Perturbative QCD in hadron collisions Gavin Salam CERN, LPTHE/CNRS (Paris) & Princeton University IX Latin American Symposium on High Energy Physics (SILAFAE 2012) S˜ ao Paolo, Brazil, 10–14 December 2012
Perturbative QCD in hadron collisions
Gavin Salam
CERN, LPTHE/CNRS (Paris) & Princeton University
IX Latin American Symposium on High Energy Physics (SILAFAE 2012)Sao Paolo, Brazil, 10–14 December 2012
An exciting past 24 months
Higgs(-like) discovery
tt asymmetry
W + dijet CDF anomaly
Exclusion of swathes of SUSY, etc.
. . .
This talk: examine recent collider-QCD developments andthe role they’re playing in some of these “headline” topics,
as well as touch on some open problems
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 2 / 35
Some of what goes into collider predictions
eventunderlying
π, K, p, etc.hadronisation
shower
u
proton proton
τ + τ −
H
u
u
hardproc.
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 3 / 35
Some of what goes into collider predictions
eventunderlying
π, K, p, etc.hadronisation
shower
u
proton proton
τ + τ −
H
u
u
hardproc.
αn + αn+1 + ...
τ + τ −
H
u
u
hardproc.
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 3 / 35
What a perturbative series should look like
E.g. QCD corrections to e+e− → hadrons cross section:
R = 1 + 0.32αs + 0.14α2s − 0.41α3
s − 0.82α4s
keep in mind αs(mZ ) ≃ 0.118
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 4 / 35
What it looks like at hadron colliders
Consider LO, NLO and their ratio K =NLO
LO
0.001
0.01
0.1
1
10
100
1000
200 400 600 800 1000 1200 1400
dσ/d
p t [p
b/G
eV]
pt [GeV]
pp, 14 TeVFastNLO, kt R=0.7
LO
NLO
NLO/LO ≃ 1.2
Adapted from Rubin, GPS & Sapeta ’10
Look at p t ofquark or gluon (jets)
gluon
proton proton
quark
1 + 2αs looks like a reasonable series
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 5 / 35
What it looks like at hadron colliders
Consider LO, NLO and their ratio K =NLO
LO
10-2
10-1
1
101
102
103
104
200 400 600 800 1000 1200 1400
dσ/d
p t,Z
[fb
/ 100
GeV
]
pt,Z [GeV]
pp, 14 TeV
LO
NLO
NLO/LO ≃ 1.5
MCFM
Adapted from Rubin, GPS & Sapeta ’10
Look at p t ofZ−boson Z−boson
proton proton
quark
1 + C × αs, with quite large C ≃ 5
To date, no generalised understanding of size of C when in range 5− 10
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 5 / 35
What it looks like at hadron colliders
Consider LO, NLO and their ratio K =NLO
LO
10-2
10-1
1
101
102
103
104
200 400 600 800 1000 1200 1400
dσ/d
p t,j1
[fb
/ 100
GeV
]
pt,j1 [GeV]
pp, 14 TeV
LO
NLO
NLO/LO ≃ 5
MCFM
Adapted from Rubin, GPS & Sapeta ’10
Look at p t ofquark (jet) Z−boson
proton proton
quark
1 + Cαs −→ C = 50 ?!! Often driven by new topologies
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 5 / 35
The NLO revolutionand one way it’s being used
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 6 / 35
SUSY example: gluino pair production
Signal
~g
~g~g
~q
~q
χ0
χ0
g
q
q
q
q
g
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 7 / 35
SUSY example: gluino pair production
Signal
~g
~g~g
~q
~q
χ0
χ0
g
q
q
q
q
g
ET/
ET/
jet
jet
jet
jet
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 7 / 35
SUSY example: gluino pair production
Signal Background
~g
~g~g
~q
~q
χ0
χ0
g
q
q
q
q
g
ET/
ET/
jet
jet
jet
jet
g
q
g
ν
ν−
g
q
q
Z
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 7 / 35
SUSY example: gluino pair production
Signal Background
~g
~g~g
~q
~q
χ0
χ0
g
q
q
q
q
g
ET/
ET/
jet
jet
jet
jet
g
q
g
ν
ν−
g
q
q
ZET/
jet jet
jet
jet
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 7 / 35
SUSY example: gluino pair production
Signal Background
g
q
g
ν
ν−
g
q
q
ZET/
jet jet
jet
jet
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 7 / 35
Complexity of NLO calculation determinedby final-state multiplicity: a 2 → 5 process.
NLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timeline
1980 1985 1990 1995 2000 2005 2010
NLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timeline
1980 1985 1990 1995 2000 2005 2010
2→
1
1979: NLO Drell-Yan [Altarelli, Ellis & Martinelli]1991: NLO gg → Higgs [Dawson; Djouadi, Spira & Zerwas]
NLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timeline
1980 1985 1990 1995 2000 2005 2010
2→
1
2→
2
1987: NLO high-pt photoproduction [Aurenche et al]1988: NLO bb, tt [Nason et al]1988: NLO dijets [Aversa et al]1993: Vj [JETRAD, Giele, Glover & Kosower]
NLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timeline
1980 1985 1990 1995 2000 2005 2010
2→
1
2→
2
2→
3
1998: NLO Wbb [MCFM: Ellis & Veseli]2000: NLO Zbb [MCFM: Campbell & Ellis]2001: NLO 3j [NLOJet++: Nagy]· · ·
2007: NLO tt j [Dittmaier, Uwer & Weinzierl ’07]· · ·
NLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timeline
1980 1985 1990 1995 2000 2005 2010
2→
1
2→
2
2→
3
2→
4(W
/Z+3j,ttbb,ttjj,...)
NLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timeline
1980 1985 1990 1995 2000 2005 2010
2→
1
2→
2
2→
3
2→
4(W
/Z+3j,ttbb,ttjj,...)
2009: NLO W+3j [Rocket: Ellis, Melnikov & Zanderighi] [unitarity]2009: NLO W+3j [BlackHat+Sherpa: Berger et al] [unitarity]2009: NLO ttbb [Bredenstein et al] [traditional]2009: NLO ttbb [HELAC-NLO: Bevilacqua et al] [unitarity]2009: NLO qq → bbbb [Golem: Binoth et al] [traditional]2010: NLO tt jj [HELAC-NLO: Bevilacqua et al] [unitarity]2010: NLO Z+3j [BlackHat+Sherpa: Berger et al] [unitarity]. . .
NLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timelineNLO timeline
1980 1985 1990 1995 2000 2005 2010
2→
1
2→
2
2→
3
2→
4(W
/Z+3j,ttbb,ttjj,...)
2→
5(W
+4j,Z+4j)
automation
2→
6(ee→
7j[LC])
2010: NLO W+4j [BlackHat+Sherpa: Berger et al] [unitarity]2011/12: NLO WWjj [Rocket: Melia et al; GoSaM+MadX Greiner et al] [unitarity]2011: NLO Z+4j [BlackHat+Sherpa: Ita et al] [unitarity]2011/12: NLO 4j [BlackHat/NGluons+Sherpa: Bern et al; Badger et al] [unitarity]2011–: first automation [MadNLO: Hirschi et al] [unitarity + feyn.diags]2011–: first automation [Helac NLO: Bevilacqua et al] [unitarity]2011–: first automation [GoSam: Cullen et al] [feyn.diags(+unitarity)]2011: e+e− → 7j [Becker et al, leading colour] [numerical loops]
W + 0,1,2,3,4 jets @NLO je
ts)
[pb]
jet
N≥(W
+
σ
1
10
210
310
410 + jetsνl→W
=7 TeVsData 2010, ALPGENSHERPAPYTHIABLACKHAT-SHERPA
-1Ldt=36 pb∫ jets, R=0.4Tanti-k
|<4.4jet y>30 GeV, |T
jetp
ATLAS
jets
) [p
b]je
tN≥
(W +
σ
1
10
210
310
410
jetNInclusive Jet Multiplicity,
0≥ 1≥ 2≥ 3≥ 4≥
The
ory/
Dat
a
0
1
jetNInclusive Jet Multiplicity,
0≥ 1≥ 2≥ 3≥ 4≥
The
ory/
Dat
a
0
1
jetNInclusive Jet Multiplicity,
0≥ 1≥ 2≥ 3≥ 4≥
The
ory/
Dat
a
0
1
Technical revolution has gonehand-in-hand with LHCmeasurements of thesecomplex processes.
