The Central Jet Veto in Vector Boson Fusion Analysis Markus Schumacher LHC-D Higgs Workshop,...

The Central Jet VetoThe Central Jet Vetoin Vector Boson Fusion Analysisin Vector Boson Fusion Analysis

Markus Schumacher

LHC-D Higgs Workshop, Freiburg October 8/9th 2007

Markus Schumacher The Central Jet Veto in VBF Analysis LHC-D Higgs WS Freiburg, Oct. 2007 2

Outline

The importance of the VBF production mode

Characteristics and Challenges of the VBF topology

Issues for the Central Jet Veto (CJV)

Overview of CJV Applications at LHC

Track Veto versus Calorimeter Jets

Comparison of MC Generators

Ideas about Efficiency Determination from Data

Conclusions and Outlook


Production of a SM Higgs Boson at LHC

dominant process: gluon fusion

factor 10 suppressed: Vector Boson Fusion (VBF)

but: additional signature in the detector


ATLAS: Hl had 3 + ll 4

HWW lqq and ll

Entdeckungspotenzial im Standardmodell

VBF dominates discovery potential at LO in ATLAS

VBF provides the only channel to see H

CMS: Hl had 3

HWW lqq and llnew


Properties of the Higgs Boson

CP Quantum NumbersRatio of Partial Width

dominated by VBF channels

(only assumed 30fb-1)

HWW used as reference

as measured most precisely

ATLAS


MSSM: VBF, esp. Hdominates discovery potential at LO

invisible Higgs: VBF cleary most sensitive channel

Discovery Potential in Extensions of the SM

30 fb-1

MSSM

30 fb-1

ATLAS

ATLAS preliminary

Invisible Higgs


ATLASprel.

300 fb-1

SM or Extended Higgs Sector e.g. Minimal SUSY ?

Discrimination via VBF

R =BR(h WW) BR(h )

assume: Mh precisely known no sys. uncerainties

Compare expected measurement

of R in MSSM with SM prediction

for same MH

=|RMSSM-RSM|exp

similar study by M. Dührssen et al. incl. 13 channels VBF dominates


VBF: Event Topology

Signature: 2 tagging jets at large rapidities

challenge: jet reconstruction no color flow between jets, rapidity gap

challenge: central jet veto Higgs decay products in central region

challenge: mass reconstruction

for H and HWWlqq

dominated by ETmiss resolution

ATLAS

=-ln tan(/2)

Tagging Jets

Higgs-Zerfall

W/Z

W/Z

degree 1 2 15 90 15 2 1


„low lumi“ 10 to20 fb-1/year „high lumi“ 100 fb-1/year

An Event at the LHC

+ ~23 overlapping pp-interactions per bunch crossing at high lumi.

~109 proton proton collisions / second ~1600 charged particles in detector per event

+ effects from „pile up“: readout time > t btw. bunch

hard collision

+ ISR,FSR

+ „underlying

event“


Jet Reconstruction

pT>20GeV

Reconstruction efficiency

- approximately confirmed by new

preliminary ATLAS studies

- optimisation w.r.t jet algo ongoing

ATLAS

90 15 2 1o

ATLAS CMS

both experiments: fake rate ~ 2% for ET>20 GeV

fake rate = probability to find jet from pileup in central part of detector

low lumi

low lumi


Basic selection cuts

ATLAS

(a) 2 tagging jets: ET(1,2)>x(y) up to |max| with 1x2<0

(b) rapidity difference (c) invariant dijet mass

CMS CMS CMS ATLAS ATLAS ATLAS ATLAS

WWll WWlqq lhad WWll WWlqq ll lhad

max 4.5 4.5 4.5 4.9 4.9 4.9 4.9

ET(GeV) 50/30 45/35 40/40 40/20 60/60 50/30 40/20

4.2 4.2 4.2 3.8 5.0 4.4 4.4

Mjj 600 1000 1000 550 1200 700 700

CMS


Central Jet Veto (CJV)

EW: color singlet exchange

QCD: color octet exchange

different radiation

pattern expected

EW QCD

born level distribution for Z + 3 jets

D. Zeppenfeld et al., Phys.Rev.D54:6680-6689,1996

Challenge: calculation of CJV efficiencies

- „matching“ of Z+2j and Z+3j calculations

- correct QCD scales

- validity of approx. for phase space region


Central Jet veto

PhD thesis D. Rainwater, hep-ph 9908378

CJV survival probabilities : no jet with ET>20GeV btw. tagging jets

two approaches: - truncated shower approximation (dotted and dashed dotted)

