# What does "non-normal parity" mean?

Nuclear physicists seem to use the term "non-normal parity" a fair bit. Googling for the term yields some 840 results, and the same search on Google Scholar indicates that about 430 of those are research papers.

Unfortunately, those searches seem to only yield research-level papers that just state the term and roll with it, assuming the reader understands it and without stopping to explain it.

So: what does "non-normal parity" mean?

• ' "Nonnormal" parity states, involving either core excitation of the excitation of a (1p) nucleon into a higher shell...' - C.J. Batty et al., Physics Letters 19(1) 35-37 (1965). Aug 2 '16 at 14:23
• @JonCuster If you find that illuminating then I would ask you to explain it in an answer; that looks as obscure to me as do all other mentions I've seen. What is "normal" parity, and how exactly are "non-normal" states different? Aug 2 '16 at 14:26
• (Moreover, the reference of Batty et al. to Lane et al. (1960) is even more mystifying: "The normal parity of a nucleus is the parity of the ground state and the nonnormal parity is simply the opposite parity". If that's the case, what's with assigning non-normal parity to a ground state?) Aug 2 '16 at 14:32
• This is a good question. I did a couple of google searches and ended up just as confused as you are. I am very interested to see what the answer will be. Aug 2 '16 at 14:49

Going back to A.M. Lane, Reduced Widths of Individual Nuclear Energy Levels one finds the quote:

At this stage we introduce a separation of the discussion which is carried through to the end of the section, namely, nuclear states are discussed according to whether they are of "normal" or "nonnormal" parity. The normal parity of a nucleus is the parity of the ground state and the nonnormal parity is simply the opposite parity. This division is natural on the shell model, where the lowest states of normal parity have the normal configuration, i.e., a number of close shells and a few "loose" particles in an unfilled orbit. Nonnormal parity states, on the other hand, have a more complicated structure since they must involve the excitation of a particle to a higher orbit or the disturbance of the closed shells.

One major difference pointed out later in the text uses C$^{18}$ as the example, where the 'normal' negative parity has one configuration that is the lowest energy, while the 'nonnormal' positive parity has some 75 different states (severely complicating the calculation of the widths of the nuclear energy levels).

Does that help?

EDIT: Looking through the TUNL A=10 nuclear data sheets, they seem to use 'unnatural parity' vs 'natural parity', pointing out for several of the various reactions used that some are more selective of 'unnatural parity' states than others. Perhaps at this point 'non-normal' has morphed into 'unnatural' in the literature.

• No. As you yourself linked, and I already pointed out, Barker and Hickey (1977) explicitly claim to produce "evidence that the ground state of $^{10}\mathrm{Li}$ has non-normal parity" (from their abstract). A ground state with non-normal parity makes absolutely no sense by Lane's definition. Aug 2 '16 at 14:45
• But the Batty paper say the non-normal comes from excitation of a nucleon... Sigh. I think at this point I may bow out gracefully, having demonstrated a near-complete lack of understanding of nuclear shell models... Aug 2 '16 at 14:48
• Cool. I don't doubt that such nuclei have non-trivial physics that makes the term sort of applicable there when suitably modified - what I want to know is what those physics and modifications are. Thanks for the help, if only in uncovering that the literature itself is not particularly consistent on the topic (which then only makes it worse that it is just thrown about unexplained even in 100p+ papers). Aug 2 '16 at 14:51

A state with parity of $$(−1)^L$$ is defined as a natural parity state, with $$L$$ being the total angular momentum of the system, and parity with value of $$+1$$ or $$−1$$ are called even or odd, respectively. A state with parity of $$(−1)^{L+1}$$ is called unnatural parity state.

Source: Ye Ning et al., Natural and Unnatural Parity Resonance States in the Positron-Hydrogen System with Screened Coulomb Interactions. https://doi.org/10.3390/atoms4010003

• This thread is quite old, so you'll have to help me refresh on the details. The question asks about non-normal parity, whereas your source talks about non-natural parity. Can you supply examples of literature where the two are stated to be equivalent? Dec 15 '20 at 10:01