
φ
1
(x
1
)φ
2
(x
2
) as a local operator, between two points x
1
, x
2
we have to choose the point
closer to x
3
. Then in the limit m`
2
1 the correlation function can be bounded with help
of (1.1)
|hφ
1
(x
1
)φ
2
(x
2
)φ
3
(x
3
)i| ≤ e
−m`
2
, m`
2
1. (1.5)
It is easy to see though that the exponential factor in (1.5) is too naive. Indeed, when
all three points are simultaneous x
0
i
= 0, without loss of generality we can choose the
coordinate system such that point ~x
3
sits at the origin, while ~x
2
= (`
1
, 0, 0, . . . ) and ~x
1
=
(a, b, 0, . . . ), where
a =
`
2
1
+ `
2
2
− `
2
3
2`
1
, b =
√
D
2`
1
, (1.6)
and
D = 2(`
2
1
`
2
2
+ `
2
2
`
2
3
+ `
2
3
`
2
1
) − `
4
1
− `
4
2
− `
4
3
= (`
1
+ `
2
− `
3
)(`
2
+ `
3
− `
1
)(`
3
+ `
1
− `
2
)(`
1
+ `
2
+ `
3
) = 16S
2
(`
1
, `
2
, `
3
). (1.7)
(Here S(`
1
, `
2
, `
3
) is the area of the triangle with the sides `
1
, `
2
, `
3
given by Heron’s for-
mula.) This is shown in figure 1, where we only keep first two components of ~x
i
, while
all others, as well as time component, are identically zero. Next, one can use Euclidean
quantization and choose time direction along (x
2
− x
3
)
µ
,
G
123
= h0|φ
2
(`
1
, 0)φ
1
(a, b)φ
3
(0, 0)|0i = h0|φ
2
(0, 0)e
−(`
1
−a)H
φ
1
(0, b)e
−aH
φ
3
(0, 0)|0i, (1.8)
resulting in the exponential suppression e
−m`
1
. This is better than (1.5). This simple
exercise shows that the exponential rate of suppressed of higher-point correlators imposed
by the two-point function is not optimal. In this paper we argue that the optimal rate of
suppression, i.e. the best rate which would universally apply to all theories and operators
φ
i
, for the three-point correlator is given by the sum of distances to the operator locations
from the Fermat-Steiner point (4.11),
hφ
1
(x
1
)φ
2
(x
2
)φ
3
(x
3
)i ∼ e
−m `
Fermat
. (1.9)
This behavior was previously established in the context of certain two-dimensional mod-
els [5–7]. When the theory is confining it readily follows from the minimal length geometry
of flux tubes [8, 9], leading to the so-called Y-law. We extend (1.9) to non-confining the-
ories in any dimensions. We also consider configurations when the three points x
µ
i
do not
lie on the same spatial plane, while all three mutual intervals are space-like, and introduce
the notion of Fermat point in that case. We argue that the suppression rate of the higher
point correlation functions is determined by the shortest tree-level graph connecting all
points — the solution of the Euclidean Steiner tree problem.
This paper is organizes as follows. In the next section we discuss three-point function
when all three points x
µ
i
belong to the same spatial plane. In section 3 we discuss possible
configurations of three points in the Minkowsi space when all mutual intervals are space-
like. Section 4 is devoted to calculation of the suppression rate of the three point function
for a general Minkowskian configuration. Section 5 concludes with a discussion of higher
point functions and implications for operator growth in relativistic theories.
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