high-q2 physics at the lhc · high-q2 physics at the lhc frank krauss institute for theoretical...
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Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
High-Q2 physics at the LHC
Frank Krauss
Institute for Theoretical PhysicsTU Dresden
DIS 2006, KEK, 23.4.2006
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Outline
1 Introduction
2 Knowledge of the initial state: PDFs
3 Knowledge of QCD evolution: Jets
4 Knowledge of background topologies: Multijets
5 Knowledge of non-factorizable QCD: Underlying event
6 Summary
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Reminder: Physics @ LHCMany interesting signals:Higgs (or alternative EWSB), SUSY, ED’s, . . .
But: Severe backgrounds in nearly all channels,(almost always with large influence of QCD)
=⇒ depend on detailed understanding of QCD.
Examples:
Central jet-veto in VBF (Higgs)
Multi-jet backgrounds for SUSY (e.g. Z+jets)
Todays signals = tomorrows backgrounds.
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Scope of this talkYES: maybe lots of interesting physics at LHC
BUT: (nearly) nothing comes for free:
A signal is what remains after background subtraction.How well do we understand bread-and-butter physics?
SO: I won’t talk too much about Higgs, BSM, etc..
I’ll talk about our understanding of QCDas far as I can tell.
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Fitting PDFs
MotivationPDFs are basic input for cross section calculations
PDFs so far mainly determined from DIS data
Uncertainties remain, typically around 5%-10%,larger for instance in high-Q2 gluons
Prepare for inclusion of LHC data in fits
Ongoing projects: FastNLO & NLO@Gridsee talks by M.Wobisch and D.Clement
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Fitting PDFs
Input of hadron collidersPDFs usually measured in DIS,Input of hadron colliders: gluon @ high Q2.
Sensitivity through
σpp→X (s) =
∫dx1dx2fi (x1, Q2
F )fj(x2, Q2F )σij→X (s, µ2
R) ,
where fi,j = PDFs, s ∼ x1x2s.
Idea for extraction of PDF: Compare σ @ NLO with data.
Problem: Duration of NLO calculation.
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Fitting PDFs
Quality of theory vs. data
1
10
10 2
10 102
103
inclusive jet productionin hadron-induced processes
fastNLOwww.cedar.ac.uk/fastnlo
DIS
pp-bar
√s = 300 GeV
√s = 630 GeV
√s = 1800 GeV
√s = 1960 GeV
100 < Q2 < 500 GeV2
500 < Q2 < 10 000 GeV2
H1 150 < Q2 < 200 GeV2
H1 200 < Q2 < 300 GeV2
H1 300 < Q2 < 600 GeV2
ZEUS 125 < Q2 < 250 GeV2
ZEUS 250 < Q2 < 500 GeV2
H1 600 < Q2 < 3000 GeV2
ZEUS 500 < Q2 < 1000 GeV2
ZEUS 1000 < Q2 < 2000 GeV2
ZEUS 2000 < Q2 < 5000 GeV2
DØ |y| < 0.5
CDF 0.1 < |y| < 0.7DØ 0.0 < |y| < 0.5DØ 0.5 < |y| < 1.0
CDF cone algorithmCDF kT algorithm
(× 100)
(× 40)
(× 8)
(× 3)
(× 1)
all pQCD calculations by fastNLO:αs(MZ)=0.118 | CTEQ6.1 PDFs | µr = µf = pT
DIS in NLO | pp in NLO + NNLO-NLL | plus non-perturbative corrections
pT (GeV/c)
data
/ th
eory
Data from CDF, PRD64 (2001) 012001
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Fitting PDFs
