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

atio to LO (x µ=1)
3
2.5
2
1.5
1
pp, 7 TeV, anti-k t R=0.7
NLOJet++, CTEQ6M
0.5
0
100 200 300 400
LO
NLO
–
– nNLO (µ)
nNLO (RLS )
500 600 700 800 900
1
2− HT,2 [GeV]
ratio to LO (x µ=1)
3
2.5
2
1.5
1
pp, 7 TeV, anti-k t R=0.7
NLOJet++, CTEQ6M
0.5
0
100 200 300 400
LO
NLO
–
– nNLO (µ)
nNLO (RLS )
500 600 700 800 900
1
2− HT,3 [GeV]
ratio to LO (x µ=1)
3
2.5
2
1.5
1
pp, 7 TeV, anti-k t R=0.7
NLOJet++, CTEQ6M
0.5
0
100 200 300 400
LO
NLO
–
– nNLO (µ)
nNLO (RLS )
500 600 700 800 900
1
2− HT [GeV]
Figure 3: The ¯nNLO and NLO K-factors for the effective mass observables in dijet events.
just scalar sums of transverse momenta of the n hardest jets above the threshold pt,min = 40 GeV.
We also denote HT ≡ HT,∞. The results are shown in Fig. 3. The uncertainties due to scales and
RLS where obtained as in the processes discussed previously. The central value of the scale was
taken at µ0 = pt,j1. Each of the three plots in Fig. 3 carries interesting information. HT,2 does
not get any at correction at ¯nNLO. This distribution, which is sensitive only to the two hardest
jets, converges already at NLO and extra jets from initial state radiation that appears at higher
orders do not affect it. HT,3 gets a substantial correction at NLO and here the LoopSim result
shows that this observable comes under control at ¯nNLO. This is because HT,3, is insensitive
to the fourth jet appearing at ¯nNLO. However, the HT distribution at ¯nNLO still receives
substantial enhancement since that observable sums up all jets in the event.
4 Conclusions
We have presented the LoopSim method for computing approximate higher order corrections
by making use of unitarity and merging NLO results with different multiplicities. The method
is supposed to work best for processes with large K-factors from LO to NLO. We have shown
that for the case of Drell-Yan process, whose distributions are known at NNLO, the predictions
obtained with LoopSim are in excellent agreement with the exact result. We have also given
approximate predictions for NNLO corrections to Z+j and dijets for a range of observables,
finding either indication of convergence or further non-negligible corrections, the latter probably
being due to additional initial state radiation appearing at higher orders.
Acknowledgments
The original results presented here were obtained with Gavin Salam and Mathieu Rubin. The
work was supported by the French ANR under contract ANR-09-BLAN-0060 and by the Groupement
d’Intérêt Scientifique “Consortium Physique des 2 Infinis” (P2I).
References
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http://www.desy.de/∼znagy/Site/NLOJet++.html