Understanding Pion Photoproduction using Chiral Perturbation Theory

Access to files

•   FULL-TEXT.PDF (pdf)

Abstract

In this thesis we present the work we have done to further the understandingof neutral pion photoproduction from the proton. Our work used heavy-baryonchiral perturbation theory to fourth order, and we explicitly include the ∆(1232)resonance using the δ power-counting scheme. We also test the effects of includingand excluding D-waves, partial waves with orbital angular momentum quantumnumber of 2.We begin our discussion in chapter 1 with a brief history of nuclear physicsbefore showing how current algebras and the partial-conservation of the axial-vector current (PCAC) can be used to describe low-energy hadronic interactions.To improve upon PCAC results, one can make use of chiral effective field theories,taking advantage of the chiral symmetry that is present in the limit of masslessquarks. We formally introduce chiral perturbation theory (χPT) in chapter 2,discussing the power-counting problem that arises once baryons are included, andpresent two different approaches to overcome this. We conclude chapter 2 showinghow the theory can be extended to include the ∆(1232) resonance.In chapter 3 we describe the model-independent framework of pion photopro-duction needed to describe both theory and experiment. We discuss the variousrecent efforts to describe pion photoproduction within χPT in the latter half ofchapter 3.We lay out the framework we have developed in chapter 4 and present ourresults in chapter 5. We have compared our work to the recent experimental datareleased by the A2 and CB-TAPS collaborations at the Mainz Microtron[22]. Ourresults show that we can accurately describe the data from threshold up to an in-coming laboratory photon energy of approximately 260 MeV, a clear improvementon previous studies [14–16, 19, 23–27]. We find that including the ∆ is necessaryto describe the data beyond E γ ≈ 200 MeV, but the case for D-waves is not soclear. We cannot pinpoint an energy where their inclusion is necessary but we dofind evidence suggesting they are important to improve the stability of the fit. Ourbest results require the inclusion of both the ∆ and D-waves.Finally, in chapter 6, we present our concluding remarks from what has beenachieved over the last four years of study and what forms possible extensions ofour work could take.

Layman's Abstract

We have studied the sub-atomic process called pion photoproduction, where a proton absorbs a photon and them emits a sub-atomic particle called a pion. This has been done using chiral perturbation theory, a theory that utilises all the known symmetries of the strong nuclear interaction but bypasses the many complexities of working with sub-nuclear particles. We explicitly include the excited state of the proton, the delta resonance, into the theory and look at the effects of describing the angular momentum of the reaction in more detail. We compare our theory to the latest data from the Mainz Microton and find that our work is in good agreement with the experiment. We conclude with predictions for other experimentally measurable quantities we do not have data for.

Keyword(s)

chiral; delta; hadron; mainz; perturbation; photoproduction; pion; proton; theory

Bibliographic metadata

Institutional metadata

Record metadata