Download - Weak Vector Boson Fusion --- a Tool to Explore the Higgs Sector at LHC Markus Schumacher SFB WS on „Massive Particle Production at LHC“ Berlin, 31 October.

Weak Vector Boson FusionWeak Vector Boson Fusion

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a Tool to Explore the Higgs Sector at LHCa Tool to Explore the Higgs Sector at LHC

Markus Schumacher

SFB WS on „Massive Particle Production at LHC“ Berlin, 31 October 2007

Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 2

Outline

Characteristics and challenges of the VBF topology

Discovery potential in H and WW in the SM

Discovery potential in extended models

Investigation of Higgs boson properties

Central jet veto efficiency from data?

Conclusions and Outlook

only discuss experimental aspects

for theoretical issues see talks by e.g. Ansgar and Malgorzata


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 theoretical knowledge of key features at ~5 to 10%

see talks by Ansgar and Malgorzata


Towards Discovery

for discovery:

- need knowledge of expected background

daten driven approaches under study

- signal efficiencies not needed

wrong modelling not optimal selection strategy more data needed

channels investigated:

ATLAS: H lep had 3and lep lep 4

H WW 2lep 2 and qq lep

(SN-ATL-2003-68 Eur.Phys.JC32(2003)19)

CMS: H lep had 3 and e 4

H WW qq lep and 2 lep 2

H (not discussed today)

(CMS TDR and CMS-2007/011 and CMS-2006/081)


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

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 pp collisions /s ~1600 charged particles/evt. in detector

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

up to now: VBF studies only perfomed for low lumi. scenario

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) trigger provided by high pt leptons

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

(c) rapidity difference (d) invariant dijet mass

CMS

(d) lepton/jet ID for Higgs decay products + some angular cuts


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

10 GeV

45 GeVCMS no agreement among channels

and two collaborations yet

need support from our theoretical friends for modelling especially of QCD background


Weak Boson Fusion: HWW

HWWe

ATLAS

10 fb-1

MH=160 GeV

dominant background: top pair (+ Wt) production

estimated background uncertainty ~ 10 %

HWWll

- no mass peak only transverse mass

+ spin

correlations

HWWqq lep

- only 1 mass peak

- mass resolution: ~ 9%

HWW

CMS 60fb-1

GeV


Comparison of Sensitivity

ll: ATLAS more powerful

lq: CMS more powerful

but: different x-sections,

MC generators, etc.

now: investigation of differences

Background from data:

build control samples using loose/reversed cuts in observables

ideally not (weakly) correlated to MT e.g. ll, ll, etc.

signal enhanced and background enhanced = signal free samples

+ in addition b-tagged samples for tt background

extrapolate from signal in BG region (MC prediction)

or simultaneous fit to signal and background regions

full validation of data driven methods under study


H tau tau

dominant background Zjj (QCD to EW 3 to 1)

estimated background uncertainty ~ 7 to 10%

mass reconstruction despite 4 in collinear approximation

mass resolution ~ 9 to 11 %

dominated by ETmiss resolution

He

ATLAS

30 fb-1


Comparison of Sensitivity

comparable sensitivity

caveat: different x-sections,

MC generators,

BG uncertainty

statistical tools

background from data:

shape: - Zjj: using jjZ and replacing decay products to look like

- tt: select b-tagged tt-sample

normalisation: - from sidebands

- free parameter in fit of data to signal + BG

(already done in CMS analysis)

full validation of data driven methods under study


Discovery Potential in Standard Model

VBF dominates discovery potential at LO in ATLAS

VBF provides the only channel to see H


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


Investigation of Properties

rates:

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

need estimate of signal efficiencies for VBF and gluon fusion „pollution“

angular distributions:

need estimate of efficiencies and correlations with selection variables

(gluon fusion is background at 5 to 10% level with diff. kin. distributions)

mass: no detailed study in VBF yet

my estimate ~ few GeV for 30fb-1

couplings / partial widths

signal rate measurements

spin and CP quantum numbers

angular distributions


Signal rates

13 analysis (incl. „cross talk“) with various systematic uncertainties

dominated by VBF channels (only assumed 30fb-1)

assumes 5% uncertainty for efficiency on tagging jets and CJV

Determination of „Couplings“

Ratio of Partial Width


ATLASprel.

300 fb-1

SM or Extended Higgs Sector e.g. MSSM ?

discrimination via VBF

R =BR(h WW) BR(h )

assume: Mh precisely known no sys. uncertainties

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


Higgs electroweak gauge boson vertex

HVV Vertex Coupling Structure

most sensitive observable:

azimuthal angle difference btw. tagging jets jj

(Plehn, Rainwater, Zeppenfeld Phys.Rev.Lett. 88(2002)05180)

investigation performed in 2 steps

1) dominant coupling term ? pure SM or CP even or CP odd

2) SM + small anomalous new contribution?

jj

Jet 1

Jet 2

+ new terms in effective Lagrangian:


Dominant Coupling Term

Signal after all cuts

Sensitivity to exclude non SM contribution from CPE/CPO:

HWW (160 GeV): ~ 5 with 10 fb-1

H(120GeV): ~ 2.5 with 30 fb-1

(MC-Generator VBFNLO D. Zeppenfeld et al.)

if SM shape dominant: scalar field has a vev (“Higgs”) if CP conserved: CP of coupling = scalar particle CP quantum number

HWW: Signal + BG 10fb-1

(Eur.Phys.J.C51(2007)385SN-ATLAS-2007-060)

ATLASATLAS


Excl.

by L3

g5E = 0.11 at mH = 160 GeV in H WW for 30 fb-1

g5E= 0.24 at mH = 120 GeV in H for 30 fb-1

systematic uncertainty from BG normalisation < 0.02 caveats: LO MC generators, gluon fusion from PYTHIA

Additional small coupling: „SM + g5E CPE“

(i) different total cross section and (ii) distorsion of jj distribution - SM+CPE: interference significant in jj distribution - SM+CPO: no interference effects in the jj distribution (with signed you regain sensitivity, see V. Hankele et al. Phys.Rev.D74(2006)095001)

only SM+ g5E CPE contribution studied here


Estimating the CJV Efficiency from Data

knowledge of central jet veto efficiency needed for investigation of properties and optimisation of selection strategy

find and select samples with similar topology as VBF signal

with reasonable rate and signal-to-background ratio

determine radiation pattern and transfer to Higgs signal process

directly or via tuning of MC generators

QCD

EW

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

loose cuts:

EW/QCD = 1:7.2 EW = 87 fb

tight cuts:

EW/QCD= 1.6 (2.9) EW = 11(5) fb

the obvious candidate: jjZee+

after basic VBf cutscompeting QCD and EW contribution


The obscure: single top (t-channel) production

(NLO)~50 pb (incl. BR tbWbl) efficiency: ~ 0.5% acc=250 fb signal: BG (mainly tt)~ 2(3):1

similar radiation pattern ? similar topology selectable? what is influence of differences?

q q

b tW

W

b

e,

Single top q q

q q

W

WH

VBF H


Conclusion and Outlook

VBF is the tool to explore the Higgs sector at LHC

VBF dominates the discovery potential at LHC (at LO)

in SM, MSSM and for Hinvivible

VBF dominates determination of couplings

and allows determination of HVV coupling structure

Current and open issues:

optimisation of jet, ETMiss, mass reconstruction

background determination from data and signal extraction

optimising CJV (tracks vs. calo, robustness against pileup, …)

determination of CJV efficiency from data

here a strong and fruitful cooperation between

theory and experiment is needed