SPIN EFFECTS IN pp → π+d REACTION AT INTERMEDIATE ENERGIES

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SPIN EFFECTS IN pp → π +d REACTION AT INTERMEDIATE ENERGIES

T. Bhatia

To cite this version:

T. Bhatia. SPIN EFFECTS IN pp→ π+d REACTION AT INTERMEDIATE ENERGIES. Journal de Physique Colloques, 1985, 46 (C2), pp.C2-375-C2-386. �10.1051/jphyscol:1985243�. �jpa-00224557�

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30URNAL DE PHYSIQUE

Colloque C2, supplement au n°2, Tome 46, fevrier 1985 page C2-375

SPIN EFFECTS IN p p •+ 7r+d REACTION AT INTERMEDIATE ENERGIES

T.S. Bhatia

Texas ASM University, U.S.A.

Résumé - Présentation de mesures expérimentales récentes de la réaction pp -*• TT d jusqu'à environ 1 GeV.

Abstract - Recent experimental measurements for the reaction pp + u d up to about 1 GeV are reviewed.

The experimental study of electroweak interactions is aided by a well established theory which makes definite quantitative predictions. Such predictions can be tested experimentally. If verified, the experiment lends additional credence to the theories; if not, it often leads to important modifications. This process has worked extremely well for the electroweak physics. Theory and experiment have complemented each other, and there have been tremendous advances in this field in the past decade or so. The search and discoveries of Z and W particles are obvious examples.

The course of strong interaction physics has been quite unlike that of electroweak interactions mainly due to the absence of an applicable theory. The progress in strong interaction physics has been largely through "surprises." The absence of a "theory" means that there are fewer predictions available to check.

An experimental program aided at looking for "surprises" is sometimes difficult to pass the scrutiny of the present day program advisory committees. Obviously, this does not mean that theory must always come first. Major advances in our understanding of physics have come about as a result of experimental discoveries that were not predicted. Successful attempts to incorporate those experimental results led to better understanding. So the essential point is that experiment and theory must go hand in hand. For the case of strong interactions, this has not always been so. A case in point: substantial spin effects have been found, first at the ZGS1 up to ~12 GeV/c and more recently at BNL2 up to ~28 GeV/c. To date, there is no explanation of the data. A successful theoretical modeling of these surprisingly large spin effects could have pointed the way to further experimentation.

The theoretical situation concerning strong interaction physics has improved somewhat in the past few years. Now, in principle, we do have a theory of strong interactions — QCD. Perturbative QCD calculations have become available. There have been questions, however, concerning the applicability of such perturbative QCD calculations to the presently feasible experiments.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985243

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Even in the absence of an applicable theory of strong interactions, there are QCD models that do make quantitative prediction^.^ For example, the bag models and the coupled channel calculations all suggest a set of 6-q states in the mass region of 2-3 G ~ v / c ~ . There are possible problems with such predictions. For the bag models, there is uncertainty about the bag radius.

For the case of coupled channel calculations, there is a corresponding matching radius, the value for which is not available. This parameter makes the values of the masses for such predicted states uncertain. If, however, a single dibaryon state can be established, thfs would go a long way towards establishing the value of this radius parameter. This, in turn, would set the mass scale for the possible 6-q structures. The spacing between the predicted dibaryon structures is an important characteristic of these model calculations. Any model consistent with the N-A mass splitting leads to a separation of about 70 MeV in the 'so and 3 ~ 1 masses due only to the color-magnetic ~plitting.~ A search for the 3 ~ 1 structure in np after a 'so structure is established in pp will serve as an important check of the model.

The expectations are, that in the few GeV range, the underlying quark gluon basis will lead to observable 6-q structures (dibaryons), if not in the direct observables themselves, then in the parameters involved in their theoretical description. One can realistically accept the view that direct manifestation of these intrinsic designs of nature seem somewhat unlikely to appear in total or differential cross sections. In order to extract the key partial waves, experiments with polarized beams and polarized targets are vital. Many such experiments have already been completed.

