Comments on np elastic scattering, 142–800 MeV

NUCLEAR PHYSICS A

Nuclear Physics A540 (1992) 449-460 Norh-Holland

Comments on np elastic scattering, 142-800 MeV D.V. Bugg Queen Mary and Westfield College, London EI 4NS, UK R.A . Bryan Texas A & M University, College Station, Texas 77843-4242, USA Received 3 June 1991 (Revised 29 January 1992) Abstract : Important new data of McNaughton et al. on np Wolfenstein parameters are added to NN phase-shift analysis . At 800 MeV, there is a dramatic improvement and one can see with confidence which way phase shifts aye heading from 500 to 800 MeV. Dispersive effects in 3Dt and 3 G3 herald the onset of 1 = 0 inelasticity . Phase shifts account naturally for the energy dependence of d aL and d QT for np scattering and do not support the claim of Beddo et al. for an 1 = 0 dibaryon resonance near 733 MeV. 1 . Introduction Extensive and precise data on Wolfenstein parameters at 788 MeV have been reported by McNaughton et al. 1 ) . These authors have also communicated privately new data at 484 MeV. All these results are added to current phase-shift analysis. Although there are no real surprises, the 788 MeV data reveal considerable detail and clarify greatly what is happening to I = 0 amplitudes between 500 and 800 MeV. This is an important energy region, approaching the I = 0 inelastic thresholds due to NN* (1420) and dd . The 484 MeV data confirm the previous analysis centred at 515 MeV and improve the accuracy of phase shifts somewhat. The new trend in 3 G5 prompts a fresh look at its behaviour below 350 Me:°V and leads to some clarification. Comments on the results fall into two classes. Sect. 2 discusses practical technicalities and sect . 3 the physics results. sect. 4 gives the conclusions. The technicalities mainly concern three points : (a) neutron beam polarisation, hence the normalisation of spin dependent data, (b) how many partial waves to free in the analysis, i.e. above what J-value to rely on pion exchange (OPE ), (c) in which I = 0 partial waves to allow inelasticity. The last of these derives clues from dispersive effects in the results themselves, i.e. it involves a riicaimentary form of analyticity constraint . Correspondence to: Prof D.V. Bugg, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX 11 OQX, UK. 0375-9474/92/$05 .00 OQ 1992 - Elsevier Science Publishers B.V. A,11 rights reserved

450

1). V Rugg, R.A . Bryan / np elastic scattering 2. Technicalities

2.1 . BEA

LARISATION

The new data establish that the neutron beam polarisation at LAMPF is - 13% higher than experimentalists had previously believed . This calibration error was predicted in a previous phase-shift analysis'- ) . The earlier calibration was based upon ANO data of Newsom et al 3); this has often been used to monitor beam polarisation via scattering near 120° centre-of-mass, where ANO reaches a maximum. The new measurements determine beam polarisation instead from KLL at 0° in p(d, 2p) n. Call this K for brevity. Measurements of elastic Wolfenstein parameters W determine an asymmetry K x W; in the phase-shift analysis K needs to go up as W goes down, and vice versa . Our analysis includes this inverse relation, using a simple method discussed in ref.' ). Results of the fit are given in table 1 . The difference between KLL for free np charge exchange and for the p(d, 2p) n reaction is a consequence of the ' So final-state interaction in the latter and the Pauli principle. It agrees quantitatively with the formalism of Bugg and Wilkin 4). (The difference near 800 MeV between their predicted correction of 0.223 and the correction of 0.166 in table l arises from changes in 1 = 0 phase shifts). 2.2. HIGH PARTIAL WAVES

The second technicality concerns which phase shifts to free in the analysis . Table 2 shows the difference in high partial waves between a free fit at 800 MeV and single pion exchange (OPE) . Where experimental error is less than this difference, it is desirable to determine the phase shift by fitting. The OFE predictions were made using g2 = 14.28 for charged pions 5 ) and g2 = 13.65 for neutral pions6 ) . Because of the different masses of charged and neutral pions, this leads to OPE amplitudes which are within about 2% of charge independence in the t-range where they are important . namely t = 0 to -M2 . Recently, there has been speculation' ) about a lower value of g2 for charged pions, but this is still a contentious issue which we do not wish to address in this paper. Because we free high partial waves as far as the data allow, this question has little relevance to our phase shifts. Changing TABLE 1

