Final answer:
Neutral particles, such as neutrons, cannot be accelerated by electric or magnetic fields in a particle accelerator because they lack charge.
Step-by-step explanation:
The particle that cannot be accelerated by electric or magnetic fields in a particle accelerator is a neutral particle, as it has no charge. Charged particles, such as protons, electrons, and positrons, can be accelerated using electric fields, as demonstrated in different types of accelerators including the linear accelerator, synchrotron, and cyclotron. Neutral particles, such as neutrons, do not interact with these fields and thus cannot be accelerated in the same way.