One of the most well known particles we detect with ATLAS is the electron. We know that it will create a track within the Inner Detector (ID) and deposit all of its energy in the Electromagnetic Calorimeter (EMCal). How do we reconstruct this?
Electrons in ATLAS
The ID is composed of multiple layers of silicon detectors plus the several layers of straw tubes. As charged particles travel through the silicon, they ionize the silicon. Because the silicon has a high voltage across it, that ionization becomes an electrical current, a signal. Nearby signals in a silicon module are formed into “hits”. A similar process happens in the straw tube tracker. Track reconstruction software starts from the hits in these tracking detectors and determines the sets that most probably belonged together, and then fits the direction and momentum of the particle that corresponded to that set of hits (the track).
Once the electron leaves the ID, it reaches the calorimeters. When the electron hits the calorimeter, it creates an “electromagnetic shower” of electrons and photons. Electrons create photons through Bremsstrahlung and photons create electrons through pair production (there are other interactions, but these are the two main ones). These processes continue until the full energy of the electron is absorbed by the calorimeter material.
These EM showers are a lot smaller than the equivalent “hadronic showers” created by hadrons, meaning they are narrow and contained in the first part of the calorimeter, which is optimized for reconstructing such showers and is hence called the EM calorimeter (ECal). EM objects (electrons and photons) can be identified by having such shorter/narrower calorimeter deposits.
We then take the information from both subsystems in order to reconstruct the electron or photon. As shown in the Object Introduction, if we can spatially connect a track to a large deposit in the calorimeter then we have an electron! … Right?
(One of) The Problem(s)
Well … our electron reconstruction and identification aren’t exactly perfect. There are a number of ways we can reconstruct a “fake” electron, the software thinks an artifact is an electron but it is not actually one. The EGamma group helps define requirements for analyzers to set in order to have confidence that the electron object is truly an electron. The Isolation and Fakes Forum also has useful tools and suggestions for dealing with these objects.
We often refer to these objects as “fakes”, but don’t write that in a paper! The most common types of fake electrons are better called non-prompt electrons (electrons that come from a heavy flavor hadron or a photon conversion), misidentified photons (e.g. with a track incorrectly associated with the photon), and misidentified hadrons (e.g. with an unusually short and narrow hadronic shower). In your analysis, you’ll want to consider the most relevant sources.
Several different “working points” are defined to allow analyzers to choose how strict a requirement they want to use when identifying electrons. These working points typically range from “Very Loose” to “Tight”. As the working points get tighter, the rate of “fake” electrons decreases, but so does the efficiency of selecting real electrons. This is always the tradeoff when doing any kind of classification.