Parameters concerning neutron beam polarisation . Errors are in parentheses. KLL (0° ), p (d, 2p) n KLL (0° ),np--> pn KLL (0° ), p(d, 2p) n KLL (01 ), np --> pn

Energy(MeV)

Experiment

Fit

788

0.720(^.0á7j

_43(0.005)

484

0.58

0.577(0.005) 0.581(0.006) 0.447(0.007)

D. V. Bugg, R.A. Bryan / np elastic scattering

451

TABLE 2 Differences in high partial waves between a free fit at 800 MeV and OPE. Units are degrees and errors are in parentheses. The prediction for the difference is from the impact parameter prescription . H-, land K-waves have respectively L = 5, 6 and 7. Wave

Experiment

OPE

F5

4 .860 (0.105 )

5 .322

-0.462(0.125)

-1 .09

-2 .554(0 .159)

-2 .332

-0 .222(0.162)

-0.49

-3 .340(0 .118)

-2 .526

-0.814(0.122)

-0.85

5.254(0 .160)

5.272

-0 .018(0 .173)

-0 .07

3 17

-0.744(0 .120)

-1 .234

0 .490(0 .121)

0.42

F6 3K6

-0.956(0 .042)

-1 .164

0.208 (0 .045)

0.11

0.418 (0 .039 )

0.473

-0 .055(0 .039)

-0 .10

315 I H5 31 6

1 16 3 K7 3Ks

Difference

Prediction

1.242 (0 .047 )

0.604

0.638 (0 .048)

0.63

-0 .957(0.069)

-1 .168

0.211(0 .071)

0.26

0.496 (0.021)

0.267

0.229 (0 .022)

0.26

2 9 for charged pions to 13 .65 alters the X2 of our fits, summed over the 6 energies we consider, by only +4 .73, an amount we consider to be insignificant. The "predictions" in table 2 for non-OPE contributions are rather rough and are derived from the classical notion of an impact parameter:

k2r2 = L(L + 1)h2 . It is assumed, for example, that I-waves (L = 6) at high momentum k, with a certain impact parameter behave like G-waves (L - 4) at lower momentum k2 but the same impact parameter: ; ~ 6 x7kt at the lower of these energies, non-OPE contributions can be taken from the Paris potential. This prescription has been discussed and illustrated by Dubois et al. 8 ). It accords tolerably well with experiment for most high partial wave. it wiii be used here in two ways: (a) as a qualitative check that phase-shift solutions are sensible, (b) to predict deviations from OPE in high partial waves where these deviations are smaller than experimental errors, e.g. 1-waves at 515 MeV. k2-

2.3 . INELASTICITY

A third point concerns I = 0 inelasticity . Up to 800 MeV, this is not well known. Yrom pd measurements, Dakhno et al. 9 ) derive (Fine, ( I = 0) = 2 f 1 mb near 800 MeV, while Andreev et al. 1° ) report results consistent with zero up to this energy . Bystricky et al. " ), however, derive values of Qine, (I = 0) rising rapidly between 600 and 800 MeV to 5 mb at the latter energy. In view of these conflicting estimates, no constraint is imposed in the present analysis on Qine, (I = 0) . A good measurement of

R.A. Baan / np elastic scattering

this quantity is the one which would now be of greatest assiç`'nce to np phase-shift analysis. We do include in the fits (I = 1) of Shimuzu et al. 12) . The next question is in which partial waves to expect I = 0 inelasticity, if any. Shortly above 800 MeV, NN* (1420) is expected to couple initially to 3S, and/or 3 DI ; this channel opens at a lab kinetic energy of 1087 MeV, though the width of N* (1420) will broaden the threshold. Likewise AA is expected to couple initially to 3D3 and 3 G3We find that the fit also suggests inelasticity in 1 P, at 800 MeV. An experimental clue to the importance of the inelastiçfy arises from dispersive effects: for partial wave amplitude fL,J 1 Re fL,, (s) = Pole terms(s) + n