Idea for accelerationhep-ph/0510324 by T.Carli, G.Salam, F.Siegert
Replace integration by summation (MC integration):
σ =∑
x1, x2, Q2
Wσ · WPDF
Bin σ in 2 dims =⇒ Wσ
Bin fi (x1, Q2F )fj(x2, Q2
F ) in 3 dims =⇒ WPDF
Interpolate in between
Pre-calculate Wσ and use for fast evaluation/fitting of PDF
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Fitting PDFs
FastNLOFlattening the PDFs
10-7
10-6
10-5
10-4
10-3
10-2
10-1
11010 2
-2 -1 0
10-410-3 10-2 10-1 0.5 0.9
x ⊗
PD
F(x)
10-7
10-6
10-5
10-4
10-3
10-2
10-1
11010 2
-2 -1 0
10-410-3 10-2 10-1 0.5 0.9
10-7
10-6
10-5
10-4
10-3
10-2
10-1
11010 2
-2 -1 0
10-410-3 10-2 10-1 0.5 0.9
10-3
10-2
10-1
1
-2 -1 0
10-410-3 10-2 10-1 0.5 0.9
w(x
) ⊗ P
DF(
x)
10-3
10-2
10-1
1
-2 -1 0
10-410-3 10-2 10-1 0.5 0.9
10-3
10-2
10-1
1
-2 -1 0
10-410-3 10-2 10-1 0.5 0.9
xparton
CTEQ6.1M
µf = 5 GeVµf = 50 GeVµf = 500 GeVµf = 5000 GeV
original (unweighted) PDFs
reweighted PDFs: w(x) = x−3/2 (1 − 0.99 x)3
gluon sum of quarks sum of anti-quarks
Accuracy of Grid interpolation
0.996
0.998
1
1.002
1.004
200 400
0.0 < η < 0.5
fast
NLO
app
roxi
mat
ion
/ ref
eren
ce
0.996
0.998
1
1.002
1.004
200 400
1.0 < η < 1.5
0.996
0.998
1
1.002
1.004
200 400
0.5 < η < 1.00.996
0.998
1
1.002
1.004
200 400
1.5 < η < 2.00.996
0.998
1
1.002
1.004
100 150 200
2.0 < η < 3.0
ET (GeV)
No. bins to cover the x-range: 8 x-bins10 x-bins12 x-bins16 x-bins
pp-bar: incl. jets at sqrt(s)=1.8 TeVDØ Collaboration hep-ex/0011036
NLO@GridAccuracy of Grid interpolation
[GeV]TE500 1000 1500 2000 2500 3000 3500 4000 4500 5000
[GeV]TE500 1000 1500 2000 2500 3000 3500 4000 4500 5000
0.9999
0.99995
1
1.00005
1.0001
1.00015
1.0002
Impact on gluon PDF
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Defining jets
Motivation(Nearly) all physics signals are with jets:
VBF: Two forward “tag”-jetsgluinos: 4 jets + ET/
Large systematic uncertainties (steeply falling spectra)
Need to define jet with good properties(better than “bunch of hadronic energy”)
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Defining jets
Theory vs. data from TevatronData from D0, PRL 94 (2005) 221801, plot by M.Zielinski
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
pT max > 180 GeV (×8000)130 < pT
max < 180 GeV (×400)100 < pT
max < 130 GeV (×20) 75 < pT
max < 100 GeV
NLOSHERPAHERWIG
D0
10-3
10-2
10-1
1
10
10 2
10 3
10 4
10 5
0.5 0.6667 0.8333 1
π/2 2π/3 5π/6 π
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Defining jets
Algorithms
Cone algorithms (iterative, midpoint)Basic idea/algorithm
Find high ET binsCone in φ-η around them with radius R
Cluster to jet (add momenta)Differences in treatment of overlapping cones
k⊥-algorithmsBasic idea/algorithm
Define “k⊥-metric” (with “radius” R)Cluster until k
ij⊥≤ kcrit
⊥
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Defining jets
Work by D.Benedetti et al., in hep-ph/0604120
CriteriaAngular distance αi
jp:∆R(pi
, j i ) of i th parton p to its jet j
Energy difference β ijp:
Distance in σ from fitted curve
(GeV)jetTE
0 20 40 60 80 100 120 140 160 180 200
part
on/E
jet
E
0
0.2
0.4
0.6
0.8
1
1.2
maxα0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
