10 - H1 - Desy

5.2 Photon selection 65
cells adjacent to the cracks are removed from the selected sample. In addition the most
energetic cell of the cluster is required to be part of the LAr calorimeter wheel in which
most of the cluster’s energy is deposited. This requirement removes any ambiguity in
assigning cluster to particular LAr calorimeter wheel.
Clusters with an electromagnetic energy fraction in the first two layers (E γ EM2
) of the
calorimeter above a certain threshold are selected. The threshold function has been chosen
such as to minimise possible efficiency losses. Figure 5.2 presents the E γ EM2
fraction for
photon initiated clusters (a) and for hadrons in example of charged pions (b) as a function
of polar angle θ γ together with the threshold function. The energy fraction for photons
depends on the polar angle due to the difference in angle of impact and thus difference
in accessible electromagnetic part of calorimeter. More importantly, it is sensitive to
differences of the structure of LAr calorimeter wheels. A significant change is visible
around θ ∼ 50 ◦ where horizontal absorber planes of CB3 change to vertical absorber
planes of FB1. The very low threshold function (20 ◦ < θ < 30 ◦ ) corresponds to FB2 wheel,
where the first layer contains G10 material instead of lead. One can note that photons
can be found generally above the cut function, while for hadrons the energy fraction in
the first two layers in the calorimeter is usually below 5%. The high electromagnetic
fractions produced by hadrons for backward detector region (∼ 140 ◦ ) correspond to the
edge of hadronic LAr calorimeter.
γ
EM2
E
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 20 40 60 80 100 120 140 160 180
γ
θ [deg]
γ
EM2
E
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 20 40 60 80 100 120 140 160 180
γ
θ [deg]
Figure 5.2: The electromagnetic energy fraction in the first two layers of LAr calorimeter
of photon clusters (a) and hadron clusters (b) together with the threshold function.
The trigger correction, as discussed in section 4.3.2 is defined by the position and energy
of the photon candidate with the assumption that the candidate itself enabled the trigger
during the online trigger selection. Since this assumption is not necessary always true,
special attention has been paid to ensure its validity. Figure 5.3 presents the difference
between candidate cluster and the closest enabled BT (see section 4.3.1) in pseudorapidity
and azimuthal angle. One can see the greatest majority of the clusters correspond to an
active BT nearby. Interesting observation can be made for a small, but still visible fraction
of events where cluster failed to trigger the BT, while the accompanying jet managed to
do it. Since jet in most cases balances the photon in azimuthal angle φ, those events can
be seen close to ∆φ ∼ π as a clearly visible peak two orders of magnitudes lower than the
main one. In order to avoid ambiguity of the trigger correction, a cut was introduced on