Theory of Fat Jets and Jet Substructure
Gavin Salam
CERN, Princeton University & LPTHE/CNRS (Paris)
Higgs Hunting 2012Orsay, France, 18–20 July 2012
Boosted massive particles → fat jets[Introduction]
Normal analyses: two quarks fromX → qq̄ reconstructed as two jets
jet 1
jet 2
X at restX
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 2 / 28
Boosted massive particles → fat jets[Introduction]
Normal analyses: two quarks fromX → qq̄ reconstructed as two jets
jet 1
jet 2
X at restX
High-pt regime: EW object Xis boosted, decay is collimated,
qq̄ both in same jet
singlefat jet
z
(1−z)
boosted X
Happens for pt & 2m/R
pt & 320 GeV for m = mW , R = 0.5
As LHC explores far above EW scale, such configurationsbecome of interest
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 2 / 28
Motivation #1[Introduction]
New heavy particles can decay to boosted W, Z, H, top, χ0 (RPV);WW scattering at high pt
◮ leptonic decays easily tagged, but rare and/or have MET
◮ hadronic decays more common and fully reconstructible
not especially Higgs oriented, except e.g.
SUSY cascades → Higgs: Butterworth, Ellis & Raklev ’07
+ Kribs, Martin, Roy & Spannowsky ’09, ’10
or H → 2a → 4g : Chen, Nojiri & Sreethawong ’10 + Falkowski et al ’10
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 3 / 28
Motivation #2[Introduction]
Hadronic decays of new EW-scale particles may be easier to see athigh pt
e.g. 125 GeV Higgs with 60% BR for H → bb̄ decayv. 0.2% for γγ and 2.6% for ZZ
Specifically for VH and tt̄H:
◮ Some relevant fraction producedat high pt (
√sLHC ≫ mEW)
◮ Backgrounds often fall faster thansignal at high pt
◮ Jet combinatorics are easier athigh pt — cleaner events
◮ Easier to organise cuts so as notto sculpt backgrounds
10-4
10-3
10-2
10-1
0 100 200 300 400 500 600 700
1/σtot dσ/dpT
pT[GeV]
ttH: pT,t
ttH: pT,H
WH: pT,HWjj: pT,j
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 4 / 28
Example improvement from boosted regime[Introduction]
Search for main decay of light Higgs boson in W/Z+H, H → bb̄
ATLAS TDR(unboosted)
(boosted)
restricting search to ptH > 200 GeV,
using the method from Butterworth, Davison, Rubin & GPS ’08
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 5 / 28
Example improvement from boosted regime[Introduction]
Search for main decay of light Higgs boson in t̄t+H, H → bb̄
(unboosted)
(boosted)
restricting search to pt,H > 200 GeV, pt,t→hadrons > 200 GeV, one leptonic top
Plehn, GPS & Spannowsky ’09
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 6 / 28
Key element #1Leading Order Structure
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 7 / 28
QCD principle: soft divergence[LO structure]
Signal Background
z
(1−z)
boosted Xz
quark
(1−z)
Splitting probability for Higgs:
P(z) ∝ 1
Splitting probability for quark:
P(z) ∝1 + z2
1− z
1/(1− z) divergence enhances background
Remove divergence in bkdg with cut on zCan choose cut analytically so as to maximise S/
√B
Originally: cut on opening angle (Seymour ’93)
or kt-distance (Butterworth, Cox & Forshaw ’02)Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 8 / 28
Common idea: undo jet clustering & cut[LO structure]
First proposed forW’s by Seymour ’93
Refined by Butter-
worth, Cox & Forshaw
’02
Refined more +showed how to useit to find H → bb̄at LHC, Butterworth,
Davison, Rubin & GPS
’08
Later in ’08: extended to top quarks by ATLAS; Thaler & Wang; Kaplan,Rehermann, Schwartz & Tweedie [Johns Hopkins top tagger].
