Another complication at LHC ist the existence of so-called
many of which occur much more frequently than the processes
for which we are looking. For events with an electron-positron or muon-antimuon
pair, the most prominent backgrounds are pairs of top-antitop quarks.
They can decay according to
where … stands for further particles in quark jets.
In order to suppress this background, one exploits
characteristic differences to events with
In contrast to events with electron-positron or muon-antimuon pairs
from Z0 decays, the background from t+t always contains two
undetectable neutrinos that take away considerable energy from the
Therefore all events with a ETMiss larger than 10 GeV should be
considered as background.
Your identified pair of particles must have a total energy corresponding to
the sum of the unknown kinetic energy of the Z0 boson plus its rest energy of 91.2 GeV,
stemming from the Z0 mass. Therefore you
should require that the sum of the energies of the pair in the Track
Momenta Window exceeds 91.2 GeV. Events which fail this requirement
are again considered as background.