Here, we report the total and differential cross sections for $J/\psi$ 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 $J/\psi$ photoproduction and its relation to the $J/\psi$-proton interaction. These measurements of $J/\psi$ 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.
The GlueX experiment at Jefferson Lab studies photoproduction of mesons using linearly polarized 8.5GeV 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 rho(770). The statistical precision achieved exceeds that of previous experiments for polarized photoproduction in this energy range by orders of magnitude. Here we confirm a high degree of s-channel helicity conservation at small squared four-momentum transfer t and are able to extract the t-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 t range. We also find that helicity amplitudes in which the helicity of the incident photon and the photoproduced rho(770) differ by two units are negligible for -t<0.5GeV2/c2.
The use of machine learning algorithms is an attractive way to produce very fast detector simulations for scattering reactions that can otherwise be computationally expensive. Here we develop a ...factorised approach where we deal with each particle produced in a reaction individually: first determine if it was detected (acceptance) and second determine its reconstructed variables such as four momentum (reconstruction). For the acceptance we propose using a probability classification density ratio technique to determine the probability the particle was detected as a function of many variables. Neural Network and Boosted Decision Tree classifiers were tested for this purpose and we found using a combination of both, through a reweighting stage, provided the most reliable results. For reconstruction a simple method of synthetic data generation, based on nearest neighbour or decision trees was developed. Using a toy parameterised detector we demonstrate that such a method can reliably and accurately reproduce kinematic distributions from a physics reaction. The relatively simple algorithms allow for small training overheads whilst producing reliable results. Possible applications for such fast simulated data include Toy-MC studies of parameter extraction, preprocessing expensive simulations or generating templates for background distributions shapes.
The spin-exotic hybrid meson $\pi_{1}(1600)$ is predicted to have a large
decay rate to the $\omega\pi\pi$ final state. Using 76.6~pb$^{-1}$ of data
collected with the GlueX detector, we measure the ...cross sections for the
reactions $\gamma p \to \omega \pi^+ \pi^- p$, $\gamma p \to \omega \pi^0 \pi^0
p$, and $\gamma p\to\omega\pi^-\pi^0\Delta^{++}$ in the range $E_\gamma =$ 8-10
GeV. Using isospin conservation, we set the first upper limits on the
photoproduction cross sections of the $\pi^{0}_{1}(1600)$ and
$\pi^{-}_{1}(1600)$. We combine these limits with lattice calculations of decay
widths and find that photoproduction of $\eta'\pi$ is the most sensitive
two-body system to search for the $\pi_1(1600)$.
We report the measurement of spin-density matrix elements of the
$\Delta^{++}(1232)$ in the photoproduction reaction $\gamma p \to
\pi^-\Delta^{++}(1232)$ with the GlueX experiment in Hall D at ...Jefferson Lab.
The measurement used a linearly polarized photon beam with
$E_\gamma=8.2-8.8$~GeV and the statistical precision exceeds the previous
measurement from SLAC by three orders of magnitude for the momentum transfer
squared region $-t < 1.4$ GeV$^2$. The data are sensitive to the previously
undetermined relative sign between couplings in existing Regge exchange models.
Linear combinations of the extracted SDMEs allow for a decomposition into
natural and unnatural exchange amplitudes, which shows that the unnatural
exchange plays an important role in the low $-t$ region.
The spin-exotic hybrid meson \(\pi_{1}(1600)\) is predicted to have a large decay rate to the \(\omega\pi\pi\) final state. Using 76.6~pb\(^{-1}\) of data collected with the GlueX detector, we ...measure the cross sections for the reactions \(\gamma p \to \omega \pi^+ \pi^- p\), \(\gamma p \to \omega \pi^0 \pi^0 p\), and \(\gamma p\to\omega\pi^-\pi^0\Delta^{++}\) in the range \(E_\gamma =\) 8-10 GeV. Using isospin conservation, we set the first upper limits on the photoproduction cross sections of the \(\pi^{0}_{1}(1600)\) and \(\pi^{-}_{1}(1600)\). We combine these limits with lattice calculations of decay widths and find that photoproduction of \(\eta'\pi\) is the most sensitive two-body system to search for the \(\pi_1(1600)\).
We report the measurement of spin-density matrix elements of the \(\Delta^{++}(1232)\) in the photoproduction reaction \(\gamma p \to \pi^-\Delta^{++}(1232)\) with the GlueX experiment in Hall D at ...Jefferson Lab. The measurement used a linearly polarized photon beam with \(E_\gamma=8.2-8.8\)~GeV and the statistical precision exceeds the previous measurement from SLAC by three orders of magnitude for the momentum transfer squared region \(-t < 1.4\) GeV\(^2\). The data are sensitive to the previously undetermined relative sign between couplings in existing Regge exchange models. Linear combinations of the extracted SDMEs allow for a decomposition into natural and unnatural exchange amplitudes, which shows that the unnatural exchange plays an important role in the low \(-t\) region.
The GlueX experiment at Jefferson Lab studies photoproduction of mesons using linearly polarized \(8.5\,\text{GeV}\) 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 \(\rho\)(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 \(s\)-channel helicity conservation at small squared four-momentum transfer \(t\) and are able to extract the \(t\)-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 \(t\) range. We also find that helicity amplitudes in which the helicity of the incident photon and the photoproduced \(\rho(770)\) differ by two units are negligible for \(-t<0.5\,\text{GeV}^{2}/c^{2}\).
