Higgs Physics at the Large Hadron Collider · Higgs Physics at the Large Hadron Collider Markus...
Transcript of Higgs Physics at the Large Hadron Collider · Higgs Physics at the Large Hadron Collider Markus...
Higgs Higgs PhysicsPhysics at at thetheLarge Large HadronHadron ColliderColliderMarkus Schumacher, Bonn University
NRW Phenomenology Meeting 2006, Bad Honnef, January 2006
Markus Schumacher Higgs Physics at the LHC NRW-Pheno-06, Bad Honnef, January 2006 2
Outline
the Higgs Mechanism and SM Higgs phenomenology at LHC
• discovery of 1 neutral scalar SM like Higgs boson
(determination of mass, spin, CP)
quantum numbers: spin and CP
BRs, total width, couplings
self coupling at SLHC
direct observation of
additional Higgs bosons
determination of Higgs profile:
deviations from SM prediction
- > 1 neutral Higgs boson
- charged Higgs boson
- exotic decays:H invisible
Higgs physics program at the LHC
•discrimination: SM or extended Higgs sectors (e.g. MSSM)
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The Higgs Mechanism in the Nut Shell
consistent description of nature seems to be based on gauge symmetry
SU(2)LxU(1) gauge symmetry no masses for W and Z and fermions
„ad hoc“ mass terms spoil
The problem:
The „standard“ solution:
new doublet of complex scalar fields
with appropiately choosen potential V
vacuum spontaneously breaks gauge symmetry
one new particle: the Higgs boson H
Φ = v + H
renormalisibility no precise calculation of observablesbad high energy behaviour WLWL scattering violates unitarity
at ECM~1.2 TeV
Higgs boson mass unknown: theory: unitarity MH<1TeV
LEP search: MH<114.4 GeV electroweak fit: MH<186 GeVexcluded at 95% CL at 95%CL
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Mass generation and Higgs couplings: Φ = v + H
effective mass = friction of particleswith omnipresent „Äther“
v =247 GeVx
fermion
gf
mf ~ gf v Yukawa coupling
MV ~ g v gauge coupling
x x
W/Z boson
g gauge
Interaction of particles with v=247 GeV
Fermions gf ~ mf / v
W/Z Bosons: gV ~ 2 MV / v
Interaction of particles with Higgs H
fermion
gf
x
W/Z boson
g gauge
Higgs
Higgs v
2
2VVH coupling ~ vev
only present after EWSB breaking !!!
unknown Higgs mass fixes Higgs profile completely
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50 100 200 100010-3
10-2
10-1
100
bb ττ cc tt gg γγ WW ZZ
Br
anch
ing
ratio
(Higgs
)
mH (GeV)
bb
WW
ZZ
ttττ
cc
gg
γγ
Higgs Boson Decays in SM
for M<135 GeV: H bb, ττ dominant
for M>135 GeV: H WW, ZZ dominant
tiny: H γγ also important
HDECAY: Djouadi, Spira et al.
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LHC
SPPS
pp collisions at the Large Hadron Collider
start in 2007 at ECM of 14 TeV
first years: low lumi running 10 fb-1/year
later (x10): high lumi running 100 fb-1/year
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Complexity of a LHC event
+ ~23 additional pp-interactions per bunch crossing at high lumi
~109 proton-proton collisions / sec
~1600 charged particles in detector per event
hard process
+ ISR,FSR
+ underlying
event
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The challenge of reconstruction
low lumi. high lumi.
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Production of the SM Higgs Boson at LHC
K = σ(N)NLO/σLO
K ~ 1.2
K ~ 2
K ~ 1.4
K ~ 1.1
Δσ ~4 to 15% + Δ from PDF (5 to15%)
signal MC at NLO only avaiable for GGF
background: NLO calculations often not avaiable
ATLAS: use K=1 and LO MC for signal and BG
CMS:K-factor for GGF + „guestimated“ K for BG
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Cross sections for background processes
Higgs 150 GeV: S/B <= 10-10
overwhelming background
trigger: 10-7 reduction
on leptons, photons, missing E
p pqq
q
p pq qq
q
HWW
l
l
ν
ν
background: mainly QCD driven
signal: often electroweak interaction
photons, leptons, …
q
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Considerations for Discovery Channels
sufficient signal rate no sensitivity to H μμ in SM
efficient trigger (γ,e,μ, …) no sensitivity for: GGF, WBF: H bb
H mass reconstructable? yes: H γγ, ZZ, bb, ττ no: H WW
control of background
- signal-to-background ratio - estimate of BG from data possible?
