Search

We report measurements of the photon beam asymmetry $\mathrm{\Sigma }$ for the reaction $\vec{\gamma }p\to K^{+}{\mathrm{\Sigma }}^0$(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 $\mathrm{\Sigma }$ was measured as a function of the Mandelstam variable $t$, and a single value of $\mathrm{\Sigma }$ was extracted for events produced in the $u$ channel. These are the first exclusive measurements of the photon beam asymmetry $\mathrm{\Sigma }$ for the reaction in this energy range. For the $t$ channel, the measured beam asymmetry is close to unity over the $t$ range studied, $-t=(0.1–1.4)$($\mathrm{GeV}/{c)}^2$, with an average value of $\mathrm{\Sigma }=1.00\pm 0.05$. This agrees with theoretical models that describe the reaction via the natural-parity exchange of the $K^{*}$(892) Regge trajectory. A value of $\mathrm{\Sigma }=0.41\pm 0.09$ is obtained for the $u$ channel integrated up to $-u=2.0$ ($\mathrm{GeV}/{c)}^2$.

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 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.

We present a search for axionlike particles, $a$, produced in photon-proton collisions at a center-of-mass energy of approximately 4 GeV, focusing on the scenario where the $a$-gluon coupling is dominant. The search uses $a\to \gamma \gamma$ and $a\to {\pi }^{+}{\pi }^{-}{\pi }^0$ decays, and a data sample corresponding to an integrated luminosity of $168{\mathrm{pb}}^{-1}$ collected with the GlueX detector. The search for $a\to \gamma \gamma$ decays is performed in the mass range of $180<m_a<480\mathrm{MeV}$, while the search for $a\to {\pi }^{+}{\pi }^{-}{\pi }^0$ decays explores the $600<m_a<720\mathrm{MeV}$ region. No evidence for a signal is found, and 90% confidence-level exclusion limits are placed on the $a$-gluon coupling strength. These constraints are the most stringent to date over much of the mass ranges considered.

The GlueX experiment at Jefferson Lab has observed $p\overline{p}$ and, for the first time, $\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ and $p\overline{\mathrm{\Lambda }}$ photoproduction from a proton target at photon energies up to 11.6 GeV. The angular distributions are forward peaked for all produced pairs, consistent with Regge-like $t$-channel exchange. Asymmetric wide-angle antibaryon distributions show the presence of additional processes. In a phenomenological model, we find consistency with a double-$t$-channel exchange process where antibaryons are created only at the middle vertex. The model matches all observed distributions with a small number of free parameters. In the hyperon channels, we observe a clear distinction between photoproduction of the $\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ and $p\overline{\mathrm{\Lambda }}$ systems but general similarity to the $p\overline{p}$ system. We report both total cross sections and cross sections differential with respect to momentum transfer and the invariant masses of the created particle pairs. No narrow resonant structures were found in these reaction channels. The suppression of $s\overline{s}$ quark pairs relative to $d\overline{d}$ quark pairs is similar to what has been seen in other reactions.

We report on the measurement of the beam asymmetry $\mathrm{\Sigma }$ for the reactions $\vec{\gamma }p\to p\eta$ and $\vec{\gamma }p\to p{\eta }^{\prime }$ 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 Laboratory. 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 quasiparticle exchange. We also compare the ratio of ${\mathrm{\Sigma }}_{\eta }$ to ${\mathrm{\Sigma }}_{{\eta }^{\prime }}$ to these models as this ratio is predicted to be sensitive to the amount of $s\overline{s}$ exchange in the production. We find that photoproduction of both $\eta$ and ${\eta }^{\prime }$ is dominated by natural parity exchange with little dependence on $-t$.

We report on the measurement of the $\gamma p\to J/\psi p$ cross section from $E_{\gamma }=11.8\mathrm{GeV}$ down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find that 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 $1.67\pm 0.39{\mathrm{GeV}}^{-2}$ 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 $\mathcal{B}(P_c^{+}\to J/\psi p)$ and cross sections $\sigma (\gamma p\to P_c^{+})\times \mathcal{B}(P_c^{+}\to J/\psi p)$.

