Initial-state radiation at LEP energies and the corrections to Higgs boson production

It is shown that for e⁺e^? scattering processes described by the exchange of one photon or vector boson the O(α) radiative corrections to the initial state completely factorize the lowest-order cross section. This factorization holds for both hard and soft (or virtual) photons. We present a formalism for applying these radiative corrections in a process-independent way, and described the corresponding computer program. As an application we study the initial-state radiative corrections to the Higgs production processes e⁺e^? → μ⁺μ^?H and e⁺e^? → e⁺e^?H.     

Photon structure functions: Are they measurable?

It is shown that 3 of the 8 independent photon structure functions are simply calculable using leading logarithm techniques in QCD. We then discuss which of these are most readily measurable in the processese ⁺e^?→e⁺e^?+X,γp→μ ⁺ μ ^? X andμp→μμ ⁺ μμ ^? X. Detailed predictions are given for the latter processes including the case where beam and target are longitudinally polarised. Given sufficient beam intensities, the contributions from the anomalous (as opposed to vector meson-like) component of the photon structure function would be easily identifiable above the Bethe-Heitler pair production background. However, currently available beam intensities make such measurements impractical at present. We discuss simple algorithms for estimating the polarised and unpolarised quark and gluon distributions inside the proton and the hadronic (?-like) piece of the photon.     

Hard photon corrections for Bhabha scattering

Numerical results for the α³ differential cross section for e⁺e^? → e⁺e^? are presented for the case of e⁺e^? colliding beams. Special care is given to the hard photon corrections. Acoplanarity distributions for e⁺e^? → e⁺e^?γ are also presented. Attention is drawn to experimental conditions which require the calculation of still higher order corrections.     

Scaling of σ(e+e− → hadrons) in asymptotically non-free theories

σ(e⁺e^? → hadrons) is calculated from the Dyson form of the photon self-energy. For the vertex of the elementary constituents the Nakanishi representation is used. A regularization scheme for the Nakanishi spectral function is taken from perturbation theoretic considerations. With simple assumptions about the spectral function, the renormalizability of the theory leads to the asymptotic form σ(e⁺e^? → hadrons) ≈ const/s. The constant depends on the anomalous dimension of the propagator. The transition to the asymptotic region occurs rather slowly. No jets are expected in the final state and the elementary constituents need not appear in the final state to an appreciable fraction.     

Photon production in Au+Au collisions at \sqrt {s_{NN} } = 130 GeV

We report raw photon, neutral pion and eta measurements at RHIC. Photons in the energy range from 100MeV ? 4GeV were detected by reconstructing e ⁺ e ^? pair production, γ+Z→e ⁺+e ^?+Z, with the STAR Time Projection Chamber (TPC). Along with the photon detection technique we discuss the purity of the photon candidates and measurements of hadronic decays via their electromagnetic decay channels. The π⁰→γ and η→γγ decay channels are addressed.     

Heavy ion e+e− pairs to all orders in Zα

The heavy ion cross section for continuum e⁺e^? pair production has been calculated to all orders in Zα. Comparison is made with available CERN SPS and RHIC STAR data. Computed cross sections are found to be reduced from perturbation theory with increasing charge of the colliding heavy ions and for all energy and momentum regions investigated. Au or Pb total cross sections are reduced by 28% (SPS), 17% (RHIC) and 11% (LHC). For very high energy (E _e ⁺, E _e ^?>3 GeV) forward pairs at LHC the reduction from perturbation theory is a bit larger (17%). Use of zero degree calorimeter triggering (and thus small impact parameter weighting) makes impact parameter representation of exact pair production useful. Preliminary exact calculations in the zero impact parameter limit show a much larger reduction from perturbation theory (about 40%) at both RHIC and LHC.     

Resonance mechanism of the production of electron-positron pairs in a strong magnetic field

The Compton-like process γe ^± → e ^± e ⁺ e ^? involving the production of an electron-positron pair in the interaction of an ultrarelativistic electron with a soft x-ray photon in the vicinity of the polar cap of a magnetar is considered. It is shown that the amplitude for this reaction has a resonance character. A simple analytic expression is obtained for the electron-absorption coefficient. Possible astrophysical implications of the resonance process γe ^± → e ^± e ⁺ e ^? are discussed.     

WWψ couplings from singleW-production in polarizede + e − collisions

Using the processeψ→Wv we study theWWψ vertex ine ⁺ e ^? collisions with polarized beams at a future 500 GeV collider. The equivalent photon approximation and its accuracy are discussed for the case of polarized electron beams. The resulting photon spectrum is then used to compute the total cross section as well as verious differential distributions for the processe ⁺ e ^?→e ⁺ W ^? v. Special emphasis is put on the effects of using polarized electron beams and their influence on the sensitivity to anomalous couplings.     

Production ofW-bosons in high energye + e − ande − p collisions

We study the processe ⁺ e ^?→W+hadrons from theZ ⁰-pole to the threshold region forW-pair production, keeping the widths of theZ ⁰ andW finite. Already slightly below threshold the cross section is strongly depressed. Also the total cross section at theZ ⁰-peak is very small. We present a simple model to account for this. We also give exact results to lowest order for the weak Compton processese ⁺ e ^?→e ⁺ W ^? v _e ande ^? p→pW ^? v _e and compare them with results obtained with the equivalent photon approximation.     

