Radiative Corrections Beyond the Soft-Photon Approximation

Abstract

This thesis calculates the bremsstrahlung correction beyond the soft-photon approximation for the decay ω → π0 l+ l- . The effect of the radiative corrections on the extraction of the transition form factor from data is investigated. Four different form-factor mod- els are used to compute corrections: vector-meson dominance, lowest-meson dominance, tree-level, and two-hadron saturation. The experimental data used for applying the cor- rections were taken from the NA60, A2, and Lepton-G experiments. The result of these corrections and their implications are evaluated and discussed.

Popular Abstract

Approximations are the lifeblood of physicists. Without them, problems in everything from Classical Mechanics to Quantum Mechanics can become impossible (or at least extremely tedious) to solve. When models are constructed, choosing right approximations in what phenomena can be ignored or not can be crucial in order to describe and imitate reality. Determining whether or not too many approximations were made (i.e. too many phenomena were ignored) or if there are new physical phenomena at work when a model no longer corresponds with reality is an essential part of physics research. This is especially vital within particle physics where the number of interactions necessitates numerous approximations. Evaluating these approximations can then provide an insight into the effectiveness of these models in describing reality.
An area of physics where this becomes apparent is in form-factor modelling. Form factors are like mystery boxes that seek to capture the full complexity of an interaction that cannot be calculated by hand. Due to their opaque nature they are determined from experimental data. This has of course not stopped physicists from developing a series of different models to try and predict the behavior of these form factors. However, when comparing these theoretical models to experiments there is sometimes a deviation for certain particle decays that cannot be explained by uncertainty in the data. This forces us to confront whether or not these models are inherently flawed or if they can be remedied.
This type of deviation is seen in the Dalitz decays of the ω vector meson. When the transition form factor has been measured and then compared with theoretical models for the form factor a deviation is clearly seen. When these form factor models were constructed, only some phenomena were included under the assumption that this would capture enough of the interaction. One way to determine the effectiveness of these models is then to determine if including more complex aspects of the interaction could eliminate this deviation.When it comes to quantum electrodynamics (QED) approximations are also made to determine the decay widths (or probability) of decays. Going beyond the simplest (or leading order) approximation can then include computing the radiative corrections one of which is the bremsstrahlung contribution. The bremsstrahlung phenomenon involves having one (or more) of the particles involved in the decay emit a photon.
This often makes the calculation for the decay width far more complicated unless, again, some approximation is introduced. One possible way to simplify the calculation is the soft-photon approximation where the energy of the photon is assumed to be small (or soft). When this calculation is attempted, a singularity (infinity) appears when the photon energy becomes small. These infinite quantities are then cancelled when other radiative corrections are included. However, this ignores the full range of possible energy values for the photon. This means when the form factor is measured, the estimate is affected by radiative corrections stemming from ignoring these energetic photons. Ignoring these corrections could then be the reason for the deviation between theory and experiment.
The goal of this thesis is to explore if computing the radiative corrections and going beyond the soft-photon approximation can eliminate the deviation seen between the different form factor models and the experimental data for the ω vector meson decays under consideration. If this deviation fails to disappear then new approaches consistent with (or not consistent with) the Standard Model are needed. Regardless of the overall effectiveness of the radiative corrections in this particular case, they should still be used when comparing form factor models and experiment.

• author: Lindahl, Jacob
• supervisor: Johan Bijnens; Tomás Husek
• organization: Theoretical Particle Physics - Undergoing reorganization
• course: FYTK02 20211
• year: 2021
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Soft-Photon Approximation, Bremsstrahlung Correction, Transition Form Factor, Radiative Corrections
• language: English
• id: 9058406
• date added to LUP: 2021-06-24 09:08:45
• date last changed: 2021-06-24 09:08:45