Production of polarized baryons in polarized and unpolarizede + e − collisions

Polarized and unpolarizede ⁺ e ^? annihilation is considered in lowest order at energies 30–150 GeV taking γ andZ ⁰ exchange in the standardSU(2)×U(1) model into account. The fragmentation of polarized quarks into polarized baryons is described by two kinds of fragmentation functions; the first is based onSU(6) weights and empirical expressions for favoured and disfavoured fragmentation; the second is based upon a Monte Carlo model of the chain decay including helicities of quarks and hadrons. From the cross sections for \(e^ + e^ - \to q\bar q\) and the fragmentation functions we obtain predictions for the baryon polarization as function of the collision energy and of kinematical variables.     

Vector boson pair production in e−e− collisions with polarized beams

The W-boson pair production in e⁻e⁻ collisions with polarized beams is investigated. The helicity amplitudes are derived for general couplings and the conditions for a good high-energy behaviour of the cross-section are given. The results are applied to the heavy vector boson production in the context of the left-right symmetric model. The Ward identities and the equivalence theorem are also discussed.     

Possibility of distinguishing the effect of neutral weak currents in e+e− → μ−μ+ experiments with longitudinally polarized clashing beams

The process e⁺e^{– – +} for the case of longitudinal polarization of the initial and final particles is studied on the basis of several models: the Weinberg-Salam model, the Lee-Prentki-Zumino model, the vector model, the X model, and under the assumption of a V-A structure of the neutral weak currents. Polarization effects tn the differential and total cross sections for this process are analyzed in detail, and various possibilities for distinguishing the contribution of neutral weak currents are found. Calculations show that neutral weak currents lead to appreciable polarization effects at attainable energies and that these effects are very sensitive to the model. It is important to note that an experimental study of these effects can yield valuable information on not only the structure of the neutral weak currents but also the sign of the weak-interaction constant.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 31–36, April, 1978.The authors thank A. A. Sokolov for a useful discussion and for interest in this work.     

Spontaneous e+e− pair creation in heavy ion collisions

We demonstrate that vibrational perturbations of supercritical “quasimolecules” which may occur in collisions of heavy ions can, within the context of first-order perturbation,theory, result in the production of coincident narrow e⁺e⁻ peaks, in place of and with a larger yield than that predicted for spontaneous positron emission from static supercritical systems. Certain features of this phenomenon coincide with those of the e⁺e⁻ peaks observed by the EPOS collaboration although some important aspects of the observations cannot be reproduced within our approximation. A detailed attempt to reconcile this theoretical model with the EPOS experiment must go beyond perturbation theory.     

Investigations of correlated ρ+ e− emission in heavy-ion collisions near the Coulomb barrier

We present the results obtained from a series of ^+–-coincidence measurements in heavy-ion collisions using the double-Orange-spectrometer at GSI. The collision systems U+U, U+Pb, and U+Ta were investigated at bombarding energies close to and slightly above (U+Ta) the Coulomb barrier. For all systems studied, very narrow (FWHM–20 keV) ^+– lines were observed in the sum-energy spectra, with kinetic energies ranging from 555 keV to 810 keV, superimposed on a continuous distribution mainly due to uncorrelated ^+– emission. Particularly in the U+Ta system, a pronounced sum-energy line appears at 634 keV, predominantly in deep-inelastic collisions. In some cases (e.g. U+Pb) the line characteristics is consistent with a two-body decay mode of an emitter which moves with the c.m. velocity of the colliding ions. However, other lines, and in particular the 634 keV line (U+Ta), exhibit a rather isotropical opening-angle distribution whereas their energy is unequally shared between positrons and electrons, thus being in clear disagreement with this scenario. In general, the data preclude an emission from the separated (moving) nuclei, and, in the latter cases, provide evidence that the e⁺e^–-pair decay occurs in the vicinity of the Coulomb field of a third heavy (positively charged) partner having only a small transverse velocity (|v|<>Dedicated to Prof. B. Povh on the occasion of his 60th birthday     

