Download pdf - Search for a Δ++ presence in the 3He nucleus by means of the 3He (p, t) Δ++ reaction

Volume 77B, number 3 PHYSICS LETTERS 14 August 1978

SEARCH FOR A A ++ PRESENCE IN THE 3He NUCLEUS

BY MEANS OF THE 3He (p, t) A ++ REACTION

B. TATISCHEFF, I. BRISSAUD, R. FRASCARIA, M. MORLET and F. REIDE Institut de Physique Nuclgaire, 91406 Orsay, France

R. BEURTEY, A. BOUDARD, J.L. ESCUDIIS, M. GARCON, L. SCHECTER x, J.P. TABET and Y. TERRIEN Centre d'Etudes Nucldaires de Saclay, DPhN/ME, 91190 Gif-sur-Yvette, France

Received 26 April 1978

The reaction 3He (p, t) A+÷ has been studied to look for evidence of (A +÷, 2n) components of the 3He wave function. The experimental conditions, proton energy and triton angles, have been chosen to emphasize such components.

The search for isobar components in nuclear wave functions has received a lot of interest lately. Except for the case of the deuteron for which there are bub- ble chamber measurements, there has been no experiment reported that shows explicit evidence of this ex- otic wave function component. A large number of calculations have been attempted, however, where the aim has been to improve the agreement between the measured quantities for some nuclei and theoretical predictions based on purely nucleonic wave functions. A complete review of the present situation has been presented recently by Green [1], by Weber and Aren- h6ver [2] and by Kisslinger [3]. Although the two- nucleon system has been studied most often, some calculations have also been tried for the three-nucleon system [4]. This nucleus should be the better one for such a study because it is the lightest with isospin 1/2 allowing single A components. The effect of isobar components on predictions [1] for the mass difference between 3He and triton, the magnetic moments of both nuclei, charge and magnetic form factors and/3 decay of the triton has been tested. The general trend of these calculations seems to be that although the agreement is improved with a 2-5% admixture of the NNA component, no real proof of such contribution has so far been demonstrated. 1 On leave of absence from Oregon University, Corvallis.

We present here results of an experiment whose ob- jective was to study the importance of the (A ++, 2n) component in the 3He wave function, by means of the 3He (p, t) reaction. The tritons produced were detected using the energy loss spectrometer SPES I and the proton beam of the Saturne Saclay Synchrotron. A cryogenic 3He target (thickness 450 mg/cm 2) was used. A time of flight measurement over an 8 m flight path defined by seven scintillator detectors allowed us to distinguish the tritons produced from a much larger number of high energy deuterons. The position in the focal plane was defined by the use of 4 double drift chambers, each 50 cm long, with a momentum range of Ap/p = +-2%. Because of decrease in solid angles at both sides of the focal plane, only a part of it corresponding to Ap/p = -+1.6% has been used. In order to avoid possible broadening of the resonance peak each measure has been divided in 11 bins each covering less than 3 X 10 -3 in Ap/p (~4 MeV in ~-~). Data have been accumulated in several different runs, generally changing the spectrometer field by Ap/p = 1.5%. An electron secondary emission counter located in the beam in front of the target and two scintillator tele- scopes have been used as a beam intensity monitor. The absolute cross sections were determined using 12C activation. The incident energy of 850 MeV was chosen so that the tritons of interest were equally

254


280 t -

n 108 c

:E

50 - ID

20

10

7~ / / p t p , .

3Fie &++ 3He p 0) (3)

p A ++ p - / / t

3He t 3H e A+ + (2) (4)

(o)

3He(p,t) &++ Ep =850 MeV J.

'~ 3He (p,3He)p Ep =800MeV~W T 'I,

+, +

+

(b) I I I I

8C.N (A++or p) 140 150 160 170 180

Fig. 1. (a) Graphs of different processes discussed; (b) experimental cross section (* corresponds to ref. [6]).

angles and transferred momenta in the center of mass at both vertices for the spectator model diagram (1), fig. la , are given. In the case of the backscattering model, diagram (2), fig. 1 a, the transferred momenta at the lower vertices is a variable quantity. The assump- tion that the transferred momentum is equally shared between both lower vertices - which will correspond to maximum overlap between initial and final 3He and 3H states - has been made. It is clear that the transferred momenta are much larger for this backscattering diagram, and explain our kinematical choice of angles. The results are shown in fig. 2. An important continuum coming from the process of graph 3 appears at all angles. When this continuum is subtracted a bump re- mains which decreases with increasing angle. The missing mass (M = 1226 + 20 MeV) is compatible with the mass of the free resonance. The width (60 + 20 MeV FWHM) is smaller than the width of the free resonance. This apparent reduced width migh be partially ac- counted for as a result of the following effect. If the A ++ is a true spectator (graph 1) then it must have ex- isted in the nucleus with the same momentum as it has in the final lab system. This latter quanti ty is defined by two-body kinematics and changes as a functi~,n of

spaced between elastically scattered protons and the momentum limit of the spectrometer. The higher energy also tends to reduce the relative spread in tri ton laboratory momentum corresponding to the width of the A + ÷ resonance.

