19 research outputs found
Measurement of the J/ photoproduction cross section over the full near-threshold kinematic region
We report the total and differential cross sections for
photoproduction with the large acceptance GlueX spectrometer for photon beam
energies from the threshold at 8.2~GeV up to 11.44~GeV and over the full
kinematic range of momentum transfer squared, . Such coverage facilitates
the extrapolation of the differential cross sections to the forward ()
point beyond the physical region. The forward cross section is used by many
theoretical models and plays an important role in understanding
photoproduction and its relation to the proton interaction. These
measurements of photoproduction near threshold are also crucial inputs
to theoretical models that are used to study important aspects of the gluon
structure of the proton, such as the gluon Generalized Parton Distribution
(GPD) of the proton, the mass radius of the proton, and the trace anomaly
contribution to the proton mass. We observe possible structures in the total
cross section energy dependence and find evidence for contributions beyond
gluon exchange in the differential cross section close to threshold, both of
which are consistent with contributions from open-charm intermediate states.Comment: 15 pages 18 figure
Beam asymmetry Σ for the photoproduction of η and η′ mesons at Eγ = 8.8GeV
We report on the measurement of the beam asymmetry
Σ
for the reactions
→
γ
p
→
p
η
and
→
γ
p
→
p
η
′
from the GlueX experiment using an 8.2–8.8-GeV linearly polarized tagged photon beam incident on a liquid hydrogen target in Hall D at Jefferson Laboratory. These measurements are made as a function of momentum transfer
−
t
with significantly higher statistical precision than our earlier
η
measurements and are the first measurements of
η
′
in this energy range. We compare the results to theoretical predictions based on
t
-channel quasiparticle exchange. We also compare the ratio of
Σ
η
to
Σ
η
′
to these models as this ratio is predicted to be sensitive to the amount of
s
¯
s
exchange in the production. We find that photoproduction of both
η
and
η
′
is dominated by natural parity exchange with little dependence on
−
t
Measurement of the photon beam asymmetry in γ⃗p→K+Σ0 at Eγ = 8.5 GeV
We report measurements of the photon beam asymmetry
Σ
for the reaction
γ⃗p→K+Σ0
(1193) using the GlueX spectrometer in Hall D at Jefferson Lab. Data were collected by using a linearly polarized photon beam in the energy range of 8.2–8.8 GeV incident on a liquid hydrogen target. The beam asymmetry
Σ
was measured as a function of the Mandelstam variable
t
, and a single value of
Σ
was extracted for events produced in the
u
channel. These are the first exclusive measurements of the photon beam asymmetry
Σ
for the reaction in this energy range. For the
t
channel, the measured beam asymmetry is close to unity over the
t
range studied,
−
t
=
(
0.1
–
1.4
)
(
GeV
/
c
)
2
, with an average value of
Σ
=
1.00
±
0.05
. This agrees with theoretical models that describe the reaction via the natural-parity exchange of the
K
∗
(892) Regge trajectory. A value of
Σ
=
0.41
±
0.09
is obtained for the
u
channel integrated up to
−
u
=
2.0
(
GeV
/
c
)
2
Measurement of Spin Density Matrix Elements in Λ(1520) Photoproduction at 8.2 GeV to 8.8 GeV
We report on the measurement of spin density matrix elements of the Λ(1520) in the photoproduction reaction γp → Λ(1520)K+, via its subsequent decay to K−p. The measurement was performed
as part of the GlueX experimental program in Hall D at Jefferson Lab using a linearly polarized
photon beam with Eγ = 8.2 GeV–8.8 GeV. These are the first such measurements in this photon
energy range. Results are presented in bins of momentum transfer squared, −(t − t0). We compare
the results with a Reggeon exchange model and determine that natural exchange amplitudes are
dominant in Λ(1520) photoproduction
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Measurement of the J/ψ photoproduction cross section over the full near-threshold kinematic region
We report the total and differential cross sections for J/ψ photoproduction with the large acceptance GlueX spectrometer for photon beam energies from the threshold at 8.2 GeV up to 11.44 GeV and over the full kinematic range of momentum transfer squared, t. Such coverage facilitates the extrapolation of the differential cross sections to the forward (t=0) point beyond the physical region. The forward cross section is used by many theoretical models and plays an important role in understanding J/ψ photoproduction and its relation to the J/ψ-proton interaction. These measurements of J/ψ photoproduction near threshold are also crucial inputs to theoretical models that are used to study important aspects of the gluon structure of the proton, such as the gluon generalized parton distribution of the proton, the mass radius of the proton, and the trace anomaly contribution to the proton mass. We observe possible structures in the total cross section energy dependence and find evidence for contributions beyond gluon exchange in the differential cross section close to threshold, both of which are consistent with contributions from open-charm intermediate states
Measurement of the photon beam asymmetry in γ - p→K+ ς0 at Eγ=8.5 GeV
We report measurements of the photon beam asymmetry ς for the reaction γ - p→K+ς0(1193) using the GlueX spectrometer in Hall D at Jefferson Lab. Data were collected by using a linearly polarized photon beam in the energy range of 8.2-8.8 GeV incident on a liquid hydrogen target. The beam asymmetry ς was measured as a function of the Mandelstam variable t, and a single value of ς was extracted for events produced in the u channel. These are the first exclusive measurements of the photon beam asymmetry ς for the reaction in this energy range. For the t channel, the measured beam asymmetry is close to unity over the t range studied, -t=(0.1-1.4)(GeV/c)2, with an average value of ς=1.00±0.05. This agrees with theoretical models that describe the reaction via the natural-parity exchange of the K∗(892) Regge trajectory. A value of ς=0.41±0.09 is obtained for the u channel integrated up to -u=2.0 (GeV/c)2. © 2020 American Physical Society
Measurement of the J / ψ photoproduction cross section over the full near-threshold kinematic region
Measurement of Spin-Density Matrix Elements in Production with a Linearly Polarized Photon Beam at
The GlueX experiment at Jefferson Lab studies photoproduction of mesons using
linearly polarized photons impinging on a hydrogen target
which is contained within a detector with near-complete coverage for charged
and neutral particles. We present measurements of spin-density matrix elements
for the photoproduction of the vector meson (770). The statistical
precision achieved exceeds that of previous experiments for polarized
photoproduction in this energy range by orders of magnitude. We confirm a high
degree of -channel helicity conservation at small squared four-momentum
transfer and are able to extract the -dependence of natural and
unnatural-parity exchange contributions to the production process in detail. We
confirm the dominance of natural-parity exchange over the full range. We
also find that helicity amplitudes in which the helicity of the incident photon
and the photoproduced differ by two units are negligible for
.Comment: 15 pages, 11 figure
Measurement of beam asymmetry for π − Δ + + photoproduction on the proton at E γ = 8.5 GeV
We report a measurement of the photoproduction beam asymmetry for the reaction using data from the GlueX experiment in the photon beam energy range 8.2--8.8 GeV. The asymmetry is measured as a function of four-momentum transfer to the and compared to phenomenological models. We find that varies as a function of : negative at smaller values and positive at higher values of . The reaction can be described theoretically by -channel particle exchange requiring pseudoscalar, vector, and tensor intermediaries. In particular, this reaction requires charge exchange, allowing us to probe pion exchange and the significance of higher-order corrections to one-pion exchange at low momentum transfer. Constraining production mechanisms of conventional mesons may aid in the search for and study of unconventional mesons. This is the first measurement of the process at this energy