precision calculations for lhc physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf ·...
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![Page 1: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/1.jpg)
DPG Fruhjahrstagung, February 2008
Precision calculations for LHC physics
Michael Kramer (RWTH Aachen)
Sponsored by:
SFB/TR 9 “Computational Particle Physics”, Helmholtz Alliance “Physics at the Terascale”, BMBF-
Theorie-Verbund, Marie Curie Research Training Network “Heptools”, Graduiertenkolleg “Elemen-
tarteilchenphysik an der TeV-Skala”
Michael Kramer page 1 DPG Fruhjahrstagung 2008
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Why precision calculations for discoveries?
We do not need theory input for LHC discoveries
8000
6000
4000
2000
080 100 120 140
mγγ (GeV)
Higgs signal
Eve
nts
/ 500
MeV
for
105
pb-
1
HSM γγ
Simulated 2γ mass plotfor 105 pb-1 mH =130 GeV
in the lead tungstate calorimeter
D_D
_105
5c
CMS, 105 pb-1
but only if we see a resonance. . .
Michael Kramer page 2 DPG Fruhjahrstagung 2008
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Why precision calculations for discoveries?
We do not need theory input for LHC discoveries
8000
6000
4000
2000
080 100 120 140
mγγ (GeV)
Higgs signal
Eve
nts
/ 500
MeV
for
105
pb-
1
HSM γγ
Simulated 2γ mass plotfor 105 pb-1 mH =130 GeV
in the lead tungstate calorimeter
D_D
_105
5c
CMS, 105 pb-1
but only if we see a resonance. . .
Michael Kramer page 2 DPG Fruhjahrstagung 2008
![Page 4: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/4.jpg)
Contents
Why precision calculations for discoveries?
– lessons from the past
– prospects: Higgs physics & BSM searches at the LHC
Theoretical tools for LHC phenomenology
– (N)NLO multi-leg calculations
– matching with parton showers & hadronization
Michael Kramer page 3 DPG Fruhjahrstagung 2008
![Page 5: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/5.jpg)
Why precision calculations for discoveries?
Tevatron jet cross section at large ET
Et (GeV)-0.5
0
0.5
1(D
ata
- The
ory)
/ The
ory
200 300 40010050
CTEQ3MCDF (Preliminary) * 1.03D0 (Preliminary) * 1.01
Michael Kramer page 4 DPG Fruhjahrstagung 2008
![Page 6: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/6.jpg)
Why precision calculations for discoveries?
Tevatron jet cross section at large ET
Et (GeV)-0.5
0
0.5
1(D
ata
- The
ory)
/ The
ory
200 300 40010050
CTEQ3MCDF (Preliminary) * 1.03D0 (Preliminary) * 1.01
⇒ new physics? + ?
Michael Kramer page 5 DPG Fruhjahrstagung 2008
![Page 7: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/7.jpg)
Why precision calculations for discoveries?
Tevatron jet cross section at large ET
Et (GeV)-0.5
0
0.5
1(D
ata
- The
ory)
/ The
ory
200 300 40010050
CTEQ3MCDF (Preliminary) * 1.03D0 (Preliminary) * 1.01
LBSM ⊇g2
M2ψγµψ ψγµψ ⇒
data − theory
theory∝ g2 E
2T
M2
Michael Kramer page 6 DPG Fruhjahrstagung 2008
![Page 8: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/8.jpg)
Why precision calculations for discoveries?
Tevatron jet cross section at large ET
(GeV)TInclusive Jet E0 100 200 300 400 500 600
(nb/
GeV
)η
d T /
dEσ2
d
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-11
10
102
CDF Run II PreliminaryIntegrated L = 85 pb-1
0.1 < |ηDet| < 0.7JetClu Cone R = 0.7
Run II DataCTEQ 6.1 Uncertainty+/- 5% Energy Scale Uncertainty
⇒ just QCD. . . (uncertainty in gluon pdf at large x)
Michael Kramer page 7 DPG Fruhjahrstagung 2008
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Why precision calculations for discoveries?
Tevatron bottom quark cross section
[D0, PLB 487 (2000)]
⇒ new physics?
Michael Kramer page 8 DPG Fruhjahrstagung 2008
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Why precision calculations for discoveries?
Tevatron bottom quark cross section
[Berger et al., PRL 86 (2001)]
10-1
1
10
10 2
10 3
10 4
10 5
10 102
SumQCDg → b~
D0 Data
CDF Data
√S = 1.8 TeV
mg = 14 GeV
mb = 3.5 GeV
mb = 4.75 GeV
~
~
pT (GeV)min
σ b(p T
≥ p
T )
(nb
)m
in
⇒ light gluino/sbottom production?
Michael Kramer page 9 DPG Fruhjahrstagung 2008
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Why precision calculations for discoveries?
Tevatron bottom quark cross section
[Mangano, AIP Conf.Proc, 2005]
⇒ just QCD. . . (αs, low-x gluon pdf, b→ B fragmentation & exp. analyses)
Michael Kramer page 10 DPG Fruhjahrstagung 2008
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Why precision calculations for discoveries?
Signals of discovery include
– mass peaks
– anomalous shapes of kinematic distributions
e.g. jet production at large ET
– excess of events after kinematic selection
e.g. bottom cross section
Michael Kramer page 11 DPG Fruhjahrstagung 2008
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Why precision calculations for discoveries?