- sometimes: shape from MC needed
excl
uded
by
LEP
main discovery channels
GGF: H γγ
GGF: H ZZ 4l+-
GGF: H WW 2(lν)
ttH: H bb
VBF: H ττ
VBF: H WW
ATLAS
since
2000
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H 2 Photons
signal: two photons background:
- reducible: pp γj+x, jj+x,..photon vs. jet seperation
- irreducible: pp γγ+Xdiphoton mass
mass resolution σM/M = 0.7%
- uniformity/understanding of ECAL
- dead material: % of unconverted γ
- pile up: vtx constraint
background estimate from sidebands
no Monte Carlo needed
exp. uncertainty: O(0.1%)
S/BG ~ 1/10
CMS 100fb-1
red./irred. BG: 2 to 1 after final cut
LO NLO: increase of S/√B ~ 50 60%(K-factor ~ 2, larger Higgs Pt)
NLO
CMS
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H ZZ(*) 4 Leptons
reducible BG: tt, Zbb 4 leptons
lepton isolation and veto against b-jets
good mass resolution σM: <~1%muon spectrometer + tracking detectors
small and flat backgroundeasy estimate from sidebandsno Monte Carlo needed
CMS
CMS
100fb-1
NLO
signal: 4 iso. leptons1(2) dilepton mass = MZ
irreducible BG: ZZ 4 leptons
four lepton mass
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signal:- 2 leptons + missing ET
- lepton spin corrleations
- no mass peak transverse mass
H WW lν lν
ATLASM=160GeV30fb-1
transverse mass
BG: WW, WZ, tt
lepton iso., missing E resolution
jet (b-jet) veto against tt
BG estimate in data from ΔΦll
normalisation rom sideband
shape from MC
NLO effect on spin corr.
gg WW contribution signal like
Dührssen, prel.
ΔΦll
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Vector Boson Fusion: pp qqH
signature:
2 forward jets with
large rapidity gap
only Higgs decay products
in central part of detector
rapidity of tag jets rapidity difference
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Vector Boson Fusion: pp qqH
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Vector Boson Fusion: challenges
0
0.2
0.4
0.6
0.8
1
0 2 4
η
Efficie
ncy
pT>20GeV
forward jet reconstruction
influence of UE and pile up?
so far only for low lumi
ATLAS
central jet veto:
influence of UE and pile up? so far only for low lumi
efficiency of jet veto at NLO.?
distributions at NLO calculatedbut: need NLO MC generator
for signal and BG
η
Zeppenfeld et al.
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Vector Boson Fusion: H tau tau ll 4 ν (or l had 3 ν)
signature: tagging jets + 2 high pt leptons + large missing ET
backgrounds: tt,Zjjcentral jet veto
reconstructionof mττ
mass resolution ~ 10%dominated bei Emiss resolution
H ττ eμ
30 fb-1ATLAS
expected BG uncertainty ~ 5 to 10%for MH > 125 GeV: flat sidebandfor MH < 125 normalisation from Z
peak, but shape?
collinear approximation
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Idea: jjZ μμ and jjZ ττ μμ
look almost the same, esp. in calos
same missing energy
only μ momenta different
Method: select Z μμ events
randomise μ momenta
apply „normal“ mass reco.
Vector Boson Fusion, H ττ: estimate of BG shape from data
PTmiss,final (GeV) Mττ (GeV)
promising prel. results from ongoing diploma thesis in BN (M. Schmitz)
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Weak Boson Fusion: H WW llνν (lνqq)
H WW eμATLAS
10 fb-1
MH=160 GeV
H 2 photons
S/BG ~ 1/1
S/BG ~1/10
VBF versus inclusive channel
ATLASM=160GeV30fb-1
H WW ll
S/BG ~ 4/1
S/BG ~ 2/3
smaller rate larger sig-to-BG ratio smaller K-factor
more challenging for detector understanding
order of significance depends on channel and Higgs mass
CMS
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Discovery Potential for light SM Higgs boson
excl
uded
by
LEP
for GGF: raise of significance by ~ 50% from LO to NLO
results for cut based analysis improvement by multivariate methods
discovery from LEP exclusion until 1 TeV
from combination of search channels with 15 fb-1 of well understood data
with individual search channels with 30 fb-1 of well understood data
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Measurement of Higgs Boson Mass
1fb300L −=∫
ATLAS
ΔM/M: 0.1% to 1%
Uncertainties considered:
“Indirect”from Likelihood fit to transverse mass spectrum: H WW lνlνWH WWW lνlνlν
Direct from mass peak:H γγ H bb H ZZ 4l
VBF with H ττ or WW
not studied yet !
i) statistical
ii) absolute energy scale
0.1 (0.02) % for l,γ ,1% for jets
iii) 5% on BG + signal for H WW
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Determination of Higgs boson couplings
couplings in production σHx= const x ΓHx and decay BR(H yy) = ΓHy / Γtot
σHx x BR ~ΓHX ΓHy
Γtot
prod decay
partial width: ΓHz ~ gHz2
tasks: - disentangle contribution from production and decay
- determine Γtot
H WW chosen as reference as best measured for MH>120 GeV
for 30fb-1 worse by factor 1.5 to 2
model independent:only ratio or partial decay widths
13 signal rates used in global fit
(incl. various syst. uncertainties)
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for MH<200 GeV, Γtot<<mass resolution
no direct determination
have to use indirect constraints on Γtot
Absolute couplings with gV < gVSM constraint
coupling to W, Z, τ, b, t
Dührssen et al.
