1. STANDARD MODEL HIGGS SEARCHES WITH ATLAS
Nikos Giokaris
University of Athens
On behalf of the ATLAS Collaboration
September 19, 2006

2. OUTLINE
MOTIVATION
THE ORIGIN OF MASS OR THE MASS PROBLEM AND THE STANDARD MODEL (SM) SOLUTION:
THE HIGGS MECHANISM
PHENOMENOLOGY OF THE SM HIGGS BOSON
Mass
Decay
Production
SM HIGGS IN ATLAS
Which channels could provide discovery
Summary of discovery potential vs mH
Measurement of mH
Measurement of Higgs Boson couplings
CONLUSIONS
Crimea, /19
Nikos Giokaris

3. MOTIVATION Why look for SM Higgs?
It is the only, still missing ingredient in the standard model
Its discovery and study of its properties might lead to an eventual understanding of the origin of mass
Crimea, /19
Nikos Giokaris

4. Particle Masses and their impact on the structure of the Universe
electron mass ( 0.5MeV ) : defines length scale of our world,
Bohr radius a=1/em me
No or small W mass: fusion in stars: p+p →D e+ν Gf~ (MW) short burning time of sun → no humans on earth
mass values of e, u, d, W and their fine tuning are
essential for creation and development of our universe
principle of mass generation  Higgs mechanism
origin of mass values  even no theoretical explanation yet
Crimea, /19
Nikos Giokaris

5. Gauge symmetries of the SM and particles masses
consistent description of nature based on gauge symmetries
electroweak SU(2)LxU(1)Y symmetry forbids „ad hoc“ masses for
gauge bosons: W and Z fermions: (l = doublet, r = singlet)
„ad hoc“ mass terms destroy
renormalisibility  no precision prediction for observables
high energy behaviour of theory  e.g. WLWL scattering
Crimea, /19
Nikos Giokaris

6. High energy behaviour of σ in WW →WW
violates unitarity
at ECM ~ 1.2 TeV
massive gauge bosons: 1 longitudinal + 2 transverse d.o.f.
massless gauge bosons: only 2 transverse d.o.f.
scalar boson H
restores unitarity, if
gHWW ~ MW
gHff ~ Mf
and MH < 1TeV
 const=f(MH)
Crimea, /19
Nikos Giokaris

7. The Higgs mechanism V = 2 || +  ||2 2 < 0  > 0
The „standard“ solution:
one new doublet of complex scalar fields (4 degrees of freedom)
with appropriately chosen potential V
V = 2 || +  ||2
2 < 0
 > 0
minimum of V not at =0  spontaneous symmetry breaking
3 massless excitations along valley  3 longitudinal d.o.f for W+- and Z
1 massive excitation out of valley  1 d.o.f for „physical“ Higgs boson
Higgs field has two „components“
1) constant background condensate with vev 247 GeV (from GF)
2) Higgs boson H with unknown mass MH ~  ~ v
Crimea, /19
Nikos Giokaris

8. Mass generation and Higgs Boson couplings: Φ= v + Hx gf
interaction with Higgs field v=247 GeV
v =247 GeV
MV ~ g v gauge coupling
mf ~ gf v Yukawa coupling
introduced „ad hoc“
Fermion x x 2
g gauge
W/Z boson
interaction with Higgs boson H
Higgs gf
fermions: gf ~ mf / v
W/Z bosons: gV ~ MV / v = g2 v 2
fermion v
VVH coupling ~ vev
only existent after EWSB
Higgs x
g gauge 2
1 unknown parameter in SM: MH
W/Z boson
Crimea, /19
Nikos Giokaris

9. Decays of the Higgs boson in the SM
HDECAY: Djouadi, Spira et al.
b b
W W ZZ tt  cc gg 
for M<135 GeV: H  bb, dominant
for M>135 GeV: H  WW, ZZ dominant
tiny: H  also important
Crimea, /19
Nikos Giokaris

10. What is the mass of the Higgs boson ?
theory: unitarity in WW scattering  MH < 1 TeV
direct search at LEP: MH<114.4 GeV excluded with 95% CL
indirect prediction in SM, e.g. t H W W W W
S. Roth b W
… mt … ln(MH) 2
MH < 186 GeV (mtop=172.7 GeV)
with 95% CL
Standard Model prefers a light Higgs boson
Crimea, /19
Nikos Giokaris

