jet physics at the lhc...2015/10/02 · higgs simplified cross sectionshiggs + jets: [les houches...
TRANSCRIPT
Jet Physics at the LHC
Frank Tackmann
Deutsches Elektronen-Synchrotron
DESY Theory WorkshopOctober 02, 2015
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 0 / 27
Introduction
Introduction.
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 0 / 27
Introduction
Why Jets?
QCD doesn’t let us observe quarks and gluons directly, only jets of hadrons
ud
u
u
u
u
u
u
u
u
uu
d
u
u
u
Jets tell us the QCD final state of the hard interaction process
36 years ago:
e−
e+
q
q
qe−
e+g
q
Today: Essentially the same (just a bit more complicated ...)
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 1 / 27
Introduction
Why Jets?
QCD doesn’t let us observe quarks and gluons directly, only jets of hadrons
ud
u
u
u
u
u
u
u
u
uu
d
u
u
u
Jets tell us the QCD final state of the hard interaction process
36 years ago:
e−
e+
q
q
qe−
e+g
q
Today: Essentially the same (just a bit more complicated ...)
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 1 / 27
Introduction
Executive Summary of Higgs Production.
∼ 2/3 of Higgs bosons are produced at low pT
0 jets
g
g
H
t
0-1 jets
g
g
H
t
g
∼ 1/3 of Higgs bosons have sizeable pT0 (normal) jets
H
Vq
q
V
1 jet
g
g
H
t
2 (quark) jetsq
H
q
q
qV
Kinematics and number of jets distinguishes different Higgs processesDiscriminates against different backgrounds (e.g. in H→WW, ττ, bb)
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 2 / 27
Introduction
Why Else Jets?
Jets are Ubiquitous
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 3 / 27
Introduction
Why Else Jets?
Jets are essential also in EW final states
Recall BR(W→ qq′) = 67%
BR(Z → qq) = 70%
Diboson excess is in dijet mass spectrumof two filtered W/Z-tagged jets[ATLAS 1506.00962] 1.5 2 2.5 3 3.5
Eve
nts
/ 1
00
Ge
V
1−10
1
10
210
310
410Data
Background model
1.5 TeV EGM W', c = 1
2.0 TeV EGM W', c = 1
2.5 TeV EGM W', c = 1
Significance (stat)
Significance (stat + syst)
ATLAS-1 = 8 TeV, 20.3 fbs
WZ Selection
[TeV]jjm1.5 2 2.5 3 3.5
Sig
nific
ance
2−
1−
0123
Search strategies increasingly rely onexploiting jet substructure
Boosted decays: top-jets, W/Z-jets, Higgs-jets b
b
H
Distinguish quark jets (from BSM cascades)from gluon jets (QCD backgrounds)
q q ℓ− ℓ+
g q χ2 ℓ+ χ1
Jet mass, shape, charge, tracks, ...
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 4 / 27
Introduction
Why Else Jets?
Jets are essential also in EW final states
Recall BR(W→ qq′) = 67%
BR(Z → qq) = 70%
Diboson excess is in dijet mass spectrumof two filtered W/Z-tagged jets[ATLAS 1506.00962] 1.5 2 2.5 3 3.5
Eve
nts
/ 1
00
Ge
V
1−10
1
10
210
310
410Data
Background model
1.5 TeV EGM W', c = 1
2.0 TeV EGM W', c = 1
2.5 TeV EGM W', c = 1
Significance (stat)
Significance (stat + syst)
ATLAS-1 = 8 TeV, 20.3 fbs
WZ Selection
[TeV]jjm1.5 2 2.5 3 3.5
Sig
nific
ance
2−
1−
0123
Search strategies increasingly rely onexploiting jet substructure
Boosted decays: top-jets, W/Z-jets, Higgs-jets b
b
H
Distinguish quark jets (from BSM cascades)from gluon jets (QCD backgrounds)
q q ℓ− ℓ+
g q χ2 ℓ+ χ1
Jet mass, shape, charge, tracks, ...Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 4 / 27
Introduction
Outline.