What can be concluded from the data so far? The recent revie4 by the Particle Data Group (April 1984) concerning dinucleon resonances states, "In summary, this reviewer feels that the evidence, both experimental and theoretical, for the and 3 ~ 3 dinucleon resonances is now very strong." On the other hand, many people working in thfs field, including some present here, would disagree with the above vivacious "feelings."

For Isospin T = 1, the important inelastic channels for study are

+ + + + +

n d + n d, pp + n d, and pp or n d + npn

.

These three channels are known to couple. Therefore for a complete understanding, one must have good data on all of the above channels. Of these three, pp + n d has been the subject of intense

+

experimental and theoretical studies. The reason for that is primarily the ease with which such experimental studies can be done, this being a two-body final state consisting of charged particles. This is not necessarily the most important pion production channel as is often stated in many published works.

Figure 1 shows the total cross sections for pion production processes as a function of incident proton kinetic energy. Up to -500 MeV pp + dnf is comparable to pp + npn+ but for higher energies, pp + npr+ is clearly the dominant pion production mode.

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Fig. 1. Single pion production cross sections.

300 500 700 900 Tp ( MeV)

A brief comment on the npn+ channel is in order. A, k L ,

ANN,

^andASL have been measured at 650 and 800 MeV at LAMPF.~ The experiment covered a limited phase space. A TRIUMF experiment7 at lower energies yielded more extensive results but with rather large statistical uncertainties. More measurements on this important channel are planned at SIN and LAMPF. One experiment presently on the floor at L A M P F ~ is n d

+

+ npn+ (no spins), while another, +p npn+ is expected to run at LAMPF in 1985. The latter is designed to be an exhaustive study of this rea~tion.~

+ + +

The other two reactions, namely, pp + n d and n d + n d are intimately connected. Since the last review by Jones,lo there has been a considerable addition to the data base from SIN, LAMF'F, GATCHINA, AND SATURNE. There has been a corresponding increase in the theoretical work

--

most notably in the partial wave analyses of the data. We are also becoming aware of the deficiencies of the theoretical treatments. In spite of its low cross section, the reaction pp + n

+

d is important. I have already mentioned the relative simplicity from both the experimental and theoretical points of view. In addition to A and Aij, this reaction also permits the experimental measurements of vector (i'rll) and tensor (t20) polarization of the deuteron. This is an important consideration. These parameters have already provided much excitement and much controversy. One last point: the low cross section for this reaction may actually enhance the possibilities for the search for narrow dinucleonic structures because of general reduction in the overall background?

SIN has-played a leading role in the study of the spin dependence for the reaction pp + r

+

d. One of the notable achievements of this group at SIN was the

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C2-378 JOURNAL DE PHYSIQUE

complete experimental determination of the six complex amplitudes for this reaction.ll Unfortunately, the top proton energy at SIN is 580 MeV, which gives a maximum of 2.14 G ~ V / C ~ for the dinucleon system. The reactions involving nd scattering at ~ 1 ~ 1 2 extend this mass range to 2.3 G~v/c*--equivalent proton beam energy of -940 MeV.

We shall briefly review the nd measurements in what follows.

iTll t20 polarization measurements

Extensive measurements of iTll and tZ0 now exist. Boschitz and his coworkers have made comprehensive measurements of iTll up to -300 MeV pion energies at SIN using a polarized target. Their earlier measurements13 showed strong oscillations, which could not be understood within conventional wisdom.

This oscillatory behavior reinforced the earlier speculation concerning dibaryons in this energy range.14 New measurements by the same group do not show the rapid changes seen earlier. There is no longer any reason to postulate nonconventional dynamics. Preliminary results from LAMPF for the deuteron polarization from the pp + n d reaction have now become available.15 This

+

parameter P is related to iTI1. The measurements of P and iTll are shown in Fig. 2, which shows the apparent disagreement. It is too early to comment on this apparent disagreement since the LAMPF analyses is not yet complete. New analyses by has shown that the SIN iTll data (510

-

940 MeV nucleon energy) has significantly improved the determination of the nd amplitudes. Bugg suggests that high accuracy measurements of KSS, the transfer of vector polarization from proton to deuteron in the sideways direction, and ASL data in the backwards hemisphere at LAMPF energies, are needed. AS,, is now available at 500, 650, and 800 MeV.17 A combination of KSS and KSL has also been measured at LAMPF, although the data does not appear to be of sufficient15 precision.

p p -. dmf Induced Vector Polarization

0.4 4

.

