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Discovery of a Quasi Dynamical Symmetry and Study of a possible Giant
Pairing Vibration
R.F. CastenWNSL, Yale
May 11,2011• Evidence for a Quasi Dynamical Symmetry along the arc of regularity with an intro to the relevant Group Theory of dynamical symmetries
• What and where is the Giant Pairing Vibration and why?
Symmetries, degeneracies, and Group theory
Illustrate using the IBA
· Valence nucleons, in pairs as bosons. Number of bosons is half number of valence nucleons – fixed for a given nucleus.
· Only certain configurations. Only pairs of nucleons coupled to angular momentum 0(s) and 2(d). N = ns + nd
· Simple Hamiltonian in terms of s an d boson creation, destruction operators – simple interactions
· Group theoretical underpinning
Group Structure of the IBAU(5)vibratorSU(3)rotor
O(6)γ-soft
U(6)
Magical group theory
stuff happens here
Symmetry Triangle of the IBA
Sph.
Def. R4/2= 2.0 R4/2= 3.33
R4/2= 2.5
6-Dim. problem
Three Dynamic symmetries, nuclear shapes
Arc of RegularityOrder amidst chaos
• An isolated region of regularity inside the triangle
• The boundary distinguishing different structural regions
• Degeneracies along the arc
• The first example of an SU(3) Quasi-dynamical symmetry in nuclei
Let’s illustrate group chains and degeneracy-breaking.
Consider a Hamiltonian that is a function ONLY of: s†s + d†d
That is: H = a(s†s + d†d) = a (ns + nd ) = aN
H “counts” the numbers of bosons and multiplies by a boson energy, a. The energies depend ONLY on total number of bosons -- the total number of valence
nucleons. The states with given N are degenerate and constitute a “representation” of the group U(6) with the quantum number N. U(6) has OTHER representations, corresponding to OTHER values of N, but THOSE states are in DIFFERENT NUCLEI.
Of course, a nucleus with all levels degenerate is not realistic (!!!) and suggests that we should add more terms to the Hamiltonian. I use this example to illustrate the
idea of successive steps of degeneracy breaking related to different groups and the quantum numbers they conserve.
Note that s†s = ns and d†d = nd and that ns + nd = N = ½ val nucleons
H = H + b d†d = aN + b nd
Now, add a term to this Hamiltonian
Now the energies depend not only on N but also on nd
States of different nd are no longer degenerate. They are “representations” of the group U(5).
N
N + 1
N + 2
nd
1 2
0
a
2a
E
0 0
b
2b
H = aN + b d†d = a N + b nd
U(6) U(5)
U(6) U(5)
H = aN + b d†d
Etc. with
furth
er term
s
Example of a nuclear dynamical symmetry -- O(6)
N
Spectrum generating algebra
Each successive term:
• Introduces a new sub-group
• A new quantum number to label the states described by that group
• Adds an eigenvalue term that is a function of the new quantum number, and thus
• Breaks a previous degeneracy
Quasi-dynamical symmetries
• Dynamical symmetries where some or all of the degeneracies and quantum numbers of the symmetry are preserved despite large changes in the wave functions from those of the symmetry itself.
• Are there such QDS in the “triangle”?• Long standing question for 20 years.
Degeneracies along the Arc of RegularityAll of them persist as well as the analytic
ratios of the 0+ “bandheads”
First example of a non-trivial
QDS in nuclei
Search for the Giant Pairing Vibration
• What is the GPV
• Where should it be and why don’t we see it
• The experimental situation and hopes
• Why it may not be where we think it should be a little known feature of mixing of bound and unbound states
Pairing in nucleiPair correlations in nucleonic motion have provided a key to
understanding the excitation spectra of even-A nuclei, including the famous pairing gap, compression of energies in odd-A nuclei,
odd-even mass differences, rotational moments of inertia, and other phenomena. Pairing “vibrations” were predicted and
discovered around 1970, confirming simple models of these 2-particle states.
