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

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Figure 2: Inclusive jet double-differential cross section as a function of jet pT in different regions of |y| for jets
identified using the anti-kT algorithm with R = 0.6.
contributions as a function of the jet pT for the central region. The total uncertainty is lower
than 4.5% for all jets with pT > 20GeV with the dominant contribution coming from the
calorimeter uncertainty.
The uncertainty for other pseudorapidity regions is assessed using in-situ di-jet intercalibration10
. Figure 1 (right) shows the fractional JES uncertainty in the forward region as a function
of jet pT. The total JES uncertainty amounts to about 14% with the intercalibration uncertainty
as a dominant source.
3 Cross section measurements
[pb/GeV]
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ATLAS measurements of inclusive jet cross sections 3 are performed as a function of jet pT in
7 bins of jet rapidity up to |y| < 4.4. The measured cross section is corrected for experimental
effects, like detector inefficiencies, resolution and trigger effects back to the hadronic final state
using bin-by-bin unfolding calculated from Monte Carlo (PYTHIA, MC10 tune). The corrected
spectrum is compared to NLO QCD theoretical predictions, calculated with the NLOJET++
program and CTEQ 6.6 NLO parton density functions (PDF) as a baseline. NLO calculations
are corrected for non-perturbative effects to account for hadronization and underlying events.
The corrections are evaluated from the ratio of the cross section with and without hadronisation
and underlying event obtained from leading-logarithmic parton shower generators (PYTHIA,
AMBT1 tune).
Figure 2 (left) shows both measured and predicted (using CTEQ 6.6 PDF set) doubledifferential
inclusive jet cross section as a function of jet pT in seven rapidity regions for anti-kT
jets with R = 0.6. The experimental uncertainty (blue band) stays within 10-40 % over the
full kinematic range studied, with the dominant contribution from the JES uncertainty. The
theoretical uncertainty (yellow band), largely coming from uncertainty of PDFs, is on the level
of the experimental one. A comparison of the measured cross section with predictions obtained
using the CTEQ 6.6, MTSW 2008, NNPDF 2.1, and HERAPDF 1.5 PDF sets is shown in Figure
2 (right). The data points and the error bands are normalized to the theoretical predictions
obtained by using the CTEQ 6.6 PDF set. Predictions using HERAPDF 1.5 appeared to follow
the data most closely, though consistency within experimental and theoretical uncertainties with
most other PDF sets is observed.
A first dedicated study of multi-jet final states 6 for events containing two or more jets with