physics beyond the standard model: higgs and supersymmetry – why, what, where and how?
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Physics beyond the Standard Model: Higgs and Supersymmetry – Why, what, where and how?. Nanyang Technological University Singapore, January 2012 John Ellis, King ’ s College London & CERN. Plan of the Lectures. The Standard Model and issues beyond it - PowerPoint PPT PresentationTRANSCRIPT
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Physics beyond the Standard Model:Higgs and Supersymmetry –
Why, what, where and how?
Nanyang Technological UniversitySingapore, January 2012
John Ellis, King’s College London & CERN
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Plan of the Lectures
• The Standard Model and issues beyond it
• Origin of particle masses: Higgs boson or?
• Supersymmetry
• Searches for supersymmetry: LHC & dark matter
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Summary of the Standard Model
• Particles and SU(3) × SU(2) × U(1) quantum numbers:
• Lagrangian: gauge interactions
matter fermions
Yukawa interactions
Higgs potential
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Gauge Interactions of the Standard Model
• Three separate gauge group factors:– SU(3) × SU(2) × U(1)
– Strong × electroweak
• Three different gauge couplings:– g3, g2, g ́
• Mixing between the SU(2) and U(1) factors:
• Experimental value: sin2θW = 0.23120 ± 0.00015
Clue for Grand Unification and supersymmetry
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Weak Interactions
• Interactions of lepton doublets:
• Charged-current interactions:
• Neutral-current interactions:
• Mixing between quark flavours:
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Status of the Standard Model
• Perfect agreement with all confirmed accelerator data
• Consistency with precision electroweak data (LEP et al) only if there is a Higgs boson
• Agreement seems to require a relatively light Higgs boson weighing < ~ 180 GeV
• Raises many unanswered questions:
mass? flavour? unification?
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Precision Tests of the Standard Model
Lepton couplings Pulls in global fit
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Parameters of the Standard Model
• Gauge sector:– 3 gauge couplings: g3, g2, g ́– 1 strong CP-violating phase
• Yukawa interactions:– 3 charge-lepton masses– 6 quark masses– 4 CKM angles and phase
• Higgs sector:– 2 parameters: μ, λ
• Total: 19 parameters
Unification?
Flavour?
Mass?
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Open Questions beyond the Standard Model
• What is the origin of particle masses?due to a Higgs boson? + other physics?solution at energy < 1 TeV (1000 GeV)
• Why so many types of matter particles?matter-antimatter difference?
• Unification of the fundamental forces?at very high energy ~ 1016 GeV?probe directly via neutrino physics, indirectly via masses, couplings
• Quantum theory of gravity?(super)string theory: extra space-time dimensions?
Susy
Susy
Susy
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Why do Things Weigh?
0
Where do the masses come from?
Newton:Weight proportional to Mass
Einstein:Energy related to Mass
Neither explained origin of Mass
Are masses due to Higgs boson? (the physicists’ Holy Grail)
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Think of a Snowfield
Skier moves fast:
Like particle without mass
e.g., photon = particle of light
Snowshoer sinks into snow,
moves slower:
Like particle with mass
e.g., electron
Hiker sinks deep,
moves very slowly:
Particle with large mass
The LHC looks for
the snowflake:
the Higgs Boson
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The Higgs Boson and Cosmology
• Changed the state of the Universe when it was about 10-12 seconds old
• May have generated then the matter in the Universe
• Contributes (too much) to today’s dark energy
• A related inflaton might have expanded the Universe when it was about 10-35 seconds old
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The Higgs Mechanism
• Postulated effective Higgs potential:
• Minimum energy at non-zero value:
• Non-zero masses:
• Components of Higgs field:
• π massless, σ massive:
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Masses for Gauge Bosons
• Kinetic terms for SU(2) and U(1) gauge bosons:
where
• Kinetic term for Higgs field:
• Expanding around vacuum:
• Boson masses:
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The Seminal Papers
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The Englert-Brout-Higgs Mechanism
• Vacuum expectation value of scalar field
• Englert & Brout: June 26th 1964
• First Higgs paper: July 27th 1964
• Pointed out loophole in argument of Gilbert if gauge theory described in Coulomb gauge
• Accepted by Physics Letters
• Second Higgs paper with explicit example sent on July 31st 1964 to Physics Letters, rejected!
