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

pQCD jets once the jet pT is greater than 400-500 GeV/c. The CDF collaboration performed
measurement of substructure of jets with pT >400 GeV/c by studing distributions of the jet
mass and measuring angularity, the variable describing the energy distribution inside the jet, and
planar flow, the variable differentiating between two-prong and three-prong decays. At small
cone sizes and large jet mass, these variables are expected to be quite robust against soft radiation
and allow, in principle, a comparison with theoretical predictions in addition to comparison
with MC results. Jets are reconstructed with the midpoint cone algorithm (cone radii R=0.4,
0.7, and 1.0) and with the anti − kt algorithm 9 (with distance parameter R=0.7). Events are
selected in a sample with 6 fb −1 based on the inclusive jet trigger. There is a good agreement
between the measured m jet1 distribution with the analytic predictions for the jet function and
with PYTHIA MC predictions. The midpoint and anti − kt algorithms have very similar jet
substructure distributions for high mass jets, see Fig. 4. The angularity distribution shown on
Fig. 5(left) in addition to reasonable agreement data and PYTHIA MC also demonstrates that
the high mass jets coming from light quark and gluon production are consistent with two-body
final states and that further rejection against high mass QCD jets can be obtained by using the
planar flow variable, Fig. 5(right).
Fraction of Events / bin of 0.002
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
τmin -2 τ-2
max
0
0 0.005 0.01 0.015 0.02 0.025 0.03
jet1
τ
-2
-1
CDF Run II, L = 6 fb
int
0.8
0.7
0.6
0.5
0.4
Midpoint
Anti-kT
0.3
0.2
0.1
0
0 0.01 0.02 0.03
Data, Midpoint, R = 0.7
QCD, Pythia 6.216
Fraction of Events / bin of 0.1
0.6
0.5
0.4
0.3
0.2
0.1
0.7
0.6
0.5
0.4
0.3
0.2
0.1
-1
CDF Run II, L = 6 fb
int
Midpoint
Anti-kT
0
0 0.2 0.4 0.6 0.8 1
Data, Midpoint, R = 0.7
QCD, Pythia 6.216
tt,
Pythia 6.216
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
jet1
Planar Flow
Figure 5: The angularity distribution for midpoint jets with pT >400 GeVc. The t¯t rejection cuts and requirement
for 90 GeV/c 2 < m jet1