Powerful validation of NLOapproach.
So do SUSY searches nowjust compare data to NLO?
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 9 / 35
Two plots from a CMS SUSY analysis
Data v. Monte Carlo backgrounds Data v. “data-driven” backgrounds
So where are the NLO predictions being used?
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 10 / 35
The CMS search did not estimate Z+jets bkgd from NLO. Instead used
dσZ+jets
dHT
=
(
dσγ+jets
dHT
)
data
×
(
dσZ+jets
dHT
/dσγ+jets
dHT
)
NLO
Example of widelyused data-drivenbkgd estimates
Combine best oftheory knowledgewith best of exper-imental knowledge.
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 11 / 35
Merging NLO and showersand the CDF W + dijet anomaly
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 12 / 35
Remember the CDF W+dijet excess?
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
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500
600
700
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) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
Z+jets 2.8%QCD 5.1%
(c)
) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
Z+jets 2.8%QCD 5.1%
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
300
400
500
600
700
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
-50
0
50
100
150) -1Bkg Sub Data (4.3 fb
Gaussian
WW+WZ
) -1Bkg Sub Data (4.3 fb
Gaussian
WW+WZ
(a)
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 13 / 35
and the D0 W+dijet non-excess?
]2Dijet Mass [GeV/c0 50 100 150 200 250 300
)2E
ve
nts
/ (
10
Ge
V/c
0
200
400
600
800
1000
1200DataDibosonW+JetsZ+JetsTopMultijetsGaussian (4 pb)
2 = 145 GeV/cjjM
-1DØ, 4.3 fb
(a)
]2Dijet Mass [GeV/c0 50 100 150 200 250 300
)2E
ve
nts
/ (
10
Ge
V/c
-50
0
50
100
150
200
250
300Data - Bkgd
1 s.d.±Bkgd
DibosonGaussian (4 pb)
2 = 145 GeV/cjjM
-1DØ, 4.3 fb(b)
) = 0.5262χP(
CDF and DØ data are not being compared to NLO (=W+partons):
They are “detector-level” data and can only be compared tohadron-level calculations + detector simulation.
In this case hadron-level = Alpgen ⊗ Pythia
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 13 / 35
Perturbative expansion: for precision.Parton Showers (PS): for realism;
To combine them: must remove double counting
Tree-level (LO) + PSDifferent tree-level multiplicites (W, W+1j, W+2j, etc.) get combined
MLM/CKKW: Alpgen+Pythia/Herwig, MadGraph, Sherpa, . . .Fully automated
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 14 / 35
eventunderlying
π, K, p, etc.hadronisation
shower
u
proton proton
τ + τ −
H
u
u
hardproc.
αn + αn+1 + ...
τ + τ −
H
u
u
hardproc.
⊗
eventunderlying
π, K, p, etc.hadronisation
shower
u
proton proton
τ + τ −
H
u
u
hardproc.
π, K, p, etc.hadronisation
shower
NLO + PS — MC@NLO, POWHEGGreater accuracy, but harder to perform than LO+PS:
NLO contains more physics than LO,so more double-counting with parton shower
Less “available” than tree+PS: until recently,➥ A single (low) multiplicity, e.g. W@NLO + PS
➥ Programmed manually for each process
Recently: move towards automation:
POWHEGBox: tt+jet, W+W++2j, . . .aMC@NLO (MadLoop + auto MC@NLO): W+2j, Z+2b, . . .
+ ideas for combining multiplicities, extending their applicabilitye.g. MENLOPS, MINLO, FxFx merging, Sherpa merging, UNLOPS, . . .
