weak vector boson fusion --- a tool to explore the higgs sector at lhc markus schumacher sfb ws on...
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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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 3
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 4
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)
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 5
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 6
„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“
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 7
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 8
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 9
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 10
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 11
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 12
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 13
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 14
Discovery Potential in Standard Model
VBF dominates discovery potential at LO in ATLAS
VBF provides the only channel to see H
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 15
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 16
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 17
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 18
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 19
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:
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 20
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 21
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 22
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 23
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
Markus Schumacher VBF: A tool to explore the Higgs sector at LHC Berlin, 31 October 2007 24
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