Electroweak radiative corrections toe + e −→HZ

The cross section fore ⁺ e ^–HZ with arbitrary lepton andZ-boson polarizations is calculated within the electroweak standard model including the completeO() virtual and soft photonic corrections. These were obtained using the computer-algebra packagesFeynArts andFeynCalc. We list the relevant analytical results and discuss the influence of the weak radiative corrections on the unpolarized as well as the polarized cross sections. For LEP200 energies the weak radiative corrections amount to several per cent. For higher energies the corrections are usually of the same order for the total unpolarized cross section. However, for large values of the Higgs mass or for the suppressed cross section of transversely polarizedZ-bosons they reach +30% and –40%, respectively. The differential cross section in the forward and backward directions gets even larger corrections for high energies, where the box diagrams constitute the dominant part of the radiative corrections.     

Supersymmetric Box Contributions toe + e −→Z 0 h 0 ande + e −→A 0 h 0

Within the MSSM we calculate the electroweak 1-loop box contributions to the processese ⁺ e ^–Z ⁰ h ⁰ ande ⁺ e ^–Z ⁰ h ⁰. We present detailed results for c.m. energies and as well as for tan =2 and tan =50. The box contributions to the processe ⁺ e ^–Z ⁰ h ⁰ are, depending on and tan , of the order –2 to –20% and toe ⁺ e ^– A ⁰ h ⁰ of 2 to 10%.     

Soft gluon radiation ine + e −→4-jets

The amplitude of a 4-jet process ine ⁺ e ^? annihilation can be used to compute the associated flux of low energy particles due to the emission of soft gluons. I calculate the radiation pattern for the two contributing process, \(e^ + e^ - \to q\bar qgg\) and \(e^ + e^ - \to q\bar qq'\bar q'\) , in a form suitable for comparison with the experimentally observed distribution of inter-jet hadrons. It is found that the dependence of the soft background radiation on the overall colour structure of the jet process can be used to differentiate between the two processes, and thus probe the 3-gluon coupling. I also suggest a suitable initial equation for extending the 4-parton cross-section to higher order gluon emission in a Monte Carlo parton shower algorithm and compare the spectrum so produced with results obtained on the parton level.     

Q^2 \to A\sqrt s Scaling law fore + e −→hadrons andpp collisions at high energy

It has been conjectured recently that multiparticle production in low momentum transfer hadronic collisions is due to gluon bremsstrahlung off highly virtual (mass \(\mu ^2 \sim \sqrt s \) ) quarks. The model gives a new scaling law: \(Q^2 \to A\sqrt s \) , for comparinge ⁺ e ^?→hadrons andpp non-diffractive inelastic data. The scaling law is tested for average multiplicities, plateau height and 〈p ₂ ^⊥ 〉. Fore ⁺ e ^? annihilation a parametrization of data based on perturbative QCD approach is used.     

Quark mass effects toO(α s ) ine + e −→hadrons asymmetries and precision tests of electroweak physics

We estimate, to first order in α _s , quark mass effects on a number of fully integrated and “two-jet” asymmetries for the processe ⁺ e ^?→ \(q\bar q \to q\bar qg\) . We find that the effects of strong interactions are appreciably decreased, especially in the case of theb quark, with respect to the massless case. For the fully integrated asymmetries these effects are sizeable. Concentrating on the “two-jet” asymmetries, we find that for small values of the parameters ε and δ defining a ”two-jet” event all the asymmetries are α _s independent. For larger values of these parameters this is no longer true. However it is still possible to define regions in the (ε, δ) plane where the strong interaction effects can be made arbitrarily small. These important features enhance the role of “two-jet” asymmetries as specially convenient candidates for high precision tests of Electroweak Physics.     

Delayed unitary cancellation and heavy particle effects in e+e−→W+W−

We examine one-loop radiative corrections to e⁺e⁻→W⁺W⁻ in the standard model with one Higgs doublet, concentrating on the effects of very heavy fermions. These disturb the delicate unitarity cancellation between s- and t-channel diagrams, raising the cross section even well below the fermion threshold and giving a clear experimental signature for the heavy sector.     

Measurement of thee + e −→π + π − π 0 ande + e −→ωπ + π − reactions in the energy interval 1350–2400 MeV

The cross sections fore ⁺ e ^?→π ⁺ π ^? π ⁰ ande ⁺ e ^?→ωπ ⁺ π ^? have been measured in the 1.35 ≦ \(\sqrt s \) ≦2.4 GeV range from 1900 nb^?1 collected by DM2 at DCI. The second process proceeds via a resonant state at ≈- 1660 MeV/c², ≈- 280 MeV/c² wide. The first one is larger than a VDM extrapolation from the ω-? peaks and, although does not show any clear structure, is compatible with the presence of the above resonance.     