- exponentiation model (solid and dashed)

(for details see, e.g.:

Phys.Rev.D54:6680-6689,1996

and PhD thesis by D. Rainwater)

or talks by Terrance Figy and

Christoph Hackstein this afternoon


Issues for the Central Jet Veto

Towards Discovery

- optimise CJV for significance

- robust against pileup, underlying event

detector noise, MC modelling, …

- if modelling wrong: kill (partially) our signal

need more data for observation

- knowledge of CJV efficiency not needed

Investigation of properties

couplings: (pollution from gluon fusion)

NOBS – NBG = (effVBF VBF + eff.GGFVBF GGF) x lumi

need knowledge of CJV eff. for VBF and GGF

HVV coupling structure from jj

need efficiencies and correlation with

observable for VBF, GGF (and background)


VBF: H

He

ATLAS

30 fb-1

comparable sensitivity in

lep had channel

caveat: different x-sections,

MC generators,

BG uncertainty, stat. tool

Zjj dominant background

QCD to EW ~ 3:1


CMS: CJV in Hlep had

No jet with: ETraw>10 GeV in min+0.5 < 3 < max-0.5

and 3= ptrack /ETraw>0.1

(sum over tracks in cone of 0.5 around jet axis)

motivation: distiniguish „signal“ from „pileup“ jets

CMS

CMS


Comparison of CJV: CMS and ATLAS

dependence of CJV efficiency

on ET of MC jet (10,15,20,..,45GeV)

10 GeV raw ~ 20 GeV MC Jet

Eff. of CJV(%) Higgs EW Zjj QCD Zjj W+jets tt

CMS lep had 81 71 40 29 24

ATLAS lep had 88 96 64 57 14

ATLAS lep lep 72 95 45 77 13

Rainwater 89 80 28 - 29

ATLAS: lep lep: no jet with ET>20 GeV ||<3.2

lep had: ‘‘ btw. tagging jets

10 GeV

45 GeVCMS

Comparison of CJV survival probabilities


HWWATLAS: HWWe CMS: HWWlqq

Comparison of significance:

ll: ATLAS more powerful

lq: CMS more powerful

But: different x-sections,

MC generators, etc.

Now: investigation of differences


Central Jet Veto in HWWllCMS: PT-Balance: <40 GeV

CJV: no jet with ET>20 GeV in |3* |< 2.0

CMS Higgs tt+j WWjj (EW) WWjj(QCD)

Eff. PTBAL.(%) 65 39 52 40

Eff. CJV(%) 68 27 62 48

Higgs tt+Wt WW+js (EW) WW+js (QCD) Z+j EW Z+j QCD

Eff. PTBAL (%) 85 24 76 66 77 67

Eff. CJV (%) 86 31 89 75 100 63

ATLAS: PT-Balance: <30 GeV

CJV: no jet with ET>20 in |3|<3.2

Higgs tt+ttj(j) WW+j (EW) WW+j (QCD) Z+j EW Z+j QCD

Eff. CJV (%) 89 46/29 75 29 - -

Rainwater no jet with ET>20 GeV btw. tagging jets


Summary of Current „Officical“ Experimental Status

no coherent picture of CJV performance (my feeling)

- different applications in channels and both collaborations

is the optimal that different for different channels?

- different MC generators

which gives most correct description?

how do they compare with the theo. calculations (TSA, Exp Mod.)?

is the use of current MC generators meaningful at all?

- disentangle effects from underlying event, pileup, noise, … ?

does the inclusion of track information really help?

- different basic VBF cuts (correlation not studied in great detail)

what is correlation with method and threshold of CJV?

