Phenomenological studies of top-pair production at NLO

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Phenomenological studies of top-pair production at NLO

Phenomenological studies of

top-pair production at NLO

Malgorzata Worek

Wuppertal University

XXXV International Conference of Theoretical Physics, Ustron’11

12-18 September 20011, Ustron, Poland 1


Outline

General motivation for NLO QCD calculations

HELAC-NLO in a nutshell

ttbb, ttjj, Htt, WWbb

Applications: WWbb & ttjj @ TeVatron and LHC

Summary & Outlook

2


HELAC-NLO Group

G. Bevilacqua (RWTH Aachen)

M. Czakon (RWTH Aachen)

M.V. Garzelli (Debrecen Uni.)

A. van Hameren (INP Krakow)

I. Malamos (Nijmegen Uni.)

C. G. Papadopoulos (INP Athens)

R. Pittau (Granada Uni.)

M. Worek (Wuppertal Uni.)







Contributors:

A. Kanaki

A. Cafarella

P. Draggiotis (Valencia Uni.)

G. Ossola (New York Uni.)

3


Introduction

8-10 partons in the final state @ LO, well separated to avoid divergences

Standard Model and beyond tools @ tree level (just few examples)

ALPGEN, AMEGIC++/SHERPA, CARLOMAT, COMIX/SHERPA,

COMPHEP, HELAC-PHEGAS, MADGRAPH/MADEVENT,

O'MEGA/WHIZARD, ...

General purpose Monte Carlo programs (parton shower, hadronization,

multiple interactions, hadrons decays, etc.)

HERWIG, HERWIG++, PYTHIA 6.4, PYTHIA 8.1, SHERPA, ...

High sensitivity to unphysical input scales

Higher order calculations are needed to improve accuracy of prediction

4


Motivation for NLO

Stabilizing the scale in the QCD input parameters:

Strong coupling constant and PDFs

More reliable theoretical error related to the scale dependence

Normalization and shape of distributions first known at NLO

Many scale processes: V+ jets, ttH, tt + jets, bb + jets, njets ...

How do we know which scale to choose ?

Improved description of jets

Jets: LO NLO Parton Hadron

Shower

Level

5


Structure of NLO

Calculations


σ NLO =


=

m

m


dσ B +


dσ B +

m+1

m+1


dσ R −

m+1


dσ A +


dσ R − dσ D

+

m

m+1


dσ A +

m

dσ V


dσ V + dσ I + dσ KP

Catani-Seymour subtraction scheme

At NLO: dσ R with m+1 partons and dσ V with m partons in the final

Separately divergent although their sum is finite

dσ A is a proper approximation of dσ R

i. same singular behavior point-by-point

ii. integrable over extra parton degrees of freedom

Individual contributions finite. MC integration can be performed !


Structure of NLO

Calculations


σ NLO =


=

m

m


dσ B +


dσ B +

m+1

m+1


dσ R −

m+1


dσ A +


dσ R − dσ D

+

m

m+1


dσ A +

m

dσ V


dσ V + dσ I + dσ KP

Three main building blocks are needed

Evaluation of one-loop amplitude - HELAC-1LOOP

Determination of coefficients via reduction method

Reduction at integrand level - OPP method - CUTTOOLS

Evaluation of scalar functions - ONELOOP

Ossola, Papadopoulos, Pittau ‘07, ’08

Draggiotis, Garzelli, Papadopoulos, Pittau ’09

Garzelli, Malamos, Pittau ’09

van Hameren, Papadopoulos, Pittau ’09

van Hameren, ’10

7


Structure of NLO

Calculations


σ NLO =


=

m

m


dσ B +


dσ B +

m+1

m+1


dσ R −

m+1


dσ A +


dσ R − dσ D

+

m

m+1


dσ A +

m

dσ V


dσ V + dσ I + dσ KP

HELAC-DIPOLES - complete, public and automatic Catani-Seymour dipoles

Extended for arbitrary polarizations

Monte Carlo over polarization states of external particles

Monte Carlo over color

Phase space restriction on the dipoles phase space

Less dipole subtraction terms per event

Increased numerical stability

Reduced missed binning problem

Czakon, Papadopoulos, Worek '09

8


One-loop amplitude

and rational part

Reduction of tensor integrals

OPP coefficients and rational part

HELAC-1LOOP

CUTTOOLS

pp → t¯tjj

HELAC-NLO

HELAC-

DIPOLES

ONELOOP

Catani-Seymour dipole subtraction

for massless and massive cases

Scalar integrals

9


Top Quark

Produced in hadron-hadron collisions through strong interactions

Decays without forming hadrons through the single mode tW b

Distinguished by its large mass, close to EW symmetry breaking scale

Unique property raises questions:

Is the top quark mass generated by the Higgs mechanism ?

Does it play more fundamental role ?

Are there new particles lighter than the top quark ?

Does the top quark decay into them ?

Could non-SM physics manifest itself in non-standard couplings of top

quark which show up as anomalies in top quark production and decays?