I can only cover a few selected topics:Higgs and jetsJet hierarchiesNew jet algorithm: XConeQuark-gluon separation
QCD corrections in jet processes are typically sizeable(N)NLO calculations are often necessary→ see Giulia Zanderighi’s talk
Jets involve multiple physical scalesFixed-order perturbation theory is often not enoughRequire understanding all-order structure, resummation, hadronic effects,...
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 5 / 27
Higgs + Jets
Higgs + Jets.
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 5 / 27
Higgs + Jets
Differential Higgs+Jets Measurements.
dσ/dpjetT
0 20 40 60 80 100 120 140
[pb
/GeV
]j1 T
p /
dσd
2−10
1−10
1
XH + STWZ
XH + JetVHeto
Hbb + Htt + VH = VBF + XH
H→pp ATLAS
data, tot. unc. syst. unc.
-1 = 8 TeV, 20.3 fbs
0≥ jetsN = 0.4, R tkanti-
[GeV]j1
Tp
0 20 40 60 80 100 120 140
ST
WZ
Rat
io to
0
1
2
3
σ(N jets)
0 1 2 3 4 5 6
[pb
]σ
1
10
210
XH8 + Y+PNLOPSNXH8 + Y+PMG5_aMC@NLO
XH + HERPA 2.1.1SXHSTWZ +
XHBLPTW +
Hbb + Htt + VH = VBF + XH
H→pp ATLASdata, tot. unc. syst. unc.
-1 = 8 TeV, 20.3 fbs
> 30 GeVjet
Tp = 0.4, R tkanti-
jetsN
1≥ 2≥ 3≥ = 0 = 1 = 2N
NLO
PS
Rat
io to
0
2
4
Combined high-resolution H → γγ and H → 4` [ATLAS arXiv:1504.05833]
Still statistics limited, but theory uncertainties will become relevant with∼20 times more statistics (∼ end of Run2)Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 6 / 27
Higgs + Jets
Higgs Simplified Cross Sections.[Les Houches & LHC Higgs WG2]
Production cross sections split into kinematic bins→ most involve jets
H→ττ· · ·· · ·· · ·
H→
γγ
VBF loose (MVA)
high pTt
VBF tight (MVA)
low pTt
ttH leptonic
VH leptonic
H→ZZ· · ·· · ·· · ·
= 0-jet
H→
WW
= 1-jet
≥ 2-jet VBF cuts
H→
bb
MVA high pT (V )
MVA low pT (V )
· · ·
· · ·
· · ·
σ(ttH)
σ(V
H) low pV
T
high pVT
very high pVT
ratios of Γγγ ΓZZ ΓWW Γbb Γττ (ΓZγ Γµµ)
= 0-jet
≥ 1-jet
σ(bbH) σ(tH)
Rest
σ(V
BF)
≥ 2-jet VBF cuts
high-q2 BSM
≃ 2-jet
& 3-jet
= 0-jet
≥ 1-jet
σ(g
gF)
≥ 2-jet VBF cuts· · ·· · ·
· · ·· · ·
· · ·µi, κi
gk
EFTcoeffs
specificBSM
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 7 / 27
Higgs + Jets
Jet Binning.
