10 - H1 - Desy

1.4 Recent prompt photon measurements 19
mass energy squared. In the second term also the LO quark-to-photon splitting function
P (0)
qγ = (1 + (1 − z) 2 )/z contributes.
The variation of the fragmentation function D γ/d (z, µ F,γ ) with the scale µ F,γ can be predicted
by the evolution equations analogue to the DGLAP evolution equations. The
fragmentation function D γ/d (z, µ F,γ ) at the scale µ F,γ can be related to the one at the
initial scale µ 0 by
(
D γ/d (z, µ F,γ ) = αe2 q µ
2
)
2π P qγ (0) (z) ln F,γ
+ D
µ 2 γ/d (z, µ 0 ). (1.38)
0
1.4 Recent prompt photon measurements
In this section some recent results on the production of prompt photons are presented.
At first some measurements using HERA data, both in deep inelastic scattering and
photoproduction are reviewed. A discussion on recent prompt photon production results
measured in hadron collisions follows.
1.4.1 Results at HERA
At HERA inclusive prompt photon and photon + jet production was measured both in
photoproduction region and in DIS.
In photoproduction both H1 [35] and ZEUS [36–38] have already reported prompt photon
measurements, which were compared to PYTHIA and HERWIG Monte Carlo (MC - see
section 2.1) as well as to NLO calculations. Figure 1.15 presents the H1 and ZEUS 3
inclusive prompt photon cross sections in bins of transverse energy of the photon E γ T
and its pseudorapidity 4 η γ compared to the MC predictions. The kinematical range
of the measurements is given by Q 2 < 1 GeV 2 , 0.2 < y < 0.7, −1.0 < η γ < 0.9 and
5 < E γ T
< 10 GeV. Slightly different ZEUS measurement phase space has been corrected
for. The ZEUS measurement was performed using 38.4 pb −1 of data integrated luminosity,
while H1 was based their results on 105 pb −1 of data integrated luminosity. The two
measurements are consistent within errors, but predictions are low in normalisation by
40 −50%. The comparison of the H1 measurement with the QCD calculation is presented
in figure 1.16a and 1.16b. After the hadronisation and multiple interaction correction,
the shape of the cross section is reasonably well described, but the normalisation is again
too low by 30 − 40%.
The prompt photon cross sections with the additional requirement on the accompanying
hadronic jet as measured by the H1 collaboration [35] are presented in figure 1.16c
and 1.16d. The measurement is somehow better described by the calculation in this case.
The better description, together with the fact that NLO corrections are in average smaller
3 The ZEUS measurement was corrected to the kinematic range used by the H1 analysis.
4 For a definition of a pseudorapidity the reference system is needed. It is introduced in section 3.2
where the η definition is also given.