10 - H1 - Desy

Chapter 10
Conclusions and outlook
The prompt photons originating from the hard interaction are a powerful test of QCD.
Compared to measurement relying on jets, they are believed to be less sensitive to the
details of hadronisation models which introduce large theoretical uncertainties. The more
difficult background situation however, which demands complex signal extraction methods
leads to an increase of the systematic error. Also statistical error is higher since the
photon cross section is suppressed by the order α as compared to the jet cross sections.
The prompt photon analysis, though challenging and intrinsically burdened with higher
uncertainty, provides a complementary measurement to results relying on jets.
The cross sections for prompt photon production in photoproduction is measured using
data taken by the H1 detector with the full HERA II integrated luminosity of 340 pb −1 .
The study provides the first ever measurement of photons in such a forward direction at
HERA. Compared to the previous measurement performed by the H1 collaboration [35],
the phase space has been also extendend into lower event inelasticities. Prompt photons
with the transverse energy 6.0 < E γ T < 15.0 GeV, pseudorapidity −1.0 < ηγ < 2.4 and
isolation z > 0.9 in photoproduction region Q 2 < 1 GeV 2 and event inelasticity region
0.1 < y < 0.7 have been studied in the photon inclusive phase space and together with
the accompanying hadronic jet with transverse energy E jet
T
> 4.5 GeV and pseudorapidity
−1.3 < η jet < 2.3. The complex unfolding method has been developed in order to
perform the measurement insensitive to possible model inaccuracies. The cross sections
are compared to QCD calculations based on collinear factorisation in NLO [39,75] and
based on the k T factorisation approach [80].
In the inclusive prompt photon phase space the dσ/dE γ T , dσ/dηγ as well as double differential
dσ 2 /dE γ T dηγ cross sections are found to be underestimated by both predictions
most significantly for low transverse energies of the photon. Furthermore, FGH calculation
fails to describe the shape of the η γ , being significantly too low for η γ < 0.9. The
cross sections predicted by the PYTHIA Monte Carlo generator is underestimates the
measurement by 40% on average, most significantly at low transverse energies.
The differential cross sections dσ/dE γ T , dσ/dηγ , dσ/dE jet
T , dσ/dηjet are studied in the
photon + jet exclusive phase space. Both calculations describe the rate of prompt photons
in the exclusive case somehow better than in the inclusive phase space. This observation