π± Elastic and inelastic scattering from 3H and 3He

N U C L EAR

Nuclear Physics A$$3 (1993) 585c-588c North-Holland, Amsterdam

PHYSICS

A

Elastic and Inelastic Scattering from 3H and 3He W.J. Briscoe a, D.B. Barlowb, B.L. Bermana, R.W. Caressa, K.S. Dhugaa, S.N. Dragic a, S.J. Greene c, D. Isenhowerd, D. Knowlesa, D. Maceka, S.K. Matthews a A. Mokhtari a, B.M.K. Nefkensb, N.J. Nicholas a, C. Pillaib, J. W. Price b, M.E. Sadler d, Ivo Slaus e, I. Supek e, M.F. Tarag n a aCenter for Nuclear Studies and Departmentof Physics,The GeorgeWashingtonUniversity, Washington, D.C. 20052 bDepartmentof Physics,The Universityof CaSfomiaat Los Angeles,Los Angeles, CaSfomia90024 cClinton P. AndersonMeson Physics Facility,LANL, Los Alamos,New Mexico 87545 dDepartment of Physics, Ahilene ChristianUniversity,Abilene,Texas79699 eRuder Boskovic Institute,Zagreb, Croatia Abstract We have measured the differential cross sections for the scattering of ~-+from 3H and 3He. The do's are normalized to ~+-d elastic scattedng measured under the same experimental conditions. Our newest measurements were performed at incident pion energies from 142 to 256 MeV and angles between 140° and 180 o. The momentum-transfer dependence of the cross sections was measured up to 11 fm -2, making it possible to explore differences in the matter form factors of 31-1and 3He.

1.

Introduction

Our program of cross-section measurements for n+-3H and ~+3He elastic scattering in the angular ran¢le between 40 ° and 110° in the region of the A I ~ resonance has yielded nte-resting resuts. 1,2 The most =mportant quantt'es measured are the superratio, R, the simple ratios, r 1 and r2, and the paired-unpaired nucleon ratios p+ and p_ (defined below). Deviation from unity beyond the Coulomb correction in R, r 1, and r2 indicates nuclearcharge-symmetry breaking (CSB). QCD implies a small intrinsic violation of charge ~ymmetry (CS) as a consequence of the up-down quark mass difference; this mass difference is responsible for a small CSB component n the N-N interaction via the mechanism of p--m mixing.3 A test of CS is made possible by measuring the superratio R = rl.r~ where r4 - dafn+3HI/da{n-3He) and r9 = da(n-3H)/da(rt+3He~. The superratio provid-es a test of CS independent of the absolute beam ca ibr,~don and of the effic=ency and acceptance of the pion detector FI s also equal to p+-p_, where p+ = da{rc+3HI/da(n+3He) and o = d~(rc-3H)/do(~'3He). If CS is valid, R is equal to unity at "eve@ angle and a t ' a l l energies after corrections have been made for e ectromagnetic effects n our experiments these are small. In pion ~cattering on ght nuclei the A resonance plays a major role, s3Decifically the A- in ~" H scattering and the A ++ in ~+3He. Quark-mode ca culations g v e a mass difference of about 5 MeV between the A ++ and the A', leading to a value of R ~ 1. i

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W.J. Briscoe et al. / ~r+ elastic and inelastic scattering,from aH alul JHe

Previous Measurements and Ca|culations

We have previously measured ~+ and n" scattering from 3H and 3He in the region of the 41232 resonance at EPICS, at Tn = 142, 180, and 220 MeV, for 61ab between 40 ° and 110o. The objectives of these experiments were to probe the nature and extent of CSB as manifested in the deviation from unity of R and to make a preliminary comparison of the ~H and UHe paired and unpaired nucleon form factors in the vicinity of the non-spin-flip dip (near 78 ° in the laboratory system). Some explanations for the deviations of R from unity are: i) dG(~*p) ~ dG0c'n), which is expected if Z~*+ and A- h=ve different masses; ii) F(UH) ~ F(UHe), where F is the matter form factor---such a difference could in turn be the consequence of different coupling constants, g(ppn o) ~ g(nnno); iii) a possible CS-violating 3-body interaction; and iv) various combinations of the above. Several model calculations for our results have been made. 4-7 Some of these succeed qualitatively in the forward hemisphere, but their predictions for backward scattering differ greatly from our findings. Kim, Kim, and Landau8 have explored the effect of a Coulomb di=tortion of the nuclear force on R. While this calcuTation shows the necessity for including the Coulomb distortion, it does not fully reproduce our results; the peak in R is predicted at a larger angle than determined by the data. Gibbs and Gibson 9 have studied ~+ and ~ elastic scattering from 3H and 3He. They argue that because the n-p force is slightly more attractive than the n-n or p-p force, the proton radius of 3H shoul~ be smaller than the neutron radius. Their Faddeev calculations yield a difference of about 0.16 fm. In the absence of the Coulomb interaction (CI) between the two protons in 3He, the 3H and 3He systems would be identical. Including the CI in the Faddeev calculations leads to an increase in the proton radius of 0.03-0.04 fm in 3He. The repulsive CI of the two protons means that the neutron is less bound. That is, the neutron distribution is also expanded, and the neutron radius is increased by 0.02-0.03 fm. Because 3He is larger than 3H, its form factor decreases faster with momentum transfer. Therefore, they predict that R > 1.

