Vector boson + jets at NLO

vs. LHC data

Lance Dixon (SLAC)

for the BlackHat collaboration Z. Bern, LD, G. Diana, F. Febres Cordero,

S. Höche, H. Ita, D. Kosower, D. Maître, K. Ozeren

LHC Theory Workshop University of Melbourne - July 4, 2012

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 2

“New physics at the LHC is a riddle,

wrapped in a mystery, inside an enigma;

but perhaps there is a key.” -W. Churchill

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 3

LHC Data Dominated by Jets

new physics

• Every process shown

also with one more jet

at ~ 1/5 the rate

• Need accurate

production rates for

X + 1,2,3,… jets in Standard Model

Jets from quarks

and gluons.

• q,g from decay of

new particles?

• Or from old QCD?

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 4

Classic SUSY dark matter signature

Heavy colored particles decay rapidly to stable

Weakly Interacting Massive Particle (WIMP = LSP)

plus jets

Missing transverse energy

MET + 4 jets

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 5

Irreducible Standard Model Background

• MET + 4 jets from

pp Z + 4 jets,

Z neutrinos

Neutrinos escape detector State of art for Z + 4 jets

was based on

Leading Order (LO)

approximation in QCD

normalization uncertain

n n LO

Now available at

Next to Leading Order,

greatly reducing

theoretical uncertainties

n n NLO

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 6

NLO = Loops

• NLO also needs tree-level amplitudes with one more parton

• Both terms infinite – combine in dim. reg. to get a finite result

tree + 1 parton 1 loop

NLO

NLO cross section needs Feynman diagrams

with exactly one closed loop

Textbook methods quickly fail, even with very powerful computers

• One-loop amplitudes were the bottleneck for a long time

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 7

A Better Way to Compute?

• Want loop-level scattering amplitudes for

many ultra-relativistic (“massless”) particles

– especially quarks and gluons of QCD

• Feynman diagrams, while very general and powerful, are not optimized for these processes

• There are much more efficient methods

8

Just one QCD loop can be a challenge

pp W + n jets (just amplitudes with most gluons)

# of jets # 1-loop Feynman diagrams

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012

Current limit with

Feynman diagrams

Current limit with

on-shell methods

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 9

Branch cut information

Generalized Unitarity (One-loop Plasticity)

Ordinary unitarity: put 2 particles on shell

Generalized unitarity: put 3 or 4 particles on shell

Trees recycled into loops!

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 10

One-Loop Amplitude Decomposition

rational part; from D-dimensional trees, or recursively

Bern, LD, Dunbar, Kosower (1994)

Known functions (integrals),

same for all amplitudes

coefficients can be determined from products

of trees using (generalized) unitarity

Missing from the old, nonperturbative analytic S-matrix

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 11

Blackhat+Sherpa: Berger, Bern, LD, Diana, Febres Cordero, Forde,

Gleisberg, Höche, Ita, Kosower, Maître, Ozeren

W + 3 jets 0902.2760, 0907.1984

Z + 3 jets 1004.1659

W + 4 jets 1009.2338

Z + 4 jets 1108.2229

W + 5 jets 12mm.nnnn

Recent progress on V + jets at NLO

Rocket: W + 3 jets Ellis, Melnikov, Zanderighi, 0901.4101, 0906.1445

MCFM: V + 0,1,2 jets Campbell, Ellis, hep-ph/0202176

• Could try to use such predictions directly for backgrounds

to experimental searches.

• However, it is generally safer to use data-driven techniques

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 12

NLO pp W + 4 jets Berger et al., 1009.2338

First hadron collider process known at NLO with 5 objects in final state.

Also important SUSY background.

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 13

NLO pp Z + 4 jets, and ratio to W±

Ita et al.

1108.2229

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 14

NLO pp W + 5 jets also feasible

+ + …

PRELIMINARY

leading color

BH+Sherpa

vs. LHC data

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L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 16

NLO pp Z + 1,2,3,4 jets vs. ATLAS 2010 data

Analysis in progress with

full 2011 data set:

> 100 times the 2010 data

ATLAS

1111.2690

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NLO pp W + 1,2,3,4 jets vs. ATLAS 2010 data

NLO undershoots badly for W + 1 jet –

production dominated by W + 2 parton

configurations. Theory can be improved here: Rubin, Salam, Sapeta 1006.2144

ATLAS

1201.1276

Issues with forward jets?

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 18

ATLAS 1201.1276

Large x gluon

too large?

Also forward jet issues at Tevatron

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D0 @ PLHC Vancouver

• Note that probability

of emitting another jet

seems OK

• Important experimentally

for assessing effects of

central jet veto on

backgrounds, in search

for Higgs in vector boson

fusion

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 20

Pure QCD: pp 4 jets vs. ATLAS data

1112.3940

4 jet events might

hide pair production

of 2 colored particles,

each decaying to a

pair of jets [e.g. talk

by B. Dobrescu]

Detailed study of

multi-jet QCD

dynamics may help

understand other

channels

ATLAS 1107.2092

Pure QCD Jet Production Ratio

L. Dixon NLO V+jets vs. LHC data U. Melbourne July 4, 2012 21

3/2 ratio

agrees well with

ATLAS data

even in regions

where it’s

quite large

Looking forward

to measurement

of 4/3 ratio

ATLAS 1107.2092

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Fixed order vs. Monte Carlo

• Previous plots NLO but fixed-order, few partons: no model of long-distance effects included;

cannot pass through a detector simulation

• Methods available for matching NLO

parton-level results to parton showers, with NLO accuracy:

– MC@NLO Frixione, Webber (2002) + SHERPA implementation

– POWHEG Nason (2004); Frixione, Nason, Oleari (2007)

• Recently implemented for increasingly complex final states

NLO MC for W + 1,2,3 jets vs. ATLAS data

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Höche et al., 1201.5882

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Conclusions

• Thanks to new methods for computing loop amplitudes, lots of NLO (fixed order, parton-level) results now available for comparison with LHC data, in particular: W/Z + 1,2,3,4,5 jets 4 jets

• Except for some issues at large jet rapidity, agrees quite well with 2010 LHC data – ATLAS analyses of both W and Z + 1,2,3,4 jets, & pure QCD 4 jets

• So far, the NLO results for the most complex processes are only at parton level, not yet embedded in a full Monte Carlo, like MC@NLO, POWHEG, or SHERPA. But this will happen before long – already done up to W + 3 jets!

• Looking forward to learning even more from comparisons to 2011 LHC data from both CMS and ATLAS