Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
QCD and the LHC
Gavin Salam
CERN, Princeton & LPTHE/CNRS (Paris)
Rutgers University, February 15 2011
The scales at play[Introduction]
The scales at play[Introduction]
tµ τneutrinos e
ud s cb
The scales at play[Introduction]
Nuc
l. P
hys.
Che
mis
try
tµ τneutrinos e
ud s cb
The scales at play[Introduction]
1 century
Nuc
l. P
hys.
Che
mis
try
Fla
v. fa
ctor
ies
Tev
atro
n
neutrinoexperiments
tµ τneutrinos e
ud s cb
The scales at play[Introduction]
1 century
Nuc
l. P
hys.
Che
mis
try
Fla
v. fa
ctor
ies
Tev
atro
n
neutrinoexperiments
LHC
tµ τneutrinos e
ud s cb
The scales at play[Introduction]
happens here...get clues as to what
The hope:
matter−antimatter asymmetry+ other questions, e.g.
nature of dark matter/energy
Nuc
l. P
hys.
Che
mis
try
Fla
v. fa
ctor
ies
Tev
atro
n
neutrinoexperiments
LHC
tµ τneutrinos e
ud s cb
The scales at play[Introduction]
cosmic rays, astrophysics, cosmologyg−2, dark matter searches, proton decay, ...
happens here...get clues as to what
The hope:
matter−antimatter asymmetry+ other questions, e.g.
nature of dark matter/energy
Nuc
l. P
hys.
Che
mis
try
Fla
v. fa
ctor
ies
Tev
atro
n
neutrinoexperiments
LHC
tµ τneutrinos e
ud s cb
The scales at play[Introduction]
cosmic rays, astrophysics, cosmologyg−2, dark matter searches, proton decay, ...
happens here...get clues as to what
The hope:
matter−antimatter asymmetry+ other questions, e.g.
nature of dark matter/energy
Nuc
l. P
hys.
Che
mis
try
Fla
v. fa
ctor
ies
Tev
atro
n
neutrinoexperiments
LHC
tµ τneutrinos e
ud s cb
extr
a
dim
ensi
ons
The Large Hadron Collider[Introduction]
The world’s largest fundamentalphysics endeavour
Designed to be 7× more powerfulthan its predecessor, Tevatron
Involving O (10 000) scientistsFrom about 60 countries
At a cost of several billion US$
This talk is about one of the facets of discovery at the LHC:
The use of Quantum Chromodynamics, i.e. the theory that governsthe behaviour of quarks and gluons
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 3 / 36
What does discovery look like?
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 4 / 36
What kinds of searches?[Introduction]
mass peak
dσ
/ dm
[log
sca
le]
mass
Signal
QCDprediction
New resonance (e.g. Z ′) where you see alldecay products and reconstruct an invari-ant mass
pp Z’
Sufficiently large, sharp signal emerges in-dependently of any knowledge of back-grounds.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 5 / 36
What kinds of searches?[Introduction]
New resonance (e.g. Z ′) where you see alldecay products and reconstruct an invari-ant mass
pp
µ+
µ−
Z’
Sufficiently large, sharp signal emerges in-dependently of any knowledge of back-grounds.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 5 / 36
What kinds of searches?[Introduction]
high−mass excess
dσ
/ dm
[log
sca
le]
mass
Signal
QCDprediction
UnreconstructedSUSY cascade.Study effective mass(sum of all transversemomenta).
~g
~g~g
~q
~q
χ0
χ0
g
q
q
q
q
g
Broad excess at high mass scales.
Knowledge of backgrounds is crucial isdeclaring discovery.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 5 / 36
What kinds of searches?[Introduction]
[GeV]effm
0 200 400 600 800 1000 1200 1400
En
trie
s / 1
00
Ge
V
-110
1
10
210
310 = 7 TeV)sData 2010 (Standard ModelmultijetsW+jetsZ+jetstt
single topDibosons
=2801/2=360 m0MSUGRA mµ1 lepton: e,
-1L dt ~ 35 pb∫
ATLAS
UnreconstructedSUSY cascade.Study effective mass(sum of all transversemomenta).
~g
~g~g
~q
~q
χ0
χ0
g
q
q
q
q
g
Broad excess at high mass scales.
Knowledge of backgrounds is crucial isdeclaring discovery.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 5 / 36
dσ
/ dm
[log
sca
le]
mass
Signal ?
CONTINUEHERE
dσ
/ dm
[log
sca
le]
mass
Signal ?
STARTHERE
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 6 / 36
dσ
/ dm
[log
sca
le]
mass
Signal ?
CONTINUEHERE
dσ
/ dm
[log
sca
le]
mass
Signal ?
STARTHERE
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 6 / 36
dσ
/ dm
[log
sca
le]
mass
Signal ?
CONTINUEHERE
dσ
/ dm
[log
sca
le]
mass
Signal ?
