The Deuteron from The Big Bang to Neutron Stars to the Charge...

Research Talk – 17 February 2012

The Deuteron – from The

Big Bang to Neutron Stars

to the Charge Distribution

of the Neutron

Blaine Norum


The Deuteron:

“…its properties are as important

to nuclear theory as the hydrogen

atom is in atomic theory.” J. Chadwick and M. Goldhaber, “A Nuclear Photo-effect:

Disintegration of the Diplon by -rays,” Nature 134 (1934) 237.

80 years on we still don’t understand it


d g ,p +g '( )nn

Structures observed at:

W = 2 M n + 9 MeV

= 2 M n + 28 MeV

= 2 M n + 48 MeV

= 2 M n + 69 MeV

= 2 M n + n +1 2( ) w ???

Final state nn in LEGS case; pn

in MMF case ® manifestly

T=1


What else did we learn?

Pions came out in polarization plane

o electric transition

o spins still parallel – S=1

Four states observed

o orbital excitations, either L related or r related

Decays accompanied by emission

o electromagnetic decay

o must involve currents as only one S possible

o neutrons have no net charge so must involve charge

distribution of neutron

Excitations of roughly equal strength

o most likely decay sequentially. That is, lowest state

decays emitting 1 , next decays emitting 2 ’s, etc.


By itself? Forget it! Other data:

d g , n( ) p

Wigner/Unitary

cusp similar to

that observed in

E0+ amplitude in

threshold p 0

photoproduction

Opening channel

is d*?

employ the Bonn potential and include meson-exchange currents, isobar currents, and relativistic corrections. Schmitt et al. suggest that improved measurements are needed in this energy region to resolve the discrepancy between theory and experiment. On the other hand, it should be noted that the calculations use the Fermi-Watson Theorem to constrain the phases of the matrix elements to be the same as those observed in np scattering, based on the assumption that the only exit channel available is simply n plus p. Given the small degree of mixing between final states of different angular momentum, this leads to a smooth variation of the polarization asymmetry throughout this region of incident photon energy.

Figure 2. The differential polarization of neutrons emitted at 45

o as measured by Nath

et al. [8]. The figure is taken from ref. 10. The theoretical curves are the OBEPQ calculation from ref. 10.

The shape of the observed break is exactly that of a Wigner or unitary cusp which can occur when a new reaction channel becomes available. Such cusps have been observed in neutral pion

photoproduction from the proton [11] when the incident photon energy corresponds to the p +

photoproduction threshold. Figure 3 shows the sharp break in the 0

E+ amplitude extracted from

measurements of the 0p pg p® reaction made at Mainz [12] and Saskatchewan [13]. While the

relatively large statistical uncertainties mask some of the sharpness of the break, there is clearly an abrupt change in slope. Figure 4 shows calculations of two direct polarization observables in the

same reaction. In one case ( )S there is an almost invisible, and surely unobservable, cusp whereas

in the other there is a very pronounced cusp. It is also instructive to note that in the total cross

section data for the 0p pg p® reaction there is no cusp visible (see Fig. 5) whereas in the 0E +

amplitude for this reaction there is a very pronounced one. By analogy, the presence (absence) of a

cusp or sharp change in slope near 12 13Eg » - MeV in the neutron polarization asymmetry (cross

section) data for the ( ),d n pg reaction are qualitatively consistent with observations of threshold

0p photoproduction.

Additional support for the conjecture of 2BT N= (where NB is the number of baryons)

resonances was provided recently [17] by the measurements of E. Ihara et al. at the RCNP of Osaka

University. They measured ( )3 , 3He p n p at Ep = 346 MeV. By comparing the differential cross

section to the L TS S ratio as a function of energy transfer it was established that there existed a

significant resonant contribution near threshold (see Fig. 6). That the resonance had 3 2T = was

required by the fact that it involved 3 protons; that it had 1 2J p -= was strongly suggested

because


d g , n( ) p at HIg S

Sawatzky Blackston

ds

dW= s 0 1+ a1P1

0 +éë a2P2

0 + a3P3

0 + a4P4

0 +e2P2

2 + e3P3

2 + e4P4

2 ùû

® Effective Amplitudes: 1S0

2

, 3P0,3P1,

3P2,3D1,

3D2,3D3

While admittedly not conclusive, these data suggest that something is happening at or near

12E MeVg = that is not accounted for by theory.