^lb*

..

. n t a r a t

gdthdd

0.3

-

^-^I⁽^-

-0.1 - Fig. 2 . The preliminary LAMPF

measurements of P com-

-02 0 pared with iTll values.

0 l O Z O 3 0 4 0 W Q 7 O Q I I

d Center o t Mass Angle

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The measurement of t20 is another story. The two groups

--

one at SIN1*

and one at L A M P F ~ ~ have not been able to resolve the discrepancies between their measurements. There is no obvious reason for doubting either of the two results. Both groups have confirmed their own measurements. Figure 3 shows the representative data. There is an obvious need for independent new measurements.

Recently a TRIUMF group has calibrated their new deuteron polarimeter with the polarized deuteron beam at the Texas A&M Cyclotron, a necessary first step towards this measurement. ~oschitz20 plans to use a polarized deuteron target at SIN for measuring t20. The relevant component of the target polarization is expected to be small, however.

Q PRESFNT WORK GRUE~LER at al.

f r ?

Fig. 3. Comparison of LAMPF measurement labelled Present Work (Holt et al.) and the SIN results (Gruebler et al.) for the t2,, parameter.

nd + pp differential cross sections

Extensive new data for n d 4- + pp differential cross section has come from

GAT CHINA.^^ for the pion kinetic energies in the range of 280

-

^{450 MeV}

corresponding to 847

-

¹¹⁸⁷MeV equivalent nucleon energies. This represents a significant extension of the data base above LAMeP energies. New phase shift analyses22 from Gatchina gives a remarkably good fit for A, all,.the way up to -1 GeV but fails completely for Am above 500 MeV.

Spin Correlation Parameters, ANN,

ALL

^{and ASL}

LAMPF data for Am, ALL, and ASL for pp + nd in the energy range of 500-800 MeV have now been completed. The analyzing powers ANO and %N are obtained from the ANN measurements. This set of new data is shown in figures 4-8 along with the current partial wave analyses and some theoretical calculation^.^^ The

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angular distributions for ANN were obtained for 500, 600, 650, 733, and 800 MeV.

The angular coverage for this phase of the experiment was 30' < O,(c.m.) <lloO.

These results together with the results from another phase of the LAMPF experiments in which ALL and AS= were measured for the same reaction at the same energies, can be used to extract the energy dependence for the moduli of one singlet and two triplet amplitudes for 90' c.m.

The values of ~~~(90') and ~~~(90') given in Table I were obtained by polynomial fits to the measured angular distributions at each energy. The errors given reflect the uncertainties for the individual points near 90'. The

TABLE 1

Experimental Results and the Singlet and Triplet Amplitudes deprived from them Tp(MeV)&

PkO

IT2( IT6[ A(S,T6)(degrees)

Fig. 4 . Analyzing power at 90°c.m.

compared with calculations as labelled.

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predictions of a recent PWA and also the results of two theoretical calculations for AN0(900) are shown in Fig 4 along with the experimental results. The data base for the PWA includes the LAMPF data ( the only spin correlation data above 590 MeV). The PWA gives good agreement but an earlier PWA without the LAMPF data in the data base gave rather poor fit above 600 MeV.

In the latest PWA solution the l ~ q partial wave continues to be dominant and peaks near 570 MeV, while the 3 ~ 3 has a broad peak centered near 640 MeV.

While the PWA analyses give a fair description of these data, there is considerable disagreement with the theoretical predictions.

