NLO Vector+Jets Predictions with BLACKHAT & SHERPA David A. Kosower Institut de Physique Théorique, CEA–Saclay on behalf of the BLACKHAT Collaboration Z. Bern, L. Dixon, Fernando Febres Cordero, Stefan Höche, Harald Ita, DAK, Daniel Maître, Kemal Ozeren [1009.2338, 1108.2229, & work in progress] ICHEP 2012, Melbourne July 5, 2012
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NLO Vector+Jets Predictions with B lack H at & SHERPA
NLO Vector+Jets Predictions with B lack H at & SHERPA. David A. Kosower Institut de Physique Th é orique , CEA– Saclay on behalf of the BlackHat Collaboration Z. Bern, L. Dixon, Fernando Febres Cordero , Stefan Höche , Harald Ita , DAK, Daniel Maître , Kemal Ozeren - PowerPoint PPT Presentation
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NLO Vector+Jets Predictions with BLACKHAT & SHERPA
David A. KosowerInstitut de Physique Théorique, CEA–Saclayon behalf of the BLACKHAT Collaboration
Z. Bern, L. Dixon, Fernando Febres Cordero, Stefan Höche, Harald Ita, DAK, Daniel Maître, Kemal Ozeren
[1009.2338, 1108.2229, & work in progress]ICHEP 2012, Melbourne
July 5, 2012
Vector+Jets
• Background to – Many searches of new physics– Measurements of Higgs properties– Measurements of top properties
• Standard candle for checking our understanding of Standard-Model processes
Next-to-Leading Order in QCD
• Precision QCD requires at least NLO
• QCD at LO is not quantitative: large dependence on unphysical renormalization and factorization scales
• NLO: reduced dependence, first quantitative prediction
• NLO importance grows with increasing number of jets
• Applications to Multi-Jet Processes: Measurements of Standard-Model distributions & cross sections
Estimating backgrounds in Searches L. Dixon’s talk
• Expect predictions reliable to 10–15%• <5% predictions will require NNLO
The On-Shell Revolution
• Ingredients to NLO calculations– Tree-level 2 V+n and 2 V+n+1… now with improved efficiency
(Britto, Cachazo, Feng, Witten; Dixon, Henn, Plefka, Schuster)– NLO parton distributions– General framework for numerical programs (Catani & Seymour)– One-loop 2 V+n
The on-shell revolution has broken the bottleneck: n=3,4,5 BLACKHAT Implementation
• Numerical implementation of on-shell methods for one-loop amplitudes
• Automated implementation industrialization• Do algebra numerically, analysis symbolically (“analytically”)• SHERPA for real subtraction, real emission, phase-space integration• Distribute results via ROOT n-tuples
• Lots of revolutionaries roaming the world– BLACKHAT– CutTools+HELAC-NLO: Ossola, Papadopoulos, Pittau,
Actis, Bevilacqua, Czakon, Draggiotis, Garzelli, van Hameren, Mastrolia, Worek & their clients
• Scale variation reduced substantially at NLO• Successive jet distributions fall more steeply• Shapes of 4th jet distribution unchanged at NLO — but first
three are slightly steeper
CMS measurements [1110.3226] — but no comparison to NLO yet
Comparisons to Atlas data 36 pb−1 [1201.1276]
Z+4 Jets
Comparison to Data
ATLAS 36 pb−1 [arXiv:1111.2690]
Cross sections for Z/γ*+≥1,2,3,4 jets, anti-kT, R = 0.4
Small scale variation, good agreement with data
Much more to come!
W+5 Jets
• Scale dependence narrows substantially at NLO
Jet Ratios
• Relaxation of kinematic restrictions leads to NLO corrections at large pT in V+3/V+2, otherwise stable
• Ratio is not constant as a function of pT — fits to α+β n will have α & β dependent on pTmin
Summary
• On-shell methods have matured into the method of choice for NLO QCD calculations for colliders
• Calculations with high multiplicity are mature for experimental comparisons
• Probe wide variety of kinematic regimes, establish confidence for extrapolations into search regions