HP2.3rd - GGI Florence High Precision for Hard Processes at the LHC - - PowerPoint PPT Presentation

• Markus Schulze

in collaboration with K. Melnikov

HP2.3rd - GGI Florence

High Precision for Hard Processes at the LHC

• Markus Schulze, Johns Hopkins University

2/16

LHC (7 TeV) : Tevatron :

σt

t ≈ 7 pb

σt

t ≈ 200 pb

events from LHC (7 TeV) with 1 fb#1 vs. events from Tevatron with 5 fb#1

≈ 200 k ≈ 30 k

• Markus Schulze, Johns Hopkins University

2/20

NLO QCD corrections to production and decay Calculation framework Top quark spin correlations Top quark mass measurements at NLO QCD

+ [S. Biswas]

• t

t

!"#

Markus Schulze, Johns Hopkins University 3/16

Literature on hadronic top production beyond leading#order is rich:

Classic NLO QCD corrections: Beenakker,Dawson,Ellis, Frixione,Meng,Nason,v.Neerven,Schuler,Smith; Czakon,Mitov Threshold resummation & Coulomb corrections: Banfi,Bonciani,Catani,Czakon,Frixione,Kidonakis,Kiyo,Kühn,Laenen,Mangano,Mitov,Moch,Nason,Ridolfi,Steinhause,Sterman,Uwer,Vogt Electroweak corrections: Beenakker,Bernreuther,Fuecker,Denner,Hollik,Kao,Kollar,Kühn,Ladinsky,Mertig,Moretti,Nolten,Ross,Sack,Scharf,Si,Uwer,Wackeroth,Yuan NNLO QCD contributions: Anastasiou,Aybat,Bonciani,Czakon,Ferroglia,Gehrmann,Körner,Langenfeld,Maitre,Merebashvili,Mitov,Moch,Rogal,Studerus,Uwer Study of non-factorizable corrections: Beenakker,Berends,Chapovsky,Fadin,Khoze,Martin,Melnikov,Yakovlev Factorizable correction to top decays: Czarnecki,Jezabek,Kühn; Bernreuther,Brandenburg,Si,Uwer Spin correlations: Mahlon,Parke; Bernreuther,Brandenburg,Si,Uwer

• stable top quarks:
• decays of top quarks:
• event generators:

MC@NLO: Frixione,Webber; Laenen,Motylinski,Nason,White POWHEG: Frixione,Nason,Oleari,Ridolfi

Markus Schulze, Johns Hopkins University 4/16

Only very recently:

Bernreuther, Si (2010); Melnikov, M.S. (2009)

NLO QCD corrections to top quark pair production and decay at hadron colliders

!"#

Allows for:

• realistic description of the final state
• implementation of arbitrary detector cuts
• accounting for all spin correlations

Top quark decays: leptonic or hadronic decays at NLO Narrow Width Approximation neglect non#factorizable corrections

Γt

• mt → 0

Markus Schulze, Johns Hopkins University 5/16

• $
• u(pt)
• ˜

u(pt) = M(t → bℓ+ν) i(pt+mt)

√2mtΓt LO

t

• t

b

• b

ν ℓ+ q q, 1 2 Mtree = ˜ u(pt) ˜ M(12 → tt) ˜ v(p

t) + O( Γt mt )

• Generate phase space of decay particles
• Generate phase space of top quarks

We calculate helicity amplitudes. Mtree =

Markus Schulze, Johns Hopkins University 6/16

• virt

real

D-dimensional generalized unitarity + OPP Dipole subtraction with alpha dependence

Production

!%$

Markus Schulze, Johns Hopkins University 6/16

• virt

real

D-dimensional generalized unitarity + OPP Dipole subtraction with alpha dependence

Production Decay

!%$

B#meson fragmentation + extra:

B b

• Measurement of the total cross section

Markus Schulze, Johns Hopkins University 7/16

The total cross section is claimed to be measured with 5#10% accuracy NLO QCD corrections: typically 10#30%

&'

t t

Markus Schulze, Johns Hopkins University 7/16

σtot = N

L

1

A

The total cross section is claimed to be measured with 5#10% accuracy Note: The total cross section is never measured in an experiment with

A = σ

σ

NLO QCD corrections: typically 10#30%

To claim that the total cross section has been measured with NLO accuracy, we need to calculate at NLO QCD. Otherwise, we introduce potential biases.