Signals of discovery include
– mass peaks
– anomalous shapes of kinematic distributionse.g. jet production at large ET
– excess of events after kinematic selectione.g. bottom cross section
Michael Kramer page 11 DPG Fruhjahrstagung 2008
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Why precision calculations for discoveries?
Signals of discovery include
– mass peakse.g. Higgs searches in H → γγ/ZZ∗ final states
– anomalous shapes of kinematic distributionse.g. SUSY searches in multijet + ET,miss final states
– excess of events after kinematic selectione.g. Higgs searches in H → WW ∗ final states
Experiments measure multi-parton final states in restricted phase space region
→ Need flexible and realistic precision calculations
– precise → including loops & many legs
– flexible → allowing for exclusive cross sections & phase space cuts
– realistic → matched with parton showers and hadronization
Michael Kramer page 12 DPG Fruhjahrstagung 2008
![Page 15: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/15.jpg)
Why precision calculations for discoveries?
Signals of discovery include
– mass peakse.g. Higgs searches in H → γγ/ZZ∗ final states
– anomalous shapes of kinematic distributionse.g. SUSY searches in multijet + ET,miss final states
– excess of events after kinematic selectione.g. Higgs searches in H → WW ∗ final states
Experiments measure multi-parton final states in restricted phase space region
→ Need flexible and realistic precision calculations
– precise → including loops & many legs
– flexible → allowing for exclusive cross sections & phase space cuts
– realistic → matched with parton showers and hadronization
Michael Kramer page 12 DPG Fruhjahrstagung 2008
![Page 16: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/16.jpg)
Searches at the LHC: outline
Examples from Higgs physics
– discovery from excess of events: pp→ H → WW ∗ → l+νl−ν
– backgrounds: beware of cuts
– beyond discovery: measuring Higgs couplings
Examples from beyond the standard model physics
– anomalous shapes: SUSY searches with multi-jets + ET,miss signatures
– backgrounds: multi-leg calculations needed
Theoretical tools for LHC physics
– LO and NLO multi-leg calculations
– matching with parton showers and hadronization
Michael Kramer page 13 DPG Fruhjahrstagung 2008
![Page 17: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/17.jpg)
Searches at the LHC: outline
Examples from Higgs physics
– discovery from excess of events: pp→ H → WW ∗ → l+νl−ν
– backgrounds: beware of cuts
– beyond discovery: measuring Higgs couplings
• Examples from beyond the standard model physics
– anomalous shapes: SUSY searches with multi-jets + ET,miss signatures
– backgrounds: multi-leg calculations needed
• Theoretical tools for LHC physics
– LO and NLO multi-leg calculations
– matching with parton showers and hadronization
Michael Kramer page 14 DPG Fruhjahrstagung 2008
![Page 18: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/18.jpg)
Searches at the LHC: pp → H → WW ∗→ l+νl−ν
g
g
H
W−
W+
ν
l−
l+
ν
– neutrinos in final state
→ no invariant Higgs mass peak
– large background from QCD
process qq → WW
– exploit lepton rapidity distributions
and angular correlations to sup-
press background(Dittmar, Dreiner) 0
100
200
300
0 50 100 150 200 250
mT (GeV)
Eve
nts
/ 5 G
eV
⇒ a precise theoretical description of the background is crucial
Michael Kramer page 15 DPG Fruhjahrstagung 2008
![Page 19: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/19.jpg)
Searches at the LHC: pp → H → WW ∗→ l+νl−ν
g
g
H
W−
W+
ν
l−
l+
ν
– neutrinos in final state
→ no invariant Higgs mass peak
– large background from QCD
process qq → WW
– exploit lepton rapidity distributions
and angular correlations to sup-
press background(Dittmar, Dreiner) 0
100
200
300
0 50 100 150 200 250
mT (GeV)
Eve
nts
/ 5 G
eV
⇒ a precise theoretical description of the background is crucial
Michael Kramer page 15 DPG Fruhjahrstagung 2008
![Page 20: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/20.jpg)
Searches at the LHC: pp → H → WW ∗→ l+νl−ν
g
g
H
W−
W+
ν
l−
l+
ν
– neutrinos in final state
→ no invariant Higgs mass peak
– large background from QCD
process qq → WW
– exploit lepton rapidity distributions
and angular correlations to sup-
press background(Dittmar, Dreiner) 0
100
200
300
0 50 100 150 200 250
mT (GeV)
Eve
nts
/ 5 G
eV
⇒ a precise theoretical description of the background is crucial
Michael Kramer page 15 DPG Fruhjahrstagung 2008
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Searches at the LHC: pp → H → WW ∗→ l+νl−ν
LO background from qq scattering
������
���
���
� � ��
� �
�����
���
���
� � ��
experimental selection cuts may enhance the importance of higher-order
corrections for background processes: gg → WW → lνl′ν′
[Binoth, Ciccolini, Kauer, MK; Duhrssen, Jakobs, van der Bij, Marquard]
W−
W+
γ, Z
ν
`
W+γ, Z, H
g
gq
q
q
g
gq
q
q
g
gd
d u
W−d
W+
ν ′
¯′
`
ν
`
ν
ν ′
¯′
ν ′
¯′
→ formally NNLO
→ but enhanced by Higgs search cuts
gg qq (NLO) corr.
