10 - H1 - Desy

22 Theoretical framework
ZEUS
dσ/E T γ (pb /GeV)
dσ/dη γ (pb)
10
1
10 -1
40
20
ZEUS (77 pb -1 )
NLO+had. (KZ) 0.5< µ R < 2
NLO+had. (FGH) µ R =1
k T -fact.+had. (LZ) 0.5< µ R < 2
PYTHIA 6.3
HERWIG 6.5
γ prompt
+ jet
(a)
6 8 10 12 14 16
E γ
T
(GeV)
(b)
Figure 1.17: Prompt photon
+ jet differential cross sections
dσ/dE γ T
(a) and dσ/dη γ
(b) in photoproduction as measured
by the ZEUS [38] collaboration
compared to predictions by
PYTHIA (solid black line), HER-
WIG (dashed-dotted black line),
NLO calculation (green shaded
band) and the calculation based
on the k T factorisation approach
(striped band).
0
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1
η γ
The importance of the radiation from the electron was shown by the study of the H1
collaboration presented in figure 1.19. The results are quoted for the phase space 4 <
Q 2 < 150 GeV 2 , y > 0.05, 3 < E γ T < 10 GeV and −1.2 < ηγ < 1.8 and have been
performed using data based on 227 pb −1 integrated luminosity. The H1 measurement in
DIS is compared to a LO calculation and to MC prediction, both including radiation
from the electron line. The additional contribution of photons radiated from the electron,
drastically changes the shape of the η γ distribution. Nevertheless, both MC and QCD
calculation underestimate the prompt photon production rate by roughly a factor two,
with the difference being most significant at low Q 2 .
The preliminary ZEUS results [42], performed with 320 pb −1 of integrated data luminosity
within the kinematic range of 10 < Q 2 < 350 GeV 2 , 4 < E γ T < 15 GeV, −0.7 < ηγ < 0.9
confirm the observations of the H1 collaboration.
1.4.2 Results in hadronic collisions
In hadron collisions prompt photons have been studied for a long time. A large variety
of experiments have measured the photon cross section at both fixed target and collider