The reaction e+e~e+e m. m has been analyzed using 97 pb ' of data taken with the Crystal Ball detector at the DESY e e+ storage ring DORIS II at beam energies around 5.3 GeV. For the first time we have measured the cross section for yy~m. m. for n m invariant masses ranging from threshold to about 2 GeV. We measure an approximately flat cross section of about 10 nb for 8'=m 0 0 (0.8 GeV, which is below 0.6 GeV, in good agreement with a theoretical prediction 'tr n' based on an unitarized Born-term model. At higher invariant masses we observe formation of the ft(1270) resonance and a hint of the fo(975). We deduce the following two-photon widths: I rr(f, (1270)) =3.19+0. 1620 z, keV and I "(fo( 975)) (0.53 keV at 90% CL. The decayangular distributions show the m~system to be dominantly spin 0 for W &0.7 GeV and spin 2, helicity 2 in the f, (1270) region, with helicity 0 contributing at most 22% (90% C.L.).

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The effect of ZrO 2 on the nucleation of lithium metasilicate crystals in lithium disilicate-based multicomponent glasses of composition in wt% x ZrO 2 + (1 À (x/100)) (73.82 SiO 2 + 3.99 K 2 O + 15.33 Li 2 O + 3.51 Al 2 O 3 + 3.35 P 2 O 5 ) with x = 0, 3, 4, 6, 8, and 12 was studied by combining the results from differential scanning calorimetry (DSC), micropenetration viscometry (MPV), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Peak shift of the first exotherm of DSC upscans indicated an increase in the temperature of maximum nucleation from 793 K (x = 0) to 873 K (x = 12), while number densities of relict structures of SEM images after nucleation at these temperatures and etching with HF led to an increase in the maximum nucleation rate from 2 9 10 17 /m 3 s (x = 0) to 6 9 10 18 /m 3 s (x = 12). MPV showed that viscosity increased with the ZrO 2 content but nucleation was fastest close to 10 9.7 Pa s for all glasses studied. XRD revealed lithium metasilicate crystals within the etched volumes of approximately one order of magnitude smaller sizes compared to SEM relict structures, which were assumed to result from early-stage phase separation and heterogeneous nucleation processes.

100 and 3000 MeV. Formation of~, g, and g' mesons is observed, and the following meson partial widths are obtained: I 0-7.7+0.5+0.5 eV, I "»-0.514+0.017+0.035 keV, and 'tl I "yy=4. 7+0.5+0.5 keV. No other narrow resonances are observed, and upper limits are given for the product I p yyB& yy. 38 1365 Q~1988 The American Physical Society 1366 D. A. WILLIAMS et al. 38 the range 10 '-10 ' sec which is difficult to measure: they have both a short decay length and a small natural width. The best determinations of the q and g' lifetimes are obtained by using measurements of I & and the branching ratio for X~yy to derive the full width. ' A precise measurement of the m. lifetime using the decaylength technique was recently achieved by Atherton et al. In this paper we describe a study of the reaction e+e~e+e yy~e+e yy, where each y* is a quasireal photon radiated from one of the beam particles. The virtual photons scatter to produce a final state consisting of two real photons. The final-state leptons scatter at very small angles and are not detected. The sum of the transverse momenta of the final-state photons with respect to the beam axis is essentially zero. %e observe three peaks in the invariant-yy-mass spectrum corresponding to the formation of m, g, and g' mesons. This is the first observation of n. production using this technique, first suggested by Low in 1960. The data used for this analysis were collected with the Crystal Ball detector running at the DORIS II e+e storage ring at DESY. The e+e center-of-mass energy varied from 9.4 to 10.6 GeV, with most of the data taken on the Y(1S), Y(2S), and Y(4S) resonances. The sample used for the study of g and g' production has an integrated luminosity of 114 pb ', of which about 40% has

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