σtot 54 fb 1373 fb 4%
σbkg 1.39 fb 4.8 fb 30%
Michael Kramer page 16 DPG Fruhjahrstagung 2008
![Page 22: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/22.jpg)
Searches at the LHC: pp → H → WW ∗→ l+νl−ν
LO background from qq scattering
������
���
��� �
� � ���
���
�����
���
��� �
� � ���
experimental selection cuts may enhance the importance of higher-order
corrections for background processes: gg → WW → lνl′ν′
[Binoth, Ciccolini, Kauer, MK; Duhrssen, Jakobs, van der Bij, Marquard]
W−
W+
γ, Z
ν
`
W+γ, Z, H
g
gq
q
q
g
gq
q
q
g
gd
d u
W−d
W+
ν ′
¯′
`
ν
`
ν
ν ′
¯′
ν ′
¯′
→ formally NNLO
→ but enhanced by Higgs search cuts
gg qq (NLO) corr.
σtot 54 fb 1373 fb 4%
σbkg 1.39 fb 4.8 fb 30%
Michael Kramer page 16 DPG Fruhjahrstagung 2008
![Page 23: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/23.jpg)
Searches at the LHC: pp → H → WW ∗→ l+νl−ν
LO background from qq scattering
������
!
"�# $
! # %&�
'()
�����
!
"�# $
! # %&�
experimental selection cuts may enhance the importance of higher-order
corrections for background processes: gg → WW → lνl′ν′
[Binoth, Ciccolini, Kauer, MK; Duhrssen, Jakobs, van der Bij, Marquard]
W−
W+
γ, Z
ν
`
W+γ, Z, H
g
gq
q
q
g
gq
q
q
g
gd
d u
W−d
W+
ν ′
¯′
`
ν
`
ν
ν ′
¯′
ν ′
¯′
→ formally NNLO
→ but enhanced by Higgs search cuts
gg qq (NLO) corr.
σtot 54 fb 1373 fb 4%
σbkg 1.39 fb 4.8 fb 30%
Michael Kramer page 16 DPG Fruhjahrstagung 2008
![Page 24: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/24.jpg)
Searches at the LHC: pp → H → WW ∗→ l+νl−ν
LO background from qq scattering
*�+*�,
-.-
/+0 1
. 0 23,
456
*+*�,
-.-
/+0 1
. 0 23,
experimental selection cuts may enhance the importance of higher-order
corrections for background processes: gg → WW → lνl′ν′
[Binoth, Ciccolini, Kauer, MK; Duhrssen, Jakobs, van der Bij, Marquard]
W−
W+
γ, Z
ν
`
W+γ, Z, H
g
gq
q
q
g
gq
q
q
g
gd
d u
W−d
W+
ν ′
¯′
`
ν
`
ν
ν ′
¯′
ν ′
¯′
→ formally NNLO
→ but enhanced by Higgs search cuts
gg qq (NLO) corr.
σtot 54 fb 1373 fb 4%
σbkg 1.39 fb 4.8 fb 30%
Michael Kramer page 16 DPG Fruhjahrstagung 2008
![Page 25: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/25.jpg)
Searches at the LHC: outline
• Examples from Higgs physics
– discovery from excess of events: pp→ H → WW ∗ → l+νl−ν
– backgrounds: beware of cuts
– beyond discovery: measuring Higgs couplings
Examples from beyond the standard model physics
– anomalous shapes: SUSY searches with multi-jets + ET,miss signatures
– backgrounds: multi-leg calculations needed
• Theoretical tools for LHC physics
– LO and NLO multi-leg calculations
– matching with parton showers and hadronization
Michael Kramer page 17 DPG Fruhjahrstagung 2008
![Page 26: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/26.jpg)
Searches at the LHC: BSM physics
Many model for new physics are addressing two problems:
– the hierarchy/naturalness problem
– the origin of dark matter
→ spectrum of new particles at the TeV-scale with weakly interacting & stable particle
→ generic BSM signature at the LHC involves cascade decays with missing energy,
eg. supersymmetry
D_D
_204
7.c.
1
q
q
q
q
q~
~
~
~
—
—
—b
b
l+
l+
g
~g
W— W+t
t
t1
ν
χ1
~χ10
~χ20
Gluino/squark production event topology al lowing sparticle mass reconstruction
3 isolated leptons+ 2 b-jets+ 4 jets
+ Etmiss
~l
+
l-
Such cascade decays allow to reconstructsleptons, neutralinos, squarks, gluinos...in favorable cases with %level mass resolutions
Michael Kramer page 18 DPG Fruhjahrstagung 2008
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Searches at the LHC: BSM physics
Many model for new physics are addressing two problems:
– the hierarchy/naturalness problem
– the origin of dark matter
→ spectrum of new particles at the TeV-scale with weakly interacting & stable particle
→ generic BSM signature at the LHC involves cascade decays with missing energy,
eg. supersymmetry
D_D
_204
7.c.
