Exclusive Measurements of Virtual Bremsstrahlung in Proton-Proton Scattering at 190 MeV

Abstract

Exclusive differential cross sections of virtual bremsstrahlung in proton-proton scattering below the pion-production threshold have been measured for the first time. The total cross section integrated over photon invariant masses of 15 to 80 MeV͞c 2 and integrated over the acceptance of the detector is 3.2 6 0.1 6 0.5 pb. The data are compared to a low-energy calculation and a fully relativistic microscopic model, which predict a virtual bremsstrahlung cross section of 3.4 and 5.9 pb, respectively. Over the entire experimentally covered phase space, the low-energy calculation gives a better description of the data than the microscopic model. [S0031-9007(99)08793-1] PACS numbers: 13.75.Cs, 25.10. + s, 25.20.LjThe study of real-photon bremsstrahlung emission during proton-proton collisions below pion-production threshold has recently received a large boost. Highquality, intense, polarized-proton beams together with high-precision detection equipment have provided accurate exclusive cross sections and polarization measurements [1,2]. With the presently achieved accuracies, one is sensitive to details of the nucleon-nucleon interaction. Stateof-the-art calculations [3-6] are now relativistic, showing the sensitivity to negative-energy states, and include nonnucleonic degrees of freedom such as the virtual excitation of the D and meson-exchange currents.The emission of virtual bremsstrahlung ͑p 1 p ! p 1 p 1 g ‫ء‬ ! p 1 p 1 e 1 1 e 2 ͒, on the other hand, has up to now not been investigated. The difficulty in the experimental study of this process arises from the fact that the cross section is lower by approximately a factor of a 1͞137, the fine-structure constant, than the already small cross section for bremsstrahlung associated with real-photon emission. The measurement of virtual photons allows the decomposition of the nucleon current into transverse and longitudinal components as well as interference terms, the so-called nucleonic response functions. Furthermore, the nucleon-nucleon-g ‫ء‬ vertex is measured in the timelike region. Calculations predict a large sensitivity of the nucleonic response functions to the details of the nucleon-nucleon interaction [7].In this paper we present the first measurements of the virtual bremsstrahlung cross section. These studies, performed at 190 MeV, obtained a total of 600 backgroundfree ppe 1 e 2 events. The angular distribution of virtual photons as well as the invariant-mass dependence was determined. The data are compared with predictions based on a relativistic low-energy theorem [8,9], as well as a relativistic microscopic calculation by Martinus et al. [7]. Within the experimental accuracy and phase-space coverage, the data are in rather good agreement with the calculation based on the low-energy theorem, which gives a cross section integrated over the experimentally covered phase space of 3.4 pb. The microscopic model seems to overestimate the experimental cross section, with a predicted cross section for virtual bremsstrahlung of 5.9 pb.The experiment was performed ...

First Determination of the Nucleon-Nucleon Response Functions in the Timelike Region

“…The difficulty in the experimental study of this process arises from the fact that the cross section is extremely small. For the experiment discussed here the total measured cross section amounts to 3.2 pb [1]. On the other hand, the response functions are well defined experimentally.…”

“…The study of virtual-bremsstrahlung emission during proton-proton collisions provides a complete set of observables to determine details of the NN interaction [1][2][3][4][5]. The virtual-photon polarizations, longitudinal and transverse, as well as their interference terms project the nucleon-nucleon current, which involves the NN T -matrix, onto six independent components, i.e., the nucleon-nucleon response functions.…”

“…In total 600 pp ! ppe 1 e 2 events were obtained [1,2], which correspond to a total integrated cross section of 3.2 6 0.1͑stat͒ 6 0.5͑syst͒ pb over the acceptance of the detector. The angular distribution of the virtual photon as well as its invariant-mass dependence have been presented in earlier publications [1,2].…”

Bremsstrahlung in the nuclear medium: Search for coherence phenomena

“…In particular, the elementary bremsstrahlung process for virtual and real photons has been studied in the pp system [13,14] and coherent bremsstrahlung in the a + p system [15]. To study the various medium effects the energy spectra and angular distributions of photons up to the kinematical limit were measured in reactions of 190 MeV protons with natural targets of C, Ni, Ag and Au.…”

“…Experimental setup of the TAPS photon spectrometer around the 70 cm diameter carbon-fibre scattering chamber and in front of the small-angle large-acceptance hadron detector SALAD at the AGOR cyclotron. Indicated are the particle tracks for virtual photon production [13] in pp scattering. In the present experiment real photons and protons were detected in the TAPS blocks.…”