" Neutrino Counting " in E+e-annihilation*

It is of great interest to study the process, e'e-+ weakly interacting particles (1) shown schematically in Fig. l(a). A final state that consists entirely of neutral particles that interact only weakly in matter is not directly observable in detectors normally used at e+e-colliding beam facilities, but we can infer the existence of such processes by detecting events accompanied by initial-state radiation of a single, hard photon [Fig. l(b)], e+e-+ 7 + weakly interacting particles. The study of single-photon production in e+e-annihilation (2) was first proposed by Ma and Okada' as a means of determining the number of massless neutrino species, but as we will discuss, measurements of single-photon rates and spectra will have implications for many theoretical ideas. e-e-3f17 (a) (b) 5465A5 Fig. 1. (a) Schematic diagram of production of weakly interacting particles by e+e-annihilation. (b) Radia-tive correction to e+e-annihilation. The first of these two lectures will start to examine what we can learn from studies of reaction (2). We will see that the physics probed by this reaction depends greatly upon the energy at which experiments are done. Most of this lecture, however, will focus on the experimental techniques used for these studies, and in particular, on the central problem of electromagnetic calorimetry. The second lecture will discuss the present status of our knowledge of single-photon production with a detailed look at the ASP experiment at PEP, and the physics interpretation of the results of that experiment. We will then go on to look at how experiments will be done at the next generation of e+e-machines SLC and LEP, and examine the importance of making a precise measurement of the cross section for reaction (2) at center of mass energies near the mass of the 2'. 1. NEUTRINO COUNTING AND EXPERIMENTAL TECHNIQUES 1.1 Physics Below the Z " Mass The standard electro-weak processes shown in Fig. 2 contribute to reaction (2) with the differential cross section,2 d20 G$CY ~(1-z7) where p: is the momentum transverse to the beam line of the detected photon, 8, is the polar angle of the detected photon, x-, = 2E,/&, gv =-i + 2sin2 8w =-.05, gA =-i, and NY is the total number of massless neutrino species that are produced through the 2' propagator. At center of mass energies far below the Z " mass the visible part of the cross section is unfortunately quite small. Notice that the photon distribution IZ;' …