We report a measurement of the ${\pi }^{-}$ photoproduction beam asymmetry for the reaction $\vec{\gamma }p\to {\pi }^{-}{\mathrm{\Delta }}^{++}$ using data from the GlueX experiment in the photon beam energy range 8.2–8.8 GeV. The asymmetry $\mathrm{\Sigma }$ is measured as a function of four-momentum transfer $t$ to the ${\mathrm{\Delta }}^{++}$ and compared to phenomenological models. We find that $\mathrm{\Sigma }$ varies as a function of $t$: negative at smaller values and positive at higher values of $\left|t\right|$. The reaction can be described theoretically by $t$-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.

We report on the measurement of spin density matrix elements of the $\mathrm{\Lambda }\left(1520\right)$ in the photoproduction reaction $\gamma p\to \mathrm{\Lambda }\left(1520\right)K^{+}$, via its subsequent decay to $K^{-}p$. The measurement was performed as part of the GlueX experimental program in Hall D at Jefferson Laboratory using a linearly polarized photon beam with $E_{\gamma }=8.2\mathrm{GeV}\text{–}8.8\mathrm{GeV}$. These are the first such measurements in this photon energy range. Results are presented in bins of momentum transfer squared, $-(t-t_{\text{0}})$. We compare the results with a Reggeon exchange model and determine that natural exchange amplitudes are dominant in $\mathrm{\Lambda }\left(1520\right)$ photoproduction.

Based on $334{\mathrm{pb}}^{-1}$ of photoproduction data collected with the GlueX detector at Jefferson Lab, we have measured for the first time the cross section of the exclusive reaction $\gamma +p\to \varphi (1020){\pi }^{+}{\pi }^{-}p$ by reconstructing the final state $K^{+}{K}^{-}{\pi }^{+}{\pi }^{-}p$ produced with a photon beam of energies between 8.0 and 11.6 GeV. Based on the measured differential cross section, we have performed a search for the strangeoniumlike exotic candidate $Y(2175)$, recently renamed to $\varphi (2170)$. This state has been reported by different $e^{+}{e}^{-}$ annihilation experiments and it is addressed here for the first time in a photoproduction experiment. We do not find evidence for this state when using the resonance parameters quoted by the Particle Data Group and provide upper limits on the photoproduction cross section. Instead, we find a structure at a mass of $m(\varphi {\pi }^{+}{\pi }^{-})=2.24\mathrm{GeV}/c^2$ with a statistical significance of about $5\sigma$. The parameters of this structure differ from those quoted by the Particle Data Group for the $\varphi (2170)$ and are consistent with a previous observation in $e^{+}{e}^{-}$ annihilation. In addition, there is evidence for a second structure at $1.82\mathrm{GeV}/c^2$.

The spin-exotic hybrid meson ${\pi }_1(1600)$ is predicted to have a large decay rate to the $\omega \pi \pi$ final state. Using $76.6{\mathrm{pb}}^{-1}$ 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 $\gamma p\to \omega {\pi }^{-}{\pi }^0{\mathrm{\Delta }}^{++}$ in the range $E_{\gamma }=8–10\mathrm{GeV}$. Using isospin conservation, we set the first upper limits on the photoproduction cross sections of the ${\pi }_1^0(1600)$ and ${\pi }_1^{-}(1600)$. We combine these limits with lattice calculations of decay widths and find that photoproduction of ${\eta }^{\prime }\pi$ is the most sensitive two-body system to search for the ${\pi }_1(1600)$.

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 $\rho \left(770\right)$ differ by two units are negligible for $-t<0.5{\text{GeV}}^2/c^2$.

We measure for the first time the differential photoproduction cross section $d\sigma /dt$ of the $a_2\left(1320\right)$ meson at an average photon beam energy of 8.5 GeV, using data with an integrated luminosity of 104 ${\mathrm{pb}}^{-1}$ collected by the GlueX experiment. We fully reconstruct the $\gamma p\to \eta {\pi }^{0}p$ reaction and perform a partial-wave analysis in the $a_2\left(1320\right)$ mass region with amplitudes that incorporate the linear polarization of the beam. This allows us to separate for the first time the contributions of natural- and unnatural-parity exchanges. These measurements provide novel information about the photoproduction mechanism, which is critical for the search for spin-exotic states.

We perform the first amplitude analysis of experimental data using deep neural networks to determine the nature of an exotic hadron. Specifically, we study the line shape of the $P_c(4312)$ signal reported by the LHCb collaboration, and we find that its most likely interpretation is that of a virtual state. This method can be applied to other near-threshold resonance candidates.

