Summary
The spectral representation for the vacuum expectation value of the axial-vector-current commutator is used to discuss the current-field identities given byg A a αμ (x)=f π∂μϕα(x)+A αμ (x). Herea αμ (x) denotes the axial-vector-meson field,g A is a properly chosen normalization constant, and the pion field ϕα(x) is defined by the relation\(\partial ^\mu A_\mu ^\alpha (x) \equiv \widehat\varphi ^\alpha (x) = f_\pi m_\pi ^2 \varphi ^\alpha (x)\), whereA αμ (x) denotes the axial vector current. We find that
and that these expressions vanish if and only if the pion field ϕα(x) is a free field. We also note that
and that these expressions vanish if and only if the axial vector currentA αμ (x) is conserved. The consequences for canonical realizations of current-field identities and PCAC are given. We also find that the vacuum expectation value of the σ-term is nonvanishing unless the current is conserved and that a nonconserved charge cannot annihilate the vacuum (as stated by Coleman's theorem).
Riassunto
Si usa la rappresentazione spettrale del valore di aspettazione del vuoto per discuter e le identità corrente-campo date dag A a αμ (x)=f π∂μϕα(x)+A αμ (x). Quia αμ (x) denota il campo del mesone vettoriale assiale,g A è una costante di rinormalizzazione opportunamente scelta, ed il campo pionico ϕα(x) è definito dalla relazione\(\partial ^\mu A_\mu ^\alpha (x) \equiv \widehat\varphi ^\alpha (x) = f_\pi m_\pi ^2 \varphi ^\alpha (x)\), doveA αμ (x) denota la corrente vettoriale assiale. Si trova che
e che queste espressioni si annullano se e solo se il campo pionico ϕα(x) è un campo libero. Si nota anche che
e che queste espressioni si annullano se e solo se la corrente vettoriale assialeA αμ (x) è conservata. Si espongono le conseguenze per le realizzazioni canoniche delle identità corrente-campo e per PCAC. Si trova anche che il valore di aspettazione del vuoto del termine σ non tende a zero a meno che la corrente sia conservata e che una carica non conservata non possa annichilare il vuoto (come è affermato dal teorema di Coleman).
Резюме
Спектральное представление для вакуумной ожидаемой величины коммутатора аксиально-векторного тока используется для обсуждения ток-полевых тождеств, заданных соотношениемg A a αμ (x)=f π∂μϕα(x)+A αμ (x). Здесьa αμ (x) обозначает поле аксиально-векторного мезона,g A есть надлежашим образом выбранная перенормировочная постоянная; пионное поле ϕα(x) определяется соотношением\(\partial ^\mu A_\mu ^\alpha (x) \equiv \widehat\varphi ^\alpha (x) = f_\pi m_\pi ^2 \varphi ^\alpha (x)\), гдеA αμ (x) представляет аксиально-векторный ток. Мы получаем, что
и что эти выражения обрашаются в нуль, если и только если пионное поле ϕα(x) представляет свободное поле. Мы также отмечаем, что
и что эти выражения обращаются в нуль, если и только если аксиально-векторный токA αμ (x) сохраняется. Приводятся следствия для канонических реализаций токполевых тождеств и РСАС. Мы также находим, что вакуумная ожидаемая величина σ-члена не обращается в нуль, если ток не сохраняется, и что несохраняющийся заряд не может уничтожить вакуум (как утверждается теоремой Колемана).
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Statement II has been partly derived inH. Genz:Zeits. Phys.,229, 206 (1369).
It also follows from this (ref. (5)H. Genz:Zeits. Phys.,229, 206 (1369)) that the σ-term has a nonvanishing vacuum expectation value and thus a nonconserved charge cannot annihilate the vacuum as stated by Coleman's theorem (ref. (7) ).
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Clearly, ifj π(0)|Ω〉 vanishes then 〈z|j π(0)|Ω〉 vanishes and thus due to analyticity 〈z 1|j π(0)|z 2〉 also vanishes. However, even though it would still follow that 〈z|j π(0)|Ω〉 is small if ‖j π(0)Ω‖ is small (since all intermediate states given a positive contribution to ‖j π(0)Ω‖) it would no longer necessarily follow that 〈z 1|j π(0)|Z 2〉 is small. The only possible conclusion seems to be that for a particlez that can decay strongly an approximate one-particle saturation implies a relatively long lifetime, since the decay is described by the matrix clement |〈z|j z(0)|Ω〉|, which is small if ‖j z(0)Ω‖ is small.
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We assume that all the equal-time limits considered exist so that we may interchange limiting producedures (differentiation, equal-time limits) with the mass intergration in the spectral representations. For a general discussion of equal-time limits and their possible nonexistence in perturbation theory we refer the reader to ref. (17).
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Genz, H., Katz, J. On current-field identities. Nuovo Cimento A (1965-1970) 70, 1–11 (1970). https://doi.org/10.1007/BF02819160
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DOI: https://doi.org/10.1007/BF02819160