Coincidence Measurements of SingleK+Electroproduction

Abstract

We have studied the reactions e" +p^ e~ +K + +A and e~ +p-» e" +K + +i: 0 by detecting the final electron and kaon in coincidence. Data are presented in the region of virtual-photon mass squared from -0.18 to -1.2 GeV 2 , virtual-photoproduction center-of-mass energy from 1.85 to 2.60 GeV, and virtual-photoproduction center-of-mass angle from 0 to 28°. The S°/A ratio tends to be smaller than in photoproduction, and the data suggest large scalar production of the K + A final state.We have used the two-arm spectrometer system at the Cambridge Electron Accelerator 1 to study the reactionse-+p-+e-+K + +X° (2) by observing the scattered electron and the kaon in coincidence.To lowest order in the electrodynamic interaction reactions, (1) and (2) can be treated as photoproduction by a virtual photon whose mass, energy, direction, and polarization density matrix are tagged by the detected electron. The virtualphotoproduction cross sections presented in this paper were obtained using the conventions of Ref. 1. The results are expressed in terms of k 2 9 the virtual-photon mass squared; W 9 the virtual-photoproduction center-of-mass energy; 0, the polar angle between the virtual photon and the kaon in the virtual-photoproduction center-ofmass frame; <p 9 the azimuthal angle between the electron scattering plane and the kaon-virtualphoton plane; and e, the parameter characterizing the polarization of the virtual photon.Data are presented for three scans. For k 2 near -0,30 GeV 2 and 0 near 0°, the W scan covers the region 1.85 < W ^2 0 62 GeV. For W near 2.17 GeV and 6 near 0°, the k 2 scan covers the range from -0.18 to -1.20 GeV 2 . For W near 2.17 GeV and k 2 near -0,39 GeV 2 , the 0 scan covers the range from 6=0° to about 20° for cp~0° and 180°. For each spectrometer setting there is a high degree of correlation between W, & 2 , 0, and e Q This is a consequence of the small angular and large momentum acceptance of the spectrometers and the strong dependence of the virtu-al-photon momentum, direction, and polarization upon the momentum of the detected electron.A combination of Cherenkov counter, time of flight, and missing-mass information was used to separate the kaons from the pions and protons. A threshold gas Cherenkov counter identified the pions. An electron-arm-hadron-arm time-offlight system with an rms resolution of 0.55 nsec separated protons and kaons. Their typical difference in time of flight was 2 nsec. A missingmass-squared plot was made of the data within a timing cut with a full width of 2 nsec centered on the kaon time of flight. The A and £° cross sections were extracted by fitting the mass spectrum with a smooth background function and A and S° fitting functions. Each fitting function was constructed by folding a Gaussian resolution function with a radiative tail. 2 ' 3 The missing-masssquared resolution function for the apparatus was

Constraints on background contributions fromK+Λelectroproduction

“…The SAPHIR collaboration at ELSA [1] has measured p(γ, K + )Λ and p(γ, K + )Σ 0 differential cross sections and recoil polarizations from threshold up to photon energies of 2 GeV. At present, the published p(e, e ′ K + )Λ data set is rather sparse with a few results from measurements in the seventies at Orsay [2], Cornell [3,4] and DESY [5] and recent data from Hall C at Jlab [6,7]. In the near future, however, concerted efforts at the Jlab facility will greatly improve on this situation.…”

Strangeness production on the nucleon

“…Thereby, the Q 2 -dependence of the electromagnetic couplings was materialized through the introduction of electromagnetic dipole form factors with a cutoff of 1.2 GeV. Both the older data [71][72][73] for dσ T + dσ L , as well as the new JLab data [70] for the separated transverse (dσ T ) and longitudinal (dσ L ) response, are reasonably well described.…”

“…4.3 against the available data from Cornell [71,72] and the CEA facility at Harvard [73]. From this figure, it is obvious again that the models A and C, which produced fair results in the real photon case, badly fail in describing the electron scattering results.…”

“…In a subsequent step, we have performed a new fit of the coupling constants, now including the available p(e, e K + )Σ 0 data from Cornell [71,72] and the CEA facility [73]. The total fit includes now 130 points for the real photon reactions γp → model calculations for the φ K -averaged virtual cross section are collected in Fig.…”

LONGITUDINAL AND TRANSVERSE CROSS SECTIONS IN THE 1H(E, E'K+)Λ REACTION

“…3). The uncertainties in these kaon electroproduction cross sections are at the same level or smaller than those in the existing world data set [8][9][10]. In order to facilitate comparison of data at different values of W and t, all data were scaled to W 2.15 GeV as outlined in Refs.…”