STARTHERE
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 6 / 36
How do you predict a background?
The LHC collides protons — made of quarks and gluons.
The theory that governs the behaviour of quarks andgluons is Quantum Chromodynamics (QCD)
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 7 / 36
QCD[QCD]
QCD, an SU(3) Yang-Mills theory, gives the rules for the interaction ofquark fields (ψ) with gluon fields (A), and gluon fields with themselves.Most simply expressed in terms of the Lagrangian
L = ψ̄a(iγµ∂µδab − gsγµtCabACµ −m)ψb −1
4FµνA F
Aµν
FAµν
= ∂µAAν − ∂νAAν − gs fABCABµACν [tA, tB ] = ifABC tC
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 8 / 36
The QCD coupling[QCD]
One key parameter: the strength ofthe interaction, i.e. the strongcoupling ‘constant’ αs = g
2s /4παs = g2s /4παs = g2s /4π
αs(Q) ≃1
b0 lnQ2/Λ2
Nobel: Gross, Politzer & Wilczek
◮ strong interactions at proton (GeV) scale
◮ weak interactions at LHC (TeV)scales:
αs(1 TeV) ≃ 0.09
QCD α (Μ ) = 0.1184 ± 0.0007s Z
0.1
0.2
0.3
0.4
0.5
αs (Q)
1 10 100Q [GeV]
Heavy Quarkoniae+e– AnnihilationDeep Inelastic Scattering
July 2009
Bethke ’09
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 9 / 36
QCD predictions[QCD]
Given small coupling, the most widespread approach to
making QCD predictions is perturbation theory.E.g. using Feynman diagrams; basic rules for QCD known for ∼40 years
recursive techniques for trees (late 80’s); unitarity for loops (90’s and 00’s)
A cross section σ is written as a series in powers of αs:
σ =︷ ︸︸ ︷
σ2 α2s︸ ︷︷ ︸
leading order (LO)
+ σ3 α3s + σ4 α
4s
︸ ︷︷ ︸
NNLO
+ · · ·
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 10 / 36
QCD predictions[QCD]
Given small coupling, the most widespread approach to
making QCD predictions is perturbation theory.E.g. using Feynman diagrams; basic rules for QCD known for ∼40 years
recursive techniques for trees (late 80’s); unitarity for loops (90’s and 00’s)
A cross section σ is written as a series in powers of αs:
σ =
next-to-leading order(NLO)︷ ︸︸ ︷
σ2 α2s︸ ︷︷ ︸
leading order (LO)
+ σ3 α3s + σ4 α
4s
︸ ︷︷ ︸
NNLO
+ · · ·
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 10 / 36
QCD predictions[QCD]
Given small coupling, the most widespread approach to
making QCD predictions is perturbation theory.E.g. using Feynman diagrams; basic rules for QCD known for ∼40 years
recursive techniques for trees (late 80’s); unitarity for loops (90’s and 00’s)
A cross section σ is written as a series in powers of αs:
σ =
next-to-leading order(NLO)︷ ︸︸ ︷
σ2 α2s︸ ︷︷ ︸
leading order (LO)
+ σ3 α3s + σ4 α
4s
︸ ︷︷ ︸
NNLO
+ · · ·
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 10 / 36
Controlling QCD for discovery[QCD]
We can only approximate QCD (e.g. LO, NLO, etc.).
Discovery comes if you have an excess with respect to a QCDprediction accounting for its uncertainties.
Standard Model
0.01
0.1
1
10
100
1000
200 400 600 800 1000
dσ/d
p t [p
b/G
eV]
pt [GeV]
QCD predictionand uncertainty
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 11 / 36
Controlling QCD for discovery[QCD]
We can only approximate QCD (e.g. LO, NLO, etc.).
Discovery comes if you have an excess with respect to a QCDprediction accounting for its uncertainties.
New Physics
0.01
0.1
1
10
100
1000
200 400 600 800 1000
dσ/d
p t [p
b/G
eV]
pt [GeV]
QCD predictionand uncertainty
data
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 11 / 36
Controlling QCD for discovery[QCD]
We can only approximate QCD (e.g. LO, NLO, etc.).
Discovery comes if you have an excess with respect to a QCDprediction accounting for its uncertainties.
New Physics
0.01
0.1
1
10
100
1000
200 400 600 800 1000
dσ/d
p t [p
b/G
eV]
pt [GeV]
QCD predictionand uncertainty
data
New Physics?
0.01
0.1
1
10
100
1000
200 400 600 800 1000
dσ/d
p t [p
b/G
eV]
pt [GeV]
QCD predictionand uncertainty
data
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 11 / 36
Controlling QCD for discovery[QCD]
We can only approximate QCD (e.g. LO, NLO, etc.).
Discovery comes if you have an excess with respect to a QCDprediction accounting for its uncertainties.
