Physics Letters B 761 (2016) 333–339
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Physics Letters B
www.elsevier.com/locate/physletb
Twist-3 T-odd fragmentation functions G
⊥
and
˜
G
⊥
in a spectator
model
Yongliang Yang
a
, Zhun Lu
a,∗
, Ivan Schmidt
b
a
Department of Physics, Southeast University, Nanjing 211189, China
b
Departamento de Física, Universidad Técnica Federico Santa María, and Centro Científico-Tecnológico de Valparaíso, Casilla 110-V, Valparaíso, Chile
a r t i c l e i n f o a b s t r a c t
Article history:
Received
7 July 2016
Received
in revised form 15 August 2016
Accepted
24 August 2016
Available
online 29 August 2016
Editor:
G.F. Giudice
We present a calculation of the twist-3 T-odd chiral-even fragmentation functions G
⊥
and
˜
G
⊥
using a
spectator model. We consider the effect gluon exchange to calculate all necessary one-loop diagrams for
the quark–quark and quark–gluon–quark correlation functions. We find that the gluon loops corrections
generate non-zero contribution to these two fragmentation function. We numerically calculate their
half-k
T
moments by integrating over the transverse momentum and also verify the equation of motion
relation among G
⊥
,
˜
G
⊥
and the Collins function.
© 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Our understanding of the hadron structure depends on what we know about the parton distribution functions and fragmentation
functions. These functions appear in the decompositions of the parton correlation functions. In recent years, several kinds of experiments
have been carried out, such as the semi-inclusive deep inelastic scattering (SIDIS), and e
+
e
−
annihilation into hadrons, which provide us
considerable information on a class of T-odd and chiral-odd fragmentation functions. A notable example is the Collins function [1] H
⊥
1
that describes the fragmentation of a transversely polarized quark to an unpolarized hadron (e.g. a pion) and can be used to analyze the
hadronic quark spin contribution. It has been widely recognized that the Collins function plays an important role in the understanding of
the transverse single spin asymmetries (SSAs). In addition, the e
+
e
−
annihilation data combined with the SIDIS data can be applied to
extract [2,3] the Collins function.
Within
the field theoretical framework of QCD, there are two approaches to interpret SSAs in high energy processes: the transverse-
momentum-dependent
(TMD) approach [1,4,5] and the twist-3 collinear factorization in terms of multi-parton correlation [6–8]. Recently,
it was suggested [9] that the fragmentation contribution in the twist-3 collinear framework may be also important for the SSA in pp col-
lision.
Later phenomenological analysis [10] showed that, besides the contribution of the twist-3 collinear distribution functions, twist-3
fragmentation functions are also necessary for describing the SSA data in both SIDIS and pp collision [11–14] in a consistent manner [15].
Three chiral-odd fragmentation functions,
ˆ
H(z), H(z) and
ˆ
H
FU
(z, z
1
), participate in those processes. The first one corresponds to the first
k
T
-moment of the TMD Collins function and has been applied to interpret the SSA in pp collisions in previous studies [16,17]. The second
one appears in the subleading order of a 1/Q expansion of the quark–quark correlator, while its TMD version H(z, k
2
T
) is also a twist-3
function. The function
ˆ
H
FU
(z, z
1
) is the imaginary part of H
FU
(z, z
1
), and is connected to another fragmentation function
˜
H(z) through
an integration over z
1
[9,18], with
˜
H(z) the collinear version of
˜
H(z, k
2
T
), which is encoded in the TMD quark–gluon–quark correlation
function. It has been found that
˜
H also plays an important role in the transverse SSA sin φ
S
in SIDIS [19].
At
the twist-3 level, apart from
˜
H(z, k
2
T
) and H(z, k
2
T
), there are two other T-odd TMD fragmentation functions for a spin-0 hadron, de-
noted
by
˜
G
⊥
(z, k
2
T
) and G
⊥
(z, k
2
T
). They appear in the decompositions of the quark–gluon–quark and quark–quark correlators, respectively.
In the TMD framework [20], the fragmentation function
˜
G
⊥
(z, k
2
T
) may give rise to longitudinal beam SSA (denoted by A
sin φ
LU
) and target
*
Corresponding author.
E-mail
addresses: zhunlu@seu.edu.cn (Z. Lu), ivan.schmidt@usm.cl (I. Schmidt).
http://dx.doi.org/10.1016/j.physletb.2016.08.053
0370-2693/
© 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.