STUDY OF THE np→ppπ- AND np→d π+ π- REACTIONS WITH THE VERTEX DETECTOR ARCOLE

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STUDY OF THE np → ppπ - AND np → d π+ π- REACTIONS WITH THE VERTEX DETECTOR

ARCOLE

Y. Terrien, G. Bruge, P. Couvert, B. Fabbro, J.-C. Faivre, C. Kerboul, M.

Rouger, F. Wellers, R. Beurtey, J. Saudinos

To cite this version:

Y. Terrien, G. Bruge, P. Couvert, B. Fabbro, J.-C. Faivre, et al.. STUDY OF THE np → ppπ- AND

np → d π+ π- REACTIONS WITH THE VERTEX DETECTOR ARCOLE. Journal de Physique

Colloques, 1990, 51 (C6), pp.C1-375-C1-378. �10.1051/jphyscol:1990632�. �jpa-00230899�

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COLLOQUE DE PHYSIQUE

Colloque C6, suppl6ment au n022, Tome 51, 15 novembre 1990

STUDY OF THE

GP+pp?r-

AND Gp+d T'T-REACTIONS WITH THE VERTEX DETECTOR ARCOLE

Y. TERRIEN, G. BRUGE, P. COUVERT, B. FABBRO, J.-C. FAIVRE, C. KERBOUL, M. ROUGER, F. WELLERS, R. BEURTEY* and J. SAUDINOS*

?PhN-SEPN, S a c l a y , France

Lab. N a t . S a t u r n e , S a c l a y , France

Resume: Nous presentons des resultats preliminaires de l'etude des

+

-+

reactions npppn- et np+dn+x- obtenus avec un detecteur d'angle solide 4n permettant la reconstruction complete de la cinematique evenements par Bvenements.

Abstract: We present preliminary results on the study of the ;)nPppx- and

-* nptdntn- reactions obtained with a quasi-4n detector. Full kinematics is obtained event by event.

The past 10 years have seen very extensive experimental studies of the elementary reactions inducedbythe interaction of two nucleons. Indeed, above the pion production threshold, it is essential to have a large body of data to be able to treat on the same footing the elastic and inelastic channels, which is the only way to get a full understanding of hadronic processes.

Besides, experimental measurement of spin observables is needed to test in detail the corresponding reaction mechanisms, which are dominated by the existence of NA and NN* intermediates states. The neutron-proton system is particularly interesting in this respect since the totalisospin I=O component cannot proceed via any A N intermediate state (total isospin conservation), and, in the 1=1 component, the A 0 p state is weakened by antisymmetrisation:

thus, one can expect to see more clearly than in pp-induced channels, the NN* intermediate state or any non-resonant contribution.

There are extremely few existing data for the simplest reactions induced by the np system, i.e. npppx- + and ;)np+dn+n-. Moreover, many of them have been

obtained by performing quasi-reactions using deuteron beam or target and most of them are more or less inclusive. Taking advantage of the intense (up to

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990632

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3x10 n/pulse), rather well defined in energy (few l6 o-') and highly polarized (60%) neutron beam obtained by polarized deuteron break-up at the Saturne National Laboratory (Saclay), we have built a new experimental set-up to study these two reactions. We use an original method based on kinematics. To define completely a 3-body final state, given the four equations of momentum and energy conservation

,

the nine kinematical independant unknows reduce to five, which means that the measurement of five of these quantities is sufficient.

our set-up allows to measure the 3x2=6 angles of the trajectories of the final state 3 particles, which provides one over-determination letting us identify the reaction, event by event. In fact, additionnal measurements of energy loss and time-of-flight (TOF) give also informations useful to select the reaction.

Our detector, ARCOLE, is essentially avertex detector supplemented with a trigger system (fig.1). It is made of three cylindrical Multi-Wire Proportionnal Chambers (MWPC) with cathode strips read-out, associated with two forward plane MWPC. The trigger system is made of a 2x2 ma plane of twenty 2-cm thick plastic scintillators covering the forward region and of a barrel of eight 1-cmthick plastic scintillators for the lateral part. These plastics are also used to get the dE/dx and TOF informations. ARCOLE is a quasi-4x detector: as an example, a 98% coverage of the total phase space is achieved for the nptppx- reaction at 1 GeV. -+

1- Experimental set-up ARCOLE.

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With this apparatus, we are able to measure the fully differential angular distributions of cross-section and asymmetry for the considered reactions, at kinetic energies of the neutron lower than 1150 MeV. The absolute normalisation of cross-sections is obtained since the neutron flux is determined by means of a neutron monitor which has been calibrated using a dedicated method.

First measurements have been performed at T = 580 and 800 MeV neutron incident kinetic energy. only the 800 MeV data have been analyzed so far.

To identify the reaction, we proceed as follow. For events where we detect 3 particles, we compute the 3 corresponding momenta from the equations of momentum conservation. At this stage, there is no assumption on the nature of the detected particle, since the mass does not enter explicitely in these equations. Then, for each particle, we look at the correlation of the momentum with the measured dE/dx or TOF; one can see from fig. 2 and 3 that proton and pion are easily distinguished, which makes easy to separe nprppx- from -P

I

PI

^I P f G e V l c ) ^-w

2- Proton-pion separation

by P/(dE/dx) spectrum. ^3-Proton-pion separation by P/TOF spectrum.

+

npdx+x-. Having thus determined masses, momenta and angles, a final kinematical check of the identification of the reaction is done by verifying that the total energy conservation law is actually satisfied.

We have then calculated some simple observables, like invariant mass histograms of the pair px- (fig. 4) or pp (fig. 5 ) .

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641

⁰1.0

A,-.

1.1 1.2 1.3 1.4 ^, ^MrGeV ^21:^41'

A _M

4- Proton-pion invariant mass.

5- Proton-proton invariant mass.

6- Angular distribution oftheasymmetry for the ;pn-(pp) reaction at 800 MeV.

The pp spectrum begins at M =2xM which corresponds to the case where PP P

the 2 protons are emitted together with a relative momentum equal to 0. Since there is no statistics for this very right case, we have obtained the analyzing power in the following way. We have cut the spectrum in bins of M and, for

PP each bin, we have extracted the analyzing power from the data. Then, extrapolation to the point M =2xM gives us the value wanted. In fact, this

PP P

procedure has been applied for bins of cos(8 ) in order to obtain angular n

distributions. Fig. 6 presents preliminary results for such an angular distribution of the analyzing power of the np+x(pp) reaction for the + extrapolation to the '"Som state M =2xM

PP P'

Further analysis and calculations are in progress. Additionnal measurements will be made at other energies.