Physics Letters B 772 (2017) 274–278
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Compton scattering off proton in the third resonance region
Xu Cao
a,d,∗
, H. Lenske
b,c
a
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b
Institut für Theoretische Physik, Universität Giessen, D-35392 Giessen, Germany
c
GSI Darmstadt, D-64291 Darmstadt, Germany
d
State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o a b s t r a c t
Article history:
Received
20 March 2017
Received
in revised form 21 June 2017
Accepted
22 June 2017
Available
online 27 June 2017
Editor:
W. Haxton
Keywords:
Coupled-channel
Compton
scattering
Beam
asymmetry
Compton scattering off the proton in the third resonance region is analyzed for the first time, owing to
the full combined analysis of pion- and photo-induced reactions in a coupled-channel effective Lagrangian
model with K-matrix approximation. Two isospin I = 3/2resonances D
33
(1700) and F
35
(1930) are found
to be essential in the range of 1.6–1.8 GeV. The recent beam asymmetry data of Compton scattering
from the GRAAL facility are used to determine the helicity couplings of these resonances, and strong
constraints are coming also from π N and K photoproduction data. The possible spin and parity of new
narrow resonances is discussed.
© 2017 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
.
Compton scattering off the nucleon, as a reaction with subtle
convolution of two different scales – electromagnetic and strong
interactions, has attracted a lot of attention since a long time. The
majority of the investigations found in the literature is devoted to
the energy range up to the (1232) region with the aim to explore
the nucleon polarizabilities [1]. However, the reaction mechanism
of Compton scattering beyond the energy region of the (1232)
resonance is rarely studied. At high energies, the inelastic channels
are emerging through coupled-channel effects and thus essential
to the description of Compton scattering, which is dominated by
the electromagnetic couplings [2]. About two decades ago, L’vov
et al. took advantage of dispersion theory with the help of single-
pion
photoproduction and resonance photocouplings from an par-
tial
wave analysis by which they could extend the range of the
model applicability into the second resonance region [3]. More re-
cently,
the Giessen coupled-channel model accounted for Compton
scattering from the very beginning on, but analyses were limited
to the energies below 1.6 GeV due to the lack of the J = 5/2par-
tial
waves and resonances at that time [4–7]. The study in the
region of third resonances becomes possible owing to the continu-
ous
updates of the Giessen model. The present version accounts for
J = 5/2 resonances [8], careful refinements of isospin 1/2 [9–12]
and
3/2 partial waves [13], respectively, and includes explicitly
*
Corresponding author at: Institute of Modern Physics, Chinese Academy of Sci-
ences,
Lanzhou 730000, China.
E-mail
address: caoxu@impcas.ac.cn (X. Cao).
2π N channels [14] based upon the experimental progress from
several groups, e.g. CLAS, CBELSA, LEPS, SAPHIR and GRAAL et al.
This
objective is reinforced by the very recent measurement of
Compton scattering off the nucleon in the center-of-mass (c.m.)
energy range of around 1.6–1.8 GeV [15,16]. The observed sharp
structures seen in the beam asymmetry data with widths of
around 25 MeV is thought to correlate with the narrow enhance-
ment
in the data of η photoproduction off the nucleon [17,18]
and
high-precision measurements of π p elastic differential cross
sections [19,20]. So, Compton scattering off the nucleon in the
resonance region is not only a suitable process to study helicity
couplings of known resonances, but also an ideal tool to search
for possible exotic states that might be weakly coupled to the π N
state.
However, as widely discussed, a solid and comprehensive
combined analysis of relevant channels on the ground of available
data is highly desirable in order to clarify the underlying nature of
those rich spectroscopic structures.
The
Giessen model is built on effective Lagrangians, treating
mesons and baryons as effective degrees of freedom and obtaining
the pion–baryon vertices according to the principles of chiral sym-
metry.
The resulting equations for the scattering amplitudes are
solved by a coupled-channel approach respecting gauge invariance.
In order to fulfill unitary, the Bethe–Salpeter equation is solved in
K -matrix approximation,
T
fi
= K
fi
+ i
a,b
K
fa
Im(G
ab
)T
bi
, (1)
http://dx.doi.org/10.1016/j.physletb.2017.06.063
0370-2693/
© 2017 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
.