One application of this progress has been to the W+dijet anomaly
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 15 / 35
CDF & DØ use Alpgen (scaled): tree level QCD + parton shower
adapted from Frederix et al 1110.5502
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 16 / 35
CDF & DØ use Alpgen (scaled): tree level QCD + parton shower
NLO has substantial shape differences: should we worry?
adapted from Frederix et al 1110.5502
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 16 / 35
CDF & DØ use Alpgen (scaled): tree level QCD + parton shower
NLO has substantial shape differences: should we worry?
NLO + parton shower (aMC@NLO) is close to Alpgen→ QCD under good control
adapted from Frederix et al 1110.5502
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 16 / 35
Instead of data−MC ⇒ data/MC
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
300
400
500
600
700
xxxxxx
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) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
Z+jets 2.8%QCD 5.1%
(c)
) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
Z+jets 2.8%QCD 5.1%
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
300
400
500
600
700
Dat
a / M
C
Mjj [GeV/c2]
Data extracted from
CD
F plots w
ith aid of g3data
CDF lνjj data (7.3 fb-1)MC = VV, V+j, ttbar, QCD
MC + Gaussian
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
100 200
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 17 / 35
Instead of data−MC ⇒ data/MC
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
300
400
500
600
700
xxxxxx
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) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
Z+jets 2.8%QCD 5.1%
(c)
) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
Z+jets 2.8%QCD 5.1%
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
300
400
500
600
700
Dat
a / M
C
Mjj [GeV/c2]
Data extracted from
CD
F plots w
ith aid of g3data
CDF lνjj data (7.3 fb-1)MC = VV, V+j, ttbar, QCD
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
100 200
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 17 / 35
Instead of data−MC ⇒ data/MC
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
300
400
500
600
700
xxxxxx
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) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
Z+jets 2.8%QCD 5.1%
(c)
) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
xxx
xxx
xxx
xxx
xxx
xxx
xxx
Z+jets 2.8%QCD 5.1%
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
300
400
500
600
700
Dat
a / M
C
Mjj [GeV/c2]
Data extracted from
CD
F plots w
ith aid of g3data
CDF lνjj data (7.3 fb-1)MC = VV, V+j, ttbar, QCD
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
100 200
aMC@NLOuncertainties: −→−→−→
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 17 / 35
Instead of data−MC ⇒ data/MC
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
300
400
500
600
700
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) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
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xxx
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Z+jets 2.8%QCD 5.1%
(c)
) -1CDF data (4.3 fbGaussian 2.5%WW+WZ 4.8%W+Jets 78.0%Top 6.3%
xxx
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Z+jets 2.8%QCD 5.1%
]2 [GeV/cjjM100 200
)2E
vent
s/(8
GeV
/c
0
100
200
300
400
500
600
700
Dat
a / M
C
Mjj [GeV/c2]
Data extracted from
CD
F plots w
ith aid of g3data
CDF lνjj data (7.3 fb-1)MC = VV, V+j, ttbar, QCD
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
100 200
aMC@NLOuncertainties: −→−→−→
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 17 / 35
“Anomaly” is a 10% effect(not clear it’s really a peak)
10% is clearly at limitof NLO accuracy
Going beyond limitations of NLO[two of the options]
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 18 / 35
High precision — NNLO — is crucial forkey processes, but not yet always available:
✓ W, Z, Higgs, γγ, VBF, VH, (tt)
✗ V V , (tt), inclusive jets, etc.
Important also to develop methods so thatwe’re less sensitive to limits on our precision.
Generally by finding ways to distinguish signalsfrom the background more efficiently, i.e.
increasing S/B .
NNLO: crucial for precision
New in 2010: NNLO VBF→H
σ (pb) at LHC√s = 7 TeV
scale choice:Q/4 ≤ µR,µF ≤ 4Q
LONLONNLO
10-1
1
10-1
1
σ(µR,µF)/σNNLO(Q)
0.92
0.96
1
1.04
1.08
100 150 200 250 300 350 400 450 500
mH(GeV)
Bolzoni, Maltoni, Moch & Zaro
New in 2011: NNLO WH (differential)
LO
NNLONLO
Ferrera, Grazzini & Tramontano
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 19 / 35
Most groundbreaking new NNLO calculation of past years:
qq → tt
Baernreuther, Czakon and Mitov 2012
First NNLO calculation with coloured particles in the initialand final state. Its new techniques may help open the way
to many other important NNLO calculations.