1
q
q
q
q
q~
~
~
~
—
—
—b
b
l+
l+
g
~g
W— W+t
t
t1
ν
χ1
~χ10
~χ20
Gluino/squark production event topology al lowing sparticle mass reconstruction
3 isolated leptons+ 2 b-jets+ 4 jets
+ Etmiss
~l
+
l-
Such cascade decays allow to reconstructsleptons, neutralinos, squarks, gluinos...in favorable cases with %level mass resolutions
Michael Kramer page 18 DPG Fruhjahrstagung 2008
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Searches at the LHC: BSM physics
• simple discriminant for SUSY
searches: effective mass
Meff = ET,miss +∑4
i=1 pjetT
• excess of events with large Meff
→ initial discovery of SUSY
LHC Point 5
10-13
10-12
10-11
10-10
10-9
10-8
10-7
0 1000 2000 3000 4000Meff (GeV)
dσ/d
Mef
f (m
b/40
0 G
eV)
Michael Kramer page 19 DPG Fruhjahrstagung 2008
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Searches at the LHC: BSM physics
• simple discriminant for SUSY
searches: effective mass
Meff = ET,miss +∑4
i=1 pjetT
• excess of events with large Meff
→ initial discovery of SUSY
Modern LO Monte Carlo tools predict much larger multi-jet background
What will disagreement between Monte Carlo and data mean?
→ Need precision NLO calculations to tell!
Michael Kramer page 20 DPG Fruhjahrstagung 2008
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Searches at the LHC: BSM physics
• simple discriminant for SUSY
searches: effective mass
Meff = ET,miss +∑4
i=1 pjetT
• excess of events with large Meff
→ initial discovery of SUSY
Modern LO Monte Carlo tools predict much larger multi-jet background
What will disagreement between Monte Carlo and data mean?
→ Need precision NLO calculations to tell!
Michael Kramer page 20 DPG Fruhjahrstagung 2008
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Searches at the LHC: outline
• Examples from Higgs physics
– discovery from excess of events: pp→ H → WW ∗ → l+νl−ν
– backgrounds: beware of cuts
– beyond discovery: measuring Higgs couplings
• Examples from beyond the standard model physics
– anomalous shapes: SUSY searches with multi-jets + ET,miss signatures
– backgrounds: multi-leg calculations needed
Theoretical tools for LHC physics
– LO and NLO multi-leg calculations
– matching with parton showers and hadronization
Michael Kramer page 21 DPG Fruhjahrstagung 2008
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Theoretical tools for LHC physics
Need flexible and realistic precision calculations
– precise → including loops & many legs
– flexible → allowing for exclusive cross sections & phase space cuts
– realistic → matched with parton showers and hadronization
Mini review of current status for LHC QCD multi-leg
– LO calculations
– LO calculations & parton showers
– NLO calculations
– NLO calculations & parton showers
Note: focus on QCD tools for LHC cross section predictions. Do not cover important QCD and elec-
troweak precision calculations for masses, couplings, decays and renormalization group evolutions in
the SM and beyond. . .
Michael Kramer page 22 DPG Fruhjahrstagung 2008
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LO calculations
Automation of LO calculations: several program packages exist for the automatic calcu-
lation of generic 2 → N cross sections (N ≤ 8) including integration over phase space:
HELAC/PHEGAS, MADGRAPH/MADEVENT, COMPHEP, GRACE, SHERPA/AMEGIC,
O’MEGA/WHIZARD, ALPGEN, ...
→ Non-trivial achievement:
consider number of Feynman graphs e.g. for gg → N gluons:
N jets 2 3 4 5 6 7 8
# diag’s 4 25 220 2485 34300 5 × 105 107
Michael Kramer page 23 DPG Fruhjahrstagung 2008
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LO calculations
Automation of LO calculations: several program packages exist for the automatic calcu-
lation of generic 2 → N cross sections (N ≤ 8) including integration over phase space:
HELAC/PHEGAS, MADGRAPH/MADEVENT, COMPHEP, GRACE, SHERPA/AMEGIC,
O’MEGA/WHIZARD, ALPGEN, ...
→ Non-trivial achievement:
consider number of Feynman graphs e.g. for gg → N gluons:
N jets 2 3 4 5 6 7 8
# diag’s 4 25 220 2485 34300 5 × 105 107
Michael Kramer page 23 DPG Fruhjahrstagung 2008
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LO calculations
Automation of LO calculations: several program packages exist for the automatic calcu-
lation of generic 2 → N cross sections (N ≤ 8) including integration over phase space:
HELAC/PHEGAS, MADGRAPH/MADEVENT, COMPHEP, GRACE, SHERPA/AMEGIC,
O’MEGA/WHIZARD, ALPGEN, ...
→ matrix element calculations crucial for the description of multi-leg processes:
(Mangano)
11
Exact, LO matrix
element estimate
Shower MC result
N.B . R eliability/system atics of M C tools: Shower M C vs M atrix elem ent results
Michael Kramer page 24 DPG Fruhjahrstagung 2008
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LO calculations with parton showers & hadronization
Include parton showers and hadronization to sum soft/collinear parton emission
and predict realistic final states
→ Beware of double counting: N -jet cross section from 2 → N parton process or from 2 →
N − (1, 2, . . .) parton process plus parton shower
→ Construct matching schemes: CKKW-L (Catani, Kuhn, Krauss, Webber; Lonnblad), MLM (Mangano), SCET (Bauer,
Schwarz), implemented in HELAC, MADGRAPH, SHERPA, ARIADNE, ALPGEN
(Alwell et al.)