g
g
H
t
g
g
H
t
g
g
g
H
t
pTpcutT
dσ/dpT
σ0(pcutT ) σ≥1(p
cutT )
Spectrum describes transition between 0-jet and ≥ 1-jet regionsNeed consistent treatment of theory uncertainties across entire spectrum
I quite nontrivial because it requires nontrivial correlations(simple factor-2-scale-variation-recipes are not good enough)
I Understood much better by now, but still open issues to figure out
Complete description from combining resummation+fixed orderFrank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 8 / 27
Higgs + Jets
Rapidity-Weighted Jet Bins.[Gangal, Stahlhofen, FT]
Generalize pjetT by defining
Tfj = pTj f(yj)
T jetf = max
j∈J(R)Tfj
Can choose different rapidityweighting functions such that T jet
f
I can be resummedI is insensitive to forward rapidities
Count jets according to Tfj0 jets: σ0(T jet
f <T cut)
≥ 1 jets: σ≥1(T jetf >T cut)
⇒ Provides different slicing through jetphase space
00
1
1 2 3 4
0.2
0.4
0.6
0.8
−1−2−3−4
TBj
TCj
pT j
yj
f(y
j)
jet
jet
00
0
1
1
2
20
40
60
80
100
120
0.5 1.5 2.5yj
pTj[G
eV]
pTj ≤ 30 GeV
TBj ≤ 30 GeV
TCj ≤ 15 GeV
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 9 / 27
Higgs + Jets
Resolving Jet Bins.(only using H → γγ here)
Pretend uncertainties are much smallerand we want to resolve the “excess”
Different slices of jet phase space addsnontrivial information
One might conclude that “excess” sits athigher pjet
T at more forward rapidities
⇒ Ultimately measure double-differential inpjetT − T
jetf
è
è è
è
è
0
10
20
30
40
50
Njets
σN
[fb]
mH =125.4GeV
pcutT =30GeV
ATLAS H→γγ
ggH(STWZ, BLPTW)+XH
XH=VBF+V H+ttH
≥ 0 = 0 ≥ 1 = 1 ≥ 2
è
è
è
è
0
1
10
10 20 30 40 500.01
0.1
T jetC [GeV]
dσ/dT
jet
C[fb/GeV]
mH =125.4GeV ATLAS H→γγ
ggH(NLL′+NLO)+XH
XH=VBF+V H+ttH
è
è
èè
è
0
1
20 40 60 80 100 120 1400.01
0.1
pjetT [GeV]
dσ/dpjet
T[fb/GeV]
mH =125.4GeV ATLAS H→γγ
ggH(NNLL′+NNLO)+XH
ggH(NLL′+NLO)+XH
XH=VBF+V H+ttH
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 10 / 27
Higgs + Jets
Resolving Jet Bins.(only using H → γγ here)
Pretend uncertainties are much smallerand we want to resolve the “excess”
Different slices of jet phase space addsnontrivial information
One might conclude that “excess” sits athigher pjet
T at more forward rapidities
⇒ Ultimately measure double-differential inpjetT − T
jetf
è
è è
è
è
0
10
20
30
40
50
Njets
σN
[fb]
mH =125.4GeV
pcutT =30GeV
ATLAS H→γγ
ggH(STWZ, BLPTW)+XH
XH=VBF+V H+ttH
≥ 0 = 0 ≥ 1 = 1 ≥ 2
è
è
è
è
0
1
10
10 20 30 40 500.01
0.1
T jetC [GeV]
dσ/dT
jet
C[fb/GeV]
mH =125.4GeV ATLAS H→γγ
ggH(NLL′+NLO)+XH
XH=VBF+V H+ttH
è
è
èè
è
0
1
20 40 60 80 100 120 1400.01
0.1
pjetT [GeV]
dσ/dpjet
T[fb/GeV]
mH =125.4GeV ATLAS H→γγ
ggH(NNLL′+NNLO)+XH
ggH(NLL′+NLO)+XH
XH=VBF+V H+ttH
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 10 / 27
Jet Hierarchies
Jet Hierarchies.
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 10 / 27
Jet Hierarchies
In More Dimensions and with More Jets...
pjet1T
pjet2T
pcutT
pcutT
0-jet1-jet
≥2-jet(excludedbypjet2
T
<pjet1
T
)
... it quickly gets more complicated
Multiscale problem
· · · �∼ pjet2T
�∼ pjet1T
�∼ QDifferent multiple hierarchiespossibleResummation often limited to LL(parton shower)
I pcutT �pjet1
T ∼mH limit isknown at NLL′ [Liu, Petriello]
Need to better understand distributionand correlations
between different kinematic variablesacross different jet multiplicities
⇒ Increasingly important as we probe higher and higher scales(and in particular if we find new heavy particles)
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 11 / 27
Jet Hierarchies
Jet Hierarchies.[Bauer, FT, Walsh, Zuberi ’11]
[Procura, Waalewijn, Zeune ’14][Larkoski, Moult, Neill ’15]
[Pietrulewicz, FT, Waalewijn (prep.)]
