The detection of the e+e ---* ~n reaction at the Adone collider by the Fenice Collaboration has allowed to measure for the first time the neutron electromagnetic form factors in the time-like region, from threshold to 2.44 GeV center of mass energy. After a brief description of apparatus and data reduction, the new data are presented and compared to the proton results. Possible future perspectives are also discussed.
1. THE FENICE DETECTOR
The detector has been designed taking into account two peculiarities of the ~n final state (common to the ffp final state):
β’ -the annihilation of antinucleons in matter that gives rise to a many prong event β’ -the low velocity of nucleon and antinucleon with respect to all other particles produced in e+e -interactions.
To achieve good tracking capability and time-of-flight resolution limited streamer tubes and scintillation counters have been employed. One basic module contains four tube layers, interleaved with thin iron plates, that are the targets where antinucleons annihilate, and one scintillator layer. This module is repeated four times (radially) and all modules are arranged with an octogonal shape around the beam axis.
In the inner part of the detector a central tracking detector and another octogonal structure reside, similar to the one described above but with thicker counters, intended for neutron detection.
Since the expected cross section is quite low (,-~ 1 nb) and the luminosity does not exceed 10Z%m-Zs -1, the entire apparatus is shielded against cosmic rays.
The trigger logics requires local activity in the counter system; this information, associated to the information on the charge emerging from the central region at beam crossing, defines the neutral and the charged triggers. In the latter case correlations between active regions are requested to identify two body reactions as e+e ---~ e+e -(luminosity monitor) and e+e -~ p+p-or multihadronic channels. No neutron signal has been requested in the neutral trigger because of the rather low neutron detection efficiency (,,~ 15%). The apparatus performances are extensively described in ref. [1].