10 - H1 - Desy

20 Theoretical framework
than in the inclusive case, suggests that higher order corrections are of less importance
for prompt photons if accompanied by an energetic jet.
Inclusive prompt photon
dσ / dE γ T (pb/GeV)
10
H1
ZEUS
PYTHIA
dir
without m.i.
HERWIG
dσ / dη γ (pb)
40
30
20
10
1
a)
10 2 6 8 10
E γ T (GeV)
0
b)
-1 -0.5 0 0.5
η γ
Figure 1.15: Inclusive prompt photon differential cross sections dσ/dE γ T
(a) and dσ/dηγ
(b) in photoproduction as measured by the H1 [35] and ZEUS [36] collaborations compared
to predictions by PYTHIA (solid black line) and HERWIG (dashed red line). The
contribution of direct events is presented with a dashed dotted line and a prediction
without multiparton interaction with blue dotted line. Cross sections of the ZEUS were
corrected to the kinematics used by the H1 analysis.
Prompt photon production with accompanying jet was also studied by the ZEUS collaboration
[38] and some results of this study are presented in figure 1.17. Monte Carlos again
predict cross sections low in the normalisation, while QCD calculations give a reasonable
description of the data. Especially the calculation based on the k T factorisation approach
(see section 2.2.2) performs well, while NLO calculations predict too low cross sections
for low transverse energies of the photon and low pseudorapidities. For an increased minimum
transverse energy of the photons to 7 GeV, the differential cross sections are well
described by all studied calculations.
In deep inelastic scattering, prompt photons were studied by both ZEUS [41, 42] and
H1 [43]. Figure 1.18 presents the differential cross sections measured by the ZEUS collaboration
[41] compared to MC predictions of PYTHIA and HERWIG. The measurement
has been performed in the phase space 5 < E γ T < 10 GeV, −0.7 < ηγ < 0.9 and
Q 2 > 35 GeV 2 . The transverse energy distribution is reasonably described by PYTHIA
scaled by factor 2.3 and by HERWIG scaled by factor 7.9. PYTHIA fails to predict even
the shape of cross section in bins of η γ , while HERWIG predicts a too steep Q 2 slope.
The contribution of photons radiated from the electron was neglected in this study.