Hard photon next-to-leading corrections to two-fermion production in e + e − collisions above the Z 0 peak

A complete next-to-leading order calculation of the initial-state O (α) photonic corrections to hadron and lepton-pair production in e ⁺ e ^?. collisions is presented. The impact of the next-to-leading order corrections on the twofermion cross section and forward-backward asymmetry is studied in detail for several event selections, with special emphasis on hard photon effects at LEP1.5 and LEP2 energies. It is shown that for typical realistic event selections, initial-state next-to-leading order corrections in the soft approximation can be safely extrapolated to energies above the Z ⁰ peak without significant loss of precision.     

Electroweak interference effects in the high energye ++e −→e ++e −+hadrons process

We examine interference effects of the electrowcak interaction in the high energye ⁺+e ^?→e ⁺ +e ^?+hadrons process. The effect provides a further test of the G-W-S model [1] and may lead to a better understanding of the photon structure functions.     

Photon damping caused by electron-positron pair production in a strong magnetic field

Damping of an electromagnetic wave in a strong magnetic field is analyzed in the kinematic region near the threshold of electron-positron pair production. Damping of the electromagnetic field is shown to be noticeably nonexponential in this region. The resulting width of the photon γ→e ⁺ e ^? decay is considerably smaller than previously known results.     

e + e −→W − e + v and non-standard gauge couplings: Another look

The cross section for singleW ^? production ine ⁺ e ^? collisions is exactly computed for an arbitraryW magnetic moment. It is found that this process may give a first look at theW Wγ vertex just below the twoW threshold. A detailed comparison is made with previous calculations that used the equivalent photon approximation.     

Radiative corrections to muon pair and quark pair production in electron-positron collisions in the Z0 region

A calculation of first-order radiative corrections to the process e⁺e^?→μ⁺μ^? is presented, which is in particular applicable to the Z₀ region. The emphasis is on a detailed treatment of hard photon effects, which affect the size of the corrections in the Z₀ region considerably. The technique used is that of a Monte Carlo simulation of μ⁺μ^? and μ⁺μ^?γ events. In an appendix the generalization to quark pair production is presented.     

Analysis of triplet production by a circularly polarized photon at high energies

The possibility in principle of determining the circular polarization of a high-energy photon by measuring the created electron polarization in the process of triplet photoproduction γ + e ^? → e ⁺ e ^? + e ^? is investigated. The respective event number, which depends on polarization states of the photon and the created electron, does not decrease as the photon energy increases, and this circumstance can ensure the high efficiency in such experiments. We study different double and single distributions of the created electron (or positron), which allow probing the photon circular polarization and measuring its magnitude (the Stokes parameter ξ₂) using the technique of Sudakov variables. Some experimental setups with different rules for event selection are studied and the corresponding numerical estimations are presented.     

Effect of triple gluon coupling in semi-inclusive polarized scattering processes

We show that the gluon helicity inside a proton and a photon can be deduced from a knowledge of a special combination of cross sections of the semi-inclusive processese+p→e+π+π+... ande ⁺+e ^?→e ⁺+e ^?+π+π+... Such a measurement could thus be used to check the QCD prediction that the gluon helicity increases linearly with lnQ ².     

A measurement of collinear and nearly collinear photon pairs produced by e+e− annihilation at the 3100 MeV resonance

We have measured collinear and nearly collinear photon pairs from e⁺e^? annihilation at total energies covering the resonance at 3100 MeV. The observed cross section is in good agreement with the e⁺e^? → γγ cross section expected from quantum electrodynamics. Upper limits for the decay modes (3100) → γγ, π^Oγ and other modes are derived.     

The production of neutral vector mesons by bremsstrahlung in electron-positron colliding beams

We study the bremsstrahlung production of the ? meson in the reaction e⁺e^? → e⁺e^?? (→ e⁺e^?π⁺π^?). This reaction gives a C = ?1 background which complicates the study of C = +1 two-photon processes at the new colliding-beam facilities. The cross section for the reaction rises from approximately 0.3 nb to 0.7 nb as the beam energy increases from 2 GeV to 15 GeV. From a study of the distributions of the final leptons and pions, we find a suitable choice of cuts which will reduce the event rate down to a small fraction of R. It is not possible to attribute the three-prong events seen at DELCO and PLUTO to this particular production mechanism.     

The π N → e +e−N reaction close to the vector-meson production threshold

The π^? p→e ⁺ e ^? n and π⁺ n→e ⁺ e ^? p reaction cross sections are calculated below and in the vicinity of the vector-meson (?⁰,ω) production threshold. These processes are largely responsible for the emission of e ⁺e^? pairs in pion-nucleus reactions and contribute to the dilepton spectra observed in relativistic heavy ion collisions. They are dominated by the decay of low-lying baryon resonances into vector-meson-nucleon channels. The vector mesons materialize subsequently into e ⁺ e^? pairs. Using πN→?⁰ N and πN→ωN, amplitudes calculated in the center of mass energy interval 1.4 < √s<1.8 GeV, we compute the π^? p→e ⁺ e ^? n and π⁺ n→e ⁺ e ^? p reaction cross sections in these kinematics. Below the vector-meson production threshold, the π⁰?ω interference in the e ⁺ e^? channel appears largely destructive for the π^? p→e ⁺ e ^? n cross section and constructive for the π⁺ n→e ⁺ e ^? p cross section. The pion beam and the HADES detector at GSI offer a unique possibility to measure these effects. Such data would provide strong constraints on the coupling of vector-meson-nucleon channels to low-lying baryon resonances.     

Radiative asymmetry around the resonance in e+e−→μ+μ−

Under the assumption that the ψ is a pure 1^?? vector particle we have calculated the angular asymmetry in the process e⁺e^?→μ⁺μ^? around the resonance. We find that the asymmetry, calculated in the soft photon limit, becomes very small around the resonance.