SPIN PHYSICS AND THE TWO NUCLEON SYSTEM AT SIN

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SPIN PHYSICS AND THE TWO NUCLEON SYSTEM AT SIN

R. Hess

To cite this version:

R. Hess. SPIN PHYSICS AND THE TWO NUCLEON SYSTEM AT SIN. Journal de Physique Colloques, 1985, 46 (C2), pp.C2-433-C2-437. �10.1051/jphyscol:1985250�. �jpa-00224565�

SPIN PHYSICS AND THE TWO NUCLEON SYSTEM AT S I N

R. Hess

Dept. of Nuolear and Particle Physics, University of Geneva, Switzerland

Résumé. On passe en revue les nesures récentes de polarisation effectuées au SIN dans le système à deux nucléons. Ces expériences sont discutées dans le contexte des résonances dibaryoniques.

Abstract. Polarization parameters measured recently at SIN in the reactions pp "*" pp, pp "*" ^ d, pp "*"

1TNN, IT d ->• II d, If d -»- pp and the spin dependent total cross section Aa of the pp-scattering are discussed in the context of the dibaryon resonances.

Introduction

A nice review on the search of dibaryon resonances in the different laboratories was pre- sented recently by K. Seth [1]. We will illustrate here the specific contributions of SIN to this field. Although some of these experiments may indicate the presence of resonances, it seems quite clear that only a general and detailed theory, explaining the whole block of the data, could solve the problem.

Many of these measurements were performed using a polarized target of a few cm built by S., Mango et al. This group is presently testing a dilution refrigerated cryostat in order to produce tensorially polarized deuterons. Other experiments are relyed on the availability of polarized proton or deuteron beams. S. Jaccard and his group is presently testing a low tem- perature source to reach an intensity of about 100 uamp [ 2 ].

The reaction Hd -» Hd

The group of Karlsruhe-SIN-Erlangen measured the vector polarization parameter iT1..

After a first positive signal, the' group presented more- precise and extensive results which weaken his conclusion on the existence of dibaryons [3]. However, "non conventional dynamics"

with D„ and G, resonances explain better the observed structure than the standard theories (see the contribution of M. Locher at this conference). These data should be extended in the near future by measurements using a deuteron target tensorially polarized.

The most clear evidence for the presence of a resonance at T„ = 134 MeV comes from the measurement of the tensor polarization T__ of the outgoing deuteron by the ETHZ-Auckland group (Fig.l) [4], This group performs a phase shift analysis on all available data which explain also the iT.. data. Of course the data are too scarce to allow a definite conclusion but a state 1 could possibly be attributed to the structure. If this is true, it could explain why this resonance is not observed in the pp-system. However, this experiment should be regarded with a serious caution since it contradicts the results of the same experiment performed at LAMPF. This discrepancy remains by now for at least two years. Both measurements were care- fully checked, calibrated and repeated and it is difficult to judge which one is the most credible. It is important to settle the problem in a near future. The Zurich group in now rebuilding the polarimeter which will allow the measurement of other tensor, polarization com- ponents of the deuteron.

Fig 1. Tensor polarization T„_ of the outgoing deuteron in the reac- tion H+d + H+d at Tfl = 134 MeV, as measured at SIN,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985250

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The reaction B + nd

The reaction pp + ~ + d was studied by the Geneva-SIN group (parameters AON, Ass. 'NN* %K

and A ) [5], the NeucMtel-SIN-Karlsruhe group (differential cross section and parameters AON and A SK ) [6] and by the Karlsruhe-SIN-Erlangen group (parameter iTI1) [7]. Among the differ-

SR . .

ent t eoretlcal works on this reaction, let us mention the relativistic calculation performed at SIN at 579 MeV using for the leading graph the exchange nucleon and a pion [a]. ^{'It was}

found that the pp spin-triplet waves are more important than expected but extra resonances are unnecessary (see also the contribution of M. Locher at this conference). At least four grc- ups fitted with success these data and the world data by a phase-shift analysis (cf.Fig.3) [ I ] . The general conclusion of these analyses is that the energy behaviour may reveal some resonance structure but again the data are insufficient to draw any firm statement. In fact, the scattering is described by 6 complex amplitudes which are related to the different observ- ables and clearly it is still necessary to measure some other kind of polarization parameters to fix experimentally the pp -t ll + d reaction. At (3 = '0 and 90' however the present data are almost sufficient to determine the non zero amplitudes [9]. Fig. 4 shows these amplitudes normalized by the square root of the differential cross section to suppress the "bell" shape observed in all the amplitudes.

Fig 2 and 3.A

-- and Ask param-

eters oggerved in the pp + lld reaction at 580 MeV (data from Geneva-SIN, black points, and from NESIKA, crosses). The curves result from the PSA fit of D. Bugg.

Fig 4 . Normalized ampli-

- -

tudes at 0 = 90° of the reaction pp -t nd.