11. Status of SM Higgs boson searches at TEVATRON
Expected sensitivity:
95% CL exclusion up to 130 GeV with 4fb-1 per experiment
3 sigma evidence up to 130 GeV with 8fb-1 per experiment
Current sensitivity::
Cross section limits at level of
~ 5 to 7 x SM cross section
Crimea, /19
Nikos Giokaris

12. The Large Hadron Collider LHC
proton proton collisions at ECM of 14 TeV, start in 2007
initial luminosity: (2)x1033 cm-2s-1  10 to 20 fb-1/year
design luminosity: cm-2s-1  fb-1/year
Crimea, /19
Nikos Giokaris

13. A Toroidal LHC ApparatuS
MC studies with fast simulation of ATLAS detector
key performance numbers from full sim.: b/tau/jet/el.// identification,
isolation criteria, jet veto, mass resolutions, trigger efficiencies, …
Crimea, /19
Nikos Giokaris

14. Production of the SM Higgs Boson at LHC
gluon fusion dominant for all masses
VBF roughly one order of magnitude smaller
HW, HZ,H tt only relevant for small MH
Crimea, /19
Nikos Giokaris

15. Cross sections for background processes
overwhelming background: mainly QCD driven
signal: often electroweak interaction
 photons, leptons, … in final state
3 level trigger system on
leptons, photons, missing energy
provides reduction by
no access to fully hadronic events
e.g. GGF, VBF with Hbb
Higgs 150 GeV: S/B <= 10-10
Crimea, /19
Nikos Giokaris

16. An event at the LHC „hard“ collision + ISR,FSR + „underlying event“
+ ~23 overlayed pp interactions per bunch crossing at high luminosity
 ~109 proton proton collisions / second
~1600 charged particles enter detector per event
+ effects from „pile up“: read out time > t btw. bunch crossings
Crimea, /19
Nikos Giokaris

17. The challenge of event reconstruction
low luminosity high luminosity
Crimea, /19
Nikos Giokaris

18. Which channels may provide discovery?
efficient trigger  no hadronic final states: e.g. GGF, VBF: Hbb
Higgs boson mass reconstructable? which mass resolution?
background reducible and controllable?
- good signal-to-background ratio
- small uncertainty on BG, estimation from data itself possible?
Status 2001
discovery channels:
inclusive:
H  2 photons
 ZZ  4 leptons
 WW  ll
exclusive:
ttH, Hbb
VBF, HZZ,WW for large M
Crimea, /19
Nikos Giokaris

19. H→ 2 Photons signal: two high Pt photons
background: irreducible pp   +x
reducible pp  j, jj, …
exp. issues (mainly for ECAL):
- , jet separation (Eff=80%, Reject. ~ few 1000)
- energy scale, angular resolution
conversions/dead material
ATLAS 100fb-1
mass resolution M: ~1 to 1.5%
S/BG ~ 1/20
precise background estimate
from sidebands (O(0.1%))
 no MC needed
preliminary NLO study:
increase of S/B by 50%
Crimea, /19
Nikos Giokaris

20. H→ ZZ(*) →4 leptons signal: 4 iso. leptons reducible BG:
1(2) dilepton mass = MZ
reducible BG:
tt, Zbb  4 leptons
 lepton isolation and
veto against b-jets
irreducible BG:
ZZ  4 leptons
 four lepton mass
good mass resolution M ~1%
 muon spectrometer + tracking detectors
small and flat background
 easy estimate from sidebands
 no Monte Carlo needed
preliminary NLO study indicates
significance increase by 25%
Crimea, /19
Nikos Giokaris

21. H → WW → l v lv signal: - 2 leptons + missing ET
- lepton spin corrleations
- no mass peak  transverse mass
ATLAS
M=170GeV
30fb-1
transverse mass
BG: WW, WZ, tt
lepton iso., missing E resolution
jet (b-jet) veto against tt
Dührssen, prel.
BG estimate in data from ll : 5%
normalisation from sideband
shape from MC
NLO effect on spin corr.
 ggWW contribution signal like
Crimea, /19
Nikos Giokaris
ll

22. only channel to see Hbb
ttH with Hbb
signal: 1 lepton, missing energy
6 jets of which 4 b-tagged
reducible BG: tt+jets, W+jets  b-tagging
irreducible BG: ttbb  reconstruct mass peak
exp. issue: full reconstruction of ttH final state  combinatorics !!!
need good b-tagging + jet / missing energy performance
mass resolution M: ~ 15%
50% correct bb pairings
difficult background estimate from
data with exp. uncertainty ~ O(10%)
 normalisation from side band
 shape from „re-tagged“ ttjj sample
ATLAS
30 fb-1
S/BG ~ 1/6
only channel to see Hbb
Crimea, /19
Nikos Giokaris