max
jpβ
0
1
2
3
4
5
6well clustered events
badly clustered events
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Defining jets: Testing the iterative cone-algorithm
Selected/well-clustered & selected events in t tH
jet cone radius0.2 0.3 0.4 0.5 0.6 0.7 0.8
Sel
ectio
n E
ffic
ienc
y (%
)
10
20
30
40
50
60
70
80
90
100
2 jets4 jets6 jets8 jets
jet cone radius0.2 0.3 0.4 0.5 0.6 0.7 0.8
Frac
Goo
d (%
)1
102 jets4 jets6 jets8 jets
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Defining jets: Testing the k⊥-algorithm
Selected/well-clustered & selected events in t tH
R-parameter0.2 0.4 0.6 0.8 1
Sel
ectio
n E
ffic
ienc
y (%
)
10
20
30
40
50
60
70
80
90
100
2 jets4 jets6 jets8 jets
R-parameter0.2 0.4 0.6 0.8 1
Frac
Goo
d (%
)1
10 2 jets4 jets6 jets8 jets
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: Motivation
Higgs searches Central jet veto in VBF
Signal/background ratiodepends on central jet veto.(rapidity gap between two “tagging jets”,
=⇒ beautiful signal at leading order)
But: How many jets come athigher orders?=⇒ currently studied.
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: Motivation
SUSY searches
Large σprod
Many hard jets.
Meff =∑
phard⊥
.
Quick Discovery?
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: MC-Comparison
Introducing SHERPAT.Gleisberg, S.Hoche, F.K., A.Schalicke, S.Schumann and J.C.Winter, JHEP 0402 (2004) 056
New event generator in C++;
Matrix elements @ LO,combined with parton shower
(S.Catani et al. JHEP 0111 (2001) 063
F.K., JHEP 0208 (2002) 015);
Hadronization by Pythia;
Underlying event a la Pythia(old version), showers added.
resummed in PS
exact MELO 5jet, but alsoNLO 4jet
L
αn
mNLLexact ME
LO 4jet
4
4
4
4
4
5
5
5
5
5
5
5
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: MC-Comparison
p⊥ of Z -bosons in pp → Z + X @ TevatronData from CDF, Phys. Rev. Lett. 84 (2000) 845
/ GeV Z P0 20 40 60 80 100 120 140 160 180 200
10-3
10-2
10-1
1
10pt Z
Z + 0 jet
Z + 1 jetZ + 2 jetCDF
GeVpb
/
dPσd
/ GeV Z P0 5 10 15 20 25 30 35 40 45 50
G
eVpb /
dP
σd
1
10
pt Z
Z + 0 jet
Z + 1 jetZ + 2 jetCDF
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: MC-Comparison
Jet rates in pp → Z + X @ Tevatron(D0-Note 5066)
0 1 2 3 4 5 6
1
10
210
310
410
0 1 2 3 4 5 6
1
10
210
310
410
data w/stat errordata w/stat & sys errorPythia range statPythia range stat & sys
D0 RunII Preliminary
Jet Multiplicity
Nr.
of E
vent
s
0 1 2 3 4 5 60.2
1
234
Jet Multiplicity
Dat
a / P
YTH
IA
0 1 2 3 4 5 6
1
10
210
310
410
0 1 2 3 4 5 6
1
10
210
310
410
data w/stat errordata w/stat & sys errorSherpa range statSherpa range stat & sys
D0 RunII Preliminary
Jet Multiplicity
Nr.
of E
vent
s
0 1 2 3 4 5 60.2
1
234
Jet Multiplicity
Dat
a / S
HE
RP
A
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: MC-Comparison
Jet spectra (1st jet) in pp → Z + X @ Tevatron(D0-Note 5066)
50 100 150 200 250 300 350
1
10
210
310
50 100 150 200 250 300 350
1
10
210
310
data w/stat errordata w/stat & sys errorPythia range statPythia range stat & sys
D0 RunII Preliminary
jet [GeV]st 1T
p
Nr.
