I am trying to understand the trigger menus of CMS(Compact Muon Solenoid). I understand the different selection cuts($E_T$ Threshold) that they use but they also keep mentioning about using isolated and non-isolated electron/photons. What does isolated and non-isolated mean in this case?
One has to think of the physics behind it. The CMS detector wants to identify the particles coming out of proton proton interactions which are deep inelastic,i.e. represent quarks, gluons and the other particles in the table, as cleanly as possible so as to be able to check standard model predictions with the measurements.
In order to do this a series of detectors is built for the experiment, each level specializing in particular detection: tracks, electrons/photons , hadrons, muons
Transverse slice through the CMS detector, showing the individual detector subsystems and particle signatures in each. The particle type can be inferred by combining the detector response in the different subdetectors. Image credit: CERN
This is a simple simulation event. It shows and isolated electron(red track) hitting and losing all energy in the electromagnetic calorimeter, emc, and an isolated gamma (small dashes line) inferred as a gamma because there is no track and there is energy in the emc.
The degree of isolation is counted with the variable η (thanks to dukwon). Triggers allow choosing events with certain topology for further evaluation. One does not want to throw out the baby with the bathwater so the limits are wide, but still there should be cuts for useless events.
High energy electrons are liable to produce a bremsstrahlung (brems) radiation of high energy, but this will be angularly connected with the electron track and may or may not be added to check with the track momentum.
Say that I want to study Higgs to gamma gamma decay, isolation of the gammas is a very necessary condition, one does not want brems from the particles going through the detectors from gluon or quark jets to be confused with the sought signal.
All this analysis is very dependent on Monte Carlo simulations, and the trigger limits are chosen after studying the sought for interactions and how they will appear in the event reconstruction.