dσ/d
E⊥
1 (p
b/G
eV)
(a)Alpgen
AriadneHelac
MadEventSherpa
10-2
10-1
100
101
102
E⊥1 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200 250 300 350 400 450 500
dσ/d
E⊥
2 (p
b/G
eV)
(b)
10-2
10-1
100
101
102
E⊥2 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200 250 300 350 400
dσ/d
E⊥
3 (p
b/G
eV)
(c)
10-3
10-2
10-1
100
101
E⊥3 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200 250 300
dσ/d
E⊥
4 (p
b/G
eV)
(d)
10-3
10-2
10-1
100
101
E⊥4 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200
Validation, systematic comparison & tuning of MC tools crucial
Michael Kramer page 25 DPG Fruhjahrstagung 2008
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LO calculations with parton showers & hadronization
Include parton showers and hadronization to sum soft/collinear parton emission
and predict realistic final states
→ Beware of double counting: N -jet cross section from 2 → N parton process or from 2 →
N − (1, 2, . . .) parton process plus parton shower
→ Construct matching schemes: CKKW-L (Catani, Kuhn, Krauss, Webber; Lonnblad), MLM (Mangano), SCET (Bauer,
Schwarz), implemented in HELAC, MADGRAPH, SHERPA, ARIADNE, ALPGEN
(Alwell et al.)
dσ/d
E⊥
1 (p
b/G
eV)
(a)Alpgen
AriadneHelac
MadEventSherpa
10-2
10-1
100
101
102
E⊥1 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200 250 300 350 400 450 500
dσ/d
E⊥
2 (p
b/G
eV)
(b)
10-2
10-1
100
101
102
E⊥2 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200 250 300 350 400
dσ/d
E⊥
3 (p
b/G
eV)
(c)
10-3
10-2
10-1
100
101
E⊥3 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200 250 300
dσ/d
E⊥
4 (p
b/G
eV)
(d)
10-3
10-2
10-1
100
101
E⊥4 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200
Validation, systematic comparison & tuning of MC tools crucial
Michael Kramer page 25 DPG Fruhjahrstagung 2008
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LO calculations with parton showers & hadronization
Include parton showers and hadronization to sum soft/collinear parton emission
and predict realistic final states
→ Beware of double counting: N -jet cross section from 2 → N parton process or from 2 →
N − (1, 2, . . .) parton process plus parton shower
→ Construct matching schemes: CKKW-L (Catani, Kuhn, Krauss, Webber; Lonnblad), MLM (Mangano), SCET (Bauer,
Schwarz), implemented in HELAC, MADGRAPH, SHERPA, ARIADNE, ALPGEN
(Alwell et al.)
dσ/d
E⊥
1 (p
b/G
eV)
(a)Alpgen
AriadneHelac
MadEventSherpa
10-2
10-1
100
101
102
E⊥1 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200 250 300 350 400 450 500
dσ/d
E⊥
2 (p
b/G
eV)
(b)
10-2
10-1
100
101
102
E⊥2 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200 250 300 350 400
dσ/d
E⊥
3 (p
b/G
eV)
(c)
10-3
10-2
10-1
100
101
E⊥3 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200 250 300
dσ/d
E⊥
4 (p
b/G
eV)
(d)
10-3
10-2
10-1
100
101
E⊥4 (GeV)
-1-0.5
0 0.5
1
0 50 100 150 200
Validation, systematic comparison & tuning of MC tools crucial
Michael Kramer page 25 DPG Fruhjahrstagung 2008
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NLO calculations
Want to test & explore models, eg. measure Higgs couplings
[Duhrssen et al.]
→ ratios of Higgs couplings can be measured with an accuracy of 10-30%
→ must be matched by theoretical accuracy in cross section and BR predictions
Michael Kramer page 26 DPG Fruhjahrstagung 2008
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NLO calculations
An accuracy δσ/σ ∼< 20% is only possible at NLO or beyond!
for example σ(pp→ ttH) ∝ g2ttH
[Beenakker, Dittmaier, MK, Plumper, Spira, Zerwas]
σ(pp → tt_ H + X) [fb]
√s = 14 TeV
MH = 120 GeV
µ0 = mt + MH/2
NLO
LO
µ/µ0
0.2 0.5 1 2 5200
400
600
800
1000
1200
1400
1
⇒ δσ/σ ≈
{
100% LO
15% NLO
Michael Kramer page 27 DPG Fruhjahrstagung 2008
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NLO calculations
cross sections are obtained from counting events after selection cuts
→ need acceptance and efficiency
→ need flexible (N)NLO calculations
modern (N)NLO calculations are set up as parton-level Monte Carlo programs
– e.g. at NLO using (dipole) subtraction [Catani, Seymour, Dittmaier, Trocsanyi, . . . ]
σNLO
=
Z
dPSm+1
»
dσreal
−dσsub
–
| {z }
finite
+
Z
dPSm
»
dσvirtual
+dσsub1
–
| {z }
finite
⇒ parton-level events (4-momenta) with positive or negative weights
– recent progress at NNLO, e.g. for Higgs production [Anastasiou, Melnikov, Petriello; Grazzini]
Note: (N)NLO parton-level Monte Carlo programs are not parton shower event generators
– they do not include parton showers and hadronization
– they do not generate unweighted events
Michael Kramer page 28 DPG Fruhjahrstagung 2008
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NLO calculations
cross sections are obtained from counting events after selection cuts
→ need acceptance and efficiency
→ need flexible (N)NLO calculations
