Multi-jet physics with BlackHat · BlackHat •Efﬁcient evaluation of one-loop QCD amplitudes (traditionally the hardest aspect of NLO predictions) •Implementation of modern generalized

Multi-jet physics with BlackHat

Kemal OzerenUCLA

Work in collaboration with:

Z. Bern, G. Diana, L. Dixon, F. Febres Cordero, S. Hoeche, H. Ita, D. Kosower, D. Maitre

1

[1106.1423], [1112,3940], [1206.6064], [forthcoming]

Tuesday, September 4, 12

Outline• quest for precision: NLO and BlackHat

• Precision QCD used in CMS jets+MET search

• new result: W+5jets @ NLO

• distributing results: ROOT ntuples

• Fernando’s talk: First NLO prediction of 4-jet

production

2


Many hard jets...num

ber

of events

10

210

310

410

510

610

data

(MadGraph)νµ → W

top

other backgrounds

CMS preliminary

= 7 TeVs at -136 pb

> 30 GeVjet

TE

exclusive jet multiplicity0 1 2 3 4 5 6

da

ta/M

C

0.5

1

1.5

To get the most from the LHC, we need to understand multi-jet final states

3


A few words on QCD Predictions• LHC workhorses for full event

simulation: Herwig, Sherpa, Pythia

• ME+PS matching when there are

many hard jets - big improvement

• But need NLO for reliable first

principles QCD prediction,

including correct normalization

• Recent exciting progress in matching NLO/PS MC@NLO [Frixione, Webber; Sherpa]

POWHEG [Nason; Frixione, Nason, Oleari]

• These tools still require the one-loop amplitude as input...

[p

b]

tot

σP

roduction

Cro

ss S

ection,

-110

1

10

210

310

410

510

CMS

W

1j≥

2j≥

3j≥

4j≥

Z

1j≥

2j≥

3j≥

4j≥

> 30 GeV jet

TE

| < 2.4 jetη|

γW

> 10 GeVγ

TE

,l) > 0.7γR(Δ

γZ

WWWZ

ZZ

ZZ→

(127)H

-136 pb -136 pb -11.1 fb -14.7 fb

JHEP10(2011)132

CMS-PAS-EWK-10-012PLB701(2011)535 CMS-PAS-EWK-11-010 CMS-PAS-HIG-11-025

theory prediction

syst)⊕CMS measurement (stat

CMS 95%CL limit

4


BlackHat

• Efficient evaluation of one-loop QCD amplitudes (traditionally the hardest aspect of NLO predictions)

• Implementation of modern generalized unitarity method

• Evaluates coefficients of integrals

• Tree amplitudes in → one-loop amplitudes out

•Opens the door to precise predictions for multi-jets, used in conjunction

with SHERPA

•Well tested and battle-hardened:

W+1,2,3,4,(5) jets, Z+1,2,3,4

• similar codes available: GoSam, HELAC-NLO, Madloop, NGluon, Rocket ...5


6

Jets+MET at CMS


7

Data-Driven Background Estimation

•New physics search in jets + MET channel

•CMS estimates Z + 3 jets background by measuring

photon + 3 jets events

�(pp � Z(� ��)) = �(pp � �) � RZ/�

background to NP measure this theory input

So what is R ? Let’s calculate it at NLO QCD...


8

Photon Isolation: theory vs experiment

!0

!

"

#

•We are interested in isolated photons

• “Fixed cone” used by experiments is not IR-safe.

Described using fragmentation functions on the

theory side.

•We employ Frixione isolation [Frixione arxiv:9801442]

(no fragmentation piece)�

i

EiT �(� � Ri�) � H(�)

Transverse Energy in Cone vanishes as δ→0

• This is ideal for theorists, problematic for experiments (detector is granular, but isolation condition is continuous)


9

Z/gamma ratio for CMS•We calculate Z+3j and γ+3j to NLO using BlackHat +Sherpa

• Critical variables:

HT =�

jets

EjetT

• Jet cut: 50 GeV

•Many different regions of interest:

study them all [1106.1423], [1206.6064]

��MET = �

�

jets

�pT,jet


10

Z/gamma ratio for CMSHow to estimate the error on a ratio?

