10 - H1 - Desy

132 Results
9.2 Prompt photon + jet production in photoproduction
The cross section for prompt photon production associated with a jet with a given kinematics
is studied. The measurement is performed in the exclusive phase space, defined in
table 1.4. The measured prompt photon plus jet cross section is
σ(ep → eγ jet X) = 50.1 ± 1.7 (stat) ± 6.5 (syst) pb.
The theoretical calculations predict cross sections of 43.8 +5.3
−1.9
pb (FGH) and 49.3+4.7 −2.1 pb
(LZ). Both are compatible with the measurement within the errors. The MC expectation
of 33.9 pb is again too low.
The differential cross sections in bins of E γ T , ηγ , E jet
T
and η jet in the exclusive prompt
photon + jet phase space are presented in figure 9.3 and tables C-4a, C-5a, C-6a and C-7a.
Both calculations give a reasonable description of the E γ T
and Ejet
T
cross sections while
only the FGH calculation describes η jet of the associated hadronic jet well. Here, the LZ
prediction is too high for jets with η jet < 0.5. As in the inclusive case, the FGH prediction
is too low for η γ < −0.5.
The presence of a jet allows to use observables which give more insight in the underlying
partonic process than in the inclusive case, as defined in section 1.5. Figure 9.4 and
tables C-8a and C-9a show the photon plus jet cross section as a function of the estimators
x LO
γ
and x LO
p
. Both distributions are well described by the calculations.
9.2.1 Photon - jet correlations
Figure 9.5 and tables C-10a and C-11a show the cross sections for two observables describing
the transverse correlation between the photon and the jet, p ⊥ and ∆Φ. Both variables
are particularly sensitive to higher order gluon emission. The sample is split into a sample
with x LO
γ > 0.8 where the direct interaction of a photon with the proton dominates and
a sample with x LO
γ < 0.8 with a significant contribution of events with a resolved photon
(see figure 5.7a). Both predictions describe the distributions for x LO
γ > 0.8 reasonably well
but tend to underestimate the tails. For x LO
γ < 0.8 the p ⊥ distribution is slightly broader
than for x LO
γ > 0.8, which reflects the increased contributions from events with a resolved
photon and from photons radiated from quarks in di-jet events. The FGH calculation
poorly describes the p ⊥ distribution but gives a reasonable description of the measurement
in ∆Φ for x LO
γ < 0.8, except for the highest bin in ∆Φ. The region ∆Φ → 180 ◦
is sensitive to multiple soft gluon radiation which limits the validity of fixed order calculations.
The LZ calculation includes multiple soft gluon radiation in the initial state
before the hard subprocess and describes ∆Φ > 170 ◦ but predicts a significantly lower
contribution of events in the tails of both distributions.