SCET+: Extension of SCET to treat additional(emerging) scale hierarchies beyond standard SCET
Adds two types of intermediate modesI collinear-soft: soft mode arising from a collinear sectorI soft-collinear: collinear mode arising from a soft sectorI Relative scaling and interactions depend on specific observable
⇒ Resummation of hierarchiesin hard jet kinematics
3 2
13
2
1
32
1
3 2
1
t ∼ u ∼ s
t� u ∼ s
t� u� s
t ∼ u� s
⇒ Resummation of double-differential spectra→ talk by Lisa Zeune
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 12 / 27
Jet Hierarchies
Jet Rates and Nonglobal Logarithms.Thrust with cone jets [Chien, Hornig, Lee 1509.04287]
2(Λ/Q)R2 ∼ τ � R2 � 1
With SCET+ can resum ln τ and also lnR
For τ ∼ R2 resum all global lnR in total jet rateI But not appearing non-global logs of Λ/Q
“Resum” nonglobal logs by explicitly resolving andsumming infinite set of emissions
Resolved soft subjets→ “dressed gluons”[Larkoski, Moult, Neill 1501.04596, 1508.07568]
I Infinite sum reproduces BMS equation[Dasgupta, Salam; Banfi, Marchesini, Smye]
I Instead: Expand in number of dressed gluons→ Systematic reorganization of pert. series
Renormalize “Color-density matrix”[Caron-Huot 1501.03754]
Renormalize matrix of bare emissions[Becher, Neubert, Rothen Shao 1508.06645]
��� ��� ��� ��� ��� ��� �������
����
����
����
����
����
����
�
���(�)/��
���-������� ����� �������������
�� ��������������
�-���� ����� �����
�-���� ����� �����
�-������� �����
�-������� �����
����� �� ��������-����� ���
(αs/π)Nc ln(mL/mR)
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 13 / 27
Jet Hierarchies
Jet Rates and Nonglobal Logarithms.Thrust with cone jets [Chien, Hornig, Lee 1509.04287]
2(Λ/Q)R2 ∼ τ � R2 � 1
With SCET+ can resum ln τ and also lnR
For τ ∼ R2 resum all global lnR in total jet rateI But not appearing non-global logs of Λ/Q
“Resum” nonglobal logs by explicitly resolving andsumming infinite set of emissions
Resolved soft subjets→ “dressed gluons”[Larkoski, Moult, Neill 1501.04596, 1508.07568]
I Infinite sum reproduces BMS equation[Dasgupta, Salam; Banfi, Marchesini, Smye]
I Instead: Expand in number of dressed gluons→ Systematic reorganization of pert. series
Renormalize “Color-density matrix”[Caron-Huot 1501.03754]
Renormalize matrix of bare emissions[Becher, Neubert, Rothen Shao 1508.06645]
��� ��� ��� ��� ��� ��� �������
����
����
����
����
����
����
�
���(�)/��
���-������� ����� �������������
�� ��������������
�-���� ����� �����
�-���� ����� �����
�-������� �����
�-������� �����
����� �� ��������-����� ���
(αs/π)Nc ln(mL/mR)
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 13 / 27
Jet Hierarchies
Phenomenology of Small-R Jets.[Dasgupta, Dreyer, Salam, Soyez, Cacciari]
Jet algorithms induce logarithms ∼ αs lnR
Use a generating functional formalism to numerically resum leading(αs lnR)n terms (LLR)
I Exploits angular ordering, evolution variable t ∼ (αs/2π) ln(R20/R
2)
Studied several jet observables, including jet trimming and filteringI Trimming is particularly sensitive to small-R effects
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 14 / 27
Jet Hierarchies
Phenomenology of Small-R Jets.
[plots stolen from F. Dreyer’s BOOST 2015 talk]
Inclusive jet spectrumLLR has noticeable effect forR . 0.4
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 15 / 27
Jet Hierarchies
Theory for Jet Substructure.
