10 - H1 - Desy

68 Prompt photon selection
reconstructed in two separate clusters the invariant mass M γγ of the photon candidate
combined with the closest electromagnetic cluster having energy above 80 MeV has been
introduced. The cut M γγ > 0.3 GeV removes as well 13% of the signal, mostly photons
that converted into electron-positron pair before reaching LAr surface and consistently
shower in two separate clusters.
All the cuts described above decrease the contamination of neutral hadrons background
to the level of roughly 50%. Determination of final prompt photon content was done on
a statistical basis by cluster shower shape analysis (see chapter 6). For a cluster to have
well defined shape, valid for the shape analysis, it is required that not all the cells are
positioned along one axis. This unlikely configuration happens for 0.7% of all photon
candidates, which are removed from the selection.
57 events, possesing more than one valid photon candidate, which amounts to 0.4% of all
the selected events, are also removed from the final sample.
All the criterias described above applied to the whole HERA II data sample select roughly
15000 prompt photon candidates. The pseudorapidity η γ and transverse energy E γ T distributions
are presented in figure 5.5. The sum of signal and background MC samples
descibes the data points reasonably well.
Events
500
400
300
Events
10
4
a. b.
3
10
Sum MC
Signal
Background
200
100
2
10
0
-1 0 1 2
γ
η
5 10 15
γ
[GeV]
E T
Figure 5.5: Distributions of pseudorapidity η γ (a) and transverse energy E γ T
(b) of the
selected prompt photon candidates plotted together with the MC prediction. The vertical
lines indicate the cut values.
5.3 Exclusive selection and its subselections
A subsample of the inclusive prompt photon selection with additional jet requirement was
used for the determination of cross sections in the exclusive phase space (see section 1.5).
Events with the photon candidate fulfilling all the photon selection criteria and with an
accompanying jet in the pseudorapidity range −1.3 < η jet < 2.3 and transverse energy
E jet
T
> 4.5 GeV are used to form the exclusive photon selection sample. The cuts are
applied on the hadronic jet with the highest energy.
The η jet and E jet
T
distributions together with MC predictions are presented in figure 5.6.