Summary
Approximate or «broken» symmetries are discussed in the framework of axiomatic field theory. A previous result is generalized to space-time symmetries:i.e., that a nonexact symmetry cannot be implemented by a time-dependent unitary operator. It is shown that this has nothing to do with the well-known difficulties of defining a charge, which exist also for exact symmetries. A further results is that if a symmetry is not exact, there is at least one observable whose VEV is maximally noninvariant.
Riassunto
Si discute il problema delle simmetrie approssimate nel quadro della teoria assiomatica dei campi. Un risultato precedente, e cioè che una simmetria non esatta non può essere realizzata mediante un operatore unitario dipendente dal tempo, viene esteso al caso di simmetrie spazio-temporali. Si rileva che tale risultato non ha alcuna relazione con le note difficoltà che sorgono quando si voglia definire una carica: tali difficoltà sussistono infatti anche per le simmetrie esatte. Si dimostra infine che se una simmetria non è esatta, c'è almeno una osservabile per il cui valor medio sul vuoto si ha non invarianza massima.
Резюме
Обсуждаются приближенные или нарушенные симметрии в рамках аксиоматической теории поля. Предыдуший результат, который состоит в том, что неточная симметрия не может быть осушествлена при помоши унитарного оператора, зависяшего от времени, обобшается для пространственно-временных симметрий. Показывается, что это не имеет ничего обшего с обшеизвестными трудностями характеристического заряда, которые сушествуют также и для точных симметрий. Дальнейшим результатом является то, что, если симметрия неточная, сушествует, по крайней мере, одна наблюдаемая, чья VEV является максимально неинвариантной величиной.
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References
E. Fabri andL. E. Picasso:Phys. Rev. Lett.,16, 408 (1966).
In this Section the usual axioms of quantum field theory are assumed. SeeR. F. Streater andA. S. Wightman:PCT, Spin and Statistics, and All That (New York, 1964).
We owe this result toS. Doplicher andD. W. Robinson: private communication byS. Doplicher.
S. Coleman:Journ. Math. Phys.,7, 787 (1966).
B. Schroer andP. Stichel: DESY preprint 66/6, February 1966.
D. Kastler, D. W. Robinson andA. Swieca:Comm. Math. Phys.,2, 108 (1966).
A. Katz:Extension of Hilbert space applied to inexact symmetries, preprint.
Here, as in Sect.2, the Wightman approach is assumed. We shall need however the stronger assumption that fields at a given time make sense (as three-dimensional distributions). See eq. (3). We owe this result toS. Doplicher andD. W. Robinson: private communication byS. Doplicher.
For sake of clarity we prefer here to use the almost synonymous terms «symmetry» and «invariance» in two sharply different meanings, A symmetry can be «exact» (in which case we shall call it «invariance») or «approximate», or even bably broken; an invariance is always exact.
For sake of simplicity, we do not consider the antiunitary case, as well as the effect of superselection rules. Both are irrelevant for the present discussion.
A physical state can be thought of in two ways: a positive functional on {ie381-1} or a ray in a representation space. Wigner's definition of symmetry clearly requires the vector approach. The difference between the two approaches is thoroughly discussed in ref. (13).
J. G. Glimm andR. V. Kadison:Pac. Journ. of Math.,10, 547 (1960) Corollary 7. See also the proof of Corollary 9.
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Fabri, E., Picasso, L.E. & Strocchi, F. Broken symmetries in quantum field theory. Nuovo Cimento A (1965-1970) 48, 376–385 (1967). https://doi.org/10.1007/BF02818013
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DOI: https://doi.org/10.1007/BF02818013