New Physics
0.01
0.1
1
10
100
1000
200 400 600 800 1000
dσ/d
p t [p
b/G
eV]
pt [GeV]
QCD predictionand uncertainty
data
New Physics?
0.01
0.1
1
10
100
1000
200 400 600 800 1000
dσ/d
p t [p
b/G
eV]
pt [GeV]
QCD predictionand uncertainty
data
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 11 / 36
A non-issue?
Even for a basic leading-order approximation, with αs ≃ 0.1,expect ∼ 10% uncertainty from missing NLO(?)
How well does QCD perturbative series converge?[QCD]
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
K ≃ 1.2
gluon
proton proton
quark
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 12 / 36
How well does QCD perturbative series converge?[QCD]
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
K ≃ 1.2
Look at p t ofquark or gluon (jets)
gluon
proton proton
quark
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 12 / 36
How well does QCD perturbative series converge?[QCD]
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
K ≃ 1.2
Look at p t ofquark or gluon (jets)
gluon
proton proton
quark
K of 1.2 is compatible with being 1 +O (αs)
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 12 / 36
How well does QCD perturbative series converge?[QCD]
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
K ≃ 1.5
MCFM
Look at p t ofZ−boson Z−boson
proton proton
quark
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 12 / 36
How well does QCD perturbative series converge?[QCD]
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
K ≃ 1.5
MCFM
Look at p t ofZ−boson Z−boson
proton proton
quark
K of 1.5 is compatible with being 1 + C × αs, with quite large C
To date, no generalised understanding of size of C when in range 5− 10
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 12 / 36
How well does QCD perturbative series converge?[QCD]
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
K ≃ 5
MCFM
Look at p t ofquark (jet) Z−boson
proton proton
quark
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 12 / 36
How well does QCD perturbative series converge?[QCD]
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
K ≃ 5
MCFM
Look at p t ofquark (jet) Z−boson
proton proton
quark
K of 5?!!! Found by several groups.Butterworth, Davison, Rubin & GPS ’08
Bauer & Lange ’09; Denner et al ’09
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 12 / 36
How well does QCD perturbative series converge?[QCD]
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
HT
,jets
[fb
/ 100
GeV
]
HT,jets [GeV]
pp, 14 TeV
LO
NLO
K ≃ 100
MCFM
Look at sum of p t ofquarks and gluons (jets)
Z−boson
proton proton
quark
Rubin, GPS & Sapeta ’10
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 12 / 36
How well does QCD perturbative series converge?[QCD]
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
HT
,jets
[fb
/ 100
GeV
]
HT,jets [GeV]
pp, 14 TeV
LO
NLO
K ≃ 100
MCFM
Look at sum of p t ofquarks and gluons (jets)
Z−boson
proton proton
quark
Rubin, GPS & Sapeta ’10
What can it possibly mean to do perturbation theory if the “O (αs)”NLO correction is so much larger than LO?
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 12 / 36
So what happens at the next order, NNLO?[QCD]
The last examples are somewhat extreme. In such cases, it’s fair toask the question:
What happens at the next order? Does QCD converge?
Despite over 10 years’ work by many people, not a single NNLOcalculation exists for a hadron-collider process with coloured particlesin the final state.
Several groups working on NNLO for this and related processes; maybeZ+jet jet process will be completed in a year or two or . . . ?
Gehrmann family, Glover, Heinrich & al
Weinzierl
Somogyi, Trocsanyi, Del Duca, et al.
Czakon, Mitov, et al.
etc.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 13 / 36
Why the large K -factor from LO to NLO?[QCD]
Leading Order
αsαEW
Next-to-Leading Order
α2sαEW
LHC probes scales well above EW scale,√s ≫ MZ .
EW bosons are light.New (logarithmically enhanced) topologies appear.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 14 / 36
In absence of a full calculation atNNLO, try to gain physical insight
Why the large K -factor from LO to NLO?[QCD]
Leading Order
αsαEW
Next-to-Leading Order
α2sαEW
LHC probes scales well above EW scale,√s ≫ MZ .
EW bosons are light.New (logarithmically enhanced) topologies appear.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 14 / 36
Why the large K -factor from LO to NLO?[QCD]
Leading Order
αsαEW
Next-to-Leading Order
α2sαEW
LHC probes scales well above EW scale,√s ≫ MZ .
EW bosons are light.New (logarithmically enhanced) topologies appear.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 14 / 36
Why the large K -factor from LO to NLO?[QCD]
Leading Order
αsαEW
Next-to-Leading Order
α2sαEW ln2 ptMZ
LHC probes scales well above EW scale,√s ≫ MZ .
EW bosons are light.New (logarithmically enhanced) topologies appear.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 14 / 36
Why the large K -factor from LO to NLO?[QCD]
Leading Order
αsαEW
Next-to-Leading Order
α2sαEW ln2 ptMZ
LHC probes scales well above EW scale,√s ≫ MZ .
