The formation of the η ′ in the reaction e + e − →e + e − η ′→e + e − π + π − γ has been measured by the L3 detector at a centre-of-mass energy of 91 GeV . The radiative width of the η ′ has been found to be Γ γγ =4.17±0.10 (stat.) ±0.27 (sys.) keV . The Q 2 dependence of the η ′ formation cross section has been measured for Q 2 ≤10 GeV 2 and the η ′ electromagnetic transition form factor has been determined. The form factor can be parametrised by a pole form with Λ=0.900±0.046 (stat.) ±0.022 (sys.) GeV . It is also consistent with recent non-perturbative QCD calculations.
The two-photon collision reaction e + e − → e + e − l + l − has been studied at 2 ≈91 GeV using the L3 detector at LEP for l = e, μ , τ . We have analysed untagged configurations where the two photons are quasi-real. Good agreement is found between our measurements and the O (α 4 ) QED expectation.
The inclusive production of D ∗± mesons in photon-photon collisions has been measured by the Aleph experiment at LEP with a beam energy of 45 GeV. The D ∗+ are detected in their decay to D 0 π + with the D 0 observed in three separate decay modes: (1) K − π + , (2) K − π + π 0 and (3) K − π + π − π + , and analagously for the D ∗− modes. A total of 33 events was observed from an integrated luminosity of 73 pb −1 which corresponds to a cross section for Σ( e + e − → e + e − D ∗± X ) of 155 ± 33 ± 21 pb. This result is compatible with both the direct production γγ → c c in the Born approximation and with a more complete calculation which includes both radiative QCD corrections and contributions in which one of the photons is first resolved into its quark and gluon constituents. The shapes of distributions for events containing a D ∗+ are found to be better described by the latter.
The reaction γγ→π0η has been investigated with the Crystal Ball detector at the DESY storage ring DORIS II. Formation of δ(980) and A2(1320) has been observed with γγ partial widths Γγγ(A2)=1.14±0.20±0.2 6 keV and Γγγ(δ)B(δ→πη)=0.19±0.07 −0.07+0.10 keV.
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