Im ƒ , (s')ds'  L s' - s

31

and the real part of the amplitude rises before the inelastic threshold. (This is a wellknown effect for I = 1 ' D2, 3 F3 and 3 P2) . The phase shifts reported here (fig. 1) display little or no such dispersive effects for 3 S, and 1 F3, a strong effect for 3D,, a weak but definite effect for 3 G3 and weak or non-existent effects in 3 D3 and 1P, . The anomalous behaviour of 3G5 is discussed below; inelasticity in this wave at 800 MeV would be surprishig and the fit rejects it strongly. We parametrise singlet S-matrix elements as S = cost p exp (2ib). Triplet coupled waves are parametrised according to the Arndt-VerWest prescription : S11 = COS2 p_ cos 2Z exp (2ib- ) ,

S,2 = S21 -_ i cos p_ cos p+ sin 2è exp i(b_ + & + 0) , S22 = COS2 p+ cos 2î exp (2ib+ ) .

Below the inelastic threshold, the parameter 0 is zero. Above threshold, it remains small, providing contributions to S-matrix elements from mixing are significantly larger than those due to inelasticity. Under these circumstances the K-matrix is nearly real and therefore 0 must be small. This is the case for all J except J = 2. For example, at 800 MeV, where the data base is very extensive, a free fit leads to 0 1 = -8*, 03 = 1 .5° and ®~ = -5°, with errors of 8-10° in each case. The small improvement of 6.02 in X 2. is an amount we judge to be insignificant for three extra parameters. Rather than allow this unnecessary freedom, we set 0 = 0 for all these waves, but we do include an allowance for this uncertainty in 0 in all other fitted parameters . For J = 2, the situation is quite different. Here, i~2 is small and inelasticity is large. In this case, 02 fits close to 0 up to 500 MeV, then rises rapidly through -45° at 650 MeV as inelasticity overtakes the mixing amplitude ; 04 settles to -90° at and above 800 MeV, with errors typically ±5" . Fitted values of p and inelastic cross sections at 800 MeV are shown in table 3. Inelasticity in 3D3 and 3G3 improves 72 at 8001 MeV by 48.3, a significant amount. However, values of p are strongly correlated, par.icularly between 1 P, and other waves. For this reason, errors, which are 20-40% of p, are at shown in 'table 3.

D. V. Bugg, R.A. Bryan / np elastic scattering

45 3

Laboratory Kinetic Energy (MeV) Fig. 1 . Phase shifts from this analysis. The dashed curves are from OPE. The full curves are to guide the eye and are used to calculate the full curve in fig. 2 . 2.4. ENERGY DEPENDENCE In fitting data, we allow for a linear variation of each phase shift with lab kinetic energy about the central energy of each fit. The gradients are obtained from values given by the smooth curves of fig. 1 . 3. Physics results After these technicalities, we turn to the physics. Phase shifts are given in fig. 1 and tables 4-6. Phase shifts for I = 1 have changed little from ref 2 ) . Coulomb barrier corrections are identical to those given there in Table 10. Although the ArndtVer'vdest formalism does not guarantee unitarity, we have checked that all coupled phase parameters in the tables satisfy the unitarity condition.

454

D. V Bugg, R.A. Bryan / np elastic scattering TABLE 3

Elasticities and np inelastic cross sections at 800 MeV. Wave 1 P, 3 D, 3 D3 3 G3

Total

p 20 .06 13.44 15.79 14.06

Q1ItC1

(mb)

