Photodisintegration of Few-Body Nuclei

Photodisintegration ofFew-Body Nuclei

Ron GilmanRutgers / 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 thresholdJefferson 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.


The py

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)


pQCD● The CCR work amazingly well (data: P. Rossi et al., hep-ph/0405207), n ~ 11, over a large angular range, once Pt ~ 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)


Models vs. Data

● At 90o, all the models and the data fall about like s-11, at sufficiently high energy


Models vs. Data II

● Data at other angles confirm the observations from the 90o data

● Since the cross sections do not clearly distinguish between the models, we turn to the polarizations


Polarizations-py

● Simple discussion: pQCD ⇨ hadron helicity conservation

●⇨py = 0● But Sargsian / HRM also predicts small py,

based on NN scatteringJefferson 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


Polarizations - Cx', Cz'

● ⇨HHC Cx',z' → 0 as 1/t,

1/t2

● HRM predicts Cx'

small, Cz' similar to

QGS● Unpublished data: 2 GeV angular distribution, should be done in a few months


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 3He → ppnspectator

● At low energies, the (pp)S=0 system in 3He has

reduced interactions, so the pp/pn cross section ratio is small (Laget)

● At high energies, different quark models varyJefferson Lab User Group The Next Seven Years June 16-18,

2004

CLAS Data on 3He → ppn● CLAS data from S. Niccolai

● Note large strength for low-momentum neutrons

● Analysis ongoing


CLAS Data on 3He → ppn● CLAS has measured 3He → 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

3He → ppn Predictions● Use pn < 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


The n Distribution● n= (E-pz)/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)


3He → ppn Oscillations● One interesting prediction from HRM: due to the oscillations in pp (vs.

pn) with energy, the 3He photodisintegration will appear to fall as ~s-10 rather than s-11 over a wide range


3He → 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?


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 3He 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


References, Acknowledgements● For more on 3He → 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 3He 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 E03-101 collaboration


Photodisintegration of Few-Body Nuclei

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