Full O(αew) electroweak corrections to e+e−→HHZ

We calculate the full $\text{O}\text{(α}{}_{\mathrm{<mtext>ew</mtext>}}\text{)}$ electroweak corrections to the Higgs pair production process e⁺e⁻→HHZ at an electron–positron linear collider in the standard model, and analyze the dependence of the Born cross section and the corrected cross section on the Higgs boson mass m_H and the c.m. energy $\text{s}$. To see the origin of some of the large corrections clearly, we calculate the QED and genuine weak corrections separately. The numerical results show that the corrections significantly suppress or enhance the Born cross section, depending on the values of m_H and $\text{s}$. For the c.m. energy $\text{s}\text{=500}$ GeV, which is the most favorable colliding energy for HHZ production with intermediate Higgs boson mass, the relative correction decreases from −5.3% to −11.5% as m_H increases from 100 to 150 GeV. For the range of the c.m. energy where the cross section is relatively large, the genuine weak relative correction is small, less than 5%.     

The e + e −→π0π0γ process below 1.0 GeV

The e ⁺ e ^?→π⁰π⁰ γreaction was studied in the energy range 0.36–0.97 GeV on a VEPP-2M e ⁺ e ^? collider with an SND detector. The decay probabilities were found to be B(ω→ π⁰π⁰γ)= (7.8 ±2.7±2.0)×10^?5 and B(π→ π⁰π⁰γ)= (4.8 _?1.8 ^+3.4 ±0.2)×10^?5.     

Limits on low scale gravity from e+e−→W+W−, ZZ and γγ

It has been proposed recently that the scale of quantum gravity (“the string scale”) can be M_S∼few TeV with n≥2 extra dimensions of size R≲mm so that, at distances greater than R, Newtonian gravity with M_Pl∼10¹⁸ GeV is reproduced if M_Pl²∼RⁿM_Sⁿ⁺². Exchange of virtual gravitons in this theory generates higher-dimensional operators involving SM fields, suppressed by powers of M_S. We discuss constraints on this scenario from the contribution of these operators to the processes e⁺e⁻→W⁺W⁻, ZZ, γγ. We find that LEP2 can place a limit M_S≈1 TeV from e⁺e⁻→W⁺W⁻, ZZ, γγ.     

Final state photon bremsstrahlung ine + e −→Z 0→hadrons as a tool for a precise measurement of the weak quark couplings

We discuss for the processe ⁺ e ^?→Z ⁰→q \(\bar q\) how final state photons can be used to disentangle the weak couplings of up and down type quarks. Since the flavour composition of multihadronic events with final state photons is different from that of all multihadrons, a combination of observables from the two samples allows the couplings to be determined. Both theZ ⁰ width and the forward-backward charge asymmetry at centre-of-mass energies aroundM _Z0 can be used to constrain the weak couplings. Taking into account the theoretical uncertainties a precision for the sum of the square axial and vector couplings of δ(v ²+a ²)～4% for up and down type quarks separately seems feasible for 10⁶ Z ⁰'s. The precision from asymmetry measurements will be less significant.     

Universal Feynman-Yang scaling for hadrons frome + e −,\bar pp andep collisions

An analysis of the single-particle distribution of inclusive at in terms of the covariant Boltzmann factor indicates that the Feynman-Yang scaling holds, the temperature ofW being 0.188 GeV. The specific central particle density ofW is found to be the same as andK ⁰ production at the same energy.     

Effective field theory analysis of new physics in e + e −→W + W − at a linear collider

The effective Lagrangian with scalar and vector resonances that might result from new strong physics beyond the SM is formulated and studied. In particular, the scalar resonance representing the recently discovered 125-GeV boson is complemented with the SU(2) _L+R triplet of hypothetical vector resonances. Motivated by experimental and theoretical considerations, the vector resonance is allowed to couple directly to the third quark generation only. The coupling is chiral-dependent and the interaction of the right top quark can differ from that of the right bottom quark. To estimate the applicability range of the effective Lagrangian the unitarity of the gauge boson scattering amplitudes is analyzed. The experimental fits and limits on the free parameters of the vector resonance triplet are investigated.     

There is no equivalence between ϱ′(1250) and inelastic effects in e+e−→π+π−

The analysis of all existing pion form factor data by a model respecting the fundamental principles, including (770), (1250) and (1600) contributions without inelastic effects and depending just on the physical parameters is carried out. The comparison of the obtained results with a previous analysis performed by means of a model with only (770) and (1600) contributions and inelastic effects confirmes the conclusions of Novosibirsk experimental group that the (1250) contribution and inelastic effects in e⁺e^–+– are indistinguishable. However, theoretically predicted behaviours of the P-wave isovector phase shift and the inelasticity from both models and their comparison with existing data unambiguously favour the model with (770), (1600) and inelastic contributions.We are indebted to Dr. D. Krupa who called our attention to the concise (in comparison with our original diagrammatized presentation) and transparent algebraic proof, given in the text by (30)–(33). One of us (A. Z. D.) would like to thank Professor Abdus Salam, the International Atomic Energy Agency and UNESCO for hospitality at the International Centre for Theoretical Physics in Trieste.     

How gluon bremsstrahlung inW-decays distorts the experimental signature ofe + e −→W + W −

The effect of gluon bremsstrahlung in the decays of theW-bosons on the measurement of the reactione ⁺ e ^?→W ⁺ W ^?→ hadrons is discussed. From simulation studies we find a substantial fraction of events with high jet multiplicities. At energies of \(\sqrt s = 200\) GeV this leads to probabilities of 25% or higher to misassign a jet to aW-boson when a realistic resolution of a detector is assumed. Some implications on the measurement of the decay and production properties of theW's are discussed.