better understanding by cooperation from theory+CMS+ATLAS

maybe good candidate for Analyis Group in Helmholtz Alliance


Track Jet Veto versus Calorimeter Jet Veto in HWWll

S. Horvat, S. Kaiser, O.Kortner MPI Munich

Idea:

disentangle jets from pileup pp collision and signal pp collision via tracks

for now: signal vertex = vertex with largest ptsum of tracks

jet finding via cone of 0.4 on tracks and calorimeter objects


Multiplicities without and with pile-up

Calo jets: increased multiplicity at low ET and large eta

Track jets: multiplicity distribution stable


Multiplicities above Threshold without and with pile-up

Calo Jets

Track Jets

||<2.5 tracking detector acceptance


Survival Probabilities

pT of track jets ~ 0.5 of calo jets

compare jet veto for

- track jets above 10 GeV

- calo jets above 20 GeV

both within || < 2.5

jet veto survival probabilities

track jet veto seems more robust ongoing study and optimisation


idea:count charged particles in rapidity gap: Jmin+0.5<trk<Jmax-0.5

Track Veto versus Central Jet veto (S. Nikitenko)

without

underlying event

with

underlying event

hadron level study with PYTHIA


Eff. veto cut Higgs Z+jets

CJV ETJ3>20 GeV 0.83 (0.81) 0.44 (0.46)

TV Ntrk > 9 0.80 0.55

* Numbers in parenthesis from CMS PTDR with full simulation

Track veto vs CJV: Survival probabilities for Hl had

Jet veto Track veto

Next steps:- check with full simulation

- compare results without and with pileup


Rapidity of non tagging jets with PT>20 GeV

Cuts applied: pt1(2) >50(30)GeV, |1,2|<4.9, 1x2< 0, mj1j2 > 700 GeV, j1j2> 4.4, j1j2 < 2.2

Comparison of MC Event Generators (M. Röder, BN)


Comparison of Different MC Generators

Survival probability depending on pt of 3rd jet in ||<3.2

significant dependence on event generator

mainly caused by different parton shower, not by underlying event


Determination of CJV Efficiency from Data

needed for investigation of Higgs boson properties

useful for optimising selection w.r.t. discovery

general interest to understand radiation pattern

goal: find and select samples with similar topology as VBF signal

with reasonable rate and signal-to-background ratio determine radiation pattern transfer to Higgs signal process (directly or via MC) tune MC generators to reproduce data

two candidates:

„the obvious“: jjZee+competing QCD and EW contribution

„the obscure“: single top (t-channel) purely EW process


The obvious jjZee(D. Zeppenfeld et al. Phys.Rev.D54:6680-6689,1996)

problem: QCD rate large compared to EW rate, still factor 7 after above cuts

EW QCD

Z

disentangle jjZ EW from QCD by VBF like selection


The obvious jjZee

good S/B only

- with hard cuts

on Mjj and jl

- at (too) small rate

small S/B

discrimination om

statisticsl basis

in fb

QCD

EW


The obvious jjZee

CJV: no jet with ETr>10GeV in min+0.7 < 3 < max-0.7

average vetos per event n calculated in exponentiation model

CJV thresholds

> 10 GeV

> 20 GeV

> 40 GeV

> 80 GeV

-

negligible correlation between veto prbabilty and jl (for <2.2)

maybe unfolding in jl may be usful

detailed experimental study needed


The obscure: single top (t-channel) production (MS, Si)

q q

b t

W

q q

q q

W

W

H

(NLO)~50 pb (incl. BR tbWbl)

Efficiency: ~ 0.5% 250 evts/fb

Signal-to-BG(tt): 2 to 3

(LO,120GeV)~0.4 pb

Efficiency: ~0.4%

similar colour structure similar radiation pattern after selection?

is it possible to prepare sample with similar topology? what is influence of differences? - initial b from gluon splitting

- different polarisations of Ws

- tbW decay

W

b

e,

Single top: qbqt VBF H


The obscure: single top (t-channel) production

samples produced with: Herwig+Jimmy+ ATLAS fast detector simulation

select tagging jets in opposite hemispheres

for single top: require b-tag (|b|<2.5 due to tracker acceptance)

invariant tag jet mass rapidity difference of tag jets


Characteristics of the third jet

with small statistics no definite conclusion possible!

at 1st glance: no significant discrepancy btw. single top and VBF

next steps: - use ME calculations for 3rd jets

- determine amount of tt background after „VBF“ selection

- use TSA or Exponentiation Model to get CJV efficiencies

transverse momentum 3*=3 - ½(1+2)


Conclusions and Outlook

None yet. This is supposed to be a workshop !

Hope for lively and interesting

discussions during this meeting.

Hope for close, good and fruitful cooperation

between theory, CMS and ATLAS people

here and in the future.

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The Central Jet Veto in Vector Boson Fusion Analysis Markus Schumacher LHC-D Higgs Workshop,...

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