0.539 0.257 0.813 0.653 2.262

Fig. 1 shows interesting trends at 800 MeV which mostly speak for themselves : steady downward movement of 3S,, 1 P,, 3D2 and 1 F3, dispersive upward movements in 3D, and 3G3 and perhaps At 515 MeV, changes from ref. 2 ) are at the level of experimental errors . The new KLL data help to eliminate the nasty correlation at this energy between 1 P, and 3G4, described in ref. 2 ) . However, to complete this task, KLL data need really to be a factor 2 more precise. Around 650 MeV, there are just enough data to attempt a phase-shift analysis . What emerges is that most of the data are predicted well by the smooth curves of fig. 1 . However, there are small discrepancies, notably in the shape and normalisation of dQ/dO. Unfortunately, different sets of dQ/dS2 data are inconsistent with one another and show different discrepancies. There are not enough accurate spin dependent data to give a secure solution and most of the phase shifts are strongly correlated, with correlation coefficients as high as 76%. This is in striking contrast to 515 and 800 MeV, where almost all correlation coefficients are below 10% . The conclusion is that the 650 MeV solution is not yet reliable, so it is not shown in the figures or tables . It is available by computer mail on request. To stabilise the solution, Wolfenstein parameter data are required of similar quality to those at 800 MeV. The behaviour of 3 G5 on fig. 1 calls for comment. It appears to turn up at 800 MeV. A dispersive effect due to impending inelasticity with J = 5 would be rather surprising at this energy. Instead, the rise is more likely to be due to attractive meson exchange contributions. This obliges us to look critically at its behaviour below 515 MeV. Free fits to data at 142, 210 and 325 MeV give values which tend to lie above the smooth curve of fig. 1 . However, it is rather surprising that a G-wave departs significantly from OPE up to 210 MeV. What we find is that the data at 142 MeV are not sufficient to determine 3G5 with any precision and fixing it at the OPE value brings a number. of other phase shifts into line with the smooth curves of fig. 1 with a negligible change in X 2- At 210 MeV, a single data point (discussed below) was having a strong influence on the value of 3G5 , but there are significant doubts over the validity of this point. When it is eliminated, all of 3 G3, 3G4 and 3 G5 move much closer to OPE values . At

TABLE 4

p

6

i,

4.02(0.20) -~ 19.04(0.28) -24.82(0.42)

3.17(0.62) -15 .05(0.39)

0 0

524 401

3G3 xz DOF

0

3.67 (0.51) -0.47 1 .22

-1 .56(0.27) -3.11

2.07(0.35) 4 .63(0 .13)

800

588 395 689

0 0 957

0 0

0 0 0 1509 885

0 1242 806

0

-1 .43(0.17) 3 .79 (0 .09 )

-7.29(0.15) 9.23(0.21)

8.01(0.13) -1 .45(0.28)

0 0

3.34(0.08) 0

2.71(0 .05) 0 0

-6.22(0.15) 8 .42 (0.15 ) -1 .07(0.14)

-5.74(0.22)

3.61(0.23) 7.95(0.11)

-34 .28(0.49) 22.85(0.19)

934

15.79 14.06 1309

20.06 13.44

4.86(0.11)

-10 .39(0.15) 10.07(0.25) -0.28(0.16)

8.17(0.21) -6.29(0.31)

4.43(0.34)

-30.18(0.35) -36.78(0.62) 20.67(0.22) 4.30 (0.24 )

-29 .92(0.50) -44 .59(0.69) 10.84(0 .41)

-11 .53(0.50) 5.48(0.36)

-6.32(0.38) 4.82 (0.25 ) -28 .65(0.26)

-32 .16(0.52) 8.18(0.43)

515

425

-6.00(0 .12) 7.37(0 .14) -1 .00

7.26(0 .07) -4.39(0.16)

3.50(0 .20)

-31 .49(0 .41) 24.60(0.17)

2.67(0.30) 5.04(0 .20) -24.95(0.17)

325

0 0

-0.75 1 .83(0.07)

-3.97 5.71(0.19)

6.06 (0.09 ) -2.94(0 .22)

24.91(0.23) 3.00(0 .21)

18.62(0.27)

31 .15(0.55)

-15 .52(0.75) 23.55(0.55)

210

142

i5 I P, 3D, 3 D3

3G4 3G5

i3 3 G3 1 F3

3 D2 3 D3

iP,

3 D,

3S,

Energy (MeV):

1 = 0 phase shifts in degrees. Errors in parentheses are statistical and true errors are probably somewhat larger. 0 = 0 for all 1 = 0 waves.

