Γγ Production Research Articles

Studies of e +e −→ γγ and γγγ reactions

We have measured the cross section for γγ production with the TOPAZ detector in the energy region √ s=50.0–61.4 GeV. The observed cross section for γγ production integrated over |cos θ| ⩽ 0.77 is 50.2±0.8±2.2 pb at 〈√ s〉=57.6 GeV and the ratio of this value to the QED prediction is 1.01±0.02±0.04. The angular distribution is in good agreement with the QED predictions, thereby setting limits on the compositeness scales, Λ L+R +=168 GeV, Λ L+R −=97 GeV, Λ L,R=141 GeV, Λ L,R −=81 GeV, and Λ L−R ±=68 GeV, at the 95% confidence level. The reaction e +e −→ γγγ was also studied and was found consistent with the QED prediction.

Lepton- and baryon-number violating collider signatures from supersymmetry

Experimental tests of supersymmetry are totally changed by the addition of lepton- or baryon-number violating Yukawa interactions at the weak scale. The basic trademark for supersymmetry is no longer missing energy, thus invalidating most previous searches. The lightest superpartner is unstable and decays into leptons or baryons yielding spectacular collider signatures never considered before. With lepton number violation these signatures at e +e − colliders include: multilepton events from γγ production, very prominent resonances in Bhabha scattering from the s channel ṽ peak, and multilepton events from rare Z decays. At hadron colliders isolated leptons become a much more important signature for supersymmetry than missing transverse energy. With baryon number violation there are new interesting multijet events at both hadron and e +e − colliders. These events are unusual because they violate baryon number and contain large strangeness. A study of the Λ and λ content of jets resulting from reactions such as e + e − → γγ → jets and e + e − → Z → vv ∗ → jets could reveal the presence of baryon-number violation at the weak scale.

QED radiative corrections to the reaction e +e − → Z γ

The one-loop QED corrections to e +e − → Z γ production are presented. In contrast to γγ production, the result contains no anomalous large double logarithms. It is pointed out that our results can also be applied to processes like e +e − → μ + μ − γ and e +e − → v vλ .

Meson pair production in γγ scattering and implications for scalar mesons

We describe a simple analytic unitary model which provides with a parametrization of low energy (from threshold up to 1.4GeV) two photon production of ππ andK\(\bar K\) pairs in term of a few parameters related to the γγ width of the scalar and tensor resonances. It can be useful for analysing data because of the large continuum found in γγπ+π− production besides thef0 (1,270) peak in recent experiments. We show that the present data already provides a strong evidence for the existence of a broad low mass scalar state, the old σ or e (600) with a γγ width of 5 to 9KeV, and that it will be difficult to get more information on the controversialS* parameters from ππ pair production only. In thef0 range we explain the downwards shift of the π+π− peak but we expect the π0π0 peak around 1.27–1.29 GeV which could raise the question either of systematic errors in mass calibration of crystal Ball experiment or of the precise value of thef0 mass.

Inelasticity effects inππ production inγγ collisions

The inelasticity effects in the production of pion pairs in the processγγ→ππ for real photons are investigated using the partial wave dispersion relations. The total cross sections for different photon helicities are calculated. It is observed that this process is dominated by theππ final state interaction. A prediction for S* (997) →γγ decay width is also made.