Model Independent Search for Deviations from the Standard Model ...

4.3. Object Identification and Selection Cuts 49
4.3.5 Missing Transverse Energy (MET)
Missing Transverse Energy is the signature of neutrinos and other non-interacting particles
inthedetectorcomputedfromthemomentumimbalanceofaneventinthetransverseplane
(see Section 1.3.2). The calculation of MET combines quantities from central tracking
(for primary vertex determination), calorimeter (for energy deposition) and muon detector
(for the muon correction to the energy of the event). MET has to be corrected for the
reconstructed objects of an event and is thus strongly dependent on this reconstruction.
The missing transverse energy is the last object computed, and it uses inputs from other
reconstruction algorithms, such as the Jet Energy Scale and the electromagnetic scale.
Uncertainties in all these values propagate, and MET reects this. MET is calculated in
several steps [40]:
First the visible energy in the calorimeter is summed up:
Ex,y vis = ∑ E x,y
i
. (4.5)
cells
To establish transverse momentum balance in the calorimeter, missing energy is
MET x = −Ex
and vis MET y = −Ey
resulting in vis
MET = √ (MET x ) 2 + (MET y ) . It is
important to stress that the calorimeter granularity is taken advantage of by using 2 cells
and primary vertex information for MET reconstruction. In this way η and ϕ can be determined
for each cell individually, and the transverse component of the measured energy
canbecalculatedforeachsinglecell. Asaconsequence, thesumoverallcellsandthus Ex,y
is more precise. In this sum, cells from the coarse hadronic (CH) calorimeter (> layer 15)
vis
are not included because of the noise present. To account for jets which deposit energy
in this part of the calorimeter, the clustered jets are corrected for this energy by using
the coarse hadronic fraction chf = E ch
E tot
. Then the raw transverse energy of the jet is
subtracted from MET x and MET y .
In addition to this correction of raw deposited jet energy in the calorimeter, MET has
to be corrected for the Jet Energy Scale. Only after the JES is applied to all clustered
calorimeter jets, their transverse energies correspond to jet energies at the particle level
and momentum balance can be applied. The same is true of the electromagnetic scale.
Finally, MET is corrected for the presence of reconstructed muons in the event, as this
minimum ionizing particle would alter the MET. The momentum of muons passing a
set of quality cuts (tight muons) is subtracted from the Missing Transverse Energy after
deduction of the expected muon energy loss in the calorimeter. The nal MET is given as
MET cal+muon =
√
(MET cal x
− p muons x ) 2 + (METy
cal − p muons y ) 2 . (4.6)
This analysis selects events with a certain amount of MET. This threshold has to be
relatively large to identify MET from particle interactions, as there is a constant MET
value of ≈ 10 GeV due to noise and MET-resolution:
• MET > 30 GeV