Search

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 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.

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 ${\pi }_1(1600)$.

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 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.

By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the $A{\text{Fe}}_2{\text{As}}_2$ $\left(A=\text{Ca},\text{Ba},\text{Sr}\right)$ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of $\left({\text{Ba}}_{0.5}{\text{K}}_{0.5}\right){\text{Fe}}_2{\text{As}}_2$ exhibits a nearly linear variation in $T$ at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.

The resistivity of ultraclean suspended graphene is strongly temperature ($T$) dependent for $5<T<240\mathrm{K}$. At $T\sim 5\mathrm{K}$ transport is near-ballistic in a device of $\sim 2\mu \mathrm{m}$ dimension and a mobility $\sim 170000{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$. At large carrier density, $n>0.5\times {10}^{11}{\mathrm{cm}}^{-2}$, the resistivity increases with increasing $T$ and is linear above 50 K, suggesting carrier scattering from acoustic phonons. At $T=240\mathrm{K}$ the mobility is $\sim 120000{\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$, higher than in any known semiconductor. At the charge neutral point we observe a nonuniversal conductivity that decreases with decreasing $T$, consistent with a density inhomogeneity $<{10}^8{\mathrm{cm}}^{-2}$.