Exclusive vector-meson production in muon-proton scattering

Abstract

From a muon-proton scattering experiment with a streamer chamber at the Stanford Linear Accelerator we present results in the ranges 0.3 < Q 2 < 4.7 GeVz and 1.7 < W < 4.7 GeV for the reactions p'p-ppV where V IS a vector meson (pO, o, or 4). It is shown that in p production the skewing parameter and the longitudinal-transverse ratio change significantly as Q increases above 1 GeVZ. The cross section for p0 production as a function of & falls below the vector-meson-dominance prediction. The ratio of the cross section for exclusive vector-meson production to the total cross section falls by a factor of 10 between photoproduction and a Q 2 of 2 GeV', yet the ratio of o to p production remains constant at the photoproduction value out to Q > 2 GeV2. I. lIVT1tODUCTBONduction of vector mesons by a generalized optical A. Theoretical background The idea that the photon-hadron coupling occurs primarily through the vector mesons (vector-meson dominance o r VMD) has had remarkable succ e s s in quantitatively explaining the behavior of the interaction of r e a l photons with nucleons, including the total yN c r o s s section, vector-meson production and decay, and Compton scattering.' On the other hand, the interaction of a virtual photon (y,) of l a r g e negative mass-squared (Q2) with a nucleon is known to exhibit behavior not easily explained2 by VMD, but r a t h e r , most easily understood in t e r m s of pointlike constituents (partons o r quarks), in particular, scaling and jet s t r~c t u r e .~ The connection between these'hvo views lies in the low-Q2 region, since a t the lower end (Q2 =0), VMD must b e a s valid a s it is f o r r e a l photons, while f o r Q2 3 1 G~v~, the simpler effects of individual partons a r e revealed.A complete picture would unify these two seemingly disparate viewpoints, perhaps by regarding the fundamental interaction a s that of the photon materializing into a quark-antiquark pair (analogous to e'e' pair production); the qq pair would exhibit the characteristics of a particular vector meson, depending on the probability that the qq wave function overlapped that of the vector meson.4 Thus the whole question of confinement, a s i t relates to the qq wave function, would have to enter the final unification. Within the yvN interaction, the total c r o s s sectheorem6 within VMD. A significant difference between photoproduction and virtual-photon scattering, aside f r o m the different Q', is the presence of a longitudinal (or s c a l a r ) component to the photon. The ratio of the total c r o s s sections f o r longitudinal and transverse virtual photons on protons, R =u,/u,, is measured by single-arm-spectrometer experiments. The r atio of the c r o s s sections f o r exclusive vector-meson production by longitudinal and transverse virtual photons, R .=o,(yVp --Vp)/u,(y,p--Vp), can be measured by observing the vector-meson decay angles, if s-channel helicity conservation (SCHC) is assumed. This assumption is associated with diffractive p production. The e x t r a degree of f r e edom...