In a proton-proton collision, it is not accurate to say that only one charged particle comes out. In fact, numerous charged particles can be produced as a result of the collision. These particles are typically detected and recorded in particle detectors surrounding the collision point.
When two protons collide, several processes can occur. One possibility is that the protons can scatter off each other, resulting in multiple outgoing particles. These particles can include charged particles, such as protons, electrons, muons, pions, and others, as well as neutral particles like photons and neutrons.
However, it is worth noting that not all of the outgoing particles are observed directly. Some particles, like neutrinos, can interact weakly and pass through matter without leaving a detectable trace. These "invisible" particles can carry away energy and momentum from the collision without being directly detected.
The reason why we often hear about a single charged particle in proton-proton collisions is because it can be easier to identify and measure the properties of a single, high-energy charged particle. Detecting and reconstructing the entire event, including all particles produced in the collision, is a complex task that requires sophisticated analysis techniques and detectors.
So, while multiple charged particles are indeed produced in proton-proton collisions, the focus is often on the most energetic and easily measurable particles, which are typically referred to as "leading" particles or "jets." These particles can provide valuable information about the underlying physics of the collision and the properties of the particles involved.