asymmetries S_{Ks p⁰}S_{K_{s} \pi^{0}}, C_{Ks p⁰}C_{K_{s} \pi^{0}} are well determined, providing a clean test for new physics.     

Spin identification of graviton resonances in the process pp → e+e? + X at the Large Hadron Collider (LHC)

Prospects for discovering heavy graviton resonances in decays to an electron-positron pair and for identifying the nature of these resonances in the ATLAS experiment at the Large Hadron Collider (LHC) are investigated. Gravitons in the Randall-Sundrum model, which features extra spatial dimensions, are considered by way of example. A comparative analysis of effects of new different-spin heavy resonances, scalar [supersymmetric neutrino (sneutrino)], vector (new gauge Z′ boson), and tensor (graviton) ones, is performed in order to identify the graviton spin. An identification of gravitons is performed by using the integrated center-edge asymmetry. For LHC, the graviton discovery (identification) reach is found to be 2.1 TeV (1.2 TeV) and 3.9 TeV (2.9 TeV) at a confidence level of 5δ (95%) for the graviton coupling constants of k/$ \bar M $ \bar M _Pl = 0.01 and 0.1, respectively. This analysis is the most general, since, for the first time, it takes into account the possible existence of scalar resonances, which affects substantially quantitative estimates of the identification reach.     

Measurement of low-mass e+e? pair production in 1 and 2?A?GeV C–C collision with HADES

HADES is a secondary generation experiment operated at GSI Darmstadt with the main goal to study dielectron production in proton, pion and heavy ion induced reactions. The first part of the HADES mission is to reinvestigate the puzzling pair excess measured by the DLS collaboration in C + C and Ca + Ca collisions at 1 A GeV. For this purpose dedicated measurements with the C + C system at 1 and 2 A GeV were performed. The pair excess above a cocktail of free hadronic decays has been extracted and compared to the one measured by DLS. Furthermore, the excess is confronted with predictions of various model calculations. Also at Panstwowa Wyzsza Szkola Zawodowa, 33-300 Nowy Sacz, Poland Also at Dipartimento di Fisica e Astronomia, Università di Catania, 95125 Catania, Italy Also at Universidade de Coimbra, Coimbra, Portugal Also at ISEC Coimbra, Coimbra, Portugal Also at Dipartimento di Fisica, Università di Milano, 20133 Milano, Italy     

Constraints on the Z-Z′ mixing angle from data measured for the process e+e? → W+W? at the LEP2 collider

An analysis of effects induced by new neutral gauge Z′ bosons was performed on the basis of data from the OPAL, DELPHI, ALEPH, and L3 experiments devoted to measuring differential cross sections for the process of the annihilation production of pairs of charged gauge W ^± bosons at the LEP2 collider. By using these experimental data, constraints on the Z′-boson mass and on the angle of Z-Z′ mixing were obtained for a number of extended gauge models.     

e+e?γ production at photon-photon colliders at complete electroweak NLO accuracy

We present the NLO electroweak radiative corrections to the begin{document}$ e^+e^-gamma $end{document} production in γγ collision, which is an ideal channel for calibrating the beam luminosity of a Photon Linear Collider. We analyze the dependence of the total cross section on the beam colliding energy, and then investigate the kinematic distributions of final particles at various initial photon beam polarizations at EW NLO accuracy. The numerical results indicate that the EW relative corrections to the total cross section are non-negligible and become increasingly significant as the increase of the beam colliding energy, even can exceed –10% in the begin{document}${{J}} = 2$end{document} γγ collision at begin{document}$ sqrt{hat{s}} = 1; {rm{TeV}} $end{document}. Such EW corrections are very important and should be taken into consideration in precision theoretical and experimental studies at high-energy γγ colliders.     

Searches for neutral Higgs bosons ine+e? collisions at LEP

A search for minimal standard model (MSM) and minimal supersymmetric model (MSSM), Higgs bosons with masses larger than 3 GeV/c² has been performed by the OPAL collaboration one ⁺ e ^– data from LEP corresponding to an integrated luminosity of 1.24 pb^–1. The limits for MSM Higgs bosons have been obtained using the channelsZ ⁰Z ^0* H ⁰,Z ^0*(v ore ⁺ e ^– or µ⁺, µ^–. The search for MSSM Higgs bosons has been performed using the channelsZ ⁰Z ^0* H ⁰,v ore ⁺ e ^– or µ⁺µ^–),h ⁰ q andZ ⁰h ⁰ A ⁰,h ⁰ A ⁰(4 jet or ^{+ –} or 4), whereh ⁰ andA ⁰ are the two lightest neutral MSSM Higgs bosons. No Higgs boson signal has been observed. The MSM Higgs boson is excluded in the mass range 3 GeV <25.3>² at the 95% confidence level; limits on the masses of the two lightest neutral MSSM Higgs bosons are obtained forh ⁰ mases up to 40.5 GeV/c².Now at Applied Silicon Inc.     

π+π+ and π+π? Colliding in Noncommutative Space

By studying the scattering process of scalar particle pion on the noncommutative scalar quantum electrodynamics, the non-commutative amendment of differential scattering cross-section is found, which is dependent of polar-angle and the results are significantly different from that in the commutative scalar quantum electrodynamics, particularly when cosθ∼±1. The non-commutativity of space is expected to be explored at around Λ_NC∼TeV.     

A study of Bose-Einstein correlations ine+e? annihilation at 91 GeV

This paper describes a study of Bose-Einstein correlations made using the ALEPH detector at LEP. The correlations are found to enhance the two particle differential cross section for pairs of identical pions by a factor which can be roughly parametrized byR(Q)=1+ exp(-Q²²), whereQ is the difference in the 3-momenta of the two pions in their centre of mass frame, =0.51±0.04±0.11 and =3.3±0.2±0.8 GeV^–1, which corresponds to a source size of 0.65±0.04±0.16 fm. The large systematic errors on these results reflect their strong dependence on the choice of the reference sample used in the analysis. This problem is believed to occur primarily because of uncertainties in the rates of resonance production and a lack of knowledge about the pion-pion strong interaction. No significant correlations are seen amongst like-charged pion-kaon pairs.Supported by CAICYT, Spain     

On the spin correlations of muons generated in the annihilation process e + e ? → μ+μ?

Using the technique of helicity amplitudes, the electromagnetic process e ⁺ e ⁻ → μ⁺μ⁻ is theoretically investigated in the one-photon approximation. The structure of the triplet states of the final (μ⁺μ⁻) system is analyzed. It is shown that in the case of unpolarized electron and positron the finalmuons are also unpolarized, but their spins are strongly correlated. Explicit expressions for the components of the correlation tensor of the final (μ⁺μ⁻) system are derived. The formula for the angular correlation at the decays of final muons μ⁺ and μ⁻, produced in the process e ⁺ e ⁻ → μ⁺μ⁻, is obtained. It is demonstrated that spin correlations of muons in the process of electron-positron pair annihilation have the purely quantum character, since one of the incoherence inequalities for the correlation-tensor components is always violated. The additional contribution of the weak interaction of lepton neutral currents through the virtual Z ⁰ boson is considered; it is established that, taking into account the weak interaction, the qualitative character of the muon spin correlations does not change. The text was submitted by the authors in English.