A

&'

• Measurement of the total cross section

t t

Markus Schulze, Johns Hopkins University 8/16

• Top quark spin correlations

J=1

q

• q
• t

t t

J=0

• t

g g

close to threshold: S-wave production (L=0)

leptons preferably parallel

ℓ+ ℓ−

leptons preferably anti-parallel

ℓ+ ℓ− Spins of production and decay mechanism are interlocked

Markus Schulze, Johns Hopkins University 9/16

These effects are only observable for top quarks

• Top quark spin correlations

spinflip time life time

∼

m Λ

Γtop

Markus Schulze, Johns Hopkins University 9/16

These effects are only observable for top quarks

• Top quark spin correlations

spinflip time life time

∼

m Λ

Γtop

So far, only one measurement exists:

requires boost into top rest frames and specification of a quantization axis

[Bernreuther et..al.]

[CDF+D0, 4.3fb-1] 1 σ dσ d cos φid cos φj = 1 4 (1 − C cos φi cos φj)

C(SM theory) = 0.78 C(Tevatron) = 0.32+0.55

−0.78

Markus Schulze, Johns Hopkins University 10/16

Mahlon, Parke: measure lepton opening angle with is not an observable Cut can be applied to the average,

mt

t < 400 GeV

mt

t

<mt

t >

Markus Schulze, Johns Hopkins University 11/16

0.22 0.24 0.26 0.28 0.3 0.32 0.34 0.36 0.38 0.5 1 1.5 2 2.5 3

LHC (7 TeV) spin correlated uncorrelated NLO NLO NLO NLO

∆φℓℓ

dσ σ d∆φℓℓ

Cuts:

pT,ℓ > 20GeV pT,bjet > 25GeV

ηℓ,ηbjet < 2.5

pT,miss > 40GeV

+

pT,ℓ < 50GeV mℓℓ < 100GeV

• ur suggestion: measure lepton opening angle

with special cuts on leptons advantage: clean observable, simpler measurement

Markus Schulze, Johns Hopkins University 12/16

New ideas (under development): define indicator that is sensitive to spin correlations

r(Φ) =

|M| |M|+|M|

r

depends on neutrino momenta which are unobservable integration of over neutrino momenta yields up to 8 solutions

r(Φ)

Φ

robs =

i r(Φi) we calculate , weighted with the cross section for spin correlated and uncorrelated top quarks

robs

Markus Schulze, Johns Hopkins University 13/16

preliminary LO result LHC(10TeV) di#leptonic final state with standard acceptance cuts

1e-05 0.0001 0.001 0.01 0.1 1 10 0.2 0.4 0.6 0.8 1 file_LHCc using ($2):(($1)==1?($3)/totLHCc:1/0) file_LHCu using ($2):(($1)==1?($3)/totLHu:1/0)

we find similar results for the Tevatron

spin correlated uncorrelated

robs

d(rσ) σdr

Markus Schulze, Johns Hopkins University 14/16

Target precision is about 1 GeV, dominated by systematics. Clean measurements involve kinematics of top quark decay products.

So far, systematics of all those studies were estimated by parton showers whose reliability at this level of precision is questionable.

• ne example:

average invariant mass of B#meson and lepton

• Top quark mass measurement at the LHC

J/Psi decay mode allows for very precise measurements already with 20 fb#1 calculate as a function of <mBℓ > mtop

Markus Schulze, Johns Hopkins University 15/16

<mBℓ >Herwig= 0.61mtop − 25.31GeV <mBℓ >Pythia= 0.59mtop − 24.11GeV <mBℓ >NLO= 0.60mtop − 26.7GeV deviations in slope lead to uncertainty ≈ 3GeV uncertanties from scale variations comparable to expected experimental errors parton show studies neglect production process

t t

Markus Schulze, Johns Hopkins University 15/16 170 172 174 176 178 180

84 80 76 72

mt [GeV]

< mBℓ >NLO [GeV]

⇒ uncertainty of 1.5 GeV on parton show studies neglect production process

t t

complete NLO study of pp → t

t → b bℓνjj

<mBℓ >Herwig= 0.61mtop − 25.31GeV <mBℓ >Pythia= 0.59mtop − 24.11GeV <mBℓ >NLO= 0.60mtop − 26.7GeV <mBℓ >prod

NLO= 0.64mtop − 32.12GeV

mtop

deviations in slope lead to uncertainty ≈ 3GeV uncertanties from scale variations comparable to expected experimental errors

