The Deuteron from The Big Bang to Neutron Stars to the Charge...
Transcript of 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
Research Talk – 17 February 2012
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
Research Talk – 17 February 2012
Research Talk – 17 February 2012
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
Research Talk – 17 February 2012
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.
Research Talk – 17 February 2012
Research Talk – 17 February 2012
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
Research Talk – 17 February 2012
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
Research Talk – 17 February 2012
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.”
Research Talk – 17 February 2012
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
Research Talk – 17 February 2012
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
Research Talk – 17 February 2012
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]
Research Talk – 17 February 2012
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
Research Talk – 17 February 2012
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
Research Talk – 17 February 2012
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
Research Talk – 17 February 2012
Polarized Target
Research Talk – 17 February 2012
Blowfish
Research Talk – 17 February 2012
Conclusions:
We need more students and have several
very interesting projects to pursue
Contact me:
Room 136
924-6789