3N AND 4N SYSTEMS AND THE Ay PUZZLE

THOMAS B. CLEGG

Department of Physics & Astronomy, University of North Carolina and TUNL

Chapel Hill, NC27599-3255 , USA*

and

Triangle Universities Nuclear Laboratory,Durham, NC27708-0308,USA

Recent work aimed at trying to define better the experimental and theoretical differences in the vector analyzing power in few nucleon scattering is discussed. New measurements underway at TUNL aimed at refining understanding of the problem are summarized.

1. What is Being Studied?

In recent years, a major theoretical effort has sought,starting with modern NN potentials (e.g Nijmegen, CD Bonn, AV-18) and including a 3N interaction (e.g. Urbana IX), to calculate scattering observables in 3N and 4N systems. Recently it has been shownthat calculations in coordinate space and. momentum space agree to within 1% for observables in p+dscattering.1 Theoretical attention is turning now toward similar calculations in 4N systems. Four-N nuclei are a fertile ‘theoretical laboratory’ because these systems are the lightest with resonant states and thresholds, and the simplest where amplitudes of isospin T=3/2 can be studied.

2. What is Learned from Comparison with Experimental Data?

In both 3N and 4N systems, excellent agreement is seen between theoretical calculations and some experimental observables, most notably the cross section and tensor analyzing powersin elastic scattering2,3. However, significant differences exist in this same system at low bombarding energiesfor the vector analyzing power, Ay. These differences are not removed, and can even be enhanced, by the addition of a 3N interaction in the theoretical calculations.At the scattering angle where the analyzing power peaks, the fractional differences between experiment and theory {expt-theory)/

expt} are ~20% for p+d and n+d,and ~40% for p+3He. New, highly accurate data from TUNL between 1.6 and 5.5 MeV for both p+3He [3]and n+3He[4]have only confirmed differences first seen nearly 20 years ago.The origin of the differences has been traced5 to incorrect splitting in the theoretical ℓ=1 phase shifts.These differences disappear for energies above 50 MeV.

*Work supported in part by the US Department of Energy under Grant # DE-FG02-97ER41041

Further explanation for this ‘Ay puzzle’ is not yet understood. One possibility is that the input NN potentials used in all the theoretical calculations may be poorly defined at these low scattering energies. This would not be surprising since so few Ay data exist there. Such low-energy NN measurements are experimentally difficult. For example, the peak magnitude of Ayfor p+p scattering is only 0.006 at 5 MeV[6]. Another possibility is that the correct 3N interaction may not yet have been found. Or is something being left out in the calculations? No agreement exists about the origin of the problem.

3. New Experimental Measurements Underway at TUNL

3.1. n+p Scattering

Recent very careful Ay measurements at 12 MeV by G. Weisel et al.7show significant differences from values predicted by the Nijmegen potential, causing some concern about the validity of that accepted potential model standard at these low energies.

3.2.n+d Scattering

Other recent measurementsby G. Weisel et al.7of Ay at En = 19 and 22.5 MeV to map out the energy dependence of the disappearance of the Ay puzzle.

3.3.n+3He Scattering

New Ay measurements underwayby J. Esterline et al.4 between En = 1.6 and 5.5 MeV will be compared with theoretical calculations mentioned above and with similar data3 recently measured for p+3He scattering.

3.4.. p+3He Scattering

Daniels et al.8areutilizing a polarized 3He target for measuringthe target analyzing power A0y and the spin correlation coefficients Axx and Ayyin p+3He scattering at energies between 2 and 5.5 MeV. These data will be used in a new energy-dependent phase shift analysissimilar to that of George et al.9to try to determine p+3He phase shifts uniquely in this low energy range.

References

1) A. Deltuva, et al., Phys. Rev. C71, 064003 (2005).

2) C. R. Brune et al., Phys Rev C63, 44013(2001).

3) B. M. Fisher et al., Phys. Rev. C74, 034001 (2006).

4) J. Esterline and W. Tornow, private communication.

5) M. H. Wood, et al., Phys. Rev. C65, 034002 (2002).

6) M.D. Barker et al., Phys.Rev. Lett.48, 918(1982).

7) G. J. Weisel and W. Tornow, private communication.

8) T. V. Daniels and T. B. Clegg, private communication.

9) E. A. George and L. D. Knutson, Phys Rev. C67, 027001 (2003).