L. Tecchio

Measurement of the proton electromagnetic form factor near threshold in the time-like region

The rate of the reaction pp→e+e− has been measured in liquid hydrogen for antiprotons at momentum values between 150 and 300 MeV/c. New precise values of the proton electromagnetic form factor in the time-like region have been obtained assuming |Ge|=|Gm| in the investigated region. The q2 dependence of the form factor close to threshold is found to have a steeper slope than indicated by existing calculations based on vector dominance models.

Precise determination of the electromagnetic form factor of the proton in the time-like region up to s = 4.2 GeV2

Abstract The s dependence of the proton form factor in the time-like region has been determined up to s=4.2 GeV2, assuming the validity of the |Ge| = |Gm| = |G| hypothesis. Data were taken in a dedicated experiment performed at the LEAR antiproton ring at CERN, increasing by one order of magnitude the available statistics on the proton form factor near threshold in the time-like region. Our result consist of cross section measurements of the p p → e + e − reaction for different beam momenta in the kinematical r 3.6⩽s⩽4.2 GeV2. The observed s dependence of the form factor close to threshold differs appreciably from the one suggested by previous experiments.

A large solid angle magnetic detector for precision studies of e.m. processes in low-energy p interactions

The detector presented is designed to measure the proton e.m. form factors by measuring e+e− pairs produced in pp annihilations at low energy or at rest. In this paper the apparatus is described in detail, with emphasis on how to achieve a suppression factor of about 1010 of the hadronic background.

A measurement of the pp→π+π− reaction for 158⩽p(p)⩽275 MeV/c

Abstract A measurement of the total cross section for the reaction p p → π+π− has been performed for seven values of the incident momentum between 158 and 275 MeV/c. The values obtained, if compared with previous results at higher momenta, agree with a 1/s dependence. The differential cross section sssumed over the whole incident momentum range has also been measured and the result of a fit by Legendre polynomials is given.

The FENICE detector at ADONE

A detector to measure the neutron e.m. form factors in the time-like region, through the e/sup +/e/sup -/ to nn reaction, is described. The FENICE experiment, at the Frascati e/sup +/e/sup -/ storage ring ADONE, has been conceived in such a way as to maximize the detection efficiency for this measurement in the c.m. energy interval from the threshold up to the J/ Psi . The apparatus is a complex array of scintillators, limited streamer tubes, and iron convertors. To reduce the cosmic-ray background a concrete shield has been built around the detector and the shield is covered by an active veto system. Antineutrons are detected by means of the charged prongs produced in annihilation events, and their energy is measured by a time-of-flight technique. Neutrons are detected by means of plastic scintillators. There are basically two types of trigger: n trigger and charged trigger. An associative memory helps to recognize the charged tracks coming from the interaction zone. The data acquisition system relies on a mu VAX 3200. A transputer module takes care of data readout and formatting. The online software is based on the Fermilab VAXONLINE system.<<ETX>>

Electron-Positron Pair Production in p\bar p Annihilation at LEAR

We report here the first results of the PS170 (APPLE) experiment [1, 2] installed on the LEAR beam, at CERN.