The total cross section for hadron production by high-energy photons has been measured from a number of nuclei ranging from hydrogen to uranium. Some shadowing is observed at a level considerably less than predicted by conventional vector-meson dominance but consistent with a modified theory. The energy dependence predicted by vectormeson dominance is observed. The shadowing in heavy nuclei shows a smooth transition from electroproduction to photoproduction.143.2± 2.7

No abstract

No abstract

We report a high-precision measurement of the ratio R of the total cross section for e+e--hadrons to that for e + e--p+p-, at a center-of-mass energy of 29.0 GeV using the MAC detector. The result is R =3.96+0.09. This value of R is used to determine a value of the strong coupling constant a, of 0.23rtr0.06, nearly independent of fragmentation models. Two different analysis methods having quite different event-selection criteria have been used and the results are in agreement. Particular attention has been given to the study of systematic errors. New higher-order QED calculations are used for the luminosity determination and the acceptance for hadrons. INTRODUCTIONPrecise knowledge of the total cross section for e+e--hadrons is of fundamental importance to the understanding of the interactions of photons and partons. In particular, the ratio of the cross section for producing hadrons to the cross section for producing pointlike fermions such as muons is one of the most important quantities in this understanding:The ratio R is expected to be nearly independent of the center-of-mass energy E,,,, and given by the quark flavors with a slowly varying correction due to QCD, and independent of the quark fragmentation process,At E,,,, =29 GeV the effect of weak interactions on the hadronic cross section is approximately 0.2%. The factor 1.4 comes from a calculation in the modified minimalsubtraction (m) renormalization scheme.'-3 For five quark flavors u, d, s, c, and b, having charges e, in units of the electron charge, and a strong coupling constant 31 1537 -@ 1985 The American Physical Society E. FERNANDEZ et al. M0 i I , --. , I --. ' .--I --, .--( a ) ( b ) FIG. 1. Cross-section views of MAC detector. (a) End view of central section; (b) side view. Key to symbols: CD, central drift chamber; SC, shower chamber; TC, timing scintillator; HC, hadron calorimeter; EC, end-cap calorimeter; MI and MO, inner and outer muon chambers.a, =0.16, one expects R = 3.87. (This value of a, is more or less consistent with determinations from studies of event topology and energy-energy correlations in e+e-annihilation, and moment analysis of neutrino scattering experiments?-9) QCD is able to predict not only the total cross section, but also the angular distribution of the thrust axis. This means that a powerful test of Q C D also may be made over a restricted angular range.The best published measurements of R have been primarily limited by systematic errors of 3 to 5% (Refs. 10-13); the statistical errors have been smaller. The systematic errors have generally been limited by uncertainties in acceptance calculations (model dependence), backgrounds (mainly two-photon interactions), and by radiative corrections (beyond a3 effects). This experiment offers two different methods of event selection, one (method A) relying heavily on calorimetry and having very large acceptance minimizing uncertainties in detection efficiency, and the other (method B) relying heavily on charged-particle tracking and using smaller angular acceptance to minimi...

The reactions e *e ~-*JU "V "y> and e*e --*/I + JLI "y have been studied at \ r s = 29 GeV with the MAC detector at the SLAC storage ring PEP. The measured cross sections, charge asymmetry, and invariant-mass distributions are in good agreement with the predictions of QED theory. Limits are derived for the production of excited muon states.