3.

Experiment

In order to differentiate among the above, we need to know R at various pion energies and angles. We probed the energy region of the • resonance, spanning the angular range from 40 ° to 173° . Our most precise measurements are those of ph+~ and p. from which we calculate R = p+.p.. We obtain r 1 and r2 by calibration of relative beam intensity using ~+-d elastic scattering, lo We assume that d o t # d ) = da(tcd), see Ref. 11. The experiment explores the full backward hemisphere at Tn = t 80 MeV. We also measured an excitation curve near 180 ° from Tz¢ = 142 to 256 MeV; at the latter energy -t = 11fm -2. At 180° the cross section is due entirely to non-spin-flip scattering; it provides a rare opportunity in nuclear physics to determine one of two scattering amplitudes without the use of a polarized target. We used the standard 160° scattering setup at EPICS but added a turntable ~o. position the.large target cells. We used target cells that were cylinders 5" in iameter and 9 high made of special aluminum (2024-T3511) with a small diffusion coefficient for tritium and a high tensile strength (65,000 psi). The walls of the cylinders are 0.073" thick and were tested up to 700 psi with helium and hydrogen. The target gases were H2, D2, T2, and 3He. The typical pressure in the cylinders was 450 psi. Not only were the pressure, vclume, and temperature measured, but arso the weight of each cylinder was measured accurately both before and after each experimental running period.

W.J. Briscoe et al. 1 lr + elastic and inelastic scauering from 31-1and 3He

4.

58'1c

N e w Results

The results of our latest measurements are shown in the figures. Figure 1 shows our complete angular distribution at 180 MeV for each of the tour measured cross sections. There is good agreement with the calculations by Gibbs and Gibson9 for x+T over the entire angular range and for ~ T in the forwaiJ hemisphere. The peculiar rise in the backward hemisphere in ~-T has also been s~en in x4He scattering and is not well understood. Figure 2 shows the superratio R at 180 MeV, again compared to the calculations of Gibbs and Gibson. The data are well represented by the middle curve, which corresponds to a difference of 0.03 fm in the radii of the paired nu~,leons in UH and UHe. We note that R is greater than one at all angles measured. 10=

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W.J, Briscoe et al, / ~r+ elastic alul inelastic scattering from 3~ atul 3He

588c

We have also obtained new data in the region of the non-spin-flip dip. The excitation function of the superratio in this region, where scattering is predominantly spin-flip, shows an interesting result: R decreases as the incident pion energy increases and, in fact, becomes less than one above 200 MeV, Figure 3. Our p+ data, see Fig. 4, show a striking difference between elastic (open boxes) and inelastic (closed boxes) scattering 1.0 - 10 MeV above breakup. This difference is due to the suppression of the spin-flip contribution in the elastic channel on the paired nucleons. Thus, measurement of the ratios p+ and p. for both the elastic and inelastic channels provides a powerful, yet simple, way for measuring the spin-flip contribution to ~'3H scattering. Also, the comparison of the superratio for elastic scattering with that for inelastic scattering is a.useful probe, among other things, it is sensitive to possible electromagnetic effects ,n R. t=5

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5.

References

1

B.M.K. Nefkens etaL Phys. Rev. Lett. 52, 735 (1984); and B.M.K. Nefkens etaL Phys. Rev. C, 41, 2770 (1990). C. Pillai et aL Phys. Lett. ;ZQ7B,389 (1988); and C. Pillai et aL Phys. Rev. C, 43, 1838 (1991), and references therein. B.M.K. Nefkens, G.A. Miller, and I. Slaus, Comments Nucl. Part. Phys. 20, 221 (1992); and G.A. Miller, B.M.K. Nefkens, and I. Slaus, Physics Reports 194, 1 (1990), and references therein. S. Barshay and L.M. Sehgal, Phys. Rev. C31, 2133 (1985). Y.E. Kim, Phys. Rev. Lett., 53, 1508 (1984). Y.E. Kim, M. Krell, and L. Tiator, Phys. Lett. B17~, 287 (1986). C. Werntz and F. Cannata, in The Three-Bodv Force in the Three- Nucleon Svstem, eds. B.L. Berman and B.F. Gibson (Springer-Verlag, Heidelberg, 1986), p391~ Kr.T. Kim, Y.E. Kim, and R.H. Landau, Phys. Rev. C, 36, 2155 (1987); and W.J. Briscoe and B.H. Silverman, Phys. Rev. C, 39, 282 (1989). W.R. Gibbs and B.F. Gibson, Phys. Rev. C 43, 1012 (1991). K. Gabathuler et aL, Nucl. Phys. A350, 253 (1980); and SIN Progress Report (1986). G. Smith eta/: Phys Rev. C, 38, 240 (1988).

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