We report measurements of the photon beam asymmetry $\Sigma$ for the reaction $\vec{\gamma} p\to K^+\Sigma^0$(1193) using the GlueX spectrometer in Hall D at Jefferson Lab. Data were collected 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 $\Sigma$ was measured as a function of the Mandelstam variable $t$, and a single value of $\Sigma$ was extracted for events produced in the $u$-channel. These are the first exclusive measurements of the photon beam asymmetry $\Sigma$ 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 $\Sigma = 1.00\pm 0.05$. This agrees with theoretical models that describe the reaction via the natural-parity exchange of the $K^{*}$(892) Regge trajectory. A value of $\Sigma = 0.41 \pm 0.09$ is obtained for the $u$-channel integrated up to $-u=2.0$~(GeV/$c$)$^{2}$.
We report measurements of the photon beam asymmetry $\Sigma$ for the reaction $\vec{\gamma} p\to K^+\Sigma^0$(1193) using the GlueX spectrometer in Hall D at Jefferson Lab. Data were collected 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 $\Sigma$ was measured as a function of the Mandelstam variable $t$, and a single value of $\Sigma$ was extracted for events produced in the $u$-channel. These are the first exclusive measurements of the photon beam asymmetry $\Sigma$ 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 $\Sigma = 1.00\pm 0.05$. This agrees with theoretical models that describe the reaction via the natural-parity exchange of the $K^{*}$(892) Regge trajectory. A value of $\Sigma = 0.41 \pm 0.09$ is obtained for the $u$-channel integrated up to $-u=2.0$~(GeV/$c$)$^{2}$.
Exclusive electroproduction of
ω
mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of
Q
2
>
1.0
GeV
2
, 3.0 GeV
<
W
<
6.3 GeV, and
-
t
′
<
0.2
GeV
2
. ...Results on the angular distribution of the
ω
meson, including its decay products, are presented. The data were accumulated with the HERMES forward spectrometer during the 1996–2007 running period using the 27.6 GeV longitudinally polarized electron or positron beam of HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio reveals that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are presented in projections of
Q
2
or
-
t
′
. Violation of
s
-channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse
ω
production by longitudinal and transverse virtual photons,
γ
L
∗
→
ω
T
and
γ
T
∗
→
ω
T
, is determined for the first time. A hierarchy of helicity amplitudes is established, which mainly means that the unnatural-parity-exchange amplitude describing the
γ
T
∗
→
ω
T
transition dominates over the two natural-parity-exchange amplitudes describing the
γ
L
∗
→
ω
L
and
γ
T
∗
→
ω
T
transitions, with the latter two being of similar magnitude. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions, which are of unnatural parity.
A series of semi-inclusive deep-inelastic scattering measurements on deuterium, helium, neon, krypton, and xenon targets has been performed in order to study hadronization. The data were collected ...with the HERMES detector at the DESY laboratory using a 27.6 GeV positron or electron beam. Hadron multiplicities on nucleus
A relative to those on the deuteron,
R
A
h
, are presented for various hadrons (
π
+
,
π
−
,
π
0
,
K
+
,
K
−
,
p, and
p
¯
) as a function of the virtual-photon energy
ν, the fraction
z of this energy transferred to the hadron, the photon virtuality
Q
2
, and the hadron transverse momentum squared
p
t
2
. The data reveal a systematic decrease of
R
A
h
with the mass number
A for each hadron type
h. Furthermore,
R
A
h
increases (decreases) with increasing values of
ν (
z), increases slightly with increasing
Q
2
, and is almost independent of
p
t
2
, except at large values of
p
t
2
. For pions two-dimensional distributions also are presented. These indicate that the dependences of
R
A
π
on
ν and
z can largely be described as a dependence on a single variable
L
c
, which is a combination of
ν and
z. The dependence on
L
c
suggests in which kinematic conditions partonic and hadronic mechanisms may be dominant. The behaviour of
R
A
π
at large
p
t
2
constitutes tentative evidence for a partonic energy-loss mechanism. The
A-dependence of
R
A
h
is investigated as a function of
ν,
z, and of
L
c
. It approximately follows an
A
α
form with
α
≈
0.5
–
0.6
.
