Physics Letters B 770 (2017) 551–555
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Bayesian extraction of the parton distribution amplitude from the
Bethe–Salpeter wave function
Fei Gao
b,c
, Lei Chang
a,∗
, Yu-xin Liu
b,c,d
a
School of Physics, Nankai University, Tianjin 300071, China
b
Department of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China
c
Collaborative Innovation Center of Quantum Matter, Beijing 100871, China
d
Center for High Energy Physics, Peking University, Beijing 100871, China
a r t i c l e i n f o a b s t r a c t
Article history:
Received
11 November 2016
Received
in revised form 24 April 2017
Accepted
25 April 2017
Available
online 10 May 2017
Editor:
J.-P. Blaizot
Keywords:
Parton
distribution function
Nakanishi
representation
Bethe–Salpeter
amplitude
Maximum
entropy method
Light-front
wave function
We propose a new numerical method to compute the parton distribution amplitude (PDA) from the Eu-
clidean
Bethe–Salpeter wave function. The essential step is to extract the weight function in the Nakanishi
representation of the Bethe–Salpeter wave function in Euclidean space, which is an ill-posed inversion
problem, via the maximum entropy method (MEM). The Nakanishi weight function as well as the corre-
sponding
light-front parton distribution amplitude (PDA) can be well determined. We confirm prior work
on PDA computations, which was based on different methods.
© 2017 The Authors. 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
The parton distribution amplitudes (PDAs) of mesons, encoding
the meson structure on the light front, play an essential role in the
hard exclusive processes. The cross sections of the processes can
be written as the convolution of the hard-scattering kernel which
can be computed perturbatively, and the PDAs of the hadrons in-
volved [1,2].
The leading twist parton distribution amplitudes of
mesons are defined by integrating out the transverse momentum
k
⊥
from the light front wave function, which are obtained through
projecting meson’s Bethe–Salpeter wave function onto the light-
front.
Although
some efforts have been made to calculate Bethe–
Salpeter
equation (BSE) directly in Minkowski space (see, e.g.,
Ref. [3]) with a simple scattering kernel, many BSE calculations
are still carried out in Euclidean space which are much easier to
handle. The challenge in the Euclidean scheme is how to project
the discrete Euclidean wave function data on the light-front to
get light-front quantities. The Nakanishi representation of the wave
function provides a natural way to solve this problem. This chal-
*
Corresponding author.
E-mail
address: leichang@nankai.edu.cn (L. Chang).
lenging question amounts to whether it is possible to compute
the weight function of the Nakanishi representation if one has an
appropriate solution of the BSE in Euclidean space. Nakanishi rep-
resentation
was proposed in Ref. [4] to parameterize the relativistic
two-particle bound state in Minkowski space.
Despite
lacking a non-perturbative proof of uniqueness of the
weight function in the Euclidean case, we suppose that the wave-
function
can still be parametrized by the following similar form,
(k, P ) =
1
−1
dz
∞
0
dγ
g(γ , 1 − 2x)
(k
2
+ zk · P +
1
4
P
2
+ M
2
+γ )
3
, (1)
where k
2
> 0is the space-like momentum and P
2
=−M
2
bs
with
M
bs
the bound state mass and M is an infrared regulated scale.
The weight function g(γ , 1 − 2x) is a two-dimensional function in
real space. The corresponding leading twist two-particle light-front
parton distribution can be defined as
ϕ(x) =
d
4
kδ(n ·k − xn · P )(k −
P
2
, P ), (2)
where n is the light-like vector n
2
= 0. We neglect the possible
spin structure for simplification at the moment. With the help of
http://dx.doi.org/10.1016/j.physletb.2017.04.077
0370-2693/
© 2017 The Authors. 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
.