Photodisintegration of Few-Body Nuclei Ron Gilman Rutgers / Jefferson Lab What have we learned? What might we learn? Jefferson Lab User Group The Next Seven Years June 16-18, 2004

Low Energy ● Low energy theory uses nucleons + π's +... ● A consistent NN force determines scattering and bound state nuclear wave functions ● Beautiful detailed calculations nicely explain data; there are also good PT calculations near threshold Jefferson Lab User Group The Next Seven Years June 16-18, 2004

Medium Energy ● Conventional theory is more complicated and less successful ● Is there good control of: ● relativity? ● the short-range nuclear structure? ● meson and baryon resonances? ● The worst case is shown. Jefferson Lab User Group The Next Seven Years June 16-18, 2004

The p y Problem ● The Arenhoevel – Schwamb theory predicts large induced polarizations, but the angle dependence is wrong. ● The older, simpler Bonn (Kang, Erbs, Pfeil, and Rollnik) calculation also has problems. Jefferson Lab User Group The Next Seven Years June 16-18, 2004

High Energy ● Conventional theory complications get worse ● 286 (+) on-shell baryon-baryon channels appear by 4 GeV ● This suggests finding good effective quark degrees of freedom to average over all the resonances. Jefferson Lab User Group The Next Seven Years June 16-18, 2004

High-Energy Motivation ● Hadrons and quarks are in principle alternate basis states, and the theory can be formulated with either ● But... is there some indication of a transition or phase change, a behavior that is simple (difficult) to understand with a quark (hadronic) model? ● The usual choice: do the cross sections fall with energy according to the constituent counting rules of QCD: d/dt ∝ s -(n-2) Jefferson Lab User Group The Next Seven Years June 16-18, 2004

pQCD ● The CCR work amazingly well (data: P. Rossi et al., hep- ph/ ), n ~ 11, over a large angular range, once P t ~ 1 – 1.3 GeV ● Why? Jefferson Lab User Group The Next Seven Years June 16-18, 2004

Models ● Most models based on upper diagram, some directly relate photodisintegration to NN scattering ● Quark-Gluon String (Regge) theory also applied (to NN scattering as well) Jefferson Lab User Group The Next Seven Years June 16-18, 2004

Models vs. Data ● At 90 o, all the models and the data fall about like s -11, at sufficiently high energy Jefferson Lab User Group The Next Seven Years June 16-18, 2004

Models vs. Data II ● Data at other angles confirm the observations from the 90 o data ● Since the cross sections do not clearly distinguish between the models, we turn to the polarizations Jefferson Lab User Group The Next Seven Years June 16-18, 2004

Polarizations-p y ● Simple discussion: pQCD ⇨ hadron helicity conservation ● ⇨ p y = 0 ● But Sargsian / HRM also predicts small p y, based on NN scattering Jefferson Lab User Group The Next Seven Years June 16-18, 2004

Polarizations-∑ ● Generally expected HHC ⇨ ∑ = -1 ● Kondratyuk et al. pointed out that limit depends on isoscalar vs isovector coupling, could range from - 1 → 1 Jefferson Lab User Group The Next Seven Years June 16-18, 2004

Polarizations - C x', C z' ● HHC ⇨ C x', z' → 0 as 1/t, 1/t 2 ● HRM predicts C x' small, C z' similar to QGS ● Unpublished data: 2 GeV angular distribution, should be done in a few months Jefferson Lab User Group The Next Seven Years June 16-18, 2004

So What Have We Learned? ● Data are not very different from pQCD expectations, even though we expect ``soft'' physics to dominate ● There are several more or less satisfactory approaches, in terms of describing the data, despite very different underlying mechanisms ● In particular, if you reproduce the NN in some model, you probably do OK on the d → pn ● This is not a very satisfying result ● How can we do better? Jefferson Lab User Group The Next Seven Years June 16-18, 2004

So What Might We Learn? ● The same models that more or less agree for pn photodisintegration give very different predictions for pp photodisintegration (Sargsian) ● Thus, we need to study 3 He → ppn spectator ● At low energies, the (pp) S=0 system in 3 He has reduced interactions, so the pp/pn cross section ratio is small (Laget) ● At high energies, different quark models vary Jefferson Lab User Group The Next Seven Years June 16-18, 2004

CLAS Data on 3 He → ppn ● CLAS data from S. Niccolai ● Note large strength for low-momentum neutrons ● Analysis ongoing Jefferson Lab User Group The Next Seven Years June 16-18, 2004

CLAS Data on 3 He → ppn ● CLAS has measured 3 He → ppn up to about 1.5 GeV ● Note the ``spectator'' neutron peak (left), vs the three body breakup (right) Jefferson Lab User Group The Next Seven Years June 16-18, 2004

3 He → ppn Predictions ● Use p n < 100 MeV/c ● RNA: pp >> pn ● HRM: pp > pn ● QGS: pp ~ pn ● TQC: pp << pn ● For the first three models, there is a dramatic change in pp / pn with energy Jefferson Lab User Group The Next Seven Years June 16-18, 2004

The n Distribution ● n = (E-p z )/m is the light cone momentum fraction ● M Sargsian showed n is ~ unaffected by FSI ● If photodisintegration takes place on low (high) momentum nucleons, as in HRM (RNA), then the distribution is narrow (broad) Jefferson Lab User Group The Next Seven Years June 16-18, 2004

3 He → ppn Oscillations ● One interesting prediction from HRM: due to the oscillations in pp (vs. pn ) with energy, the 3 He photodisintegration will appear to fall as ~s -10 rather than s -11 over a wide range Jefferson Lab User Group The Next Seven Years June 16-18, 2004

3 He → ppn Predictions ● Furthermore, the interesting spin physics in pp elastic scattering might be reflected in the pp photodisintegration spin observables ● If the high energy spin physics in pp arises from charm threshold, then there should also be interesting spin physics in the photodisintegration near strangeness threshold, about 1.6 GeV (Brodsky) ● No existing data would test this – perhaps we need a new proposal? Jefferson Lab User Group The Next Seven Years June 16-18, 2004

Conclusions ● d → pn is the most promising exclusive reaction for a transition to quark degrees of freedom... ● but soft physics remains important, and we have several more or less okay quark models – so what is the correct way to think about the problem? ● pp → pp using 3 He gives several handles on the underlying physics, through the ratio pp / pn, the shape of the n distribution, and the possible oscillations. ● With good luck and scheduling of E03-101, we will know the answer in a few years Jefferson Lab User Group The Next Seven Years June 16-18, 2004

References, Acknowledgements ● For more on 3 He → ppn: ● S. J. Brodsky et al., Phys. Lett. B 578, 69 (2003) ● Hall A E03-101: E. Piasetzky, R. Gilman et al. ● For a review of the deuteron photodisintegration (including the references I could not fit in here): R. Gilman and F. Gross, J. Phys. G 28, R37 (2002) ● The 3 He program is largely due to M Sargsian and E Piasetzky, with additional large contributions from Brodsky, Frankfurt, Hiller, Miller, and Strikman, and Radyushkin, de Sanctis, Kondratyuk, and the E collaboration Jefferson Lab User Group The Next Seven Years June 16-18, 2004