The particle that can exist alone and is smaller than any other is the electron.

Here's why:

* Exists Alone: Electrons are fundamental particles, meaning they are not made up of smaller components. They can exist independently.

* Smaller than any other: Electrons are point particles, meaning they have no size. This means they are smaller than protons, neutrons, quarks, and any other known particles.

While there are theories about even smaller particles, such as strings or preons, these are hypothetical and have not been experimentally confirmed.