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 20 / 35
LO, NLO , NNLO res
Baernreuther, Czakon & Mitov NNLO qq → tt cross-section
NNLO: yet not always reassuring
New in 2011: NNLO γγ
LO
NLO
NNLO
Catani, Cieri, de Florian, Ferrera & Grazzini
Some key processes see large orgiant NLO/NNLO corrections.
Can’t help but wonder if we’remissing something,
especially in the gg → H case.
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 23 / 35
One series that’s ugly: gluon fusion Higgs cross section
For 8 TeV pp collisions, mH = 125 GeV:
σgg→H = 6.8 pb(
1 + 9.9αs + 36α2s + · · ·
)
= 6.8 pb (1 + 1.23 + 0.56 + · · · ) = 19.0 pb
for µR = µF = 12mH , αs(µR) = 0.124
σgg→H = 5.6 pb(
1 + 11.4αs + 63α2s + · · ·
)
= 5.6 pb (1 + 1.27 + 0.79 + · · · ) = 17.2 pb
for µR = µF = mH , αs(µR) = 0.112
There are explanations: threshold logarithms, π2 terms from analyticcontinuation. A problem is perhaps that there are too many explanations. . .
Baglio & Djouadi have raised the convergence issue before
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 24 / 35
Consequences of a bad series
Everything else you try about (gluon-fusion) Higgs production suffers as aresult of the original bad series: e.g. jet veto efficiency
FIXED ORDER PREDICTION
ε(p
t,ve
to)
pt,veto [GeV]
Higgs production (mH = 125 GeV), NNLO
pp, 7 TeVmH/4 < µR,F < mH
MSTW2008 NNLO PDFsanti-kt, R = 0.5
scheme a
scheme b
scheme c
0
0.2
0.4
0.6
0.8
1
10 100
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 25 / 35
Consequences of a bad series
Everything else you try about (gluon-fusion) Higgs production suffers as aresult of the original bad series: e.g. jet veto efficiency
FIXED ORDER PREDICTION
ε(p
t,ve
to)
pt,veto [GeV]
Higgs production (mH = 125 GeV), NNLO
pp, 7 TeVmH/4 < µR,F < mH
MSTW2008 NNLO PDFsanti-kt, R = 0.5
scheme a
scheme b
scheme c
0
0.2
0.4
0.6
0.8
1
10 100
RESUMMED PREDICTION
ε(p t
,vet
o)
gg → H, mH = 125 GeV
NNLO
NNLL+NNLO
HqT-rescaled POWHEG + Pythia 0.2
0.4
0.6
0.8
1
pp, 8 TeVmH /4 < µR,F , Q < mH , schemes a,b,cMSTW2008 NNLO PDFsanti-kt, R = 0.5Pythia partons, Perugia 2011 tune
ε(p t
,vet
o) /
ε cen
tral
(pt,v
eto)
pt,veto [GeV]
0.8
0.9
1
1.1
1.2
10 20 30 50 70 100
Banfi, Monni, GPS & Zanderighi ’12
see also Becher & Neubert ’12Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 25 / 35
Looking at data differently
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 26 / 35
H → bb (57% of decays) v. hard to see
Best hope is pp → W±H (and ZH), W±→ ℓ±ν, H → bb.
e,µ
b
νb
H
W
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 27 / 35
H → bb (57% of decays) v. hard to see
Best hope is pp → W±H (and ZH), W±→ ℓ±ν, H → bb.
pp → WH → ℓνbb + bkgds
ATLAS TDR 1999
Conclusion (ATLAS TDR):
“The extraction of a signal from H → bbdecays in the WH channel will be verydifficult at the LHC, even under the mostoptimistic assumptions [...]”
