# Mattias Sjö

Doctoral Student, Master's degree, Bachelor's Degree## Research areas and keywords

### UKÄ subject classification

**Subatomic Physics**

### Keywords

- effective field theory, mesonic physics, low-energy particle physics, chiral perturbation theory

### Research

I currently work with Johan "Hans" Bijnens and study processes involving mesons, i.e. light but unstable bound states of quarks and antiquarks. They are described by an effective field theory, *chiral perturbation theory*, which arises from the low-energy limit of the strong sector of the Standard Model. I work with higher orders in this expansion, so that the low-energy approximation gains higher precision and holds for somewhat higher energies. My interests can thus be described tongue-in-cheek as high-energy low-energy high-energy physics.

My master's project and a subsequent doctoral project was about the development of methods for complicated calculations in a simplified version of chiral perturbation theory. This resulted in the publication *Higher-order Amplitudes in the Nonlinear Sigma Model* with Hans Bijnens and Karol Kampf.

My present project is about constraining the values of the many free parameters in chiral perturbation theory using fundamental properties such as unitarity and crossing symmetry.

I mainly work with exact mathematical calculations rather than numerics. The often extremely large expressions are handled using the FORM language. I also employ Mathematica as well as scripts and numerical programs written in various languages. I like representing mathematical objects graphically.

## Recent research outputs

Research output: Contribution to journal › Article