Differential cross sections were measured at 6 ° , 10 ° and 15 ° (in the lab. system). Forward angles were chosen to favour graph 1 rather than the product ion graph 2 (see fig. 1). In table 1 some experimental conditions: tr i ton momenta, corresponding recoil isobar

Table 1 Triton momenta (in MeV/c), recoil z~ ++ angles (in degrees) and transfer momenta Q and R (in fm -1) c.m. corresponding re- spectively to lower and higher vertices for diagrams (1): spectator model, and (2): backscattering model.

A + + 01tab P[ab 0c.m. Diagram 1 Diagram 2

IQI LRI IQll =IQ21 IRI

6 1776 165,3 1.67 3.81 2.87 8.62 10 1748 155.4 1.93 3.91 2.83 8.50 15 1694 143 2.35 4.08 2.75 8.25

o I ,1300 1200 100 l ) ! WL En=860MeV

' I , * 30

8 10

30 - - 0 20

~t -q5 "~ + , - ~ 10

1300 1200 1100 Missing Moss (MeV)

Fig. 2. Measured missing mass distributions of the 3He (p, t )X reaction at Ep = 850 MeV.

255


the detected triton energy (or missing mass). Thus as one crosses the region of the bump one is sensitive to different parts of the momentum distribution of the A++ in 3He. The relative probability of these different momentum components thus must be convoluted with the free A shape and could give rise to an apparent decrease in the width detected. There are different other causes for broadening or reducing the width which are discussed in ref. [7].

Some measurements at a slightly higher incident energy Ep = 900 MeV have also revealed a bump for 0~ ab = 6 °, 7 ° and 15 °, corresponding again to a mass close to that of the free ~ resonance but showing a smaller width. Because of some normalization prob- lems at this energy, the corresponding data are not given except for 15 ° where da/d~2 cm = 15 -+ 9 nb/sr.

That the bump corresponds to the reaction 3He(p, t)A ++ and not to a quasi free d(p, t)Tr + reaction, can be seen most clearly from the kinematic shift both with changing angle and changing incident proton energy. One could also argue that as the backward pion production cross section for the free d(p, t)Tr + seems to be rather flat one would not expect it to yield the rather sharp angular variation of the cross section measured at 850 MeV.

The decrease of the cross section with increasing angle could indicate that the second graph (of the im- pulse approximation production) is being inhibited by the large momentum transfer and that it may be ne- glected. The triangular graph (graph 4 of fig. la), however, cannot be distinguished from graph 1 and its contribution must be calculated. The importance of triangular graphs in backward elastic scattering was first pointed out experimentally and theoretically [5] for elastic pd scattering and then shown experimentally for p 3 He backward elastic scattering [6]. In the latter case their importance was seen to have diminished al- ready at an energy of Ep -- 850 MeV. Moreover the cross section variation seen in the same angular range as the present experiment was very small.

The continuum corresponding to the three-body

final state (graph 3) can be calculated using the measured d (p, 7r+)t cross sections and some assumptions on the off-shell matrix elements. The low momentum side of the A bump can also be affected by contribu- tions from broad N* resonances, whose importance of course is unknown, as well as from double pion emission which gives rise to a four-body final state.

In summary, we have presented preliminary results for the reaction 3He (p, t)A ++ and compared these data with backward elastic scattering. We observe different shapes but the same order of magnitude for the differential cross sections for both reactions. The con- nection between both experiments appears clear if we consider all processes (in first order) which can inter- fere to produce these reactions. Of course all these different processes are not independent and one must be careful not to introduce some double counting. In con- clusion it seems the A ++ probability in the 3He wave function can only be extracted from theoretical calculations of the contribution of different graphs to the experimental spectra.

The authors are indebted to S. Buhler and J. Momm~jat for cryogenic 3He target and to J. Habault, C. Legr61e and J. Lemeur for their assistance during the experiment. They also thank Professor J. Cameron for his helpful suggestions in reading the manuscript.

References

[1] A.M. Green, Rep. Prog. Phys. 39 (1976) 1109, and references therein.

[2] H.J. Weber and H. Arenh6vel, Phys. Rep. 36C (1978) 277. [3] L. Kisslinger, to be published. [4] A.M. Green and T.H. Schucan, Nucl. Phys. A188 (1972)

289. [5] N.S. Craigie and C. Wilkin, Nucl. Phys. B14 (1969) 477;

V.M. Kolybasov and N.Ya. Smorodinskaya, Yad. Fiz. 17 (1973) 1211.

[6] R. Frascaria et al., Phys. Lett. 66B (1977) 329. [7] W. Weise, Nucl. Phys. A278 (1977) 402;

H. Arenh6vel, Phys. Lett. 49B (1974) 329.

256