Reinhard A. Schumacher

The experiment at Jefferson Lab has observed p p ¯ and, for the first time, Λ Λ ¯ and p Λ ¯ 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 Λ Λ ¯ and p Λ ¯ systems but general similarity to the p 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 s ¯ quark pairs relative to d d ¯ quark pairs is similar to what has been seen in other reactions.

We measure the spin-density matrix elements (SDMEs) for the photoproduction of ϕ ( 1020 ) off of the proton in its decay to K S 0 K L 0 , using 105 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 GeV 2 , providing the first measurement of their t dependence for photon beam energies of E γ = 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 ϕ ( 1020 ) differ by two units are negligible.

The spin-exotic hybrid meson π_{1}(1600) is predicted to have a large decay rate to the ωππ final state. Using 76.6 pb^{-1} of data collected with the GlueX detector, we measure the cross sections for the reactions γp→ωπ^{+}π^{-}p, γp→ωπ^{0}π^{0}p, and γp→ωπ^{-}π^{0}Δ^{++} in the range E_{γ}=8-10 GeV. Using isospin conservation, we set the first upper limits on the photoproduction cross sections of the π_{1}^{0}(1600) and π_{1}^{-}(1600). We combine these limits with lattice calculations of decay widths and find that photoproduction of η^{'}π is the most sensitive two-body system to search for the π_{1}(1600).

Cognitive neuroscience methods can identify the fMRI-measured neural representation of familiar individual concepts, such as apple, and decompose them into meaningful neural and semantic components. This approach was applied here to determine the neural representations and underlying dimensions of representation of far more abstract physics concepts related to matter and energy, such as fermion and dark matter, in the brains of 10 Carnegie Mellon physics faculty members who thought about the main properties of each of the concepts. One novel dimension coded the measurability vs. immeasurability of a concept. Another novel dimension of representation evoked particularly by post-classical concepts was associated with four types of cognitive processes, each linked to particular brain regions: (1) Reasoning about intangibles, taking into account their separation from direct experience and observability; (2) Assessing consilience with other, firmer knowledge; (3) Causal reasoning about relations that are not apparent or observable; and (4) Knowledge management of a large knowledge organization consisting of a multi-level structure of other concepts. Two other underlying dimensions, previously found in physics students, periodicity, and mathematical formulation, were also present in this faculty sample. The data were analyzed using factor analysis of stably responding voxels, a Gaussian-naïve Bayes machine-learning classification of the activation patterns associated with each concept, and a regression model that predicted activation patterns associated with each concept based on independent ratings of the dimensions of the concepts. The findings indicate that the human brain systematically organizes novel scientific concepts in terms of new dimensions of neural representation.