modern (N)NLO calculations are set up as parton-level Monte Carlo programs
– e.g. at NLO using (dipole) subtraction [Catani, Seymour, Dittmaier, Trocsanyi, . . . ]
σNLO
=
Z
dPSm+1
»
dσreal
−dσsub
–
| {z }
finite
+
Z
dPSm
»
dσvirtual
+dσsub1
–
| {z }
finite
⇒ parton-level events (4-momenta) with positive or negative weights
– recent progress at NNLO, e.g. for Higgs production [Anastasiou, Melnikov, Petriello; Grazzini]
Note: (N)NLO parton-level Monte Carlo programs are not parton shower event generators
– they do not include parton showers and hadronization
– they do not generate unweighted events
Michael Kramer page 28 DPG Fruhjahrstagung 2008
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NLO calculations
cross sections are obtained from counting events after selection cuts
→ need acceptance and efficiency
→ need flexible (N)NLO calculations
modern (N)NLO calculations are set up as parton-level Monte Carlo programs
– e.g. at NLO using (dipole) subtraction [Catani, Seymour, Dittmaier, Trocsanyi, . . . ]
σNLO
=
Z
dPSm+1
»
dσreal
−dσsub
–
| {z }
finite
+
Z
dPSm
»
dσvirtual
+dσsub1
–
| {z }
finite
⇒ parton-level events (4-momenta) with positive or negative weights
– recent progress at NNLO, e.g. for Higgs production [Anastasiou, Melnikov, Petriello; Grazzini]
Note: (N)NLO parton-level Monte Carlo programs are not parton shower event generators
– they do not include parton showers and hadronization
– they do not generate unweighted events
Michael Kramer page 28 DPG Fruhjahrstagung 2008
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(QCD-) NLO calculations at the LHC: status
2 → 2: anything you want. . . (cf. MCFM (Campbell, Ellis, . . . ))
2 → 3: pp → jjj (Bern, Dixon, Kosower; Kunszt, Signer, Troscanyi; Giele, Kilgore, Nagy)
pp → V jj (Campbell, Glover, Miller)
pp → Hjj[GF] (Campbell, Ellis, Zanderighi)
pp → Htt (Beenakker, Dittmaier, MK, Plumper, Spira, Zerwas; Dason, Reina, Wackeroth, Orr, Jackson;)
pp → γγj (de Florian, Kunszt; Del Duca, Maltoni, Nagy, Troscani, Binoth, Guillet, Mahmoudi)
pp → Hjj(j)[WBF] (Figy, Hankele, Oleari, Zeppenfeld)
pp → HHH (Plehn, Rauch; Binoth, Karg, Kauer, Ruckl)
pp → V V jj[WBF] (Jager, Oleary, Zeppenfeld)
pp → ZZZ (Lazopoulos, Melnikov, Petriello)
pp → WWZ (Hankele, Zeppenfeld)
pp → ttj (Dittmaier, Uwer, Weinzierl)
pp → V V j (Dittmaier, Kallweit, Uwer; Campbell, Ellis, Zanderighi; Binoth, Guillet, Karg, Kauer, Sanguinetti)
2 → 4: no LHC process yet. . .
Michael Kramer page 29 DPG Fruhjahrstagung 2008
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An experimenters wishlist. . .
Theorists reply: In your dreams. . .
Michael Kramer page 30 DPG Fruhjahrstagung 2008
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An experimenters wishlist. . .
Theorists reply: In your dreams. . .
Michael Kramer page 30 DPG Fruhjahrstagung 2008
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An experimenters wishlist. . .
Main bottleneck for multi-leg NLO calculations:
tensor-reduction of Pentagon/Hexagon/Heptagon loop integrals, e.g.
77
89
:8
;:;
89
:8
<=> ?
With current techniques
pp→ 3 partons @ NLO is “straightforward but tedious”
pp→ 4 partons @ NLO is possible
Need breakthrough in technology for pp→ 4 partons @ NLO
spinors & twistors, on-shell methods. . . ?
⇒ NLO (parton-level) Monte Carlo programs will not be available for all potentially
relevant (background) processes in the near future
Michael Kramer page 31 DPG Fruhjahrstagung 2008
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An experimenters wishlist. . .
Main bottleneck for multi-leg NLO calculations:
tensor-reduction of Pentagon/Hexagon/Heptagon loop integrals, e.g.
@@
AB
CA
DCD
AB
CA
EFG H
With current techniques
pp→ 3 partons @ NLO is “straightforward but tedious”
pp→ 4 partons @ NLO is possible
Need breakthrough in technology for pp→ 4 partons @ NLO
spinors & twistors, on-shell methods. . . ?
⇒ NLO (parton-level) Monte Carlo programs will not be available for all potentially
relevant (background) processes in the near future
Michael Kramer page 31 DPG Fruhjahrstagung 2008
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An experimenters wishlist. . .
Main bottleneck for multi-leg NLO calculations:
tensor-reduction of Pentagon/Hexagon/Heptagon loop integrals, e.g.
II
JK
LJ
MLM
JK
LJ
NOP Q
With current techniques
pp→ 3 partons @ NLO is “straightforward but tedious”
pp→ 4 partons @ NLO is possible
Need breakthrough in technology for pp→ 4 partons @ NLO
spinors & twistors, on-shell methods. . . ?
⇒ NLO (parton-level) Monte Carlo programs will not be available for all potentially
relevant (background) processes in the near future
Michael Kramer page 31 DPG Fruhjahrstagung 2008
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Need for NNLO at the LHC?