of E
vent
s
50 100 150 200 250 300 3500.2
1
234
jet [GeV]st 1T
p
Dat
a / P
YTH
IA
50 100 150 200 250 300 350
1
10
210
310
50 100 150 200 250 300 350
1
10
210
310
data w/stat errordata w/stat & sys errorSherpa range statSherpa range stat & sys
D0 RunII Preliminary
jet [GeV]st 1p
Nr.
of E
vent
s
50 100 150 200 250 300 3500.2
1
234
jet [GeV]st 1T
p
Dat
a / S
HE
RP
A
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: MC-Comparison
Jet spectra (2nd jet) in pp → Z + X @ Tevatron(D0-Note 5066)
20 40 60 80 100 120 140 160 180
1
10
210
310
20 40 60 80 100 120 140 160 180
1
10
210
310data w/stat errordata w/stat & sys errorPythia range statPythia range stat & sys
D0 RunII Preliminary
jet [GeV]nd 2T
p
Nr.
of E
vent
s
20 40 60 80 100 120 140 160 1800.2
1
234
jet [GeV]nd 2T
p
Dat
a / P
YTH
IA
20 40 60 80 100 120 140 160 180
1
10
210
310
20 40 60 80 100 120 140 160 180
1
10
210
310data w/stat errordata w/stat & sys errorSherpa range statSherpa range stat & sys
D0 RunII Preliminary
jet [GeV]nd 2T
p
Nr.
of E
vent
s
20 40 60 80 100 120 140 160 1800.2
1
234
jet [GeV]nd 2T
p
Dat
a / S
HE
RP
A
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: MC-Comparison
Jet spectra (3rd jet) in pp → Z + X @ Tevatron(D0-Note 5066)
20 40 60 80 100 120
1
10
210
20 40 60 80 100 120
1
10
210
data w/stat errordata w/stat & sys errorPythia range statPythia range stat & sys
D0 RunII Preliminary
jet [GeV]rd 3T
p
Nr.
of E
vent
s
20 40 60 80 100 1200.2
1
234
jet [GeV]rd 3T
p
Dat
a / P
YTH
IA
20 40 60 80 100 120
1
10
210
20 40 60 80 100 120
1
10
210
data w/stat errordata w/stat & sys errorSherpa range statSherpa range stat & sys
D0 RunII Preliminary
jet [GeV]rd 3T
p
Nr.
of E
vent
s
20 40 60 80 100 1200.2
1
2345
jet [GeV]rd 3T
p
Dat
a / S
HE
RP
A
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: MC-Comparison
φ1.jet,2.jet in pp → Z + X @ Tevatron(D0-Note 5066)
0 0.5 1 1.5 2 2.5 30
50
100
150
200
250
0 0.5 1 1.5 2 2.5 30
50
100
150
200
250 data w/stat errordata w/stat & sys errorPythia range statPythia range stat & sys
D0 RunII Preliminary
(jet,jet)φ ∆
Nr.
of E
vent
s
0 0.5 1 1.5 2 2.5 30.2
1
234
(jet,jet)φ ∆
Dat
a / P
YTH
IA
0 0.5 1 1.5 2 2.5 30
50
100
150
200
250
0 0.5 1 1.5 2 2.5 30
50
100
150
200
250data w/stat errordata w/stat & sys errorSherpa range statSherpa range stat & sys
D0 RunII Preliminary
(jet,jet)φ ∆
Nr.
