1. L. D. Landau andE. M. Lifshitz:Sow. Phys.,6, 244 (1934). This article is also found inCollected Works of L. D. Landau, edited byD. Ter Haar (New York, 1965)
2. For the experimental situation see the following article:D. Schildknecht, H. J. Willutzki andG. Wolf: DESY preprint 71/28 (1971).
3. V. F. Balakin, V. M. Budnev andI. F. Ginzberg:JETP Lett.,11, 388 (1970).
4. S. J. Brodsky, T. Kinoshita andH. Terazawa:Phys. Rev. Lett.,25, 972 (1970).
5. N. A. Romero, A. Jaccarini, P. Kessler andJ. Parisi:Phys. Rev. D,3, 1569 (1971).