Low efficiency, huge backgrounds, e.g. tt
NB: Evidence of this channel seen recently
at Tevatron, but similar difficulties
e,µ
b
νb
H
W
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 27 / 35
H → bb (57% of decays) v. hard to see
Best hope is pp → W±H (and ZH), W±→ ℓ±ν, H → bb.
pp → WH → ℓνbb + bkgds
ATLAS TDR 1999
Conclusion (ATLAS TDR):
“The extraction of a signal from H → bbdecays in the WH channel will be verydifficult at the LHC, even under the mostoptimistic assumptions [...]”
Low efficiency, huge backgrounds, e.g. tt
Analysis of signal/bkgd suggests:
◮ Go to high pt (ptH , ptW > 200 GeV)◮ Lose 95% of signal, but more efficient?◮ Maybe kill tt & gain clarity?
W
H
bb
e,µ ν
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 27 / 35
pp → ZH → ννbb, @14TeV, mH=115GeV
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Cluster event, C/A, R=1.2
Butterworth, Davison, Rubin & GPS ’08
also earlier work by Seymour; Butterworth et al
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 28 / 35
pp → ZH → ννbb, @14TeV, mH=115GeV
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Fill it in, → show jets more clearly
Butterworth, Davison, Rubin & GPS ’08
also earlier work by Seymour; Butterworth et al
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 28 / 35
pp → ZH → ννbb, @14TeV, mH=115GeV
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Consider hardest jet, m = 150 GeV
Butterworth, Davison, Rubin & GPS ’08
also earlier work by Seymour; Butterworth et al
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 28 / 35
pp → ZH → ννbb, @14TeV, mH=115GeV
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
split: m = 150 GeV, max(m1,m2)m
= 0.92 → repeat
Butterworth, Davison, Rubin & GPS ’08
also earlier work by Seymour; Butterworth et al
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 28 / 35
pp → ZH → ννbb, @14TeV, mH=115GeV
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
split: m = 139 GeV, max(m1,m2)m
= 0.37 → mass drop
Butterworth, Davison, Rubin & GPS ’08
also earlier work by Seymour; Butterworth et al
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 28 / 35
pp → ZH → ννbb, @14TeV, mH=115GeV
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
check: y12 ≃pt2pt1
≃ 0.7 → OK + 2 b-tags (anti-QCD)
Butterworth, Davison, Rubin & GPS ’08
also earlier work by Seymour; Butterworth et al
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 28 / 35
pp → ZH → ννbb, @14TeV, mH=115GeV
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Rfilt = 0.3
Butterworth, Davison, Rubin & GPS ’08
also earlier work by Seymour; Butterworth et al
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 28 / 35
pp → ZH → ννbb, @14TeV, mH=115GeV
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Rfilt = 0.3: take 3 hardest, m = 117 GeV
Butterworth, Davison, Rubin & GPS ’08
also earlier work by Seymour; Butterworth et al
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 28 / 35
pp → ZH → ννbb, @14TeV, mH=115GeV
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Rfilt = 0.3: take 3 hardest, m = 117 GeV
Butterworth, Davison, Rubin & GPS ’08
also earlier work by Seymour; Butterworth et al
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 28 / 35
ATLAS and CMS H → bb are high-pt, but 2-jet based
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 29 / 35
Some taggers and jet-substructure observables
Jet Declustering
Jet Shapes
Matrix−Element
Seymour93
YSplitter
Mass−Drop+Filter
JHTopTagger TW
CMSTopTagger
N−subjettiness (TvT)
CoM N−subjettiness (Kim)
N−jettiness
HEPTopTagger(+ dipolarity)
Trimming
Pruning
Planar Flow
Twist
ATLASTopTagger
Templates
Shower Deconstruction
Qjets
EEC
Multi−variate tagger
apologies for omitted taggers, arguable links, etc.