TABLE 5 1 - 1 phase shifts 6 in degrees for np scattering ; for pp scattering, phase shifts differ by the Coulomb barrier corrections given in rd, 2 ) . Errors in parentheses are statistical and true errors are probably somewhat larger. Up to and including 515 MeV the inelastic mixing parameter 02 is 0 and at 800 MeV it is fitted to be -90 f 5° . For other waves it is set to 0. 800 425 515 Energy (MeV) : 142 210 325 -43.55(0.71) -27.69(0.23) -13.55(0.24) -21 .90(0 .21) 5.64(0.43) -2.13(0.30) 3 Po I SO -41 .48(0.55) -18.94(0.24) -25.33(0 .26) 16.12(0.55) 4.01(0.30) -9.19(0 .24) 3p, -50.71(0.47) -41 .55(0.17) -30.57(0.15) -36.19(0.15) -17.28(0.13) -21 .86(0.15) 19.24(0.10) 16.14(0.28) 17.11(0.11) 17.75(0.12) 13.94(0.09) 16.11(0.12) 3 P2 -1 .31(0.12) -1 .74(0.11) -1 .07(0.10) -2.80(0.07) -2.51(0.07) -2.35(0.08) ~2 -1.62(0.11) -4.84(0.16) 0.07(0.10) 0.94(0.19) 1 .36(0.16) 0.88(0 .10) 3 F2 13.37(0.09 ) 3 .51(0.18) 9.79 (0.08 ) 12.33(0.10) 5 .20(0.18) 7.69(0.10) D2 -6.74(0.17) -1.45(0.09) -2 .86(0 .10) -2.70(0.09) -2 .45(0.09) -1 .99(0.16) 3 F3 4.39(0.05) 6.09(0.06) 3.58(0.07) 0.86 (0.09) 1 .79(0.11) 2.94(0.09) 3 F4 -1 .31(0.08) -1.88(0.05) -1 .10(0.04) -1 .48(0.04) -1 .68(0.05) -0.83(0.03) E4 0.37(0.05) 0.33(0.06) 0.49 (0.05 ) 0.51(0.06) 0.24 (0.03 ) 0.31(0.05) 3 H4 4.58(0 .06) 2.93 (0.05 ) 1 .60(0.06) 2 .39 (0.06) 0.73(0.06) 0.99(0.06) 1 G4 -1.56(0.08) -1 .25(0 .09) -1 .12(0.05) -1 .31(0.07) -0.50(0.03) -0.78(0.04) 3H5 1.65(0.04) 0.76 (0.04) 0.49 (0.05 ) 0 .77(0.05) 0.13(0.03) 0.20(0.04) 3116 -0.96(0.04) -0.698 -0.874 -0.35 -0.56 -0 .22 iF6 3K6 0.277 0.42(0.04) 0.148 0.206 0.048 0.085 1.24(0.05) 0.601 0.84(0.04) 0.172 0.269 0.444 116 -0.756 -0.957 -0.437 -0.580 -0.141 -0.246 3 K7 0.285 0 .50(0.02) 0.126 0.195 0.024 0.050 3 Ks

~o

s

nA hh

b

a

áx

á

04

C

D. V. Brug, R.A. Bryan / np elastic scattering

45 7

TABLE 6

1 = 1 parameters p, wherc n = cost p. Values refer to np scattering and pp values are related by the Coulomb barrier corrections given in ref. 2 ). Units are degrees and errors in parentheses are statistical; true errors are probably somewhat larger. Energy (MeV ): 3 Po So 3Pt 3P2 3 F2 1 D2 3 F3 3F4 3H4 G4 3H5 3 H6 1 I6

325 0.24 0 3.77(1 .00) 0.45 0.23 3.79(1 .20) 0.44 0 0 0.05 0 0 0

425 1.92 0.70 5.46(1.30) 5.81(0.88) 1.84 13.44(0.34) 3.60 0.22 0 0.96 0 0 0

515 6.37 2.34 9 .43(1 .30) 8.12(1 .10) 3.86 24 .56(0.22) 8.56(0.41) 0.66 0.72 2.60 1.00 0 0.60