EW bosons are light.New (logarithmically enhanced) topologies appear.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 14 / 36
Why the large K -factor from LO to NLO?[QCD]
Leading Order
αsαEW
Next-to-Leading Order
α2sαEW ln2 ptMZ
LHC probes scales well above EW scale,√s ≫ MZ .
EW bosons are light.New (logarithmically enhanced) topologies appear.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 14 / 36
Physical understanding → better predictions[QCD]
Since dominant contribution comes from Z+2partons, try combining NLOZ+parton with NLO Z+2partons. LoopSim: Rubin, GPS & Sapeta ’10
ptptpt of jet 1
0
2
4
6
8
10
250 500 750 1000 1250
K-f
acto
r w
rt L
O
pt,j1 (GeV)
MCFM 5.7, CTEQ6Mpp, 14 TeV
anti-kt, R=0.7pt,j1 > 200 GeV
LONLO–nNLO (µ dep)–nNLO (RLS dep)
Never achieved previously
We call this n̄NLO
Not quite full NNLO
But has many of its qualities
◮ ptj distribution seems toconverge at n̄NLO
◮ scale uncertainties reduced by∼ factor 2
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 15 / 36
Physical understanding → better predictions[QCD]
Since dominant contribution comes from Z+2partons, try combining NLOZ+parton with NLO Z+2partons. LoopSim: Rubin, GPS & Sapeta ’10
HT, jets =∑
jets pt,jHT, jets =∑
jets pt,jHT, jets =∑
jets pt,j
1
10
100
1000
500 1000 1500 2000 2500
K-f
acto
r w
rt L
O
HT,jets (GeV)
MCFM 5.7, CTEQ6Mpp, 14 TeVanti-kt, R=0.7pt,j1 > 200 GeV
LONLO
–nNLO (µ dep)–nNLO (RLS dep)
Never achieved previously
We call this n̄NLO
Not quite full NNLO
But has many of its qualities
◮ Signficiant furtherenhancement for HT ,jets
◮ n̄NLO brings clear message:
HT ,jetsHT ,jetsHT ,jets is not a goodobservable!
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 15 / 36
0.01
0.1
1
10
100
1000
200 400 600 800 1000
dσ/d
p t [p
b/G
eV]
pt [GeV]
QCD predictionand uncertainty
data
Really New Physics?
What went into the QCDprediction?
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 16 / 36
As if “giant” K -factors weren’t bad enough. . .
How about infinite K -factors?
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 17 / 36
Quarks & gluons? You never see them[Jets]
q
q
Start off with quark and anti-quark, qq̄
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 18 / 36
Quarks & gluons? You never see them[Jets]
In perturbative quantum chromody-namics (QCD), probability that aquark or gluon emits a gluon:
∼ dEE
dθ
θ
Diverges for small gluon energies E
Diverges for small angles θ
θ
q
q
A quark never survives unchangedit always emits a gluon (usually low-energy, at small angles)
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 18 / 36
Quarks & gluons? You never see them[Jets]
In perturbative quantum chromody-namics (QCD), probability that aquark or gluon emits a gluon:
∼ dEE
dθ
θ
Diverges for small gluon energies E
Diverges for small angles θ
q
q
Each gluon radiates a further gluon
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 18 / 36
Quarks & gluons? You never see them[Jets]
In perturbative quantum chromody-namics (QCD), probability that aquark or gluon emits a gluon:
∼ dEE
dθ
θ
Diverges for small gluon energies E
Diverges for small angles θ
q
q
And so forth
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 18 / 36
Quarks & gluons? You never see them[Jets]
In perturbative quantum chromody-namics (QCD), probability that aquark or gluon emits a gluon:
∼ dEE
dθ
θ
Diverges for small gluon energies E
Diverges for small angles θ
q
q
Meanwhile the same happens on other side of event
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 18 / 36
Quarks & gluons? You never see them[Jets]
In perturbative quantum chromody-namics (QCD), probability that aquark or gluon emits a gluon:
∼ dEE
dθ
θ
Diverges for small gluon energies E
Diverges for small angles θ
q
q
And then a non-perturbative transition occurs
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 18 / 36
Quarks & gluons? You never see them[Jets]
q
q
π, K, p, ...
Giving a pattern of hadrons that “remembers” the gluon branchingHadrons mostly produced at small angle wrt qq̄ directions or with low energy
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 18 / 36
Quarks & gluons? You never see them[Jets]
jet
jet
q
q
π, K, p, ...
Giving a pattern of hadrons that “remembers” the gluon branchingHadrons mostly produced at small angle wrt qq̄ directions or with low energy
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 18 / 36
Quarks & gluons? You never see them[Jets]
jet
jet
q
q
π, K, p, ...
Giving a pattern of hadrons that “remembers” the gluon branchingHadrons mostly produced at small angle wrt qq̄ directions or with low energy
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 18 / 36
Instead of quarks and gluons, one sees “jets” of hadrons.