The moduli of the singlet and the two triplet amplitudes at 90' are completely determined by ALL(900) and ~~~(90') along with the 90' cross-sections. The values of the moduli obtained from the LAMPF measurements are given in Table I, and compared with the predictions of Blankleider and

~ f n a n ~ ~ in Pig. 5 The predictions overestimate the singlet and underestimate the triplet peaks. These peaks are expected from the partial wave feeding an S-wave NA intermediate state which has a threshold near 575 MeV, and from the five NN triplet waves feeding the P-wave NA intermediate states with thresholds near 650 MeV.

Fig. 5. Energy dependence of the moduli of the singlet and two triplet amplitudes at 900 c.m. SIN data (crosses) is also shown. Solid curves are from Blank- leider and Afnan (ref. 23).

The theoretical treatment due to ~iskanen2~ differs from that of Blankleider and AfnanZ3 but they both fail to treat spin relativistically.25 Blankleider and Afnan treat the P I 1 nN amplitude as a combination of a pole and

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C2-382 JOURNAL DE PHYSIQUE

a nonpole part whereas ~ a r c i l a z o ~ ~ had moderate success reproducing the vector and tensor polarization data by using an empirical Pll amplitude. Finally, in view of Foroughi's claim26 that the magnitude of T6 is not affected by a possible coupled triplet P dibaryon but the magnitude of T2 is, these measurements along with vector polarization results provide the experimental input necessary to distinguish between threshold effects,and resonances.

We now turn to the ALL and ASL measurements. Once again it should be noted that some of the first measurements of this type were indeed performed at SIN.

However the SIN experiments were confined to energies below 590 MeV, corresponding to invariant mass below 2148 MeV, not the energy region of greatest interest. The first measurements of ALL and ASL above 582 MeV are from the recent _LAMPFexperiments covering the energy range 500-800 MeV (invariant mass 2112-2241). Figure 6 shows ~ ~ ~ ( 8 ) for three energies along with PWA fits

A s L ( ~ )

Tp = 500 MeV

-0.4 -0.6

Fig. 6. Angular distributions A SL(O) compared with calculation from Blankleider and Afnan (Ref. 2 3 ) , Lyon group (Ref. 28) and

Hiroshige's PWA solution (Ref. 27).

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and theoretical predictions. Angular distributions A ~ ~ ( B ) are shown in Fig. 7.

Spin-dependent cross sections obtained from the measured ALL data are shown for three angles in Fig 8. The characteristic feature of the data is a broad peak near 600 MeV for aSM and near 670 MeV for a T

.

These shapes can be understood qualitatively, without invoking dibaryone, as arising from threshold effects in the intermediate NA S state for singlet transitions and in the intermediate NA P state for triplet transitions. PWA fits2' are quite good. This is not surprizing since this data is in the data base. Theoretical calculations*3 28 without dibaryons, do quite well for the singlet case but fail for the triplet case.

The Lyon group28 used relativistic kinematics for all of the particles, whereas Blankleider and Afnan treated only the pion kinematics relativistically.

The handling of the intermediate pion was not truly relativistic in either calculation, since virtual pions going backwards (in time) were not included.

The heavier vector meson exchange diagrams were not included. It is quite possible that refinements of these calculations may lead to better agreement for the triplet case also. The need for hypothesizing dibaryons to explain the data is not yet clear. The only conclusion that can safely be drawn at this stage is that more complete and reliable theoretical calculations are needed.

1

⁶⁰⁰^{M e V}

4 -

-

/ 7 0 0 M e V

-'-.

+

**tkvGy:**

/ t r

. .*

, ?

40' 50' 60' 70- 80.

Fig. 7. Angular distributions

-

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JOURNAL DE PHYSIQUE

I

' eA -

^5b0

^'

- -

L

-

I I l I I I

I 17001 I

-

. - -

I I I I I I

I I lp0ol I I

- -

,--..

I I I I I I

0.0 0.0

500 600 700 800 500 600 700 800

Tp (MeV) Tp (MeV)

Fig. 8. Spin dependent cross sections as a function of energy.