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 9 / 28
Common idea: undo jet clustering & cut[LO structure]
First proposed forW’s by Seymour ’93
Refined by Butter-
worth, Cox & Forshaw
’02
Refined more +showed how to useit to find H → bb̄at LHC, Butterworth,
Davison, Rubin & GPS
’08
Later in ’08: extended to top quarks by ATLAS; Thaler & Wang; Kaplan,Rehermann, Schwartz & Tweedie [Johns Hopkins top tagger].
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 9 / 28
Common idea: undo jet clustering & cut[LO structure]
First proposed forW’s by Seymour ’93
Refined by Butter-
worth, Cox & Forshaw
’02
Refined more +showed how to useit to find H → bb̄at LHC, Butterworth,
Davison, Rubin & GPS
’08
Later in ’08: extended to top quarks by ATLAS; Thaler & Wang; Kaplan,Rehermann, Schwartz & Tweedie [Johns Hopkins top tagger].
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 9 / 28
Noise reduction
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 10 / 28
Noise & different kinds of event[Noise reduction]
Plain pythia event
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 11 / 28
Noise & different kinds of event[Noise reduction]
Plain pythia event + last year’s pileup
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 11 / 28
Noise removal from jets — a boosted top example[Noise reduction]
Key idea:
◮ Look at jet on smallerangular scale
◮ Discard its softer parts
◮ Filtering Butterworth et al ’08
◮ Pruning Ellis, Vermillion and Walsh ’09
◮ Trimming Krohn, Thaler & Wang ’09
[With earlier methods by Seymour ’93 and Kodolova et al ’07;
also Soper & Spannowsky ’10, ’11]
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 12 / 28
Noise removal from jets — a boosted top example[Noise reduction]
Key idea:
◮ Look at jet on smallerangular scale
◮ Discard its softer parts
◮ Filtering Butterworth et al ’08
◮ Pruning Ellis, Vermillion and Walsh ’09
◮ Trimming Krohn, Thaler & Wang ’09
[With earlier methods by Seymour ’93 and Kodolova et al ’07;
also Soper & Spannowsky ’10, ’11]
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 12 / 28
Noise removal from jets — a boosted top example[Noise reduction]
Key idea:
◮ Look at jet on smallerangular scale
◮ Discard its softer parts
◮ Filtering Butterworth et al ’08
◮ Pruning Ellis, Vermillion and Walsh ’09
◮ Trimming Krohn, Thaler & Wang ’09
[With earlier methods by Seymour ’93 and Kodolova et al ’07;
also Soper & Spannowsky ’10, ’11]
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 12 / 28
Noise removal from jets — a boosted top example[Noise reduction]
[Boost 2010
writeup]
◮ Filtering Butterworth et al ’08
◮ Pruning Ellis, Vermillion and Walsh ’09
◮ Trimming Krohn, Thaler & Wang ’09
[With earlier methods by Seymour ’93 and Kodolova et al ’07;
also Soper & Spannowsky ’10, ’11]
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 12 / 28
Overview of methods
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 13 / 28
Some taggers and jet-substructure observables[Methods]
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/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 14 / 28
Handles for distinguishing signal v. background[Methods]
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→bb̄ H
→bb̄ t→qqq̄
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/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 15 / 28
Comparing top taggers: QCD fakes rate v. signal eff.[Methods]
Herwig, 500 < pt < 600 GeV Herwig++, 200 < pt < 800 GeV
From the extensive “Boost 2011” report, which reviewed taggersdiscussed software, determined performance on MC, etc.
Bottom line: some taggers clearly better than others.But many taggers behave similarly & details depend on analysis
(+ MC choice)
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 16 / 28
Shower deconstruction for ZH[Methods]
Matrix-element method on steroids
showerdeconstruction
BDRSmethod
For each event estimate the probabil-ity that event is signal-like or back-ground like.
Break event into many mini-jets;use Monte-Carlo type Sudakovs andsplitting functions to get estimate ofmulti-parton matrix element for S &B hypotheses.
Intelligently combines full info aboutLO splitting, radiation, b-tags, etc.
Soper & Spannowsky ’11
cf. also multivariate (BDT) type methods
from Cui & Schwartz ’10
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 17 / 28
Experimental validation(two brief examples)
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 18 / 28
CMS single-jet W mass peakin events with a lepton andseparate b-tagged jet.
