4 M. Chaichian et al. / Nuclear Physics B 950 (2020) 114846
is NC QFT. We prove that classical results, such as the irreducibility of the set of field operators,
the theorems of Reeh and Schlieder [1–4] remain valid in the noncommutative case. It should
be emphasized that the results obtained in this paper do not depend on the SO(2) invariance of
the theory in the v
ariables x
1
and x
2
and therefore can be extended to more general cases. The
irreducibility of the set of field operators remain valid in any theory, which is translation invariant
in commutative variables, if only eq. (27)is fulfilled. The first theorem of Reeh and Schlieder is
valid, if the Wightman functions are analytical in the v
ariables x
0
and x
3
in the primitive domains
of analyticity (“tubes”).
In the SO
(1, 3) invariant theory new consequences of the generalized Haag’s theorem are
found, without analogues in NC QFT. At the same time it is proven that the basic physical
conclusion of Haag’s theorem is valid also in the SO(1, 1) invariant theory, and it is sufficient
that spectrality
, local commutativity condition and translational invariance be fulfilled only for
the transformations concerning the commutating coordinates. The analysis of Haag’s theorem
reveals essential distinctions between commutative and noncommutative cases, more precisely
between the SO(1, 3) and SO(1, 1) invariant theories. In the commutative case, the conditions
(59) and (60), whose consequence is generalized Haag’
s theorem, lead to the equality of Wight-
man functions in two theories up to four-point ones. In the present paper it is shown that in the
SO(1, 1) invariant theory, unlike the commutative case, only two-point Wightman functions are
equal and it is sho
wn that from the equality of two-point Wightman functions in two theories it
follows that if in one of them the current is equal to zero, it is equal to zero in the other as well
and under weaker conditions than the standard ones. It is also shown that for the deri
vation of
eq. (60)it is sufficient to assume that the vacuum vector is a unique normalized vector, invariant
under translations along the axis x
3
. It is proven that from the equality of four-point Wightman
functions in two theories, the equality of their elastic scattering amplitudes follows and, owing
to the optical theorem, the equality of total cross sections as well. In derivation of this result the
local commutativity (LCC) is not used.
The study of W
ightman functions leads still to new nontrivial consequences also in the com-
mutative case.
2
The paper is arranged as follows. In section 2 the basic properties of Wightman functions in
space-space NC QFT are formulated; in section 3 the irreducibility of the set of field operators
is proven; in section 4 generalizations of the theorems of Reeh and Schlieder to NC QFT are
obtained; section 5 is devoted to generalized Haag’
s theorem; in section 6 it is shown that in the
commutative case, the conditions of weak local commutativity (WLCC) and of local commuta-
tivity (LCC), which are valid in the noncommutative case ((24) and (22)), appear to be equivalent
to the usual WLCC and LCC, respectively.
2. Basic properties of Wightman functions in space-space NC QFT
As in the commutative case, we assume that every vector from the space of the complete set
of all physical states, J , can be approximated with arbitrary accuracy by the vectors of the
ϕ
f
1
··· ϕ
f
n
0
. (8)
In other words the vacuum vector
0
is cyclic, i.e. the axiom of cyclicity of vacuum is fulfilled.
2
A partial result on the subject had been previously communicated in [40].