[NB: many of the tools available in FastJet & SpartyJet]
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 30 / 35
Handles for distinguishing signal v. background
their colour (q v. g)
radiation offprongs sensitive to
z
(1−z)
boosted X
softer prong mom. fraction z
radiation off X sensitive to its colour charge
large−angle (>> 2m/pt)
g→gg(g) q→qg(g) g→bb H
→bb t→qqq
softer prong z soft soft hard hard hard
prong colour factors 2×CA CF+CA 2×CF 2×CF 3×CF
system colour factor CA CF CA 0 CF
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 31 / 35
Boosted Ws and tops in single jets: data!
W’s in a single jet
with Pruning + Mass Drop requirement
NB: combined in IR unsafe way. . .
tops in a single jet
with HEPTopTagger
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 32 / 35
Some BSM searches with jet-substructure techniques
A range of techniques being used for varied BSM scenarios
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 33 / 35
Closing
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 34 / 35
0
0.2
0.4
0.6
0.8
1fr
act
ion
of A
TL
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& C
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pa
pe
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Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 35 / 35
0
0.2
0.4
0.6
0.8
1fr
act
ion
of A
TL
AS
& C
MS
pa
pe
rs th
at ci
te th
em Papers commonly cited by ATLAS and CMS
as of 2012−06−28, from ’papers’, excluding self−citations
Plo
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ase
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t alg
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6 P
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Her
wig
6 M
CA
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EN
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Q6.
6 P
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MS
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08 P
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MC
@N
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MM
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2010
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Mad
Gra
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Her
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today’s progress in QCD = tomorrow’s workhorse
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 35 / 35
EXTRAS
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 36 / 35
NLO bottleneck: 1-loop part
TraditionalDraw all Feynman diagrams with 1loop. Work out formulae for them.
Work hard to reduce integrals toknown forms (+ tricks).
Recursive/unitarity methodsAssemble loop-diagrams from indi-vidual tree-level diagrams.
Build trees by sticking togethersimpler tree-level diagrams
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 37 / 35
NLO bottleneck: 1-loop part
TraditionalDraw all Feynman diagrams with 1loop. Work out formulae for them.
Work hard to reduce integrals toknown forms (+ tricks).
Recursive/unitarity methodsAssemble loop-diagrams from indi-vidual tree-level diagrams.
Build trees by sticking togethersimpler tree-level diagrams
Some main ideas:
Bern, Dixon & Kosower ’93[sewing together trees]
Britto, Cachazo & Feng ’04[on-shell complex loop momenta]
Ossola, Pittau & Papadopoulos ’06[handful of loop momentum choices givefull amplitude]
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 37 / 35
CDF Wjj: difference wrt MC v. ratio to MC
CDF difference
Dat
a -
MC
(E
vent
s/(8
GeV
/c2 ))
Mjj [GeV/c2]
Data extracted from
CD
F plots w
ith aid of g3data
CDF lνjj data (7.3 fb-1)
-150
-100
-50
0
50
100
150
200
100 200
CDF ratio
Dat
a / M
C
Mjj [GeV/c2]
Data extracted from
CD
F plots w
ith aid of g3data
CDF lνjj data (7.3 fb-1)MC = VV, V+j, ttbar, QCD
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
100 200
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 38 / 35
Wjj ratio to MC, DØ v. CDF
DØ ratio
Dat
a / M
C
Mjj [GeV/c2]
Data extracted from
CD
F plots w
ith aid of g3data
D0 (4.3 fb-1)MC = VV, V+j, ttbar, QCD
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
100 200
CDF ratio
Dat
a / M
C
Mjj [GeV/c2]
Data extracted from
CD
F plots w
ith aid of g3data
CDF lνjj data (7.3 fb-1)MC = VV, V+j, ttbar, QCD
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
100 200
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 39 / 35
0
0.2
0.4
0.6
0.8
1fr
actio
n of
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LAS
& C
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cite
them
Papers commonly cited by ATLAS and CMSas of 2012-02-18, from ’papers’, excluding self-citations
Plo
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ased
on
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from
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LAS
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i-kt j
et a
lg.
CT
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PD
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CT
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Her
wig
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eavy
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wig
++
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Tun
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hia
8.1
Gavin Salam (CERN) Perturbative QCD in hadron collisions SILAFAE 2012-12-10 40 / 35