23. Vector boson fusion VBF: pp→qqH
Jet
Forward tagging jets
signature:
2 forward jets
with large rapidity gap
only Higgs decays
in central part of dector  
Higgs Decay
=-ln tan(/2)
ATLAS
ATLAS
Crimea, /19
Nikos Giokaris

24. VBF: Challenges ATLAS reconstruction of taggings jets influence of
- „underlying event“ (UE) ?
- overlapping events (OE) ?
- „pile up“ (PU) ?
 so far only low lumi considered
pT>20GeV
central jet veto:
influence of UE, OE, PU?
efficiency of jet veto at NLO?
but: no NLO MC-Generator yet
now: study started using SHERPA
Zeppenfeld et al.
Crimea, /19
Nikos Giokaris

25. collinear approximation
VBF: H  ll 4 
signature: tagging jets + 2 leptons + large missing tranvsere energy
background: QCD processes tt,Zjj  central jet veto
 reconstruction of m
He
collinear approximation
ATLAS
expected BG ~ 5 to 10%
for MH > 125 GeV: side band
for MH < 125 normalisation from
Z-peak, shape from Z
30 fb-1
M /M ~ 10%
dominated by Emiss
Crimea, /19
Nikos Giokaris

26. VBF, H: determination of background from data
Idea: jjZandjjZwith identical topology
muons are MIPS  same energy deposition in calorimeters
only difference: momentum spectra of muons
Method: select Z   events
„randomise“ -momenta according to Z MC
apply „usual“ selection and mass reconstruction
shape of background
can be extracted precisely
from data itself
(M. Schmitz, Diplomarbeit BN 2006)
Crimea, /19
Nikos Giokaris

27. HWWll: VBF versus inclusive channel
ATLAS
M=170GeV
30fb-1
ATLAS
10 fb-1
HWWe
S/BG ~ 3.6 Signal = S/BG ~ 0.7 Signal = 144
VBF with respect to gluon fusion
smaller rate larger sig-to-BG ratio smaller K-factor
more challenging for detector understanding
order of significance depends on channel and Higgs mass
Crimea, /19
Nikos Giokaris

28. Discovery potential in SM
10 fb-1
30 fb-1
excluded by LEP
excluded by LEP
VBF dominates discovery potential for low mass (at least at LO)
with 15 fb-1 and combination of channels: discovery from LEP to 1TeV
prel. NLO studies: increase of signifcance up to 50% for incl. channels
so far: cut based  improvement with multivariate techniques
Crimea, /19
Nikos Giokaris

29. Measurement of Higgs boson mass
ATLAS
Direct from mass peak:
HHbb HZZ4l
(energy scale 0.1 (0.02)% for l,,1% for jets)
“Indirect”
from transverse mass spectrum:
HWWllWHWWWlll
S. Roth
300 fb-1
M/M: 0.1% to 1%
Higgs boson mass
determines Higgs sector in the SM
is precision observable of the SM
Crimea, /19
Nikos Giokaris

30. Determination of Higgs boson couplings
coupling in production Hx= const x Hx and decay BR(Hyy)= Hy / tot
Prod.
Decay
HX Hy 2
Partial width: Hz ~ gHz
Hx x BR ~
tot
goal: - disentangle contribution from production and decay
- determine total width tot
model independent:
only ratio of partial width
13 final states in global fit
(including various syst. uncertainties)
H WW used as reference as most precise determination for MH>120 GeV
Crimea, /19
Nikos Giokaris

31. EXCITING TIMES LIE AHEAD !!!
CONCLUSIONS
Need to understand mass of particles and its origin
Higgs mechanism provides a consistent description
The SM Higgs particle should be discovered by ATLAS at LHC, if it exists
Its mass, width, spin & CP will be determined
Partial width and absolute ratio of couplings need theoretical input
ATLAS collaboration is now:
Refining background & trigger eff., id eff. estimates
Improving reconstruction & MC simulation
EXCITING TIMES LIE AHEAD !!!
Crimea, /19
Nikos Giokaris

32. Acknowledgements M.Schumacher A. Kalogeropoulos Crimea, 2006 9/19
Nikos Giokaris