lower limit from observable rates:
Γtot > ΓW+ΓZ+Γt+Γg+....
upper limit needs input from theory:
mild assumption: gV<gVSM
rate(VBF,H WW)~ΓV2/Γtot<(ΓV
2 in SM)/Γtot
Γtot< rate/(ΓV2 in SM)
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ΔMH2 = αΛ2 =αMPlanck
Motivation for Supersymmetry from Higgs Sector
Higgs mass problem in SM:2
“solves” hierarchy problem: why v=246 GeV << MPl=1019GeV ?
ΔMH2 = α(MSM-MSUSY)
2 2
W W+ HH natural value ~ MPl
vs electroweak fit MH~O(100GeV)
SUSY solution:- partner with spin difference by ½ cancel divergence exactly if same M- SUSY broken in nature, but hierarchy still fine if MSUSY~1 TeV
H Hχ+
χ−-
SUSY breaking in MSSM:parametrised by 105 additional parameterstoo many constrained MSSM with O(5) additional parameters
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The MSSM Higgs sector in a tiny Nut Shell
SUSY: 2 Higgs doublets 5 physical bosonsreal MSSM: 2 CP even h, H, 1 CP odd A, charged H+, H-
at Born level 2 parameters: tanβ, m A mh < MZ
large corrections: mh < 133 GeV for mtop=175 GeV, MSUSY=1TeV
corrections depend on 5 SUSY parameters: Xt, M0 , M2, Mgluino, μfixed in the benchmark scenarios e.g. MHMAX scenario
maximal Mh conservative LEP exclusion
Mtop=174.3 GeVmain questions for LHC:
at least 1 Higgs boson observablein the entire parameter space?
how many Higgs bosons can be observed?
can the SM be discriminated from
extended Higgs sectors?
calculated with
FeynHiggs
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Light Higgs Boson h
large area covered by many channelssure discovery and parameter
determination possible
small area uncovered @ mh~95 GeV
300 fb-130 fb-1
VBF dominates observation
small area from bbh,h μμ
for small Mh
ATLAS preliminaryATLAS preliminary
VBF, H ττ covers hole plane
from either H or h observation
with 30fb-1
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Overall Discovery Potential: 300 fb-1
at least one Higgs boson
observable for all parameters
in all CPC benchmark scenarios
significant area where only lightest Higgs bosonh is observable
can H SUSY decays orHiggs from SUSY decaysprovide observation?
discrimination via h profiledetermination?
similar results in other benchmark scenarios
VBF channels , H/A ττ only used with 30fb-1
300 fb-1
ATLAS preliminary
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Higgs Decays to Gauginos
Msleptons=250 GeV
e.g. H0 → χ20 χ2
0 → χ10 χ1
0+4l
dominant background from SUSY
• results for most optimisticscenarios shown fine tuned
• require small slepton massesfor large BR(χ2 → χ1 +2 leptons )
• reduced sensitivity for other channels, consistent interpretation needed
CMS 100fb-1
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ATLASprel.
300 fb-1
SM or Extended Higgs Sector e.g. Minimal SUSY ?
discrimination via rates from VBF
R = BR(h WW) BR(h ττ)
assume Higgs mass well measuredno systematic errors consideredsame contribution from GGF
compare expectedmeasurement of R in MSSM with prediction from SMfor same value of MH
Δ=|RMSSM-RSM|/σexp
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CP-Violating MSSM: Overall Discovery Potential
MH1: < 70 GeVMH2: 105 to 120 GeVMH3: 140 to 180 GeV
small masses below 70 GeVnot yet studied in ATLAS
300 fb-1yet uncovered areasize and location of „hole“ depends on Mtop and program for calculation
ATLAS preliminary
at Born level: CP symmetry conserved in Higgs sector
complex SUSY parameters mixing between neutral CP eigenstates
no absolute mass limit from LEP
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Conclusion and Outlook
Standard model: discovery of SM Higgs boson with 15 fb-1
• requires good understanding of whole detector• multiple channels with larger luminosity Higgs profile investigaton
CP conserving MSSM: at least one Higgs boson observable• only h observable in wedge area at intermediate tanβ• maybe Higgs to SUSY or SUSY to Higgs observable?• discrimination via Higgs parameter determination seems promising
CP violating MSSM: probably a „hole“ with current MC studies• promising MC studies on the way see 2nd next talk
more realistic MC studies:• influence of miscalibrated and misaligned detector• improved methods for background estimation from data• use of NLO calcluations and MCs for signal and background
additional extended models + seach channels:• CPV MSSM, NMSSM, 2HDM• Higgs SUSY, SUSY Higgs• VBF, H bb (b-trigger at LV2), VBF,H inv. (add. forward jet trigger)