The Giant Pairing Vibration – a pairing mode in the next higher major shell – should also exist, roughly at ~ 14 MeV in heavy nuclei, and be strongly populated in Q-matched two-nucleon
transfer reactions.
As pairing is such a fundamental feature of nuclei, searches for the GPV are extremely important. However, they have never been
seen, despite extensive searches.
Pairing Vibrations 0+ states
132Sn(t,p) 134Sn
Collective mode: The two nucleons occupy many final 0+ levels with
coherent wave functions
Strongly populated in 2-nucleon transfer reactions like (p,t) and (t,p).
This PAIR transfer is exactly analogous to the quadrupole phonon
creation in the GEOMETRIC VIBRATOR model. Multi-phonon
states.
Extensively studied in 1970’s. Model verified
Pairing Vibrations 0+ states
132Sn (p,t) 130Sn
Pair Removal mode(different cross section
than pair addition)
Concept of Pairing Vibrations(analogy to geometrical vibrational phonon model
except there are two modes – pair creation and pair removal)
A
2A
3A
2B
3B
B
BA
Giant Pairing Vibration
90 Zr(t,p) 92Zr
Collective mode TWO shells up: The two nucleons occupy many final 0+
levels with coherent wave functions. Predicted at energies from ~ 7 to ~14
Mev by different models
Should be strongly populated in 2-nucleon transfer reactions like (p,t)
and (t,p).
Never found – why?
• Despite efforts using conventional pair transfer reactions, such as (t, p) and (p,t) (G. M. Crawley et al., Phys. Rev. Lett. 39 (1977) 1451), the GPV has never been identified.
• Fortunato et al. (Eur. Phys. J. A14 (2002) 37) suggeststhat beams, such as t or 14C, do not favor excitation of high-energy collective pairing modes due to a large energy mismatch.
• Q-values in a stripping reaction involving weakly bound 6He are muchcloser to the optimum.
Why has the GPV never been observed?
• pp RPA calculations on 208Pb.
• Two-neutron transfer form factors from collective model
• DWBA (Ptolemy) calculation of sGPV
L. Fortunato et al., somewhereL. Fortunato, Yad. Fiz. 66 (2003) 1491
Some Recent Calculations
What we did and why(progress report, not final story)
Basic idea is that mixing with UNbound levels with widths, has two effects:
• Gives width to GPV, making it hard to see –spread out in energy
• Increases its energy relative to mixing with bound levels
2-Level mixing
Normal mixing of
bound levels
Mixing of bound
level with unbound
level
Lower level (GPV) gets width and is lowered LESS
GPV
Principal Collaborators
QDS
Dennis Bonatsos
Libby McCutchanJan JolieRobert Casperson
GPV
Augusto MacchiavelliRod ClarkMichael Laskin
With huge help fromPeter von Brentano and Hans Weidenmuller who actually understand the
mixing of bound and unbound levels
Summary: QDS + GPV THANKS !
• Fundamental excitation mode of the nucleus predicted nearly 30 years ago (R. A. Broglia and D. R. Bès, Phys. Lett. B69 (1977) 129), but never seen.
• Schematic of the dispersion relation. The two bunches of vertical lines representthe unperturbed energy of a pair of particles placed in a given potential. The GPV is the collective state relative to the second major shell.
• The GPV is a coherent superposition of pp excitations analogous to the morefamiliar Giant Shape Vibrations based on ph excitations.
~ 20 G
~ 60 70MeV /A1/ 3
Investigating the Giant Pairing Vibration
6He beams with EBEAM~4-7 MeV/A at intensities of I≥106 particles/sec.
Experimental set-up is simple:
6He
Thin Aufoil
RutherfordCounter
a
208Pb target~10 mg/cm2
Annular SiDE-E detectors
Rate estimate:
sGPV = 3mb Target = 10 mg/cm2
IBEAM = 106 p/s eDET = 25%2000 counts/day in GPV
Experimental Considerations
L=0 transfer favors forward scattering angles