Recently the Geneva group measured the polarization of the outgoing deuteron using the same apparatus employed for the study of the ppscattering. The deuteron is scattered on a target of 3 cm carbon. No attempt was made to separate the inelastic scattering behind the C-target but a single charged particle was required. The azimuthal distribution behind the

where for ex.

and cZC = -- A22-L22 (1 + A20.T20)

The carbon scattering is described by the deuteron polarizations and by the carbon parameters A20- A21* A22 and

"d,,

For the time being, these data are analyzed using a phase shift analysis which contains addi- tional parameters describing the carbon properties (fig. 5 ) .

E E P S - I C E P S - I S

EX?. S I N R-82-06

Fig 5. Asymmetries produced by the outgoing deuteron of the reaction pp + (Id as observed be- hind a 3 on carbon target and using a transversally polarized proton beam of T = 445 and 580

MeV. P

Spin dependent total Cross sections

The structure of q, first observed by the Argonne group in pp scattering, has been con- firmed by several experiments. The Geneva-Saclay- SIN results [9] are in good agreement with the data of Argonne, W Fand Saclay, but are snaller than the results of D. Bugg et al. at TRIXW. The SIN experiment neasured also the spin correlation parameter+ApK(pp -+ (fd) and AWKK(pp -+ pp at mall angle) but with a reduced accuracy. Combining these ata the previous spln correlation data for pp -+ pp [lo] and pp + (d'I [Ill, the individual components of the structure were found. The figure 6 shows in particular the A (pp -+ llNN) cross section and large discrepancies with the present theories were observed.

7,

The elastic scattering

-

-

The goal of the Geneva-SIN group was the direct experimental determination of the 5 coni- plex amplitudes of the pp elastic scattering at T = 4 4 5 , 4 9 0 , 515, 540 and 579 MeV. This

P

C2-436 JOURNAL DE PHYSIQUE

a

program is now completed and data at 579 MeV have been published [12]. Data are availablee at 445, 515 and recently at 540 MeV. The amplitudes have been presented at 445 MeV [ 1 3 ] and at 579 MeV [14]. The figure 8 shows the 3-spin parameters observed at 540 and 560 MeV [ 1 5 ] . This last energy was chosen in the context of the structure observed in the deuteron polariza- tion T20 of the q+d scattering at 134 MeV. No indication for a narrow resonance was found for the pp system in these scattering data and in other spin transfer parameters.

Ihe NESIKA group measured recently the asymnetry parameter P at 579 MeV with a high accu- racy [161. Data are in good agreement with the previous measurement.

PP-PP

0 . 5 : ..-.. MU*,"

0 . 3 -

0.1 -

- 0 . 1 -

560 MeV

Fig 8. The 3-spin parameters M and M measured at 540 and 560 MeV in the pp elastic wDsn wokn

scattering.

Outlooks.

Spin physics with the two nucleon system is well alive at SIN. More data are expected in the near future in all the above reactions, i.e., for the reaction ld'l -+ lT+d : measurements with a tensorially polarized target or experiments analysing the different polarization compo- nents of the outgoing deuteron, for the reaction pp + ll + d : a complete determination of the scattering amplitudes at T = 445, 515 and 580 MeV, for the reaction pp + 7 f pn : measurements of the spin transfer par%neters at T = 515 and 580 MeV, and for the elastic scattering pp +

pp : polarization data below 445 MeV here the world data are now relatively scarce and/or inaccurate in comparison to the high accurate and extended set of data produced recently by L4MPF and SIN above 445 MeV. It is somewhat unfortunate that TRIUMF did not consider serious- ly this lack of data between 100 and 300 MeV. It seems also important to study the npsystem with a degree of accuracy similar to that of the ppsystem.

By lack of time, other specific polarization exberiments like the parity violation meas- urement in ppscattering or the studies of the low energy reactions have not be reported here.

[ 11 K. Seth, Proc. INS Intern. Conf. on Photonuclear and Related Physics, Tokyo 1983.

[ 21 P.A. Schmelzbach et al., contribution submitted to this conference.

[ 31 G.R. Smith et al., Phys. Rev. C29(1984)2206.

[ 41 W. Gruebler, Invited talk to this Conference.

[ 51 E. Aprile et al., Nucl. Phys. A415(1984)365.

[ 61 J. Hoftiezer et al., Nucl. Phys. ~412(1983)273 and 286.

[ 71 R.G. Smith et al., to be published in Phys. Rev. C.

[ 81 W. Grein et al., SIN PR 83-08 and Annals of Physics 1984.

[ 91 E. Aprile et al., Nucl. Phys. A415(1984)391.

[lo] E. Aprile et al., in press in Nucl. Phys. A (19%).

[ll] E. Aprile et al., Phys. Rev. D28(1983)21.

[12] E. Aprile et al., Phys. Rev. D27(1983)2600.

[13] E. Aprile et al., SIN News letters no 15 (1983)NL19.

[14] R. Hausarmnann, Ph.D. Thesis no 2038, University of Geneva (1982).

[15] E. Aprile et al., Contribution to the 10th Intern. Conf. on Few Body Problems in Phy- sics, Karlsruhe, Germany (1983).

[lb] A. Berdoz et al., J. Phys. G.: Nucl. Phys. 9(1983)L251.