10 - H1 - Desy

136 Results
9.3 NLO corrections
The leading order Feymann diagrams for direct and resolved prompt photon production
is of order (α 2 α 0 s ). The FGH calculation (see 2.2.1) is performed to the order (α2 α s ),
which includes already the next-to-leading order correction. The correction increases the
calculation by roughly 25% in the inclusive prompt photon phase space and by 10% in the
exclusive photon + jet phase space. More differential information is presented in figure 9.6
where few chosen measurements are presented together with the LO calculation, the NLO
calculation and the calculation without the box diagram (see 1.13a) considered.
The cross section in the inclusive phase space (figures 9.6a and 9.6b) are affected by
the correction most significantly at low transverse energies (correction of 32%) and for
photons in the forward direction (43%). For the cross sections in the exclusive phase space
a very strong positive correction (up to 70%) can be seen for events with the hadronic jet
going into the forward direction (figure 9.6d). At the same time, NLO slightly decreases
cross sections in the low η jet range. The NLO influence is strongest for the cross sections
in bins of x LO
γ
(figure 9.6e), where additional gluon radiations shift direct events towards
values. The correction significantly decreases the cross section in the highest
bin (by 40%) but increases it in the other bins.
lower x LO
γ
x LO
γ
The box diagram increases calculated cross sections on average by 11%. Its influence is
the strongest for low pseudorapidities of the photon, where it increases the cross sections
by almost 20%. Low values of x LO
p are also strongly affected (difference of 20%). An
interesting feature can be observed for the cross section in bins of x LO
γ variable. Even
though the box diagram can not be unambiguously assigned to the direct event class, it
contributes only at x LO
γ ≈ 1. Since in the final state there are only two particles leaving
the hard interaction, the x LO
γ definition is properly balanced in the same way as it is the
case for leading order direct events.
Figure 9.7 presents the same comparison for the cross sections in bins of photon - jet
correlation variables in the direct and resolved enhanced phase space separately. In leading
order, only back-to-back configuration exists. The photoproduction criteria Q 2 < 1 GeV 2
together with small intrinsic 〈k T 〉 of partons in the proton do not yield any sizeable
momentum imbalance which could be seen outside of the lowest p ⊥ bin of 2 GeV width.
The next order correction smears out both distributions. The box diagram contributes
mainly in the back-to-back configuration in the direct enhanced phase space.