How can you compare a calculation of quarks and gluonsto a measurement of hadrons?
What happens to the divergences that arise inperturbative QCD beyond leading order?
Jets made systematic: jet definitions[Jets]
jet 1 jet 2
LO partons
Jet Def n
jet 1 jet 2
Jet Def n
NLO partons
jet 1 jet 2
Jet Def n
parton shower
jet 1 jet 2
Jet Def n
hadron level
π π
K
p φ
LHC events may be discussed in terms of quarks, quarks+gluon, or hadrons
A jet definition provides common representation of different “levels” ofevent complexity.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 19 / 36
QCD jets flowchart[Jets]
Jet (definitions) provide central link between expt., “theory” and theory
And jets are an input to almost all analyses
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 20 / 36
QCD jets flowchart[Jets]
Jet (definitions) provide central link between expt., “theory” and theory
And jets are an input to almost all analyses
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 20 / 36
A $100 000 000, 20-year old problem[Jets]
A significant community of QCD theorists has spent the past ten yearsmaking accurate calculations of signals and backgrounds at the LHC (withremarkable advances in field theory on the way)
O (100) people × 10 years ≃ $100 000 000
Problem 1: the jet definitions previously used by LHC experiments werenot compatible with these calculations — they “leaked” infinities:
σ = c1 αs + c2 α2s +∞∞∞α3s + · · ·
Problem 2: the jet definitions advocated by theorists since 1990’s hadbeen mostly shunned by proton-collider experiments
a) bad response to experimental noiseb) severe computational issues (1 minute/event ×1010 recorded events)
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 21 / 36
Solving the jets problem[Jets]
Discovered a link between QCD jet-finding and problems
of 2D computational geometryCacciari & GPS ’05
Many techniques could be carried over from comp. geom field
Developed a theory of the interplay between jet-finding,QCD radiation and experimental noise
Cacciari, GPS & Soyez ’08
A crucial element was linearity of response
Proposed a new jet-definition based on what we’d learnt
anti-ktCacciari, GPS & Soyez ’08
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 22 / 36
[Jets]
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
Island Beach State Park
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.0492063
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.052381
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.0904762
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.103175
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.14127
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.169841
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.181222
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.421097
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.478454
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.947691
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} = 0.949818
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} > 1
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
DeltaR_{ij} > 1
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
You can cluster anything[Jets]
pt/GeV
40
20
00 1 2 3 4 y
30
10
How anti-kt works:
◮ Define pairwise i–j distances
dij = min
(
1
p2ti,1
p2tj
)
∆R2ij
◮ Define single-particle distances
diB =1
p2ti
◮ If smallest is dij merge iand j
◮ If smallest is diB call i a jet
A non-intuitive successor to
kt alg of Catani et al. ’91
How does anti-kt fare?[Jets]
Timing v. particle multiplicity 2005
10-4
10-3
10-2
10-1
1
101
102
100 1000 10000 100000
t / s
N
KtJe
t k t
CDF M
idPoin
t (see
ds >
0 GeV
)
CDF M
idPoin
t (seed
s > 1 G
eV)
CDF J
etClu (
very u
nsafe)
3.4 GHz P4, 2 GB
R=0.7
Tevatron LHC lo-lumi LHC hi-lumi LHC Pb-Pb
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 24 / 36
How does anti-kt fare?[Jets]
Timing v. particle multiplicity 2008
10-4
10-3
10-2
10-1
1
101
102
100 1000 10000 100000
t / s
N
KtJe
t k t
CDF M
idPoin
t (see
ds >
0 GeV
)
CDF M
idPoin
t (seed
s > 1 G
eV)
CDF J
etClu (
very u
nsafe)
FastJe
t
Seedl
ess IR
Safe C
one
(SISC
one)
Cam/Aac
hen
R=0.7
anti-k t
LHC lo-lumi LHC hi-lumi LHC Pb-Pb
k t
in critical region of N ∼ 2000− 40001000 times faster than previous attempts with similar jet algorithms
FastJet code available publicly at http://fastjet.fr/
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 24 / 36
http://fastjet.fr/
How does anti-kt fare?[Jets]
Experimental sensitivity to noise
As good as, orbetter than allpreviousexperimentally-favouredalgorithms.
Essentially becauseanti-kt has linearresponse to softparticles.
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 24 / 36
How does anti-kt fare?[Jets]
Coefficient of “infinity”
10-5 10-4 10-3 10-2 10-1 1
Fraction of hard events failing IR safety test
JetClu
SearchCone
PxCone
MidPoint
Midpoint-3
Seedless [SM-pt]
Seedless [SM-MIP]
Anti-Kt, SISCone
50.1%
48.2%
16.4%
15.6%
9.3%
1.6%
0.17%
0 (none in 4x109)
Safe for perturbative QCDpredictions:
No “leakage” of infinitiesto higher orders
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 24 / 36
Anti-kt contained in FastJet[Jets]
A program that brings together theoretical physics and computationalgeometry. Cacciari, GPS & Soyez ’06-
In 2009: about 40 000 page loads from 3 300 IP addresses, in & 1000 locations.
