While leptons are technically the name given to particles that interact with the elec- troweak force and not the strong force; in the chapters to come, the use of the word lepton refers to only electrons and muons.
This is because it is useful to have a term to separate electrons and muons from the other leptons3as these are objects that have high reconstruction efficiencies.
Neutrinos are only detected by momentum imbalances seen within the detector; and taus are heavy enough to decay hadronically meaning that their signature usually re- quires its own analysis outright.
Similar to the track selections for ID tracks, electrons and muons can be designated loose or tight. In both cases the particle candidates are required to have a pT of > 7 GeV, a
small impact parameter, and to be constructed within the region of the detector devoted to precision physics (| η | < 2.474).
7.3.1
Electrons
Electrons are constructed from topo-clusters that are matched to ID tracks [118, p8]. Instead of simply building electrons from these deposits from a cut-based method, the constituent objects are fed into a multivariate analysis (MVA) which constructs the like- lihood that the input features resemble something that is known to be an electron. In this there are additional possible requirements on this probability in the form of ‘LooseLH’, ‘MediumLH’ and ‘TightLH’ [120, p11] computed from ‘shower shape’ and track qual- ity variables.
The analyses make use of both loose and tight quality electrons. The tight requirement will be used when only a single electron track is expected in an event and the loose requirement will be used when more than one electron track (usually two) is expected.
3Yet more proof that in general physicists are terrible at naming things. I say why not use ‘emu’
instead! It sounds funny because it’s the name of bird and best of all it’s technically correct.
In addition to the properties already mentioned above, loose electrons are required to have tracks that are isolated from those that go into other objects. The quantity used to determine the track isolation is the scalar sum of the transverse momentum (pT) of the
tracks with pT > 0.4 GeV in a cone of ∆R around the electron5, excluding the track of the
electron itself. The tracks to be considered must be of good quality and originate from same the primary vertex as the electron track [120, p12]. and fulfil a LooseLH quality criteria. Tight electrons are selected to TightLH criteria and have stricter isolation but based on calorimeter deposits.
7.3.2
Muons
Since the muons have their own sub-detector, muon reconstruction can happen inde- pendently in both the ID and the MS. The information from the two sub-systems is then combined to form muon tracks. The ID muons are constructed much like their electron counterparts, but the requirements in the MS are are slightly different. While the pTstill
has to be above the same threshold of≥ 7 GeV, the tolerated acceptance is increased to | η | < 2.7 to cover the entire pseudo-rapidity of the MS [1, p4].
Loose and Tight muons are used in the analyses, and similarly to their electron counter- parts the tight requirement is used when only a single muon track is expected and the loose requirement is be used when more than one track is expected.
Also similarly to electrons there are four different types of muon quality selections that can be built: ‘Loose’, ‘Medium’, ‘Tight’ and ‘High-pT’ [121, p6]6 with ‘Medium’ being
the ATLAS default.
5∆R measures the size of a resulting object in the (rapidity-φ plane)
6Each criterion is designed to maximise a certain aspect of muon reconstruction: ‘Loose’ is designed to
maximise efficiency; ‘Medium’ for the determination of statistical uncertainties; ‘Tight’ for muon purity at the cost of some efficiency; and ‘High-pT’ for momentum resolution.
Loose muons are selected for the analyses using the ‘Loose’ [121, p7] quality criterion and are required to have a similar level of track isolation to that of electrons. Tight muons fulfil ‘Medium’ [121, p7] quality criteria and have a stricter track isolation re- quirement.
7.3.3
Taus
Taus are the heaviest known lepton (in the original sense) and while the analysis will not use taus to look for signal, they are massive enough to decay into quarks as well as into other leptons. Leptonically decaying taus will decay into muons and electrons, but these can be distinguished from the ones that are produced from the decay of heavier objects, as they typically will come from displaced vertices and have large impact pa- rameters. Hadronically decaying taus decay into quarks which form jets in the detector. These jets, however, are not easily distinguished from deposits from other hadronically decaying particles.
Taus, therefore, feature in the analysis in the form of a ‘veto’. Events that reconstruct tau candidates are removed from the analysis to avoid taus being misidentified as jets. The information from these tau candidates also goes into the missing transverse energy calculation (see Section 7.4).
When taus decay they mostly do so hadronically (∼65%) [29, p17] into pions (and kaons). Pions can have charges of +1, 0 and -1 and have masses of about 135 MeV (kaons are heavier at 493 MeV). Neutral pions do not leave tracks in the detector so the ones that are mostly useful for the construction of taus are the charged ones. Since taus have a charge of ±1, and are many times heavier than pions there are two main types of decay mode that are interesting for forming the tau veto. These are classified with the number of charged tracks in the final state: 1-prong (τ− → π−ν
(τ−→ π−π−π+ν τ)7.
Tau-leptons constructed within the analysis must have pT≥ 20 GeV and | η | < 2.5 but
lie outside of the transition region between the barrel and end-cap ECal, 1.37 < η < 1.52 [1, p4].
Like the lighter charged leptons, tau candidates are reconstructed using MVA tech- niques. The quality labels ‘loose’, ‘medium’ and ‘tight’8are based on the target recon-
struction efficiency and hence are often referred to as working points.
The working points for tau candidate reconstruction in the ‘loose’ : ‘medium’ : ‘tight’ scheme are 0.60 : 0.50 : 0.45 for 1-prong and 0.50 : 0.40 : 0.30 for 3-prong [122, p8]. The analyses use ‘medium’ quality taus [1, p4].