361 Page 4 of 18 Eur. Phys. J. C (2019) 79 :361
¯
J (q
1
, q
2
) is still UV divergent. The global UV infinities are
subtracted by separating physical and non-physical scales in
¯
D
p
. That means using the identity
1
¯
D
p
=
1
¯q
2
1
−
2(q
1
· p)
¯q
2
1
¯
D
p
, (15)
and noticing that the second term is more UV convergent
than the original propagator. The same expansion has to be
applied to the other propagators in 1/
¯
D, until
¯
F/
¯
D is written
as follows
¯
F
ˆρ ˆσ
(q
1
, ¯q
2
1
)
¯
D
=
¯
F
ˆρ ˆσ
(q
1
, ¯q
2
1
)
¯
D
V
+
¯
F
ˆρ ˆσ
(q
1
, ¯q
2
1
)
¯
D
F
,
(16)
where [
¯
F/
¯
D]
V
do not depend on physical scales. Since also
¯
G
ρσ
does not contain physical scales, [
¯
F/
¯
D]
V
defines the
global UV divergent behavior of
¯
J (q
1
, q
2
):
[
¯
J (q
1
, q
2
)]
GV
:=
¯
F
ˆρ ˆσ
(q
1
, ¯q
2
1
)
¯
D
V
¯
G
ρσ
. (17)
[
¯
J (q
1
, q
2
)]
GV
is called a Global Vacuum (GV) and is written
between square brackets, that is the standard FDR notation
to indicate the vacuum part of an object. Note that (
¯
F/
¯
D)
F
in (16) gives rise to a subtracted integrand which is globally
UV convergent but still divergent when q
2
→∞:
¯
F
ˆρ ˆσ
(q
1
, ¯q
2
1
)
¯
D
F
¯
G
ρσ
. (18)
This is fixed by subtracting the Sub-Vacuum (SV) from
¯
G
ρσ
by means of the expansion
1
¯q
2
12
=
1
¯q
2
2
−
q
2
1
+ 2(q
1
· q
2
)
¯q
2
2
¯q
2
12
. (19)
The final result has the form
¯
G
ρσ
=
¯
G
ρσ
SV
+
¯
G
ρσ
F
, (20)
so that the fully UV subtracted integrand
¯
F
ˆρ ˆσ
(q
1
, ¯q
2
1
)
¯
D
−
¯
F
ˆρ ˆσ
(q
1
, ¯q
2
1
)
¯
D
V
¯
G
ρσ
−
¯
G
ρσ
SV
(21)
is integrable in four dimensions.
3
Upon integration, the vacua
subtracted in (21) induce the appearance of logarithms of
μ
2
of UV origin, so that both IR and UV singularities are
regulated by the same regulator.
3
The described procedure is quite general. For example, despite the
fact that subtracting sub-divergences via (19) potentially re-introduces
UV singularities due to q
1
, the expressions can be arranged in such a way
that fully subtracted integrands can be always written down. Indeed, this
has been automated in the case of two-loop off-shell QCD amplitudes
in [22].
The procedure leading to (21) is conveniently encoded in
a linear integral operator
[d
4
q
1
][d
4
q
2
], (22)
whose action on a two-loop integrand is defined by three
subsequent operations:
– subtract the vacua;
– integrate over q
1
and q
2
;
– take the asymptotic limit μ
2
→ 0.
The last operation means retaining only the logarithmic
pieces in the asymptotic expansion, neglecting O(μ
2
) terms.
Thus, the FDR two-loop integration over
¯
J (q
1
, q
2
) in (11)is
defined as follows
4
¯
I :=
[d
4
q
1
][d
4
q
2
]
¯
N
¯
D ¯q
2
2
¯q
2
12
=
d
4
q
1
d
4
q
2
¯
F
ˆρ ˆσ
(q
1
, ¯q
2
1
)
¯
D
F
¯
G
ρσ
F
. (23)
In the following, we will often omit terms that integrate to
zero in globally prescribed numerators. Hence, it is conve-
nient to introduce a notation for that, that is
¯
N
¯
N if both
numerators give the same result upon FDR integration.
Equation (23) defines a gauge-invariant object, in which
the necessary gauge cancellations are preserved by the GP
operation. By “gauge-invariant object” we mean that a cal-
culation of
¯
I in a different gauge will give the same result. It
is instructive to check that no change in
¯
I is produced if one
shifts the numerator of the gluon propagator as
g
ρ ˆρ
→ g
ρ ˆρ
+ λ
1
q
ρ
1
q
ˆρ
1
¯q
2
1
, g
σ ˆσ
→ g
σ ˆσ
+ λ
2
q
σ
1
q
ˆσ
1
¯q
2
1
, ∀λ
1,2
.
(24)
Thus,
¯
I gives the same result when computed in any gauge.
Another consequence of the WIs is that the term proportional
q
ρ
2
q
σ
1
+ q
ρ
1
q
σ
2
in (13) should not contribute to
¯
I when con-
tracted with
¯
F
ˆρ ˆσ
. That is,
[d
4
q
1
][d
4
q
2
]
¯
F
ˆρ ˆσ
(q
1
, ¯q
2
1
)
¯
D ¯q
2
2
¯q
2
12
q
ρ
1
q
σ
2
+ q
ρ
2
q
σ
1
= 0. (25)
After taking into account the vanishing of scaleless integrals,
this corresponds to the WI depicted in Fig. 3. Nevertheless,
we keep this piece in (13), as we will explicitly show in our
calculation that it never contributes.
4
Due to the vacuum subtraction, q
2
i
/ ¯q
2
i
= 1 under FDR integration.
This is the reason why we perform the explicit gauge cancellations of
(14).
123