[After exclusion of robots]
ATLAS & CMS use anti-kt for all their jet-finding[Jets]
ATLAS & CMS use anti-kt for all their jet-finding[Jets]
Among the handful of LHC searches so far, anti-kt jets have probedthe highest scales, ∼ 2 TeV, about twice as high as Tevatron.
Anti-kt solves a long-standing
problem, crucial in providing acommon language to compare
theory and experiment. dσ
/ dm
[log
sca
le]
mass
Signal ?
But is QCD really about noth-ing other than comparing the-ory and experiment? dσ
/ dm
[log
sca
le]
mass
Signal ?
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 27 / 36
The quest for the Higgs
Using jets better, in order to make discoveries possible
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 28 / 36
Higgs mass and Higgs decays?[Discovery]
[GeV]HM50 100 150 200 250 300 350 400
[G
eV]
top
m
150
155
160
165
170
175
180
185
190
WAtop band for mσ1
WAtop
68%, 95%, 99% CL fit contours excl. m
LE
P 9
5% C
L
Tev
atro
n 9
5% C
L
WAtop
68%, 95%, 99% CL fit contours incl. m
68%, 95%, 99% CL fit contours incl. WA and direct Higgs searchestopm
[GeV]HM50 100 150 200 250 300 350 400
[G
eV]
top
m
150
155
160
165
170
175
180
185
190
G fitter SM
Nov 10
Likely Higgs Mass
There’s some likelihood thatthe Higgs boson will be“light”, MH ∼ 120 GeV
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 30 / 36
Higgs mass and Higgs decays?[Discovery]
[GeV]HM50 100 150 200 250 300 350 400
[G
eV]
top
m
150
155
160
165
170
175
180
185
190
WAtop band for mσ1
WAtop
68%, 95%, 99% CL fit contours excl. m
LE
P 9
5% C
L
Tev
atro
n 9
5% C
L
WAtop
68%, 95%, 99% CL fit contours incl. m
68%, 95%, 99% CL fit contours incl. WA and direct Higgs searchestopm
[GeV]HM50 100 150 200 250 300 350 400
[G
eV]
top
m
150
155
160
165
170
175
180
185
190
G fitter SM
Nov 10
Likely Higgs Mass
BR(H)
bb_
τ+τ−
cc_
gg
WW
ZZ
tt-
γγ Zγ
MH [GeV]50 100 200 500 1000
10-3
10-2
10-1
1
Higgs decayproducts v. MHMHMH
There’s some likelihood thatthe Higgs boson will be“light”, MH ∼ 120 GeV
If it is, crucial test of whetherit is the Higgs, will comefrom measuring several dif-ferent decays
Remember: Higgs couplings
intimately related to origin
of particle masses
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 30 / 36
H → bb̄ (main light-Higgs decay) v. hard to see[Discovery]
Best hope is pp → W±H, W± → ℓ±ν, H → bb̄.
e,µ
b
νb
H
W
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 31 / 36
H → bb̄ (main light-Higgs decay) v. hard to see[Discovery]
Best hope is pp → W±H, W± → ℓ±ν, H → bb̄.
pp → WH → ℓνbb̄ + bkgds
ATLAS TDR
Conclusion (ATLAS TDR):
“The extraction of a signal from H → bb̄decays in the WH channel will be verydifficult at the LHC, even under the mostoptimistic assumptions [...]”
Low efficiency, huge backgrounds, e.g. tt̄
e,µ
b
νb
H
W
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 31 / 36
H → bb̄ (main light-Higgs decay) v. hard to see[Discovery]
Best hope is pp → W±H, W± → ℓ±ν, H → bb̄.
pp → WH → ℓνbb̄ + bkgds
ATLAS TDR
Conclusion (ATLAS TDR):
“The extraction of a signal from H → bb̄decays in the WH channel will be verydifficult at the LHC, even under the mostoptimistic assumptions [...]”
Low efficiency, huge backgrounds, e.g. tt̄
Try a long shot?
◮ Go to high pt (ptH , ptW > 200 GeV)◮ Lose 95% of signal, but more efficient?◮ Maybe kill tt̄ & gain clarity?
W
H
bb
e,µ ν
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 31 / 36
H → bb̄ (main light-Higgs decay) v. hard to see[Discovery]
Best hope is pp → W±H, W± → ℓ±ν, H → bb̄.
pp → WH → ℓνbb̄ + bkgds
ATLAS TDR
Conclusion (ATLAS TDR):
“The extraction of a signal from H → bb̄decays in the WH channel will be verydifficult at the LHC, even under the mostoptimistic assumptions [...]”