Jet substructure methods
Typically involve complex algorithms and cuts, very sensitive toresummation and nonperturbative effects
I Vast majority of studies and applications heavily rely (necessarily) on partonshower Monte Carlos
In past few years focus increased on gaining a better analytic andsystematic understanding in two important and complementary ways[many groups/authors ...]
I Better analytic understanding, tools, calculationsI Instead of optimizing solely for performance, also taking theory controlability
into account in designing new algorithms and methods
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 16 / 27
Jet Hierarchies
Analytic Boosted Boson Discrimination.[Larkoski, Moult, Neill]
First factorization and analytic resummation of a 2-prong variable D2
(similar to 2-subjettiness but based on energy correlation functions)
For boosted boson signal and QCD background (latter is the hard part)
Use SCET+ to resum and combine different subjet limits
e(�)2 ⇠
⇣e(↵)2
⌘�/↵
e(↵)3 ⌧
⇣e(↵)2
⌘3
jet axis
R
Collinear Subjets
Jna
Jnb
S+nanbnt
Sntnt
e(↵)2 ⇠ e
(�)2
e(↵)3 ⌧
⇣e(↵)2
⌘3
jet axis
RJn
Soft Subjet
Snnnsj
Snsj nsj
Jnsj
e(↵)2 ⇠ e
(�)2
jet axis
R
e(↵)3 ⇠
⇣e(↵)2
⌘2
Jn
Soft Haze
Snn
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 17 / 27
Jet Hierarchies
Analytic Boosted Boson Discrimination.[Larkoski, Moult, Neill]
��� ��� ��� ��� ��� ������
���
���
���
��(���)
�������������������
��(���)
��������
����� �+�- → ������
� ����� ��������� ����� ������ ������ ���������� ��� ������������� ��� ������ ������ �����
�� ∈ [��� ���] ���� �=�
������� �
���������
��� ��� ��� ��� ��� �����-�
��-�
��-�
��-�
� ���������������������
��(���)� � ����� ��� ���
�������������������� �� ������������� ���������� �������������++
����� �+�- → ������
�� ∈ [��� ���] ���� �=�
Region relevant for discrimination is very sensitive to hadronizationI Described by nonperturbative soft function with field-theoretic definitionI Good description with single-parameter shape function
Should be possible to extend to other substructure observables and ppcollisions
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 18 / 27
XCone
XCone.
Substructure without substructure
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 18 / 27
XCone
XCone: An eXclusive Cone-Jet Algorithm.[Stewart, FT, Thaler, Vermilion, Wilkason]
Basic constructionStart from standard N-jettiness
TN({nk}) =∑
i
min {ρjet(pi, n1), . . . , ρjet(pi, nN), ρbeam(pi)}
I Given N jet axes {nk}, partitions event into N jet regions and beam regionI Shape and size of jet regions depend on ρjet and ρbeam measures
(New XCone measure yields conical jets and maintains theory control)
Minimize over all jet axes
TN = minn1,n2,...,nN
TN({nk})I Finding exact global minimum is computationally expensiveI Finding approximate (local) minimum is sufficient and inexpensive
Key featuresExclusive: Returns preselected number of jets
I Yields best interpretation of the event in terms of the signal one is looking forI Final TN value provides additional quality measure for free
Smoothly transitions between well-separated and boosted regimesFrank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 19 / 27
XCone
Comparison to anti-kT .