Annihilation mechanisms of e+e− and μ+μ− production in relativistic nuclear collisions

We revise the μ⁺μ^? and $q\bar q$ annihilation mechanisms of dilepton production during relativistic nuclear collisions. We focus on the modifications caused by the specific features of in-medium pion and quark states rather than by medium modification of the ρ-meson spectral density. The main ingreditent emerging in our approach is a form-factor of the multi-pion (multi-quark) system. Replacing the usual delta-function the form-factor plays the role of distribution which, in some sense, “connects” the total 4-momenta of the annihilating and outgoing particles. The difference between the c.m.s. velocities attributed to annihilating and outgoing particles is a particular consequence of this replacement and results in the appearance of a new factor in the formula for the lepton pair production rate. We obtained that the form-factor of the multi-pion (multi-quark) system causes broadening of the rate which is most pronounced for small invariant masses, in particular, we obtain a growth of the rate for the invariant masses below two masses of the annihilating particles.     

Limiting soft particle emission in e + e −, hadronic and nuclear collisions

In e ⁺ e ^? collisions the particle spectra at low momenta reflect the properties of the underlying “soft” QCD gluon bremsstrahlung: the particle density, in the limit p→0, becomes independent of the incoming energy $\sqrt{s}$ and directly proportional to the colour factors C _A , C _F for primary gluons or quarks respectively. We find that experimental data from the pp and nuclear reactions reveal the same behaviour: in the limit p _T→0 the invariant particle spectra become independent of the collision energy, and their intensities in e ⁺ e ^?, pp and nuclear reactions are compatible with the expected colour factors C _F : C _A : (N _part/2)C _A for N _part nucleons, participating in the interaction. Coherent soft gluon bremsstrahlung is, therefore, suggested to be the dominant QCD mechanism for the soft particle production in all these reactions. These “soft” particles probe the very early stage of hadron formation in the collision. Future measurements at the LHC will provide crucial tests on the contributions from possible incoherent multi-component processes.     

Meson-photon transition form factors and resonance cross-sections in e+e− collisions

Meson-photon-photon transition form factors for S-, P-, and D-wave states are calculated, the meson being treated as a non-relativistic heavy-quark-antiquark pair. The full dependence on both photon virtualities is included. Cross-section formulas for charge-conjugation even mesons with J^p = 0⁻, 0⁺, 1⁺, 2⁺, and 2⁻ in electron-positron collisions are presented and numerical results for LEP energies are given. In particular, we find two-photon event rates for χ_c1, η_c(2S), and η_b(1S) within reach of LEP.With minor modifications to incorporate SU(3)-flavour breaking we estimate rates for 18 light mesons as well, based on the observation that their two-photon decay widths agree remarkably well with measured data. Finally we point out that e⁺e⁻ cross sections for 1⁺ states do not vanish at low Q², the Landau-Yang suppression factors of the two-photon cross sections being compensated by the photon propagators.     

Hard bremsstrahlung amplitudes fore + e − collisions with polarized beams at LEP/SLC energies

Cross sections are presented for the processese ⁺ e ^?→e ⁺ e ^?γ ande ⁺ e ^?→f \(\bar f\) γ (wheref is any light fermion) in the form of helicity amplitudes in which all complex phase information is kept. Owing to a spcial choice of the bremsstrahlung polarization and the use of spinor techniques the resultant expressions are extremely compact and lend themselves to fast numerical evaluation. An algorithm is given to obtain from the helicity amplitudes the cross section for arbitrary polarization of the incominge ⁺ e ^? beams. Special attention is given to the case of bremsstrahlung at very small angles to the beams, where a complicated dependence on the electron mass has to be taken into account.     