We report on the measurement of the $\gamma p \rightarrow J/\psi p$ cross section from $E_\gamma = 11.8$ GeV down to the threshold at $8.2$ GeV using a tagged photon beam with the GlueX experiment. ...We find the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section $d\sigma /dt$ has an exponential slope of $1.67 \pm 0.39$ GeV$^{-2}$ at $10.7$ GeV average energy. The LHCb pentaquark candidates $P_c^+$ can be produced in the $s$-channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions $\mathcal{B}(P_c^+ \rightarrow J/\psi p)$ and cross sections $\sigma(\gamma p \to P_c^+)\times\mathcal{B}(P_c^+ \to J/\psi p) $.
We report on the measurement of the $\gamma p \rightarrow J/\psi p$ cross section from $E_\gamma = 11.8$ GeV down to the threshold at $8.2$ GeV using a tagged photon beam with the GlueX experiment. ...We find the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section $d\sigma /dt$ has an exponential slope of $1.67 \pm 0.39$ GeV$^{-2}$ at $10.7$ GeV average energy. The LHCb pentaquark candidates $P_c^+$ can be produced in the $s$-channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions $\mathcal{B}(P_c^+ \rightarrow J/\psi p)$ and cross sections $\sigma(\gamma p \to P_c^+)\times\mathcal{B}(P_c^+ \to J/\psi p) $.
In this work, we report on the measurement of the beam asymmetry $\Sigma$ for the reactions $\vec{\gamma}p\rightarrow p\eta$ and $\vec{\gamma}p \rightarrow p\eta^{\prime}$ 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 Lab. These measurements are made as a function of momentum transfer $-t$, with significantly higher statistical precision than our earlier $\eta$ measurements, and are the first measurements of $\eta^{\prime}$ in this energy range. We compare the results to theoretical predictions based on $t$--channel quasi-particle exchange. Furthermore, we compare the ratio of $\Sigma_{\eta}$ to $\Sigma_{\eta^{\prime}}$ to these models, as this ratio is predicted to be sensitive to the amount of $s\bar{s}$ exchange in the production. We find that photoproduction of both $\eta$ and $\eta^{\prime}$ is dominated by natural parity exchange with little dependence on $-t$.
The storage cell of the internal, longitudinally polarized, atomic gas target of the HERMES experiment is presented. The polarized atoms of hydrogen or deuterium are accumulated in an open-ended thin ...walled storage cell through which the circulating electron or positron beam of the HERA accelerator passes. The target areal density is
10
14
atoms/cm
2
, two orders of magnitude larger than without the cell. The construction details of the cell are described and the cell's performance during HERMES run of 1997–2000 is discussed.
Exclusive electroproduction of π+ mesons was studied by scattering 27.6 GeV positrons or electrons off a transversely polarized hydrogen target. The single-spin azimuthal asymmetry with respect to ...target polarization was measured as a function of the Mandelstam variable t, the Bjorken scaling variable xB, and the virtuality Q2 of the exchanged photon. The extracted Fourier components of the asymmetry were found to be consistent with zero, except one that was found to be large and that involves interference of contributions from longitudinal and transverse virtual photons.
The first detailed measurement of the dependence on target nuclear mass of the average squared transverse momentum 〈pt2〉 of π+, π−, and K+ mesons from deep-inelastic lepton scattering is obtained as ...a function of several kinematic variables. The data were accumulated at the Hermes experiment at Desy, in which the Hera 27.6 GeV lepton beam was scattered off several nuclear gas targets. The average squared transverse momentum was clearly observed to increase with atomic mass number. The effect increases as a function of Q2 and x and remains constant as a function of both the virtual photon energy ν and the fractional hadron energy z, except that it vanishes as z approaches unity.
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