• Correlated scale variation leads to tiny errors. Should we trust this?

•We study NLO - MEPS and take this as a guide to the uncertainty,

finding around 5-10% error

• Other issues: large QCD logs, large EW logs [0508253]. What is their

impact?

Can use jet ratios as a diagnostic for

large logs

/ [GeV]TH0 100 200 300 400 500 600 700 800

- M

ET /

[GeV

]TH

-400

-200

0

200

400

600

800

1000

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1, R = 0.5 [anti-kt]2

TH’ = µ

Z+3j / Z+2j (NLO)

BlackHat+Sherpa

LHC 7 TeV

Z+3j/Z+2j


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Outcome

• We worked closely with groups from

CMS

• Fruitful cross-talk between theory and

experiment

• This search was very constraining...

CMS [1207.1898]/ GeV

0 100 200 300 400 500 600 700 800 900 10000

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

LONLOME+PS

, R = 0.5 [anti-kt]2TH'

= µ

+3jγZ+3j /

Set 1 cuts

BlackHat+SherpaLHC 7 TeV

[GeV]0m500 1000 1500 2000 2500 3000

[GeV

]1/

2m

100

200

300

400

500

600

700

800

900

1000

± l~ LEP2 ± 1χ∼ LEP2 No EWSB

= L

SPτ∼

Non-Convergent RGE's) = 500g~m(

) = 1000g~m(

) = 1500g~m(

) = 2000g~m(

) = 1000q~m(

) = 1500q~m(

) = 2000

q~m(

) = 2500

q~m()=10βtan(

= 0 GeV0A > 0µ = 173.2 GeVtm = 7 TeVs, -1 L dt = 4.98 fb∫CMS,

Observed signal theoryσ1±Observed exp.σ1±Expected

-1CMS, 36 pb

LM5

Good example of utility of high-precision

theory: ratio = input to data-driven

method

See [1106.1423] and [1206.6064] for

many plots and numerical results


W+5j Production at NLO

this talk: preliminary results

12


W + 5jets Production at the LHCFirst 2→6 NLO calculation at a

hadron collider

Preliminary

T T �; FSWSR� ��

PITXSR + 1)8 + �Nq

g

l

!q

g

g

g

g

sample Feynman diagram (octogon!)

For searches: background for various NP signatures

(also to top pair production)

For measurements: check theory vs experiment

leading colour approx. in the virtual part only: expect 2-3%13


Theoretical Tools

•BlackHat for virtual part [Bern, Dixon, Febres Cordero, Hoeche, Ita,

Kosower, Maitre, KJO]

•COMIX (part of Sherpa) for real emission [Hoeche]

•Sherpa - organizational framework [Hoeche, Hoeth, Krauss,

Schoenherr, Schumann, Siegert, Winter]

•LHAPDF for parton distributions [Whalley, Bourilkov, Group]

•FASTJET for jet clustering [Cacciari, Salam, Soyez]

•ROOT for analysis and storing events [see later]14


W+5jets: reduced scale variation at NLO

• Plot shows effect of varying μ up and down by a factor of 2

Strong reduction in scale

uncertainty

µ* = µ6

15

/ [GeV]T

First jet p0 50 100 150 200 250 300 350 400

[pb

/ GeV

]T

/ d

pσd

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014 LO

NLO

PRELIMINARYBlackHat+Sherpa

+ 5 jets-W, R = 0.5 [anti-kt]2

TH' = µ


Jet Ratios: key observables

W+3j / W+2j

W+4j / W+3j

W+5j / W+4j

dash=LOsolid=NLO

• Note - they are not constant!• Both theory and experimental errors are minimized in the ratio• Can be important input to data-driven methods for backgrounds (compare Z/γ + 3jets used in SUSY search )

