Chiral Symmetry Breaking in Nuclei J.H. Hamilton 1, S.J. Zhu 1,2,3, Y.X. Luo 1,4,, A.V. Ramayya 1, J.O. Rasmussen 4, J.K. Hwang 1, S. Frauendorf 5, V. Dimitrov 5, G.M. Ter-Akopian 6, and A.V. Daniel 6 1 Vanderbilt Univ., 2 Tsinghua Univ., 3 JIHIR, ORNL, 4 LBNL, Berkeley 5 Univ. Notre Dame, 6 JINR, Dubna

Two Chiral Molecular States Mirror

Chirality in particle physics Spin parallel or antiparallel to a particle’s momentum. The two orientations corresponds to a right- handed and left-handed system. Left-handed neutrinos introduced chiral asymmetry into the world. P S

Nuclei with only two types of particles should be achiral. Frauendorf et al. noted chiral bands can be associated with angular momentum in triaxial nuclei when the total angular momentum is not along any axis. Chirality is a geometrical concept that derives only from the orientation of the angular momentum with respect to the triaxial shape. R Z : the rotation about the Z axis, T : Time reversal

Chiral doubling When the angular momentum has substantial components along all three axes of the tri- axial nucleus, then there are 2 energetically equivalent orientations of the angular momentum vector with the short, intermediate and long axes forming right or left handed systems with respect to the angular momentum. This can give rise to two  I = 1 bands with same parity and very close energy.

Examples of chiral doublets Odd-odd nuclei (Z~59, N~75) where the angular momentum is composed of odd h 11/2 proton along short axis, h 11/2 neutron hole along long axis and collective rotation along intermediate axis, 134 Pr. Recently reported “The best chiral properties observed to date were discovered in the 104 Rh nucleus involving the h 11/2 x  g 9/2 -1 configuration where the valence proton and neutron play opposite roles to those in the A=130 region.” (PRL92, 2004)

In order to demonstrate the general nature of chirality it is important to find examples of chiral sister bands with a different qp composition. Here we report the first observation of a pair of chiral vibrational bands in several even- even nuclei with A=

Experimental Details 1.Source : 252 Cf with T 1/2 = 2.6 y, SF : 3 % 2.Strength : 62  Ci 3.Sandwiched between two 10 mg/cm2 Fe foils inside a 3 in diameter plastic ball. 4.Detectors : Gammasphere with 102 Comp. supressed Ge detectors. 5.6X10 11 triple and higher fold coincidence events. 6.Compressed and less compressed cubes and time-gated cubes for analysis with Radware program.

Evidence for Triaxiality in A= region One and two phonon gamma bands in 104,106 Mo Guessous et al., PRL 75 (1995). Energies of gamma bands in Ru isotopes, Shannon et al., PL B336 (1994). Hua et al., PR C69 (2004) reported evidence for triaxiality in odd A 103,105,107 Mo but with 101,103 Zr having more axially symmetric shapes. Our studies have provided new insight into tri-axiality in this region. Extended the one and two phonon gamma bands in 104,106 Mo and gamma bands in 108,110,112 Ru. Studied 99,101 Y, 103,105,107 Nb, 105,107,109 Tc and 109,111,113 Rh. Carried out Tri-axial-Particle-plus-Rotor calculations for Y, Nb, Tc and Rh nuclei.

Energy levels of the asymmetric rotor 

Gamma band staggering

 7/2 + [413] band level staggering

Theory :  2 ~ 0.3,  = o

(4) (5) (12 - ) (14 - ) 106 Mo    Chiral doublet bands Ground  

2 +  0 + and 4 -  3 +  Red : band 4 Green : band 5

TAC predicts Chiral Vibrational Bands. Band-(4) : Zero Phonon Band-(5): One phonon

Tilted axis cranking calculations yield  = 0.29 and  = 31 o. TAC calculations indicate the chirality is generated by h 11/2 particle coupled to the short axis and a mixed d 5/2,g 7/2 hole coupled to the long axis with collective rotation along the intermediate axis. The angular momentum vector moves with increasing rotational frequency from the plane spanned by the intermediate and long axis through an aplanar orientation into the plane spanned by the short and intermediate axis, in contrast to odd-odd nuclei where it moves from the long-short plane toward the intermediate axis.