Top quark physics tries to answer all these questions

Properties of the top quark have already been examined at the Tevatron

LHC is a top factory, producing about 8 million tt pairs per

experiment per year at low luminosity (10 fb 1 /year)

10


Top Pair Production

Complete off-shell effects @ NLO

Double-, single- and non-resonant

top contributions of the order O( s3 4 )

Complex-mass scheme for unstable top

W gauge bosons are treated off-shell

pp(p¯p) → e + ν e µ − ν µ b¯b + X

Sum over helicities and color via MC

LO + V obtained by reweighting of tree

level unweighted events

Dipole channels for subtracted real part

Check of Ward identity for virtual part

Cancellation of divergences

max independence test for real part

Bevilacqua, Czakon, van Hameren, Papadopoulos, Worek ’11

11


Top Pair Production

TeVatron

LO & NLO scale dependence

σ LO = 34.922 +40%

−26% fb

σ NLO = 35.727 −4%

−8% fb

K = 1.023

LHC

σ LO = 550.538 +37%

−25% fb

σ NLO = 808.665 +4%

−9% fb

K = 1.47

Bevilacqua, Czakon, van Hameren, Papadopoulos, Worek ’11

12


Top Pair Production

TeVatron

Differential cross section

Fixed scale m t

Not rescale LO shapes

Distortions 15% - 80%

For angular distributions

corrections 5% - 10%

Bevilacqua, Czakon, van Hameren, Papadopoulos, Worek ’11

13


Top Pair Production

LHC

Differential cross section

Fixed scale m t

Corrections 50% - 60 %

Relatively constant

H T distorted up to 80%

Bevilacqua, Czakon, van Hameren, Papadopoulos, Worek ’11

14


TeVatron

Top Pair Production

A t FB =0.051

A b FB =0.033

A +

FB =0.034

top quarks are emitted in the

direction of incoming protons


A t y>0

FB =

N t(y) − y0 N t(y)+ y


Top Pair + 2 Jets

Important background for Higgs searches @ Tevatron and @ LHC

H WW ∗ produced via weak boson fusion

For m H ~ 130 GeV, i.e. when BR(H WW ∗ ) is large enough

Higgs boson mass peak cannot be directly reconstructed

Background processes can not be measured from the side bands

H bb produced via associated production with a tt pair

Process useful only in the low mass range m H < 135 GeV

Unique access to the top and bottom Yukawa couplings

Reconstruction of the H bb mass peak difficult

The bb pair can be chosen incorrectly

b-tagging efficiency, two b-jets can arise from mistagged light jets

Very precise knowledge of QCD backgrounds

ttjj, ttbb, WWjj is necessary !

16


Top Pair + 2 Jets

Partonic subprocesses

Number of Feynman diagrams

Number of dipoles

As a measure of complexity

dσ R − dσ D

dσ V

Bevilacqua, Czakon, Papadopoulos, Worek ’10 ‘11

17


Top Pair + 2 Jets

TeVatron

σ LO =0.3584 +94%

−45% pb

σ NLO =0.2709 +0.5%

−21% pb

94% 21%

K = 0.76 -24%

Total cross sections

with different jets

separation cuts R jj

and the jet resolution

parameters R

K = 0.86 -14%

Bevilacqua, Czakon, Papadopoulos, Worek ’10 ‘11

18


TeVatron

Top Pair + 2 Jets

Corrections to p T and m tt

are substantial

Do not simply rescale the

LO shapes

Induce distortions 40%

For H T even 50% 60%

deformations

Bevilacqua, Czakon, Papadopoulos, Worek ’10 ‘11

19


TeVatron

Top Pair + 2 Jets

Angular distributions

Negative and moderate

corrections

15% 30%

Relatively constant

Bevilacqua, Czakon, Papadopoulos, Worek ’10 ‘11

20


Top Pair + 2 Jets

A t FB,LO = −0.103 +0.003

−0.004

A t FB,NLO = −0.046 +0.005

−0.006

anti-top quarks are emitted in the

direction of incoming protons

(forward direction by definition)

with a consistent expansion in s

A t FB,NLO = −0.058 +0.014

−0.042

unexpanded ratio of the NLO cross sections

Bevilacqua, Czakon, Papadopoulos, Worek ’10 ‘11

21


Top Pair + 2 Jets

LHC

σ LO = 13.398 +87%

−43% pb

σ NLO =9.82 −15%

−15% pb

87% 15%

K = 0.73 -27%

Total cross sections

with different jets

separation cuts R jj

and the jet resolution

parameters R

K = 0.86 -14%

Bevilacqua, Czakon, Papadopoulos, Worek ’10 ‘11

22


LHC

Top Pair + 2 Jets

m tt 20% 30% corrections

p T up to 60% distortions

H T even 80% deformations

Corrections are substantial

and do not simply rescale

LO shapes

Bevilacqua, Czakon, Papadopoulos, Worek ‘10 ’11

23


LHC

Top Pair + 2 Jets

NLO QCD corrections

are negative and

relatively constant

Angular distributions

20% 30% corrections

Bevilacqua, Czakon, Papadopoulos, Worek ’1o ‘11

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Summary & Outlook

Already calculated by HELAC-NLO: ttbb, ttjj, Htt, WWbb

ttbb, WWbb completed by two groups

Permille level agreement in both cases!

Much wider study for ttjj: variation of the center of mass energy, jet

algorithms, cone size in jet algorithm, transverse momentum cuts

HELAC-NLO

Complete tool at NLO built around HELAC‐PHEGAS:

HELAC‐1LOOP, CUTTOOLS, ONELOOP, HELAC‐DIPOLES

Other processes from NLO Wishlist under attack

Constant improvements in speed and functionality

Interfaced to PYTHIA and HERWIG showers via POWHEG-BOX: ttj, ttH

Decays of massive particles, showering and hadronization

Final results at the hadron level

http://helac-phegas.web.cern.ch/helac-phegas/

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