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

– Rocket: Ellis, Giele, Kunszt, Lazopoulos, Melnikov, Zanderighi

– Samurai: Mastrolia, Ossola, Reiter, & Tramontano– NGluon: Badger, Biedermann, & Uwer– MadLoop: Hirschi, Frederix, Frixione, Garzelli, Maltoni,

& Pittau– Giele, Kunszt, Stavenga, Winter

• Ongoing analytic work– Almeida, Britto, Feng & Mirabella

W+4 Jets

• 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