800 17.68 (1 .73) 14.70 (1.63) 14.89(1.11) 35.32(0.34) 15.66 (0.66) 34.38(0.30) 37.07(0.32) 4.48 3.90 ' 35(0.50) 7.85(0.40) 0.63 7.07(0.43)

325 MeV, fixing 3G5 to a value taken from the smooth curve of fig. 1 again helps bring 3G3 and ' P, into line with the smooth curves . With these smoothings, the qualitative behaviour of 3G5 makes good sense. It moves positive of OPE in similar fashion to the behaviour of 3D3 in the 50-200 MeV range. As was remarked by Bryan and Gersten many years ago' 3 ), 3 D3 starts off negative at threshold and rapidly moves positive of OPE. The questionable data point near 210 MeV is the value of Rl determined by Axen et al. ' 4 ) . It was inferred from the polarisation of a neutron beam produced by the p (d, n) reaction . There was one unknown parameter in its experimental deteh- nination, which was taken from the phase-shift analyses of those days. Recent phase shifts are significantly different. It turns out that values of Rt at 332, 434 and 506 MeV of Axen et al. contribute little to present phase-shift analyses. Our recommendation is that these values should now be dropped. The experimental determination of 3 G5 below 425 MeV is not easy to improve. The real sensitivity to it lies in the spin-orbit combination ANOda/dQ, which has a positive peak near 25° and a negative one near 140° ; the relative magnitudes of these peaks is sensitive to 3 G5 . However, measurements of da/dQ are notoriously difficult over such a wide angular range, and measurements with an accuracy better than 1 % would be needed to do much good. This is a daunting task. We now turn to a different topic, namely A QL and AQT . Fig. 2 compares experimental values' 5-20 ) for np scattering with those derived from the smooth curves of fig. 1 . Even

D. V Bugg, R.A. Bryan / np elastic scattering

45 8

10 0

I 0

I

I

400

i

I

800

I

I

1200

Laboratory Kinetic Energy (MeV)

and AuT for np. Squares and triangles denote recent data from SIN/PSI (ref. 16) ) and SACLAY (reE 17 ) ); earlier SACLAY data (refs. 18,19 )) are indicated by diamonds and data from LAMPF (ref 15 )) by crosses. The full line is calculated from the smooth curves of fig. 1. Fig. 2.

AQL

if the experi iental values of fig. 2 are omitted from the fits, predictions change very little. The conclusion is that elastic data alone are adequate to predict AQL and AaT well and there are no surprises here. Incidentally, this supplies direct evidence that I = 0 inelasticity is small, since A aL and A aT depend on the imaginary parts of forward scattering amplitudes . Beddo et aL 15 ) remarked in their abstract on a small bump in AQL (1 = 0), which they sought to associate with an I = 0 dibaryon resonance at a mass of 2213 MeV (beam energy T = 733 MeV) with a width T = 74 MeV. The height, of their bump required elasticity x = (2J + 1)T1 lr = 0.913 . This resonance would require anticlockwise looping in the Argand diagraiTt, consequently a significant inelasticity and a dispersive effect in the phase shift a extending down to 515 and even 425 MeV. The partial wave amplitudes show no hint of this behaviour. The magnitudes of possible loops in the Argand plots for all waves up to J = 5 are significantly smaller than required by the hypothesis of Beddo et al. Partial waves contributing to A at are: AQL

oc Im[ (2J + 1)Rj - (2J + 1)Rj,j -1- Rj+l,r - Rj-l,r + 4

J(J + 1)Rj] ,

where R,, is the singlet wave and Rj the mixing amplitude; RL, are triplet amplitudes . A positive peak in AaL is most likely to be associated with a singlet wave. The one

