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GlueX Experiment Document 4131-v1

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Dissertation: Measurement of Pseudoscalar Beam Asymmetry for $\pi^-\Delta^{++}$ Photoproduction on the Proton at $E_\gamma$=8.5 GeV with the GlueX Experiment

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Jonathan Zarling
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Jonathan Zarling
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26 Jul 2019, 11:49
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26 Jul 2019, 11:49
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26 Jul 2019, 11:49
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The past decade has seen a multitude of new experimental candidates for tetraquark and pentaquark particles. Such additions to the accepted constituent quark picture also call into question the gluon: can it form one of the constituents of a hadron just as quarks do? Theorized particles known as “exotic hybrid mesons” with $q\bar{q}g$ constituents are expected to exist in the spectrum of light mesons with unconventional quantum numbers. Their detection would be clear indication of a new type of meson. Current experimental evidence supports the existence of at most one such state, yet many are predicted by state-of-the-art Lattice QCD calculations.

The GlueX experiment seeks to observe these particles via the photoproduction process: i.e. the interaction of a high energy photon and a proton. The linear polarization of the GlueX experiment’s beam offers additional observable quantities to exploit and a different production mechanism to contribute to the worldwide search for new types of hadronic states. %GlueX also enjoys good geometrical acceptance and high statistics.

Not much is known about how hadrons are produces at photon energies $E_\gamma=6-~12$~GeV. Studying the production of well-established mesons offers the best insight into such production mechanisms; the simplest class of particles to study are pseudoscalar mesons. These reactions can be described by the $t$-channel exchange of quasi-particle Reggeons. Neutral $\pi^0$ and $\eta$ mesons have been analyzed by the GlueX experiment, however reactions involving charge exchange have not been rigorously investigated at 8.5 GeV previously. We measure the beam asymmetry in the reaction $\gamma p \rightarrow \pi^- \Delta^{++}$, the highest statistics charge exchange reaction available with GlueX. The way in which the $\pi^-$ is produced is dependent on the momentum imparted to the recoiling $\Delta^{++}$. At low momentum transfer, negative naturality pion exchange dominates. At higher values, natural vector $\rho$ and tensor $a$ exchanges are needed to explain observed behavior. We report precise results in a wide range of momentum transfer. These will inform future searches for hybrid mesons in charge exchange reactions such as $\gamma p \rightarrow \pi^- \eta^{(\prime)} \Delta^{++}$.

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