# ATLAS events

Beams of protons are accelerated around the Large Hadron Collider (LHC) and are brought to collision at the centre of the ATLAS detector. The collisions produce debris in the form of new particles which fly out in all directions. Over a billion particle interactions take place in the ATLAS detector every second.

The protons within the two beams are grouped in bunches which are squeezed down in size to increase the chances of a collision. In the released data, the bunches crossed every 50 ns. There were about 30 collisions on average per bunch-crossing.

An event is the data resulting from a particular bunch-crossing.

Pile-up is defined as the average number of particle interactions per bunch-crossing. It is directly correlated with the instantaneous luminosity.

Luminosity is one of the most important parameters of the LHC. The higher the luminosity, the more data the experiments can gather to allow them to observe rare processes. However, increasing luminosity increases pile-up. This presents a challenge for physics analyses as it makes successfully identify collisions of interest harder.

The primary vertex is defined as the inelastic collision of two protons with the highest overall transverse momentum. In a typical collision event, several vertices along the beam are produced. It is important to correctly identify them to associate the observed particles to the correct vertex to suppress the effects from pile-up. The many low transverse momentum proton-proton interactions occuring are called minimum bias events.

The above event display shows a candidate $Z$ boson decaying into two muons with 11 reconstructed vertices. This event was recorded on April 24th and is typical for the 2011 environment with high pile-up.

The reconstruction of vertices is important for many physics studies. This includes searches for new particles, identifying jets containing $b$-quarks or taus, and reconstruction of exclusive $b$-quark decays.