Higgss-channel production ine + e − collisions below theW + W − threshold

The cross section for direct neutral Higgs production in the reaction is calculated in theE _CM energy range of 40 to 160 GeV and compared to the corresponding Electro-Weak (EW) process . Neglecting radiation effects, a signal of the order of 10^–4 to 10^–3 over the EW can be expected outside theZ ⁰ region for the Minimal Standard Model Higgs in its decay to a state. For \sqrt s - M_Z $$ " align="middle" border="0"> , thes-channel Higgs formation can surpass the Bjorken Bremsstrahlung process and thus may afford at LEP 2 a realistic search method for high mass, say 100 GeV Higgs, given enough luminosity. For a non-Standard Model Higgs, in some cases, significantly higher signals are expected. The effects of initial state radiation and-machine energy resolution are evaluated and the gain in using longitudinal polarized electron beams is discussed.Supported in part by the Israel Ministry for Science and Technology     

Physics opportunities with polarized e − and e + beams at TESLA

Beam polarization at e⁺-e^– linear colliders will be a powerful tool for high precision analyses. Often it is assumed that the full information from polarization effects is provided by polarization of the electron beam and no further information can be obtained by the simultaneous polarization of the positrons. In this paper we point out the advantages of polarizing both beams, and summarize the polarization-related results of the Higgs, Electroweak, QCD, SUSY and Alternative Theories working groups of the ECFA/DESY workshop for a planned linear collider operating in the energy range s = 500-800 GeV.     

Signals of left-right supersymmetry ine + e − collisions

We study chargino and neutralino pair production at the Next Linear e⁺ e⁻ Collider in a supersymmetric left-right model. We investigateon- and off-resonance signals via the processes: and . These reactions could lead to observable signals as they compare with the background signal from W^± -pair production.     

Production of singleW bosons ine + e − ande − p collisions

The production ofW bosons in electronpositron colliding beams below theW ⁺ W ^? threshold is discussed. The corresponding reactions \(e^ + e^ - \to l^ \pm W^ \mp \mathop {\nu _l }\limits^{( - )} \) yield total cross-sections of the order of 10^?37 cm² (forl=e) and 10^?38 cm² (forl=μ). The total cross-sections as well as the angular and energy distributions of theW boson in \(e^ + e^ - \to e^ \pm W^ \mp \mathop {\nu _e }\limits^{( - )} \) are sensitive to the magnetic moment of theW. So, with this reaction, not onlyW bosons can be produced but also the gauge nature of the interaction can be tested. The decay \(Z^0 \to l^ \pm W^ \mp \mathop {\nu _l }\limits^{( - )} \) is discussed in view of its observation ine ⁺ e ^? experiments. Finally the crosssections ofe ^- p→ν _e W ^- X ⁺ are computed. They are of the order of 10^?38 cm² for \(\sqrt s \simeq 200\) GeV.     

Limits on a lepton-number violating process from e+e− collisions

If a new hypothetical particle induces a lepton-number violating process, it could also affect the reactions e⁺e⁻→μ⁺μ⁻ and e⁺e⁻→τ⁺τ⁻ through the t-channel exchange of the particle. A fit of the cross sections and forward-backward asymmetries measured by experiments at PEP and PETRA yields the 90% confidence level limits on the coupling of the partile: G_eμ < 0.119G_F and G_eτ < 0.085G_F, where G_F is the Fermi coupling constant. These limits exclude certain regions of couplings allowed by other lepton-number violating searches.     

Discovering and studying bileptons with e−e− collisions

We analyze the prospects for discovering and unraveling the nature of doubly charged bileptons at a linear collider of the next generation running in its e⁻e⁻ mode. We stress the importance of initial state radiation, beam spread and polarization, and compute the discovery bounds. The gauge nature of vector bileptons can be determined by studying hard photon emission.     

Multijet production in e+e− collisions as a phase transition of a quark-gluon system

The process of multijet production in e⁺e⁻ collisions is interpreted as a critical phenomenon in the quark-gluon system formed at small distances and distributed along the hyperbolas t² − x² = τ² during its space-time evolution. The appearance of colourless hadronic jets in the critical region τ ≈ τ_c ± Δτ is assumed to imply Kadanoff scaling in the one-dimensional system specified by the rapidity distribution of jets along the original qq̄ direction. The consequences of the model for multijet production at present and future experiments is discussed.