The
J
/
ψ
-
p
scattering length,
α
J
/
ψ
p
, can be extracted from the
J
/
ψ
photoproduction cross section near threshold using the Vector Meson Dominance (VMD) model to relate the reaction
γ
p
→
J
/
...ψ
p
to
J
/
ψ
p
→
J
/
ψ
p
. Such estimates based on experimental data result in values for
|
α
J
/
ψ
p
|
, which are much lower than most of the theoretical predictions. In this work, we study the relations between the different results, depending on the use of the total or the differential cross sections, and the method of extrapolating the data to threshold in the case of a low-statistics data sample, such as the near threshold
J
/
ψ
photoproduction dataset. We estimate a range for
|
α
J
/
ψ
p
|
of 0.003–0.025 fm as extracted from experimental data within the VMD model and discuss possible reasons for such lower values compared to the theoretical results.
Threshold π- photoproduction on the neutron Briscoe, W. J.; Kudryavtsev, A. E.; Strakovsky, I. I. ...
The European physical journal. A, Hadrons and nuclei,
08/2020, Letnik:
56, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Recent data from the PIONS@MAX-lab Collaboration, measuring the total cross section of the incoherent pion photoproduction reaction,
γ
d
→
π
-
p
p
, near threshold, have been used to extract the
E
0
...+
multipole and total cross section of the reaction
γ
n
→
π
-
p
, also near threshold. These are the first measurements of the reaction
γ
d
→
π
-
p
p
in the threshold region. The value of
E
0
+
is extracted through a fit to the deuteron data in a photoproduction model accounting for final-state interactions. The model takes an
S
-wave approximation for the elementary reaction
γ
n
→
π
-
p
with
E
0
+
=
const in the threshold region. The fit over all the 6 deuteron data points gives the value
E
0
+
=
-
31.86
±
0.8
(in units
10
-
3
/
m
π
). We explore the dependence of our results on the choice of data subsets included in the fit. The obtained values of
E
0
+
, for different subsets, have overlapping errors and agree with previous determinations.
The unpolarized and polarized Beam Charge Asymmetries (BCAs) of the
e
→
±
p
→
e
±
p
γ
process off unpolarized hydrogen are discussed. The measurement of BCAs with the CLAS12 spectrometer at the ...Thomas Jefferson National Accelerator Facility, using polarized positron and electron beams at 10.6 GeV is investigated. This experimental configuration allows to measure azimuthal and
t
-dependences of the unpolarized and polarized BCAs over a large
(
x
B
,
Q
2
)
phase space, providing a direct access to the real part of the Compton Form Factor (CFF)
H
. Additionally, these measurements confront the Bethe-Heitler dominance hypothesis and eventual effects beyond leading twist. The impact of potential positron beam data on the determination of CFFs is also investigated within a local fitting approach of experimental observables. Positron data are shown to strongly reduce correlations between CFFs and consequently improve significantly the determination of
R
e
H
.
We report on the results of the first search for the production of axion-like particles (ALPs) via Primakoff production on nuclear targets, γA→aA, in the “SRC-CT” experiment using the GlueX detector ...at Jefferson Lab. This search uses an integrated luminosity of 100 pb⋅−1nucleon on a 12C target with a real photon beam of energies 6<Eγ<10.8 GeV, and explores the mass region of 200<ma<450 MeV via the decay a→γγ. This mass range is between the π0 and η meson masses, which enables the use of the measured η meson production rate to obtain absolute bounds on the ALP production with reduced sensitivity to experimental luminosity and detection efficiency. We find no evidence for an ALP, consistent with previous searches in the quoted mass range, and present limits on the effective photon coupling scale of O(1TeV−1). We further find that the ALP production limit we obtain is hindered by the peaking structure of the non-target-related dominant background the in GlueX spectrometer, which we treat by using data on 4He to estimate and subtract it. We comment on how this search can be improved in a future higher-statistics dedicated measurement.
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