Low efficiency, huge backgrounds, e.g. tt̄
Try a long shot?
◮ Go to high pt (ptH , ptW > 200 GeV)◮ Lose 95% of signal, but more efficient?◮ Maybe kill tt̄ & gain clarity?
W
H
bb
e,µ ν
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 31 / 36
Question:
What’s the best strategy to identify thetwo-pronged structure of the boosted
Higgs decay?
3 QCD principles help guide our analysis[Discovery]
◮ QCD radiation from a boosted Higgs decay is limited by
angular ordering
◮ Higgs decay shares energy symmetrically, QCDbackground events with same mass share energy
asymmetrically
◮ QCD radiation from Higgs decay products is point-like,noise (UE, pileup) is diffuse
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 32 / 36
pp → ZH → νν̄bb̄, @14TeV, mH=115GeV[Discovery]
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Cluster event, C/A, R=1.2
Butterworth, Davison, Rubin & GPS ’08
SIGNAL
Zbb BACKGROUND
arbitrary norm.G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 33 / 36
pp → ZH → νν̄bb̄, @14TeV, mH=115GeV[Discovery]
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Fill it in, → show jets more clearly
Butterworth, Davison, Rubin & GPS ’08
SIGNAL
Zbb BACKGROUND
arbitrary norm.G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 33 / 36
pp → ZH → νν̄bb̄, @14TeV, mH=115GeV[Discovery]
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Consider hardest jet, m = 150 GeV
Butterworth, Davison, Rubin & GPS ’08
SIGNAL
0
0.05
0.1
0.15
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
Zbb BACKGROUND
0
0.002
0.004
0.006
0.008
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
arbitrary norm.G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 33 / 36
pp → ZH → νν̄bb̄, @14TeV, mH=115GeV[Discovery]
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
SIGNAL
0
0.05
0.1
0.15
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
Zbb BACKGROUND
0
0.002
0.004
0.006
0.008
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
arbitrary norm.G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 33 / 36
pp → ZH → νν̄bb̄, @14TeV, mH=115GeV[Discovery]
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
SIGNAL
0
0.05
0.1
0.15
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
Zbb BACKGROUND
0
0.002
0.004
0.006
0.008
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
arbitrary norm.G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 33 / 36
pp → ZH → νν̄bb̄, @14TeV, mH=115GeV[Discovery]
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
SIGNAL
0
0.05
0.1
0.15
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
Zbb BACKGROUND
0
0.002
0.004
0.006
0.008
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
arbitrary norm.G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 33 / 36
pp → ZH → νν̄bb̄, @14TeV, mH=115GeV[Discovery]
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Rfilt = 0.3
Butterworth, Davison, Rubin & GPS ’08
SIGNAL
0
0.05
0.1
0.15
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
Zbb BACKGROUND
0
0.002
0.004
0.006
0.008
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
arbitrary norm.G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 33 / 36
pp → ZH → νν̄bb̄, @14TeV, mH=115GeV[Discovery]
Herwig 6.510 + Jimmy 4.31 + FastJet 2.3
Rfilt = 0.3: take 3 hardest, m = 117 GeV
Butterworth, Davison, Rubin & GPS ’08
SIGNAL
0
0.05
0.1
0.15
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
Zbb BACKGROUND
0
0.002
0.004
0.006
0.008
80 100 120 140 160mH [GeV]
200 < ptZ < 250 GeV
arbitrary norm.G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 33 / 36
How well-designed jet-finding helps you[Discovery]
Search for main decay of light Higgs boson, W/Z+H, H → bb̄(The only way of seeing this decay — other than the next slide)
using the method from Butterworth, Davison, Rubin & GPS ’08
Other recent work focuses on v. high background rejection (lower efficiency)
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 34 / 36
How well-designed jet-finding helps you[Discovery]
Recovering the ttH, H → bb̄ Higgs channel
Plehn, GPS & Spannowsky ’09
based in part on Johns Hopkins top tagger ’08
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 34 / 36
How well-designed jet-finding helps you[Discovery]
Dijet mass reconstruction for new heavy resonance X → gg
1/N
dn/
dbin
/ 2
dijet mass [GeV]
gg, M = 2000 GeV
arXiv:0810.1304
0
0.02
0.04
0.06
0.08
1900 2000 2100
kt, R=0.4Qwf=0.13 = 190 GeV
1/N
dn/
dbin
/ 2
dijet mass [GeV]
gg, M = 2000 GeV
arXiv:0810.1304
0
0.02
0.04
0.06
0.08
1900 2000 2100
C/A-filt, R=1.3Qwf=0.13 = 53 GeV
(subtr.)