of E
vent
s
0 0.5 1 1.5 2 2.5 30.2
1
234
(jet,jet)φ ∆
Dat
a / S
HE
RP
A
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: MC-Comparison
Extrapolation to LHC: pZ ,jet⊥
in Z+ jets
0 50 100 150 200 250 300 350 400pT (e+e-) [GeV]
10-6
10-5
10-4
10-3
10-2
10-1
1/σ
dσ/d
p T [1
/GeV
]
MC@NLOPythiaSherpa
e+e- + X @ LHC
50 100 150 200 250 300pT (first jet) [GeV]
10-3
10-2
10-1
100
101
dσ/d
p T [p
b/G
eV]
MC@NLOPythiaSherpaSherpa 2jet
e+e- + X @ LHC
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Multijets: MC-Comparison
Extrapolation to LHC: pZ ,jet⊥
in Z+ jets
20 40 60 80 100 120 140pT (second jet) [GeV]
10-3
10-2
10-1
100
101
dσ/d
p T [p
b/G
eV]
MC@NLOPythiaSherpaSherpa 2jet
e+e- + X @ LHC
20 40 60 80 100pT (third jet) [GeV]
10-3
10-2
10-1
100
dσ/d
p T [p
b/G
eV]
MC@NLOPythiaSherpaSherpa 2jetSherpa 3jet
e+e- + X @ LHC
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Underlying event
In the following: Data from CDF, PRD 65 (2002) 092002, plots partially from C.Buttar
Multiple parton interactionsModel:
Incoherent parton-parton interactions
Ordered in p⊥ (or similar)=⇒ scales with Q2 of hard interaction
Proton AntiProton
Multiple Parton Interactions
PT(hard)
Outgoing Parton
Outgoing Parton
Underlying EventUnderlying Event
Observables
Charged Jet #1
Direction
∆φ∆φ∆φ∆φ
�Transverse� �Transverse�
�Toward�
�Away�
�Toward-Side� Jet
�Away-Side� Jet
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Underlying event
Compare 3 regionsNchg versus PT(charged jet#1)
0
2
4
6
8
10
12
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<N
ch
g>
in
1 G
eV
/c b
in
1.8 TeV |ηηηη|<1.0 PT>0.5 GeV/c
"Toward"
"Away"
"Transverse"
CDFdata uncorrected
PTsum versus PT(charged jet#1)
0
10
20
30
40
50
0 5 10 15 20 25 30 35 40 45 50
PT(charged jet#1) (GeV/c)
<P
Ts
um
> (
Ge
V/c
) in
1 G
eV
/c b
in
1.8 TeV |ηηηη|<1.0 PT>0.5 GeV/c
"Toward"
"Away"
"Transverse"
CDFdata uncorrected
p⊥ @ Tevatron, transverse
"Transverse" PT Distribution (charged)
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
1.0E+01
0 2 4 6 8 10 12 14
PT(charged) (GeV/c)
dN
ch
g/d
PT
(1
/Ge
V/c
)
CDFdata uncorrected
1.8 TeV |ηηηη|<1
PT(chgjet1) > 2 GeV/c
PT(chgjet1) > 5 GeV/c
PT(chgjet1) > 30 GeV/c
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Underlying event
From Tevatron . . .
2
4
6
8
CDF data
PYTHIA6.214 - ATLAS (CTEQ5L)
JIMMY4.1 - UE (CTEQ6L)
PYTHIA6.323 - UE (CTEQ6L)
< N
chg >
- tr
ansv
erse
reg
ion
0
1
2
0 10 20 30 40 50Pt leading jet (GeV)
Rat
io (M
C/d
ata)
. . . to LHC . . .
2
4
6
8
10
12
0 10 20 30 40 50
CDF data
PYTHIA6.214 - ATLAS (CTEQ5L)
JIMMY4.1 - UE (CTEQ6L)
PYTHIA6.323 - UE (CTEQ6L)
LHC prediction
Pt leading jet (GeV)
< N
chg >
- tr
ansv
erse
reg
ion
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
Underlying event
. . . or is it rather this?
F. Krauss TUD
High-Q2 physics at the LHC
Introduction Fitting PDFs Defining Jets Multijets Underlying event Summary
SummarySuccess of LHC probably depends on detailedunderstanding of QCD
The first few years of LHC running are a great time forQCD-addicts
There is leeway for an improved understanding of QCD -on all levels between theory and experiment
There are still puzzles and problems to be resolved -
from technicalities:sufficient precision in PDFs, jets and their definitions,multijets (a personal selection)to basics:underlying event, interplay of soft & hard QCD . . .
F. Krauss TUD
High-Q2 physics at the LHC