Markus Schulze, Johns Hopkins University 16/16

(

• NLO QCD corrections to production and decay

NLO QCD corrections to production

t t t t + jet

• realistic description of the final state incl. spin correlations

is crucial for a precise understanding

• allows for new improved studies of

e.g. top mass measurement or spin correlations

• I left out:
• important contributions have been neglected in the past

Markus Schulze, Johns Hopkins University

)%

Markus Schulze, Johns Hopkins University

* +

t t

Major background for Higgs WW in VBF

Markus Schulze, Johns Hopkins University 16/16

$* +

t t

comprises 2/3 of all backgrounds (80% from )

0.05 0.1 0.15 0.2 0.25 50 100 150 200 250

mT(llν) (GeV/c2) Arbitrary units

• 0.02

0.04 0.06 0.08 1 2 3

∆φll (rad) Arbitrary units

t t + X t t + jet

Markus Schulze, Johns Hopkins University 17/16

Dittmaier,Uwer,Weinzierl (2007) Bevilacqua,Czakon,Papadopoulos,Worek (2010) Melnikov,S. (2010)

σNLO = 375.8 ± 1.0 pb σNLO = 376.2 ± 0.6 pb σNLO = 376.6 ± 0.6 pb

$* +

t t

LHC:

Cross check with DUW (stable top quarks):

• pt
• σ

p(t) [ / ]

• yt
• σ

y(t) [ ]

( = mt)

Markus Schulze, Johns Hopkins University 18/16

$* +

t t

We include LO decays into leptons and jets:

Tevatron: (semi#lept.) LHC, signal: (7 TeV, di#lept.) LHC, VBF bkgrd.: (14 TeV, di#lept.)

− − − −

• 50

100 150 200 250 σ p(ℓ+) [ / ] p(ℓ+) [ ]

• LO band

NLO band NLO ( = mtop) − − −

• 100

200 300 400 500 σ M (ℓ−, ℓ+) [ / ] M(ℓ−, ℓ+) [ ]

• LO band

NLO band NLO ( = mtop) 100 200 300 400 500 600 700 0.2 0.4 0.6 0.8 σ m [ / ] m [ ]

• LO

NLO 20 30 40 50 60 70 80 1 2 3 σ ϕ(ℓ−, ℓ+) [ ] ϕ(ℓ−, ℓ+)

• LO

NLO 14 13 12 11 1 2 3 4 50 100 σ y(ℓ−) [ ] y(ℓ−)

• LO band

NLO band NLO ( = mtop) − − 300 400 500 600 700 σ H [ / ] H [ ]

• LO band

NLO band NLO ( = mtop)

Markus Schulze, Johns Hopkins University 18/16

$* +

t t

We include LO decays into leptons and jets:

Tevatron: (semi#lept.) LHC, signal: (7 TeV, di#lept.) LHC, VBF bkgrd.: (14 TeV, di#lept.)

− − − −

• 50

100 150 200 250 σ p(ℓ+) [ / ] p(ℓ+) [ ]

• LO band

NLO band NLO ( = mtop) − − −

• 100

200 300 400 500 σ M (ℓ−, ℓ+) [ / ] M(ℓ−, ℓ+) [ ]

• LO band

NLO band NLO ( = mtop) 100 200 300 400 500 600 700 0.2 0.4 0.6 0.8 σ m [ / ] m [ ]

• LO

NLO 20 30 40 50 60 70 80 1 2 3 σ ϕ(ℓ−, ℓ+) [ ] ϕ(ℓ−, ℓ+)

• LO

NLO − − 300 400 500 600 700 σ H [ / ] H [ ]

• LO band

NLO band NLO ( = mtop) 14 13 12 11 1 2 3 4 50 100 σ y(ℓ−) [ ] y(ℓ−)

• LO band

NLO band NLO ( = mtop)

• yℓ−
• m

σ y(ℓ−) [ ]

Markus Schulze, Johns Hopkins University 18/16

$* +

t t

We include LO decays into leptons and jets:

Tevatron: (semi#lept.) LHC, signal: (7 TeV, di#lept.) LHC, VBF bkgrd.: (14 TeV, di#lept.)