D. V. Bugg, R.A. Bryan / np elastic scattering

459

which is hardest to determine and which admits the largest value ofx is 'P l . However, even in this wave, the fitted value at 800 MeV gives a value of x a factor 4.5 smaller than required by Beddo et al. If inelasticity is put into ' F3 instead of ' P, , X 2 deteriorates and the fitted value of q is 0.978 ; this gives a value of x a factor 10 smaller than Beddo et al. suggest. In neither ' P, nor ' F3 is there evidence for the strong dispersive effect required by a resonant loop. In 'PI, a dispersive effect of about +21* would be expected at 515 MeV and -20° at 800 MeV; in ' F3 the effects would be about f7°, far beyond what the partial-wave analysis allows. Similar remarks apply to other 3 partial waves, notably 3D,, 3D3 and G3 . There is direct experimental evidence against a narrow dibaryon. The unpolarised 2' total cross sections of Lisowski et al. ) from 500 to 800 MeV, made at fine energy steps with a broadband neutron beam, show no indication of a narrow resonance; their statistical errors are 1%. They give a figure illustrating the large effect a 1 P, resonance would have . There is a small systematic discrepancy between the smooth curves of fig. 2 and the experimental data. Plotted values are as supplied by the experimental groups. However, it s~-emc likely that the-se from LAMPY need to be renormalised downwards in magnitude by about 13% to. allow for the newly determined polarisation of the neutron beam. In any case, it must be remembered that normalisation uncertainties due to beam and target polarisations are at least 5% and there is the additional experimental problem of determining the packing density of material in the polarised target. In the considerably easier pp measurements, results of different groups cover a range of about ±10%.

4. Conclusions Data of McNaughton et al. on np Wolfenstein parameters improve decisively the determination of I = 0 phase shifts at 800 MeV. The trend of their movements from 500 to 800 MeV is instructive (fig. 1). It heralds the approach of inelasticity in 3D, and 3G3. The phase-shift analysis does not support the claim of Beddo et al. for an 1 = 0 dibaryon resonance. The behaviour of AuL can be attributed quite naturally (fig . 2) to the energy dependence of the elastic phases, which are dominantly repulsive approaching 800 MeV.

It is a pleasure to thank Dr. M. McNaughton for conveying experimental results prior to publication. References 1) M. McNaughton et al., submitted to Phys . Rev. C. 2) D. V. Bugg, Phys. Rev. C41 (1990) 2708

460 3) 4) 5) 6) 7)

8) 9) 10) 11) 12) 13) 14) 15 i 16) 17) 18) 19) 20) 21)

D, V. Bugg, R.A. Bryan / np elastic scattering C.R. Newsom et at., Phys. Rev. C39 (1989) 965 D.V. Bugg and C. Wilkin, Nucl. Phys. A467 (1987) 575 R. Koch and E. Pietarinen, Nuci. Phys. A336 (1980) 331 J.R. Bergervoet et al., Phys. Rev. C41 (1980) 1435 R.A. Arndt, Z. Li, L.D. Roper and R.L. Workman, Phys. Rev. Lett. 65 (1990) 157; F Gross, J.W. Van Orden and K. Holinde, Phys. Rev. C41 (1990) 1909; J.J. de Swart, R. Klomp, R. Timmermans and Th.A. Rijken, Nijmegen University preprint THEFNYM-90.14, presented at the Shanghai Interna!ional Workshop 1990 R. Dubois et al., Nucl. Phys. A377 (1982) 554 L.G. Dakhno et al., Phys. Lett. B114 (1982) 409 V.P. Andreev et al., Zeit. Phys. A371 (1988) 329 J. Bystricky, C. Lechanoine-Leluc and F. Lehar, J. de Phys. 48 (1987) 199 F. Shimuzu et al., Nucl. Phys. A386 (1982) 571 R.A. Bryan and A. Gersten, Phys. Rev . D6 (1972) 341 D. .Azen et al., Phys. Rev. C21 (1980) 998 M. Beddo et al., Phys. Lett. ái258 (1991) 24 R. Binz et al., PSI preprint PSI-PR-90-36 (1990) and SIN Newsletter 20 (1987) 10 J.M. Fontaine et al., Nucl. Phys . B358 (1991) 297 F. Perrot et al., Nucl. Phys. B278 (1986) 881 F. Lehar et al., Phys. Lett. B189 (1987) 241 F. Lehar et al., Nucl. Phys. A478 (1988) 533c P.W. Lisowski et al., Phys. Rev. Lett. 61 (1983) 529