NNLO calculations are needed
– for high-precision measurements (αs, MW , pdf’s,. . . )
– if there are large corrections & scale dependence at NLO, eg. for gg → H
– if NLO accuracy is reduced after cuts:
Ht
g
g
g
is
{
NLO for σinclusive
LO for dσ/dpT
Michael Kramer page 32 DPG Fruhjahrstagung 2008
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Need for NNLO at the LHC?
NNLO calculations are needed
– for high-precision measurements (αs, MW , pdf’s,. . . )
– if there are large corrections & scale dependence at NLO, eg. for gg → H
– if NLO accuracy is reduced after cuts:
Ht
g
g
g
is
{
NLO for σinclusive
LO for dσ/dpT
Michael Kramer page 32 DPG Fruhjahrstagung 2008
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Need for NNLO at the LHC?
NNLO calculations are needed
– for high-precision measurements (αs, MW , pdf’s,. . . )
– if there are large NLO corrections & scale dependence, eg. for gg → H
– if NLO accuracy is reduced after cuts:
(Anastasiou, Dissertori, Stockli)
Michael Kramer page 33 DPG Fruhjahrstagung 2008
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NLO calculations with parton showers & hadronization
Michael Kramer page 34 DPG Fruhjahrstagung 2008
![Page 54: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/54.jpg)
NLO calculations with parton showers & hadronization
Fixed-oder calculations have limitations
– they do not predict realistic final states
experimentally theoretically at LO
Michael Kramer page 35 DPG Fruhjahrstagung 2008
![Page 55: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/55.jpg)
NLO calculations with parton showers & hadronization
Fixed-oder calculations have limitations
– they do not predict realistic final states
experimentally theoretically at NLO
Michael Kramer page 36 DPG Fruhjahrstagung 2008
![Page 56: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/56.jpg)
NLO calculations with parton showers & hadronization
Fixed-oder calculations have limitations
– they break down for certain kinematic configurations
[MK, Mrenna, Soper]
-0.1
0
0.1
0 5 10 15 20
e+e– → 3 jets: jet-mass distribution df3/dM [GeV-1]
NLO
(√s = MZ; µ = √s/6, kT algorithm, ycut = 0.05)
M [GeV]
⇒ wrong jet structure for M → 0
Michael Kramer page 37 DPG Fruhjahrstagung 2008
![Page 57: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/57.jpg)
NLO calculations with parton showers & hadronization
Fixed-oder calculations have limitations
⇒ add parton showers → summation of multi-parton emission
[MK, Mrenna, Soper]
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20
e+e– → 3 jets: jet-mass distribution f -13 df3/dM [GeV-1]
NLO+Pythia
NLO
(√s = MZ; µ = √s/6, kT algorithm, ycut = 0.05)
M [GeV]
⇒ realistic jet structure and accurate (NLO) prediction for rate
Michael Kramer page 38 DPG Fruhjahrstagung 2008
![Page 58: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/58.jpg)
NLO calculations with parton showers & hadronization
NLO⊕PS matching formalism
– avoid double counting: parton showers include part of the short-distance physics already
included in the NLO calculation
– various schemes have been proposed (Frixione, Webber, Nason, Oleari, Collins, Zu, Soper,MK, Nagy, Weinzierl, Giele, Kosower, Skands, Bauer, Schwarz,. . . )
MC@NLO (Frixione, Webber, . . . ) is still top of the class
– well tested and supported
– includes many processes: pp → V, H, ll, lν, QQ, HV, single top, V V
(Near?) future: variety of general NLO⊕PS matching formalisms
– independent of specific shower algorithm → combine NLO with any MC
– in terms of commonly used dipole subtraction formalism→ easy to use for NLO practitioner
Note: There are also new development in parton showers designed for NLO matching and showersincluding quantum interference (Gieseke, Stephens, Webber, Seymour, Siodmok,. . . ; Sjostrand, Skands,. . . ; Nagy, Soper; Dinsdale,
Ternick, Weinzierl, Schumann, Krauss; Giele, Kosower)
Michael Kramer page 39 DPG Fruhjahrstagung 2008
![Page 59: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/59.jpg)
NLO calculations with parton showers & hadronization
NLO⊕PS matching formalism
– avoid double counting: parton showers include part of the short-distance physics already
included in the NLO calculation
– various schemes have been proposed (Frixione, Webber, Nason, Oleari, Collins, Zu, Soper,MK, Nagy, Weinzierl, Giele, Kosower, Skands, Bauer, Schwarz,. . . )
MC@NLO (Frixione, Webber, . . . ) is still top of the class
– well tested and supported
– includes many processes: pp → V, H, ll, lν, QQ, HV, single top, V V
(Near?) future: variety of general NLO⊕PS matching formalisms
– independent of specific shower algorithm → combine NLO with any MC
– in terms of commonly used dipole subtraction formalism→ easy to use for NLO practitioner
Note: There are also new development in parton showers designed for NLO matching and showersincluding quantum interference (Gieseke, Stephens, Webber, Seymour, Siodmok,. . . ; Sjostrand, Skands,. . . ; Nagy, Soper; Dinsdale,
Ternick, Weinzierl, Schumann, Krauss; Giele, Kosower)
Michael Kramer page 39 DPG Fruhjahrstagung 2008
![Page 60: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/60.jpg)
NLO calculations with parton showers & hadronization
NLO⊕PS matching formalism