− − − −

• 50

100 150 200 250 σ p(ℓ+) [ / ] p(ℓ+) [ ]

• LO band

NLO band NLO ( = mtop) − − −

• 100

200 300 400 500 σ M (ℓ−, ℓ+) [ / ] M(ℓ−, ℓ+) [ ]

• LO band

NLO band NLO ( = mtop) 100 200 300 400 500 600 700 0.2 0.4 0.6 0.8 σ m [ / ] m [ ]

• LO

NLO 20 30 40 50 60 70 80 1 2 3 σ ϕ(ℓ−, ℓ+) [ ] ϕ(ℓ−, ℓ+)

• LO

NLO − − 300 400 500 600 700 σ H [ / ] H [ ]

• LO band

NLO band NLO ( = mtop) 14 13 12 11 1 2 3 4 50 100 σ y(ℓ−) [ ] y(ℓ−)

• LO band

NLO band NLO ( = mtop)

• ϕℓ−, ℓ
• σ

ϕ(ℓ−,ℓ) [ ]

Markus Schulze, Johns Hopkins University 19/16

$* +

t t

Forward#Backward Asymmetry at the Tevatron At

t =

0% LO Aexp

t t

= +19% ± 8% +5% ± 2% NLO

[Kühn,Rodrigo]

CDF: (2.3 fb#1)

Markus Schulze, Johns Hopkins University 19/16

$* +

t t

Forward#Backward Asymmetry at the Tevatron in agreement with DUW At

t =

0% LO CDF: (2.3 fb#1) Aexp

t t

= +19% ± 8% +5% ± 2% NLO −8.3% ± 0.1% LO −2.3% ± 0.5% NLO −5.1% ± 0.1% LO −0.5% ± 0.7% NLO

[Kühn,Rodrigo]

At

t+jet(p⊥jet >30GeV) =

Aℓℓ−+jet(p⊥jet >20GeV) =

Markus Schulze, Johns Hopkins University

• If precise measurements are available, NLO describes data best.

Sabine Lammers (U-Indiana, D0) comparison of different MC generators with D0 data for Z+jet (Run II, 1fb-1 )

TEV O(1000) events LHC O(10000) events already with 1/fb at Tevatron ∼ at LHC

t t

Z+jet

Markus Schulze, Johns Hopkins University

0.5 1 1.5 100 200 300 400 500 600 NLO/LO pT(ℓ+) [ GeV ] Tevatron LHC

K - factor

Markus Schulze, Johns Hopkins University 10−1 100 101 50 100 150 200 dσ dMℓ+b [ fb/GeV ] Mℓ+b [ GeV ] (c) 10 20 30 100 110 120 130 140 LO NLO NLO (LO decay)

LHC

• NLO induces a tail
• boundary is top mass dependent
• spin studies for BSM particles

invariant mass of lepton and b-jet

max(M2

ℓb) = m2 top − m2 W

Markus Schulze, Johns Hopkins University

• 1
• 0.5

0.5 1 0.4 0.5 0.6 dσ σ d cos(ϕℓ+ℓ−) cos(ϕℓ+ℓ−) (d) LO NLO NLO (LO decay)

• 1
• 0.5

0.5 1 0.4 0.5 0.6 dσ σ d cos(ϕℓ+ℓ−) cos(ϕℓ+ℓ−) (d) LO NLO NLO (LO decay)

LHC Tevatron leptons preferably parallel leptons preferably anti-parallel

ℓ+ ℓ−

• substantial angular correlations, even at NLO
• NLO effects at Tevatron are significant

typical observable: angle between the directions of flight of leptons in the corresponding top rest frame

ϕℓℓ−:

1 σ dσ d cos(ϕℓℓ− )

ℓ+ ℓ−

Markus Schulze, Johns Hopkins University

• 1
• 0.5

0.5 1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 cos(ψℓ+ℓ−) (b) LHC LO NLO NLO (LO decay) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

• 1
• 0.5

0.5 1 dσ σ d cos(ψℓ+ℓ−) cos(ψℓ+ℓ−) (a) Tevatron LO NLO NLO (LO decay)

LHC Tevatron simpler observable:

• pening angle of the leptons in the

laboratory frame

ψℓℓ−:

• top quark rest frames need not to be reconstructed
• angular correlations remain, stronger NLO effects at LHC

1 σ dσ d cos(ψℓℓ− )

Markus Schulze, Johns Hopkins University 14/16

Virtual corrections:

$* +

t t

Runtime

gg → t tg

5000min/0.65Mevents = 460msec/event

( Intel Xeon 2.8GHz, events after cuts,

• incl. QuadPrec stabilization )

Real corrections:

( Intel Xeon 2.8GHz, events after cuts,

• incl. Dipoles )

2400min/7Mevents = 21msec/event

gg → t tgg

with a handful of quad#core processors ⇒ distributions in 4 days DUW: ≈ 10x faster for virtual corrections. However, we compare a mostly analytic reduction with a fully numerical approach.

α=10−2