Boosted tt→ hadrons event (from BOOST 2010 sample)
Well-separated jetsXCone jets practically the sameas leading anti-kT jets
Adjacent/overlapping jetsanti-kT merges signal jets, picksup ISR, FSR jetsXCone still finds jets, dynamicallysplit by nearest-neighbor
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 20 / 27
XCone
Case Study: Boosted Higgs Reconstruction.[Thaler, Wilkason]
Boosted H → bb with increasing pT
XCone provides stable performanceacross kinematic regimes
I Effectively provides automatic transitionfrom 2-jettiness to 2-subjettiness
I Further improvements possible, e.g. withexplicit ISR veto
(GeV)T,min
p200 300 400 500 600 700 800 900 1000
Sig
nal
Eff
icie
ncy
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
= 2β
= 1β
Tak
R = 0.5
[100,150] GeV∈m
Higgs Efficiency for N = 2
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 21 / 27
XCone
Case Study: Boosted Higgs Reconstruction.[Thaler, Wilkason]
(GeV)jjm0 50 100 150 200 250
Rel
ativ
e O
ccurr
ence
0
0.05
0.1
0.15
0.2
0.25
> 200 GeV)T
Higgs N = 2 (p
= 2β
= 1β
Tak
R = 0.5
> 200 GeV)T
Higgs N = 2 (p
(GeV)jjm0 50 100 150 200 250
Rel
ativ
e O
ccurr
ence
0
0.05
0.1
0.15
0.2
0.25
> 500 GeV)T
Higgs N = 2 (p
= 2β
= 1β
Tak
R = 0.5
> 500 GeV)T
Higgs N = 2 (p
(GeV)jjm0 50 100 150 200 250
Rel
ativ
e O
ccurr
ence
0
0.05
0.1
0.15
0.2
0.25
> 800 GeV)T
Higgs N = 2 (p
= 2β
= 1β
Tak
R = 0.5
> 800 GeV)T
Higgs N = 2 (p
Boosted H → bb with increasing pTXCone provides stable performanceacross kinematic regimes
I Effectively provides automatic transitionfrom 2-jettiness to 2-subjettiness
I Further improvements possible, e.g. withexplicit ISR veto
(GeV)T,min
p200 300 400 500 600 700 800 900 1000
Sig
nal
Eff
icie
ncy
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
= 2β
= 1β
Tak
R = 0.5
[100,150] GeV∈m
Higgs Efficiency for N = 2
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 21 / 27
XCone
Case Study: Boosted Top Reconstruction.
Classic example of jet substructure: pp→ tt→WWbb→ qqqqbb
(GeV)T, min
p200 300 400 500 600 700
Sig
nal
Eff
icie
ncy
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
= 2β = 1β
(Bst)Tak (Res)Tak
R = 0.5 [150,200] GeV∈m
> 50 GeVW+ m
3×Top Efficiency for N = 2
(GeV)T, min
p200 300 400 500 600 700
Bac
kgro
und M
ista
g
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
= 2β = 1β
(Bst)Tak (Res)Tak
R = 0.5 [150,200] GeV∈m
> 50 GeVW+ m
3×QCD Mistag for N = 2
(GeV)T,min
p200 300 400 500 600 700
Im
pro
vem
ent
BS
/
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
= 2β = 1β
(Bst)Tak (Res)Tak
R = 0.5 [150,200] GeV∈m
> 50 GeVW+ m
3×Signal Significance for N = 2
Use XCone with N = 2× 3 (R2 � 1, R3 = 0.5)
Compare toI “Resolved”: 2 kT jets (R� 1) with 3 anti-kT subjetsI “Boosted”: 2 anti-kT jets (R = 1.0) with 3 kT subjets
⇒ Better performance across all pTFrank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 22 / 27
Quark-Gluon Discrimination
Quark-Gluon Discrimination.
a.k.a. Hunting the White Whale of Jet Substructure
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 22 / 27
Quark-Gluon Discrimination
Quark-Gluon Discrimination.
QCD background jets from ISR and FSR are dominantly “gluon-jets”
Signal jets are dominantly “quark-jets”I Color singlets couple to quarks only
⇒ Would be powerful discriminator if it could be exploitedI See b-taggingI Long history, but still (almost embarassingly) not well under controlI Initiated a comprehensive study at Les Houches this year
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 23 / 27
Quark-Gluon Discrimination
What Are We Even Talking About?
Jesse Thaler Ñ Report of the Les Houches Quark/Gluon Subgroup 3
What is a Quark Jet?!From lunch/dinner discussions
A quark parton!
A Born-level quark parton!
The initiating quark parton in a Þnal state shower!
An eikonal line with baryon number 1/3"and carrying triplet color charge!
