The first measurement of the neutron electromagnetic form factors in the time-like region

NUCLEAR

PHYSICS A ELSEVIER

Nuclear Physics A623 (1997) 333c-339c

The first measurement of the neutron electromagnetic form factors in the time-like region C.Voci Dipartimento di Fisica and INFN, Padova on behalf of Fenice Collaboration 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. T H E F E N I C E D E T E C T O R The detector has been designed taking into account two peculiarities of the ~n final state (common to the ffp final state): • - the annihilation of antinucleons in matter that gives rise to a many prong event • - 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]. 0375-9474/97/$17.00 © 1997 - Elsevier Science B.V. All rights reserved. PII: S0375-9474(97)00452-1

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About 2 • 10s triggers have been collected at the center of mass energies Ec.,,.=1.9, 1.92, 2.0, 2.1, 2.44 GeV. Since a few tens of events are expected at each energy, a rather complex analysis has been carried out. After eliminations of machine backgrounds and residual cosmic rays, a topological analysis followed by a neural network approach looks for the many prong signature. Finally all ~n and ~p candidates are usually inspected in a three level scanning procedure. The efficiency of the whole process is ~ 0.3 for ~p and 0.2 for ~n.

2. T H E ~p C H A N N E L S The e+e - --+ ~p cross section has been studied by the Fenice experiment in order to have a reference reaction, useful as a check. The ~p events are very cleanly extracted from the multihadronic sample, requiring two collinear charged tracks originating from the interaction region; one of these tracks must end with a many prong interaction (~ annihilation). After the visual scanning of the selected events, a clear signal practically without background emerges in the 1//3 variable, where/3 is the antiproton velocity (fig.l), computed as distance of flight, from the interaction vertex to the annihilation point, divided by the time-of-flight, that is deduced from the analysis of the times of the various counters hit. The cross section at the various energies is shown in fig.2; the angular distribution and the form factors are shown later together with the neutron data. The agreement between previously published data and Fenice data is good and this fact gives confidence in the absolute normalization of the experiment[2].

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3. T H E ~n C H A N N E L The search for the ~ n events is more difficult. Since we require only the antineutron annihilation (i.e. we cannot rely on a two body signature because of the quoted low neutron detection efficiency), it is not possible to reject completely these cosmic ray events, most likely originated by neutrons of high energy, that simulate ~ annihilations. A n d this is true even after a very careful scanning performed by different skilled observers. However cosmic events are casually distributed in time and give an almost flat 1/fl distribution, that we have checked also in specific runs below the ~ n threshold. Good events are instead peaked around the 1/fl value that is computed from the current Ec.,n. in the hypothesis of a e+e - ~ ~ n reaction. The final samples of events are shown in fig.3. The distributions are fitted as a sum of a gaussian curve for the signal and a polinomial curve for the background; the number of ~n events is given by the area under the ganssian. The cross section is shown in fig.4; it is systematically larger t h a n the ffp cross section. Also the angular distributions of the two classes of events are different. Due to the low statistics the events at all energies are groupped together; the ffp distribution looks flat in the full acceptance region while the ~n one shows a 1 + cos 2 8 behaviour (fig.5a,b).

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OF T H E F O R M F A C T O R S

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They are shown in fig.6, with all other existing data. In the neutron case our data suggest a different situation. At threshold the two form factors, that must be equal, have a small value; then, while [ GE[ stays small, I GM [ increases, giving the main contribution to cross section. Fitting the data of fig.5b with a function like (2), one obtains that ] GE I< 0.1 ] GM I. In fig.7 [ GM ] is shown, calculated

from (1) assuming I Co I= O. Summing up, our measurement for the proton is in agreement with previous measuremerits, showing a sharp rise near threshold and that [ Go I=l GM [ at least up to ~ 6 GeV u. The first measurement for the neutron, performed with the same apparatus shows instead a sharp dip near threshold and that I Go I is about in order of magnitude lower with respect to I GM I" The absolute values of] GM I are about 50% larger than the proton ones. In both cases the time-like values are consistently higher than the space-like values at the same I q2 ]. Analyticity assumption and the fact that at large q2 in the space-like region I GP~ I /#p "~1 G ~ ] / I P,, I w°uld give in the time-like region I a ~ 12 _ ~

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F U T U R E

P E R S P E C T I V E S

Fenice data on neutron form factors reveal interesting features, partially unexpected. While we are reasonably confident that the average cross section for ~n is about at the same level of the ~p one, much more statistics would be necessary to solve the question of the relation between i GE i and I GM I as a function of energy and of the ratio At 2 GeV symmetric e+e - facilities no longer exist or are unsuitable (low luminosity). Indeed in the Feuice case the low peak luminosity has been the only drawback: the ~n signal, although clear, was only two-three times larger than the cosmic background. A possible solution would be to use one existing ring with high energy (El) positrons against one existing linac with low energy (E2) electrons, giving rise to asymmetric collisions at E ~ = 2 Ev/-E~IE~. Calculations show that the luminosity can be reliably higher than 103°cm-Ss-1. In addition the neutron and the antineutron are forward produced, along the positron direction, and can be both detected in the same calorimeter, drastically reducing the cosmic ray background. R E F E R E N C E S

1. A. AntoneUi et al., Nucl. Instr. and Meth. A 337 (1993) 34. 2. A. Antone]li et al., Phys. Left. B 334 (1994) 431;