– avoid double counting: parton showers include part of the short-distance physics already
included in the NLO calculation
– various schemes have been proposed (Frixione, Webber, Nason, Oleari, Collins, Zu, Soper,MK, Nagy, Weinzierl, Giele, Kosower, Skands, Bauer, Schwarz,. . . )
MC@NLO (Frixione, Webber, . . . ) is still top of the class
– well tested and supported
– includes many processes: pp → V, H, ll, lν, QQ, HV, single top, V V
(Near?) future: variety of general NLO⊕PS matching formalisms
– independent of specific shower algorithm → combine NLO with any MC
– in terms of commonly used dipole subtraction formalism→ easy to use for NLO practitioner
Note: There are also new development in parton showers designed for NLO matching and showersincluding quantum interference (Gieseke, Stephens, Webber, Seymour, Siodmok,. . . ; Sjostrand, Skands,. . . ; Nagy, Soper; Dinsdale,
Ternick, Weinzierl, Schumann, Krauss; Giele, Kosower)
Michael Kramer page 39 DPG Fruhjahrstagung 2008
![Page 61: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/61.jpg)
NLO calculations with parton showers & hadronization
NLO⊕PS matching formalism
– avoid double counting: parton showers include part of the short-distance physics already
included in the NLO calculation
– various schemes have been proposed (Frixione, Webber, Nason, Oleari, Collins, Zu, Soper,MK, Nagy, Weinzierl, Giele, Kosower, Skands, Bauer, Schwarz,. . . )
MC@NLO (Frixione, Webber, . . . ) is still top of the class
– well tested and supported
– includes many processes: pp → V, H, ll, lν, QQ, HV, single top, V V
(Near?) future: variety of general NLO⊕PS matching formalisms
– independent of specific shower algorithm → combine NLO with any MC
– in terms of commonly used dipole subtraction formalism→ easy to use for NLO practitioner
Note: There are also new development in parton showers designed for NLO matching and showersincluding quantum interference (Gieseke, Stephens, Webber, Seymour, Siodmok,. . . ; Sjostrand, Skands,. . . ; Nagy, Soper; Dinsdale,
Ternick, Weinzierl, Schumann, Krauss; Giele, Kosower)
Michael Kramer page 39 DPG Fruhjahrstagung 2008
![Page 62: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/62.jpg)
Conclusions & outlook . . .
Signal cross sections in the SM and MSSM are (or will rather soon be) known at
NLO accuracy
– theoretical uncertainty ≈ 15% for inclusive cross sections
– larger uncertainty for exclusive observables
– can predict distributions and observables with cuts
– in general no parton showers/hadronization
Progress in matching (N)LO calculations with parton showers and hadronization
Many backgrounds (multi-leg processes) are only know at LO
(Need breakthrough in techniques to do pp→> 4 partons at NLO)
First LHC data will allow us to focus on the relevant processes. . .
Michael Kramer page 40 DPG Fruhjahrstagung 2008
![Page 63: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/63.jpg)
Conclusions & outlook . . .
Signal cross sections in the SM and MSSM are (or will rather soon be) known at
NLO accuracy
– theoretical uncertainty ≈ 15% for inclusive cross sections
– larger uncertainty for exclusive observables
– can predict distributions and observables with cuts
– in general no parton showers/hadronization
Progress in matching (N)LO calculations with parton showers and hadronization
Many backgrounds (multi-leg processes) are only know at LO
(Need breakthrough in techniques to do pp→> 4 partons at NLO)
First LHC data will allow us to focus on the relevant processes. . .
Michael Kramer page 40 DPG Fruhjahrstagung 2008
![Page 64: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/64.jpg)
Conclusions & outlook . . .
Signal cross sections in the SM and MSSM are (or will rather soon be) known at
NLO accuracy
– theoretical uncertainty ≈ 15% for inclusive cross sections
– larger uncertainty for exclusive observables
– can predict distributions and observables with cuts
– in general no parton showers/hadronization
Progress in matching (N)LO calculations with parton showers and hadronization
Many backgrounds (multi-leg processes) are only know at LO
(Need breakthrough in techniques to do pp→> 4 partons at NLO)
First LHC data will allow us to focus on the relevant processes. . .
Michael Kramer page 40 DPG Fruhjahrstagung 2008
![Page 65: Precision calculations for LHC physicsweb.physik.rwth-aachen.de/~mkraemer/mkraemer_dpg.pdf · 2008-03-03 · DPG Fruhjahrstagung,¤ February 2008 Precision calculations for LHC physics](https://reader033.vdocument.in/reader033/viewer/2022050516/5f9ff4c4d90c027d95131b24/html5/thumbnails/65.jpg)
Conclusions & outlook . . .
Signal cross sections in the SM and MSSM are (or will rather soon be) known at
NLO accuracy
– theoretical uncertainty ≈ 15% for inclusive cross sections
– larger uncertainty for exclusive observables
– can predict distributions and observables with cuts
– in general no parton showers/hadronization
Progress in matching (N)LO calculations with parton showers and hadronization
Many backgrounds (multi-leg processes) are only know at LO
(Need breakthrough in techniques to do pp→> 4 partons at NLO)
First LHC data will allow us to focus on the relevant processes. . .
Michael Kramer page 40 DPG Fruhjahrstagung 2008