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This is not intended to be an exhaustive glossary. You will find answers to many of your questions by using a search engine. However, here are a few ATLAS physics terms explained to start you off.

Pseudorapidity (η)Describes the angle of a particle relative to the beam axis.
Azimuth angle (φ)Describes the angle of a particle measured from the x-axis, around the beam.
Event displaysAre like snap-shots, tracing the paths of particles produced in a collision.
Branching fractionIn general, a particle can decay in several modes or decay channels. For example, a Z boson can decay into a pair of neutrinos, a pair of charged leptons, or a pair of quarks (i.e., all the standard model fermions lighter than half the Z mass). The probability for a Z to decay into a neutrino pair is about 20%, into a pair of charged leptons (electrons, muons, or taus) is about 10%, and into a pair of quarks (u,d,c,s,b) is about 70%. These probabilities are called branching fractions.
ChannelThe decay channel signifies a certain route a physics process has taken. For example, the W boson may decay to either a pair of hadrons or a pair of leptons. The signature for these W bosons in the detector are therefore either two hadrons or two leptons.
Electron Volt (eV)One electron volt is equal to ~1.6 x 10-19 joules. MeV (106) and TeV (1012) are units of energy used in particle physics. 1 TeV is about the energy of motion of a flying mosquito.
AntiparticleSubatomic particle having the same mass as one of the particles of ordinary matter but opposite electric charge and magnetic moment.
Pile-upAverage number of particle interactions per bunch-crossing.