A quark operator appearing in a hard matrix element"in the context of a factorization theorem!
A parton-level jet object that has been quark-tagged using a soft-safe ßavored jet algorithm (automatically collinear safe if you sum constituent ßavors)!
A phase space region (as deÞned by an unambiguous hadronic Þducial cross section measurement) that yields an enriched sample of quarks (as interpreted by some suitable, though fundamentally ambiguous, criterion)
Ill-DeÞned
Well-DeÞned
[stolen from Jesse Thaler, Les Houches]
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 24 / 27
Quark-Gluon Discrimination
What Are We Even Talking About?
Jesse Thaler Ñ Report of the Les Houches Quark/Gluon Subgroup 3
What is a Quark Jet?!From lunch/dinner discussions
A quark parton!
A Born-level quark parton!
The initiating quark parton in a Þnal state shower!
An eikonal line with baryon number 1/3"and carrying triplet color charge!
A quark operator appearing in a hard matrix element"in the context of a factorization theorem!
A parton-level jet object that has been quark-tagged using a soft-safe ßavored jet algorithm (automatically collinear safe if you sum constituent ßavors)!
A phase space region (as deÞned by an unambiguous hadronic Þducial cross section measurement) that yields an enriched sample of quarks (as interpreted by some suitable, though fundamentally ambiguous, criterion)
Ill-DeÞned
Well-DeÞned What we mean
What people
sometimes
think we mean
[stolen from Jesse Thaler, Les Houches]
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 24 / 27
Quark-Gluon Discrimination
Les Houches Quark/Gluon Performance Study.[Les Houches q/g subgroup]
Compare e+e−→ uu (“quark-tagged”) to e+e−→ gg (“gluon-tagged”)
Generalized jet angularities: λκβ =∑i∈jet z
κi θ
βi
Separation power:1
2
[S(λ)−B(λ)]2
S(λ) +B(λ)
“Les Houches Angularity” λ(κ = 1, β = 0.5), R = 0.6, Q = 200 GeV
quarksPythia-8205Sherpa-2.1.1Vincia-1201Herwig-2 7 1
0
1
2
3
4
5
λ(κ=1, β=0.5), R=0.6
1/N
dN
/dλ
0 0.2 0.4 0.6 0.8 1
0.6
0.8
1
1.2
1.4
λ
MC
/Dat
a
gluons
Pythia-8205Sherpa-2.1.1Vincia-1201Herwig-2 7 1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
λ(κ=1, β=0.5), R=0.6
1/N
dN
/dλ
0 0.2 0.4 0.6 0.8 1
0.6
0.8
1
1.2
1.4
λ
MC
/Dat
a
separation power
Pythia-8205Sherpa-2.1.1Vincia-1201Herwig-2 7 1
0
0.5
1
1.5
2
λ(κ=1, β=0.5), R=0.6
1/N
dN
/dλ
0 0.2 0.4 0.6 0.8 1
0.6
0.8
1
1.2
1.4
λ
MC
/Dat
a
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 25 / 27
Quark-Gluon Discrimination
Quark/Gluon Performance: Some Lessons Learned.total separation power
Preliminary
gluon rejection at 50% quark eff.
Preliminary
Improving quark/gluon robustness seems synonymous with controllingfinal state shower uncertainties (LEP measured quark not gluon event shapes)
Hadronization can be important, even for IRC safe angularities⇒ Comparison to analytic predictions, uncertainties, stay tuned ...
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 26 / 27
Quark-Gluon Discrimination
Summary.
Jets are our window onto the hard interaction
Jets inevitably involve nontrivial QCD corrections and several physicalscales
I Fixed-order calculations are often necessary but seldom sufficient
As experimental analyses are getting more sophisticated they make moredetailed use of jets
I More differential, more exclusive binning, more detailed substructure, ...I Important to keep theory under control
Calculations and methods are constantly being refined and developedI But there are also many open issues
Many things I have not covered (apologies ...)→ See e.g. many interesting talks at BOOST 2015
Frank Tackmann (DESY) Jet Physics at the LHC 2015-10-02 27 / 27