/ GeVTH0 100 200 300 400 500 600 700 800

+(n-

1)j

-+n

j / W

-W

0

0.2

0.4

0.6

0.8

1

1.2NLO32NLO43NLO54

BlackHat+SherpaPreliminary

LHC 7 TeV

LO

NLO

16


/ GeVT

lead jet p50 100 150 200 250

+nj

-+n

j / W

+W

0

0.20.40.60.8

11.21.41.6

1.82

2.2

NLO 2-jetNLO 3-jet x 0.8NLO 4-jet x 0.6NLO 5-jet x 0.4

More Ratios: W+ / W-

W+ / W- + 2jet

W+ / W- + 3jet

W+ / W- + 4jet

dash=LOsolid=NLO

x 0.8

x 0.6

NOTE scalings for visual clarity

x 0.4

W+ / W- + 5jetBlackHat+SherpaPreliminary

LHC 7 TeV

17


18

100 200 300 400 500

0.0001

0.001

0.01

0.1

1

10

100

dσ /

dpT

[ pb

/ G

eV ]

LONLO

100 200 300 400 500First Jet pT [ GeV ]

0.5

1

1.5

2

2.5

3

3.5 LO / NLO

100 200 300 400 500

100 200 300 400 500Second Jet pT [ GeV ]

100 200 300 400 500

100 200 300 400 500Third Jet pT [ GeV ]

100 200 300 400 500

100 200 300 400 500Fourth Jet pT [ GeV ]

100 200 300 400 500

0.0001

0.001

0.01

0.1

1

10

100

100 200 300 400 500Fifth Jet pT [ GeV ]

0.5

1

1.5

2

2.5

3

3.5

NLO scale dependence

BlackHat+Sherpa

LO scale dependence

pTjet > 25 GeV, | ηjet | < 3

ETe > 20 GeV, | ηe | < 2.5

ETν > 20 GeV, MT

W > 20 GeV

R = 0.4 [anti-kT]

√%s = 7 TeV

µR = µF = HT^ ’ / 2

W- + 5 jets + X

nth jet pT in events with at least 5 jets [preliminary]

1st jet 2nd 3rd 4th 5th

Preliminary


Distributing the Results - ROOT ntuples

• NLO calculations often very computationally intensive

• don’t want to run again and again with different cuts etc.

• solution: store events and apply analysis cuts later

• ROOT ntuple files ideal for this purpose

• store coefficients of ln μ. For example, for the virtual piece:

1PSST = %+ & ln µ + ' ln� µ

• Major benefit 1: can change scales, PDF, add new observables...

• Major benefit 2: can hand over the ntuples to experimentalists

We are working towards release of ntuples, including a library of code for their analysis

19


20

Using the Ntuples

• events stored in ntuple are parton level

• user should perform clustering into jets [FASTJET]

• at generation level, event selection is such as to permit only certain jet algorithms (we choose siscone, kt, anti-kt R=0.4, 0.5, 0.6, 0.7)

• four parts to be added [c.f. Catani-Seymour subtraction]:

Born Virtual Int Sub Real-Sub� ��

n-parton events (n+1) and n-parton events

+ + +

• cancellations within RS piece - need to be careful when evaluating statistical error


ROOT ntuples in Action ATLAS W+jets [1201.1276]

• ntuples created with BlackHat+Sherpa [1009.2338]

• experimenters perform their own analysis of the NLO results

• we are currently working also with CMS people

... we look forward to similar comparisons of our W+5jet NLO results with data!

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Summary

• I stressed the importance of NLO precision in achieving our physics goals at the LHC

• Application of BlackHat+Sherpa to Jets+MET new physics search

• new result: W+5jets → highest multiplicity NLO calculation at a hadron collider

• described how our results are passed to experimenters and compared to data: ROOT ntuples

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Multi-jet physics with BlackHat · BlackHat •Efﬁcient evaluation of one-loop QCD amplitudes (traditionally the hardest aspect of NLO predictions) •Implementation of modern generalized

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