We measure the spin-density matrix elements (SDMEs) for the photoproduction of $\varphi \left(1020\right)$ off of the proton in its decay to $K_S^0{K}_L^0$, using 105 ${\mathrm{pb}}^{-1}$ of data collected with a linearly polarized photon beam using the GlueX experiment. The SDMEs are measured in nine bins of the squared four-momentum transfer $t$ in the range $-t=0.15-1.0$ ${\mathrm{GeV}}^2$, providing the first measurement of their $t$ dependence for photon beam energies of $E_{\gamma }=8.2-8.8$ GeV. We confirm the dominance of Pomeron exchange in this region and put constraints on the contribution of other Regge exchanges. We also find that helicity amplitudes where the helicity of the photon and $\varphi \left(1020\right)$ differ by two units are negligible.

It is found that to parametrize the $\overline{p}$ invariant cross section in low-energy $p-p$ collisions, a better scaling variable $\Delta X_R=X_R-X_{Rmin}$ can be used instead of the usual radial variable $X_R$, where $X_{Rmin}$ is the minimum of $X_R$ for a given $s$ and $p_t$. The ratio of the $\overline{p}$ invariant cross section to the corresponding radial-scaling limit is seen to be independent of $p_t$ explicitly, and is monotonically and quickly approaching to one from above for increasing $s$.

We present an application of the Weyl-van der Waerden spinor calculus to the computation of the polarized graviton-scalar and graviton-fermion Compton scattering amplitudes. We demonstrate that with the minimal gauge choice the number of contributing diagrams are minimal; hence, the amount of work is reduced considerably in the calculation. We allow the scalar and fermion fields to be massive, and the technique is not only applicable in the high-energy limit.

Solution-processed molybdenum disulfide (${\mathrm{Mo}\mathrm{S}}_2$) shows promise for tunable photonics and optoelectronics development. However, though scalable, it often leads to devices with inferior performance as a result of its random, discrete nature. In this study, we show via density-functional-theory calculations that the electronic structure of the individual solution-processed nanosheets can be modulated collectively by external electric fields. Particularly, the nanosheets can form Stark ladders, giving variations in the underlying optical processes and thus tunable collective optical properties. We confirm this electro-optical modulation experimentally using solution-processed ${\mathrm{Mo}\mathrm{S}}_2$ films with ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] incorporated, and prove that the local polarization fields from P(VDF-TrFE) can modulate the collective optical properties of the ${\mathrm{Mo}\mathrm{S}}_2$, specifically, the optical absorption and photoluminescence. Given the scalability of solution processing, our results underpin the potential of electro-optical modulation of solution-processed ${\mathrm{Mo}\mathrm{S}}_2$ for tunable photonics and optoelectronics development. To illustrate this potential, we demonstrate solution-processed electroabsorption modulators.

In the modeling of infectious disease spread within explicit social contact networks, previous studies have predominantly assumed that the effects of shifting social associations within groups are small. These models have utilized static approximations of contact networks. We examine this assumption by modeling disease spread within dynamic networks where associations shift according to individual preference based on three different measures of network centrality. The results of our investigations clearly show that this assumption may not hold in many cases. We demonstrate that these differences in association dynamics do yield significantly different disease outcomes both from each other and also from models using graph-theoretically accurate static network approximations. Further work is therefore needed to explore under which circumstances static models accurately reflect constantly shifting natural populations.

We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO $5\sigma$ and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process.

Using x-ray photoelectron spectroscopy we observe the breaking of the strong interatomic bond in molecular CO at low temperature on a stepped Cu surface. Since the electronic structure of Cu does not allow for the splitting of CO at such low temperatures it suggests that there may be a less obvious pathway for the process. Through x-ray photoelectron spectroscopy we can clearly identify products associated with the dissociation of CO and the subsequent formation of stable graphitic carbon on the surface. However, the dissociation of CO can be inhibited when the stepped Cu surface is kept clean from surface carbon. These observations imply that the reaction is driven by the presence of small amounts of weakly bound carbon at the surface. Density-functional theory calculations confirm that carbon atoms on a stepped Cu surface indeed are the preferred adsorption sites for CO, which increases the stabilization of CO on the surface and weakens the C–O bond. This results in the breaking of the C–O bond at the step edge via the Boudouard reaction ($2{\mathrm{CO}}_{\text{ads}}\to {\mathrm{C}}_{\text{ads}}+{\mathrm{CO}}_2$) with a barrier of 0.71 eV.