Cacciari, Rojo, GPS & Soyez ’08
Other recent work: Krohn, Thaler & Wang ’09
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 34 / 36
How well-designed jet-finding helps you[Discovery]
Supersymmetry with R-parity violating decays χ̃01 → qqqOne of its most difficult incarnations
0
10000
20000
30000
40000
50000
60000
70000
0 50 100 150 200
m1/(
100G
eV
) dN
/dbin
per
fb-1
m1 [GeV]
Herwig 6.5 + Jimmy 4.3
Cam/Aachen R=0.7pt1 > 500 GeV
signal + background
background (just dijets)
signal
0
200
400
600
800
0 50 100 150 200m
1/(
100G
eV
) dN
/dbin
per
fb-1
m1 [GeV]
Cam/Aachen + filt, R=0.7pt1 > 500 GeV, zsplit > 0.15, mbc > 0.25 mabc
pt3, pt4 > {100,80} GeV, |∆y13|, |∆y14| < 1.5
signal + background
background (just dijets)
signal
Butterworth, Ellis, Raklev & GPS ’09
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 34 / 36
How well-designed jet-finding helps you[Discovery]
Supersymmetry with R-parity violating decays χ̃01 → qqqOne of its most difficult incarnations
0
10000
20000
30000
40000
50000
60000
70000
0 50 100 150 200
m1/(
100G
eV
) dN
/dbin
per
fb-1
m1 [GeV]
Herwig 6.5 + Jimmy 4.3
Cam/Aachen R=0.7pt1 > 500 GeV
signal + background
background (just dijets)
signal
0
200
400
600
800
0 50 100 150 200m
1/(
100G
eV
) dN
/dbin
per
fb-1
m1 [GeV]
Cam/Aachen + filt, R=0.7pt1 > 500 GeV, zsplit > 0.15, mbc > 0.25 mabc
pt3, pt4 > {100,80} GeV, |∆y13|, |∆y14| < 1.5
signal + background
background (just dijets)
signal
Butterworth, Ellis, Raklev & GPS ’09
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 34 / 36
Establishing the rules for systematically makingbetter discoveries with jets is work in progress
But the evidence for its potential is clearly there
How well-designed jet-finding helps you[Discovery]
Supersymmetry with R-parity violating decays χ̃01 → qqqOne of its most difficult incarnations
0
10000
20000
30000
40000
50000
60000
70000
0 50 100 150 200
m1/(
100G
eV
) dN
/dbin
per
fb-1
m1 [GeV]
Herwig 6.5 + Jimmy 4.3
Cam/Aachen R=0.7pt1 > 500 GeV
signal + background
background (just dijets)
signal
0
200
400
600
800
0 50 100 150 200m
1/(
100G
eV
) dN
/dbin
per
fb-1
m1 [GeV]
Cam/Aachen + filt, R=0.7pt1 > 500 GeV, zsplit > 0.15, mbc > 0.25 mabc
pt3, pt4 > {100,80} GeV, |∆y13|, |∆y14| < 1.5
signal + background
background (just dijets)
signal
Butterworth, Ellis, Raklev & GPS ’09
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 34 / 36
Conclusions
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 35 / 36
Conclusions[Conclusions]
QCD is unavoidable at LHC
The simplicity of its formulation contrasts with the
richness of its phenomenology
Even after 20 years of planning for the LHC,QCD still has surprises for us
Both in its own rightAnd as a tool for discovery
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 36 / 36
EXTRAS
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 37 / 36
JetClu (& Atlas Cone) in Wjj @ NLO[Extras]
W
jet jet
α2sαEW α3sαEW α
3sαEW
1-jet +∞+∞+∞2-jet O (1) −∞ 0
With these (& most) cone algorithms, perturbative infinities fail tocancel at some order ≡≡≡ IR unsafety
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 38 / 36
JetClu (& Atlas Cone) in Wjj @ NLO[Extras]
W
jet jet
soft divergence
W
jet jet
α2sαEW α3sαEW α
3sαEW
1-jet +∞+∞+∞2-jet O (1) −∞ 0
With these (& most) cone algorithms, perturbative infinities fail tocancel at some order ≡≡≡ IR unsafety
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 38 / 36
JetClu (& Atlas Cone) in Wjj @ NLO[Extras]
W
jet
W
jet jet
soft divergence
W
jet jet
α2sαEW α3sαEW α
3sαEW
1-jet +∞+∞+∞2-jet O (1) −∞ 0
With these (& most) cone algorithms, perturbative infinities fail tocancel at some order ≡≡≡ IR unsafety
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 38 / 36
JetClu (& Atlas Cone) in Wjj @ NLO[Extras]
W
jet
W
jet jet
soft divergence
W
jet jet
α2sαEW α3sαEW α
3sαEW
1-jet +∞+∞+∞2-jet O (1) −∞ 0
With these (& most) cone algorithms, perturbative infinities fail tocancel at some order ≡≡≡ IR unsafety
G. Salam (CERN/Princeton/LPTHE) QCD and the LHC 38 / 36
IntroductionQCDJetsDiscoveryConclusionsExtras