# Future Prospects for Probing Scalar-Tensor Theories with Gravitational Waves from Mixed Binaries

## TL;DR

This paper explores how gravitational wave observations from mixed black hole-neutron star binaries can test and constrain scalar-tensor theories beyond general relativity, with current bounds improvable and future detections promising significant advancements.

## Contribution

It analyzes the potential of gravitational wave data to constrain scalar-tensor theories, including those with spontaneous scalarization and string theory motivations, highlighting prospects for future improvements.

## Key findings

- Current bounds can be improved with existing detectors for small black hole masses.
- Future detections can enhance constraints by several orders of magnitude.
- Combining multiple events significantly boosts the testing power.

## Abstract

The extreme-gravity collisions of binaries with one black hole and one neutron star provide for excellent tests of general relativity. We here study how well one can constrain theories beyond general relativity with additional scalar fields that allow for spontaneous scalarization of neutron stars, and those motivated from string theory. We find that existing bounds can be improved with current gravitational-wave detectors if the black hole mass is sufficiently small. Bounds will further improve by many orders of magnitude with future detections, especially by combining multiple events.