Thus the 106 Mo structure and motion of the equilibrium position of the angular momentum are quite different from the odd-odd nuclei. The angular momentum moves rather rapidly from one into another principal plane with a very shallow minimum of the aplanar orientation. These indicate the chirality has a dynamical character. It appears as a low energy vibration which correspond to slow excursions of the angular momentum vector into left handed and right handed regions rather than substantial tunneling between the left and right handed configurations in odd-odd nuclei. The two bands in 106 Mo correspond to the zero and one phonon states of the chiral vibration. The 106 Mo case is unusual in that zero phonon chiral band decays only to the one phonon gamma band and the one phonon band decays only to the two phonon gamma band. This is an interesting unsolved question for theory.

104 Rh 106 Mo S(I) = 1/2J moi = [E(I)-E(I-1)]/2I, Vaman et al., PRL (2004)

1.New type chiral bands 2.Chiral vibrational bands in even-even nucleus 3.TAC calculations indicate that in 106 Mo chirality has a dynamical character 4.Two bands are low energy zero and one phonon chiral vibrations 5.This different mechanism of generating chirality helps prove the general nature of chirality in nuclei

Crossover to cascade transition strengths and transitions between the pairs of doublets. In some cases cascade transitions are stronger and then in others the  I=1 transitions are stronger. In some cases there are crossing transitions going up in spins( 104 Rh) and in other cases ( 106 Mo) crossing transitions occur only at the bottom. There is an unknown phase factor that connects the right -and left-hand terms. Differences in phase strongly influence both of the above properties. So E2/M1 ratios and crossing transitions between doublet pairs are not unique signatures of chirality.

Examples of transition energies and relative intensities in 110 Ru EnergiesRel. IntensitiesEnergiesRel. Intensities  bands  bands  bands   bands  bands  bands  bans  bands  bands

8 -  8 + and 8 +  6 + g

S(I) = 1/2M.O.I.

100 Zr

Ground band strongly axial symmetric prolate deformed  2 =0.37 An excited band from 0 + to 12 + is nearly spherical  2 =0.12. The two new doublet bands can be a) axial symmetric b) tri-axial. The 5 - band heads for the doublets decay to the 4 + and 6 + ground band states. We measured the half-lives of both 5- states to be < 8nsec by triple coincidence technique for time windows 4, 8, 16, 20, 28, 48, 72, 100, 300, 500 nsec. These short life times are not consistent with  K=5 if these are axially symmetric states. Thus the life times and branching ratios indicate these bands are tri-axial.

(spin) Exp. Theory 100 Zr

Energy differences between states of same spin 106 Mo and 110 Ru have very similar energy differences. 100 Zr has the largest most nearly zero (degenerate) levels observed in any nucleus. Thus it could have the best chiral doublet bands. In 112 Ru the energy differences decrease rapidly toward zero as in 104,105 Rh and 134 Pr and then cross zero as in 134 Pr. This crossing could indicate that these nuclei are soft chiral vibrations away from the crossing point where they pass through a region of chiral rotation.

The energy difference to first order is Plank’s constant times tunneling frequency between the right and left handed terms. However, Frauendorf has recently noted there is a phase factor connecting the two terms and it could change with spins. It is possible that a phase factor that slowly varies with spin could explain the crossing of zero. This phase factor also strongly influences the intensity ratios of cross-over to cascade transitions and transitions between bands.

Summary Observed first pairs of degenerate doublet bands in even- even neutron-rich nuclei, 106 Mo, 110,112 Ru and 100 Zr. In tri-axial 106 Mo and 110 Ru, these bands have properties of soft chiral vibrations where neutron particles and holes provide the conditions for tri-axial shape. In tri-axial 112 Ru they look more like the chiral rotors observed in other nuclei. These data on chiral vibrations provide evidence that chiral behavior is a more general property of nuclei. The 100 Zr doublets are likely also chiral vibrational bands. This is unexpected since it’s ground state is axially symmetric and strongly deformed. Thus there is new physics here. 100 Zr is the first case of coexistence of strongly deformed and nearly spherical shapes coexisting with triaxial shape. Problem of how to explain the crossing of zero of the energy differences is an important major problem. This may be related to the phase factor between the right- and left- symmetry states.