Eur. Phys. J. C (2019) 79 :685 Page 3 of 11 685
Two effects are embodied in the DL kernel dw
DL
i
through
the replacement in Eq. (2). The QCD coupling α
S
is evalu-
ated at the scale of the soft-gluon transverse momentum q
T
[17,18]: this accounts for the resummation of the LL terms.
The gluon coupling acquires a correction of O(α
2
S
) (which
is controlled by the coefficient K in the
MS renormalization
scheme): this produces the resummation of NLL terms. Since
the replacement takes place in the exponent of the Sudakov
form factor, it is produced by the correlated radiation of soft
partons (both two soft gluons and a soft q ¯q pair), whereas
the independent emission of soft gluons is taken into account
through the exponentiation. We also note that A
CMW
i
(α
S
) is
an effective coupling at the inclusive level, since it is obtained
by integrating over the momenta of the final-state correlated
partons. The coupling A
CMW
i
(α
S
) refers to radiation that is
both soft and collinear. The Sudakov form factor includes
other NLL terms due to soft wide-angle (i.e., non-collinear)
radiation and hard (i.e., non-soft) collinear radiation: we post-
pone some comments on these terms.
Since the CMW Sudakov kernel refers to soft and collinear
radiation, it can be viewed as obtained by considering the
soft limit of multiple collinear radiation. In this respect it
is natural to compare it with the DGLAP kernel [19,20]
that controls the collinear evolution of the parton distribu-
tion functions (PDFs). In the soft limit, z → 1, the flavour
diagonal DGLAP kernel P
ii
(α
S
; z) (1− z is the longitudinal-
momentum fraction that is radiated in the final state) has the
following behaviour [21]:
P
ii
(α
S
; z) =
1
1 − z
A
i
(α
S
) +··· ,(z < 1), (4)
where the dots on the right-hand side denote terms that are
less singular than (1 − z)
−1
(we have also neglected contact
terms, proportional to δ(1 − z), of virtual origin). The soft
behaviour in Eq. (4) also applies to the collinear evolution of
the parton fragmentation functions.
The perturbative function A
i
(α
S
) in Eq. (4)isusually
called (light-like) cusp anomalous dimension, since it can
also be related to the renormalization of cusp singularities
of Wilson loops [22,23]. In the context of our discussion,
A
i
(α
S
) directly refers to the soft limit in Eq. (4), indepen-
dently of any relations with Wilson loop renormalization.
The perturbative expansion of A
i
(α
S
) reads
A
i
(α
S
) =
∞
n=1
α
S
π
n
A
(n)
i
. (5)
where α
S
is the renormalized MS coupling. The perturbative
coefficients A
(1)
i
, A
(2)
i
[24,25] and A
(3)
i
[19,20] are explicitly
known. Using the
MS factorization scheme for PDFs and
fragmentation functions, these coefficients are
A
(1)
i
= C
i
, (6)
A
(2)
i
=
1
2
KC
i
, (7)
A
(3)
i
= C
i
245
96
−
67
216
π
2
+
11
720
π
4
+
11
24
ζ
3
C
2
A
+
−
209
432
+
5
108
π
2
−
7
12
ζ
3
C
A
n
F
+
−
55
96
+
1
2
ζ
3
C
F
n
F
−
1
108
n
2
F
, (8)
where ζ
k
is the Riemann ζ -function. The fourth-order coef-
ficient A
(4)
i
is known in approximate numerical form [26,27]
(the calculation in full analytic form is under completion),
and a first numerical estimate of A
(5)
q
has been presented
recently [28]. By direct inspection of Eqs. (6)–(8) we note that
the dependence on i (the type of radiating parton) of the per-
turbative function A
i
(α
S
) is entirely specified up to O(α
3
S
) by
the overall colour factor C
i
. This overall dependence on C
i
,
which is customarily named as Casimir scaling relation, fol-
lows from the soft-parton origin of A
i
(α
S
) [29], and it is vio-
lated at higher perturbative orders [30], starting from O(α
4
S
).
From Eqs. (5)to(7) we see that, up to the second per-
turbative order, A
i
(α
S
) coincides with the CMW coupling
A
CMW
i
(α
S
) in Eqs. (2) and (3). One may be tempted to con-
clude that the cusp anomalous dimension provides a sensible
definition of a physical (though effective) soft-gluon cou-
pling beyond O(α
2
S
). The equivalence between A
i
(α
S
) and
soft-gluon coupling, however, cannot hold in general. Indeed
A
i
(α
S
) depends on the factorisation scheme of collinear sin-
gularities, while the physical coupling should not.
We add more comments on this point, since there are
conceptual analogies (and differences) between the soft-
collinear part of the Sudakov kernel and the soft limit of the
DGLAP kernel. The DGLAP kernel is related to the proba-
bility of correlated emission of collinear partons with com-
parable values of transverse momenta (independent collinear
emission is instead taken into account through the perturba-
tive iteration of the kernel). To obtain the DGLAP kernel, the
transverse momenta are integrated up to some value of the
evolution (or factorization) scale. The transverse-momentum
integral is collinear divergent in the low-momentum region:
within the
MS factorization scheme, the divergences are han-
dled by using dimensional regularization in d = 4−2 space
time dimensions, and the DGLAP kernel is defined as the
coefficient of the ensuing 1/ pole [see related comments
after Eq. (22)]. This is an unphysical procedure, although it
is perfectly well defined for factorization purposes (a differ-
ent factorization procedure would lead to a different DGLAP
kernel). In contrast, the q
T
integration of the Sudakov ker-
nel does not lead to collinear divergences since the low-q
T
region is ‘physically’ regularized by the definition of the
measured observables. Nonetheless, the equality between the
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