Uses pruning (+ mass-dropcondition on split jet)
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 19 / 28
ATLAS validation showing av-erage MD-F (BDRS) jet massas robust against pileup.
Trimming, with suitable pa-rameters, is also robust.
NB: Pileup now 2× higherCould get 4× worse?
Further improvements maybeneeded (and possible)
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 20 / 28
Calculations(Just those for VH & for single-jet properties)
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 21 / 28
Fully differential WH @ NNLO[Calculations]
Ferrera, Grazzini & Tramontano ’11
WH production with H → bb̄
Fat-jet pt distribution at
LONLONNLO
shows good stability fromNLO to NNLO
it’s the top-killing jet veto that
causes the K -factor to be < 1
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 22 / 28
WH @ NLO in production and decays[Calculations]
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
250 300 350 400 450 500
dσ/d
p T,j
[fb/G
eV]
LO
NLO prod.
NLO
NLO prod. (veto)
NLO (veto)
0 0.2 0.4 0.6 0.8
1 1.2 1.4 1.6
250 300 350 400 450 500
K-f
acto
r
pT,j [GeV]
LHC 14TeV
(mH+mW)/2 ≤ µR(p) = µF
(p) ≤ 2(mH+mW)
µR(d) = mH
Banfi & Cancino ’12
See also Richardson & Winn ’12for NLO WH production and
decay in Herwig++Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 23 / 28
Resummed subjettiness for boosted Z (just decay)[Calculations]
Distribution of τ21 subjettiness ratio
� �
0.00 0.05 0.10 0.15 0.20 0.25 0.300.2
0.5
1.0
2.0
5.0
10.0
20.0
Τ21
1
Σ
d Σ
d Τ21
Pythia vs. N3LL�NNLL
Q � 0 GeV
Q � 1000 GeV
Bands: NNLL, N3LL Lines: Pythia 8
UNBOOSTED
BOOSTED (Q=1 TeV)
Feige, Schwartz, Stewart & Thaler ’12 (adapted)
Precise resummedcalculations for thrust
e+e− → Z → qq̄ can becarried over to hadronic
boosted Z τ21 subjettinessratio (because it’sbasically the same
observable)
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 24 / 28
Calculations of backgrounds[Calculations]
0
2
4
6
8
10
12
14
0 0.05 0.1 0.15 0.2 0.25 0.3
1/σ
dσ
/ d
ζ
ζ = mJ/pTJ
Z+jet, R=0.6, pTJ > 200 GeV
NLL+LO with shift α= 1.5 GeVNLL+LO with shift α= 2.0 GeV
Sherpa with hadronisationPythia 8 with hadronisation
Herwig++ with hadronisation
NLL+LO jet mass calculation
Dasgupta et al ’12
Wbb̄ background in POWHEG
Oleari & Reina ’11
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 25 / 28
Long term aim? Concordance of different tools[Calculations]
Jet veto efficiency for gg → H
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
cf. also Becher & Neubert ’12
Illustrate with an examplefrom standard gg → H: jetveto efficiency, where various
tools agree well:
pure NNLO
NNLL+NNLO jet-vetoresummation
POWHEG reweightedwith HqT NNLL+NNLO
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 26 / 28
Outlook
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 27 / 28
Outlook[Calculations]
The next two talks will probably not show fat jet H → bb̄ searches!
(a) they do use the boosted Higgs idea (i.e. high pt), but lumi so farinsufficiect to go to pt ’s where fat-jet methods perform well.
(b) at intermediate pt ’s “traditional” jet analyses can be adapted tomimic fat-jet searches.
Recent experimental validation has provided maturity needed forfat-jet methods to come “on-line” as higher luminosities are delivered
to ATLAS and CMS.
Advanced theory tools not always as mature as for gg → H, butdeveloping rapidly.
Gavin Salam (CERN/Princeton/CNRS) Theory of Fat Jets Higgs Hunting 2012-07-19 28 / 28