2011 QCD and High Energy Interactions - Rencontres de Moriond ...

The results of two new measurements of inclusive γ + 3 jetand γ + 2 jet events (see Fig.1
b) and c)) were presented by D0 during last two years 5,6 . They are based on new methods
proposed by CDF for studing the DP processes 4 . High integrated luminosity L =1.02 ± 0.06
fb−1 collected in RunII allowed to extend the mentioned above p jet2
T interval of 2 GeV width,
used in 4 , up to the range 15 ≤ p jet2
T ≤ 30 GeV and to select thee(two) jets γ + 3(2) jet + X
events with high photon transverse momentum, 60 < p γ
T < 80 GeV (vs. pγ
T > 16 GeV in
CDF, where ”photon” was taken to mean either a single direct photon or multiple photons from
neutral meson decay in jet fragmentation 4 ) with a large photon purity which is well known
from previous D0 RunII measurements 8 . The high p γ
T scale (i.e. the scale of the first parton
interaction) allows a better separation of the first and second parton interactions in momentum
space.
a)
p
p
σA
σB
b)
1
PT
p
p
γ
T
jet1
T
ΔS
p
p
jet3
T
Figure 1: a) Schematical diagram of a double parton scattering event; b) Diagram showing the two possible sum
vectors of the γ + 1 jet and dijet systems in γ + 3 jet events; c) Diagram showing the two possible sum vector of
in γ + 2 jet event.
jet2
T
2
PT
the γ + 1 jet system and p jet2
T
Three DP signal diagrams (DP Type I, DP Type II, DP Type III), which give the most
conribution to the cross section, are shown in plot a) of Fig.2 together with the diagram of
a background single parton-parton (SP) collision with hard gluon bremsstrahlung qg → qγgg,
q¯q → gγgg that has the same γ+ 3jets signature. In plot a) the light and bold lines correspond
to two separate parton interactions. The dotted line represents unreconstructed jet. The
SP events together with double interaction (DI) events (i.e. events produced in two distinct
hard interactions occurring in two separate p¯p collisions in the same beam crossing) provide an
essential background.
a)
DP Type I DP Type II DP Type III SP
jet2
p
T
jet1
p
T
γ
p
T
p
jet3
T
jet3
p
T
jet2
p
T
jet1
p
T
γ
p
T
jet1
p
T
jet2
p
T
γ
p
T
jet3
p
T
jet3
p
T
jet1
p
T
γ
p
T
jet2
p
T
(mb)
eff
σ
25
20
15
10
5
0
-1
D0, L = 1.0 fb
int
c)
15 20 25 30
jet2
p (GeV)
T
b)
A
PT
Fraction of DP events
0.6
0.5
0.4
0.3
0.2
0.1
0
p
p
γ
T
jet1
T
from ΔS
φ
from ΔSp
T
from ΔSp’
Δφ
-1
DØ, L = 1.0 fb
int
p
jet2
16 18 20 22 24 26 28 30
jet2
p (GeV)
T
c)
Figure 2: a) Diagrams of DP Types I, II, III and SP events. For DP events, the light and bold lines correspond
to two separate parton interactions. The dotted line represents unreconstructed jet; b) σeff values in the three
”p jet2
T ” intervals; c) Fractions of DP events in the three ”p jet2
T ” intervals.
D0 experiments 5,6 collected γ+3 jets and γ+2 jets events which were selected with leading (in
pT) jet pT > 25 GeV, while the next-to-leading (second) and third jets must have pT > 15 GeV.
Each event must contain at least one γ in the rapidity region |y| < 1.0 or 1.5 < |y| < 2.5 and at
least three jets with |y| < 3.0. The events used for the analysis should first pass triggers based on
the identification of high pT clusters in the EM calorimeter with loose shower shape requirements
> 35 GeV. Jets are reconstructed using the
for photons. These triggers are 100% efficient for p γ
T
T
T