10E MeVg = 14E MeVg =

3

0P 3

1,2,3D 3

0P 3

1,2,3D

Solution HA HI Sg HA HI Sg HA HI Sg HA HI Sg

1 0.87 0.88 ( ), ,+ - + ( ), ,+ - + 0.82 0.81 ( ), ,+ - + ( ), ,- + -

2 0.69 0.71 ( ), ,- + + ( ), ,- + + 0.71 0.69 ( ), ,+ - + ( ), ,- + -

3 0.60 0.59 ( ), ,+ - + ( ), ,+ - + 0.57 0.57 ( ), ,- + + ( ), ,+ - +

4 0.43 0.41 ( ), ,+ - + ( ), ,+ - + 0.47 0.45 ( ), ,- + - ( ), ,+ - +

Table 3. Effective Transition Matrix Elements

Lending support to the observation that something anomalous is happening in the N-N system at

rather low energies are observations by Huisman et al. [33] of p-p bremsstrahlung at an incident proton

energy of 190 MeV. They observed that the best available theories could not reproduce the angular

distributions, with the discrepancy being dependent on the energy of the final p-p state. Figure 8 shows

this discrepancy. Note that there is a sharp increase in the discrepancy at about 12 MeV. This energy is

for the p-p system; to obtain the corresponding energy at which such a discrepancy would appear in the

n-p system one must subtract the Coulomb energy, on the order of 1-2 MeV. This yields an energy, relE ,

of about 10 MeV, close to the energy of the

discontinuity in the ( ),d n pg data discussed

above. A theoretical analysis of the pp ppg®

results and discrepancies by Cozma, Tjon, et al.

[34] yielded the conclusion that “at least an

important part of the problem resides in the

description of the low energy NN interaction.”

If there is a significant problem with our

understanding of the N-N interaction at such a

low energy, the implications could be dramatic.

Perhaps it is the source of the longstanding Ay

problem in light nuclei? [35] A detailed study

of the photo-disintegration of the deuteron

promises to shed much light on this issue.

Figure 8. Discrepancy in pp ppg®

It is interesting to note that the discrepancies at an rel

E of about 10 MeV appear to manifest

themselves as peaks in processes in which a pion is created but as discontinuities when only photons are

involved. Accordingly, it is important to investigate them with both reactions involving only photons as

proposed here as well as with reactions involving pions. A proposal to study the reaction


Proton-Proton Bremsstrahlung at Einc

lab =190 MeV :

Conclusion of Tjon et al. - “… an important part of the problem

resides in the description of the low energy NN interaction.”


So who might care if very rare d* state exists?

Astrophysicists:

Neutron stars have magnetic fields of ~1012

G

Many mechanisms proposed, run from very complicated to

very exotic (i.e., strange quark condensates)

To 1st approximation core is cold Fermi gas of neutrons

n + n® nn( )S=1

* can’t decay as neutron states occupied

If even small paramagnetic coupling exists, then large

magnetic fields possible


Cosmologists:

Minutes after Big Bang, universe full of neutrons, protons,

photons, neutrinos

<E> ~ few MeV and deuterons starting to form

N + N ® N + N has no effect on statistics/thermodynamics

n + p® d +g balanced by d +g ® n + p so no net effect

N + N ® NN( )T =1

*+g 1 ® N + N +g 2[ ]+g 1 has two effects:

Entropy increased

Average nucleon kinetic energy decreased

® deuteron formation accelerated


Who are we? Blaine Norum, Prof. and P.I. Accelerator Physics

Richard Lindgren, Res. Prof. Ryan Bodenstein, GS

Pil-Neo Seo, Res. Scientist James McCarter, GS

Charles Hanretty, Res. Associate

Slava Tkachenko, Res. Associate

Khem Chirapatpimol, GS

Ryan Duve, GS

Peng Peng, GS

Siqi Lin, UGS [summer]


What are we doing?

Active Experiments:

1. Analyzing , 'd e e p n at threshold; JLAB HALL A

2. Analyzing 0, 'p e e p at threshold; JLAB HALL A

3. Analyzing ,d n p at 30 MeV; HIGS

4. Meson production from neutron; JLAB HALL B


In preparation:

1. ,d n p GDH Sum Rule; HIGS

2. ,d d and ,p n p ; HIGS

3. ,d n p ; HIGS

4. ,Z e e Bethe-Heitler; HIGS

5. 33 ,He e e He for n

EG ; JLAB HALL C

6. , ' 'd e e ; Mainz, Germany


Development:

1. Developing polarized proton/deuteron target for use at

HIGS

2. Upgrading Blowfish detector system at HIGS

New electronics

New mounting

3. Developing analyzer/detector package for ,d n p


Polarized Target


Blowfish


Conclusions:

We need more students and have several

very interesting projects to pursue

Contact me:

Room 136

924-6789

[email protected]

The Deuteron from The Big Bang to Neutron Stars to the Charge...

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