A for pp + dn upto 2.3 GeV

+

Angular distributions for the analyzing power A for $p + IT

+

d reaction have been measured up to -2.3 GeV proton kinetic energy at SATURNE. The preliminary data suggest a back-angle peak at -1.9 GeV. This is a possible candidate for the 6-q, 'so structure predicted by the cloudy bag model near this energy (2.66 G ~ v / c ~ ) although the width appears to be somewhat larger than predicted.4 An extremely important test for the 6-q-state interpretation could come from the 3 ~ 1 structure for np scattering near 1.7 GeV kinetic energy (2.59 G~v/c~).

Conclusions

There are important unresolved discrepencies for the tZ0 measurements.

These need to be resolved by additional measurements, preferably by totally new

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experimental efforts. Spin measurements above 800 MeV may prove to be extremely useful.

Extensive new spin data have now become available upto 800 MeV. To be sure, not all of the data can be satisfactorily explained by theory. But since the theoretical efforts are known to have easily identifiable deficiencies, it is not prudent to postulate exotic phenomenon in this energy region.

REFERENCES

See for example, A. D. Krisch, Proceedings of the Third LAMPF I1 Workshop, LA-9933-C, July 18-28, 1983, p. 280.

R. Raymond, et al, Conf. on the Intersections between Particle and Nuclear Physics, Steamboat Springs, Colorado, May 23-30, 1984; K. Brown, et al, to be published.

P. J. Mulders, Phys. Rev. m ,3039 (1982). E. L. Lomon, Phys. Rev. m , 576 (1982).

E. L. Lomon, 10th Int. Conf. on Particles and Nuclei, July 30-August 3, 1984, Heidelberg.

Review of Particle Properties, Particle Data Group, Reviews of Mod. Phys. 56, S290 (1984)

T. S. Bhatia, et al, Phys. Rev. C g 2071 (1983).

R. Shypit, et al, Phys. Lett.

124B

314 (1983).

Gordon Mutchler, private communication.

LAMPF Expt. 815, D. Bugg, spokesman.

G. Jones, AIP Conf. Proc. 3, 15 (1982). Also Nucl. Phys.

m,

¹⁵⁷

(1984).

E. Aprile-Giboni, et al, Nucl. Phys.

e,

391 (1984).

G. Smith, et al, KFK Preprint 84-2, June 1984, submitted to Phys. Rev. C.

J. Bolger, et al, Phys. Rev. Lett. 46, 167 (1981). J. Bolger, et al, Phys. Rev. Lett. 5, 1667 (1982).

W. Grein and M.P. Locher, J. Phys. G.

7,

1355 (1981). M. P. Locher and M. G. Samio, Phys. Lett.

m,

227 (1983).

S. Turpin, et al, to be published.

D. Bugg, private communication and to be published.

G. Glass, et al, Preprint, submitted for publication.

For a recent review see W. Gruebler, Conf on the Intersections between Part. Nucl. Phys., Steamboat Springs, Colorado, May 23-30, 1984.

R. Holt, et al. Phys. Rev.

67

472 (1981).

E. Boschitz, private communication.

M. J. Borkowski, et al, 10th Int. Conf. Particles and Nuclei, July 30-August 3, 1984, Heidelberg.

A. V. Kravtsov, et al, Preprint 963, July 1984, Leningrad, to be published.

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C2-386 JOURNAL DE _PHYSIQUE

23. B. Blankleider and I. R. Afnan, Phys. Rev. C* 1572 (191).

24. J. A. Niskanen, Phys. Lett. 79B 190 (1978), 17 (1982).

25. H. Garcilazo, Phys. Rev. Lett. 52 652 (1984).

26. F. Foroughi, J. Phys. 6 8 1345 (1982).

27. N. Hiroshige e t al, Prog. Th. Phys. 8 2074 (1982), and N. Hiroshige, private communication.

28. T. Mizutani et al, Phys. Lett. 177 (1981).