• Revised version (Aug. 31st 1964) accepted by PRL
• Guralnik, Hagen & Kibble (Oct. 12th 1964)
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The Englert-Brout-Higgs Mechanism
•Englert & Brout •Guralnik, Hagen & Kibble
• •
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The Higgs Boson
• Higgs pointed out a massive scalar boson
• “… an essential feature of [this] type of theory … is the prediction of incomplete multiplets of vector and scalar bosons”
• Englert, Brout, Guralnik, Hagen & Kibble did not comment on its existence
• Discussed in detail by Higgs in 1966 paper
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Nambu, EB, GHK and Higgs
Spontaneous breaking of symmetry
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A Phenomenological Profile of the Higgs Boson
• Neutral currents (1973)
• Charm (1974)
• Heavy lepton τ (1975)
• Attention to search for W±, Z0
• For us, the Big Issue: is there a Higgs boson?
• Previously ~ 10 papers on Higgs bosons
• MH > 18 MeV
• First attempt at systematic survey
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• Higgs decay modes and searches in 1975:
A Phenomenological Profile of the Higgs Boson
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• Couplings proportional to mass:– Decays into heavier particles favoured
• But: important couplings through loops:– gluon + gluon → Higgs → γγ
Higgs Decay Branching Ratios
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Constraints on Higgs Mass
• Electroweak observables sensitive via quantum loop corrections:
• Sensitivity to top, Higgs masses:
• Preferred Higgs mass: mH ~ 80 ± 30 GeV• Compare with lower limit from direct searches:
mH > 114 GeV
• No conflict!
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The State of the Higgs in Mid-2011
• High-energy search:– Limit from LEP:
mH > 114.4 GeV
• High-precision electroweak data:– Sensitive to Higgs mass:
mH = 96+30–24 GeV
• Combined upper limit:
mH < 161 GeV, or 190 GeV including direct limit
• Exclusion from high-energy search at Tevatron:
mH < 158 GeV or > 173 GeV
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Combining the Information from Previous Direct Searches and Indirect Data
mH = 125 ± 10 GeV •Gfitter collaboration
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Latest Higgs Searches @ Tevatron
Exclude (100,109); (156,177) GeV
Standard Model
prediction
Experimental
upper limit
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Higgs Production at the LHC
A la recherche du Higgs perdu …
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Higgs Hunting @ LHC: Status reported on Dec. 13th, 2011
Exclude 112.7 GeV to 115.5 GeV,
131 GeV to 237 GeV,
251 GeV to 453 GeV
Exclude 127 to 600 GeV
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Has the Higgs Boson been Discovered?
Interesting hints around Mh = 125 GeV ?
CMS sees broad
enhancement
ATLAS prefers
125 GeV
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Has the Higgs Boson been Discovered?
Interesting hints around 125 GeV in both experiments
- but could also be 119 GeV ?
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ATLASSignals
• γγ: 2.8σ• ZZ: 2.1σ• WW: 1.4σ• Combined: 3.6σ
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CMS Signals
Combined: 2.6σ
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Has the Higgs Boson been Discovered?
Unofficial blogger’s combination
NOT ENDORSED BY EXPERIMENTS
but he was right last time !
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Combining the Information from Previous Direct Searches and Indirect Data
mH = 125 ± 10 GeVErler: arXiv:1201.0695
mH = 124.5 ± 0.8 GeV
Assuming the Standard Model
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The Spin of the Higgs Boson @ LHC
Higher mass: angular correlations in H → ZZ decays
Low mass: if H →γγ,It cannot have spin 1
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Do we already know the ‘Higgs’ has Spin Zero ?
• Decays into γγ, so cannot have spin 1
• 0 or 2?
• If it decays into ττ or b-bar: spin 0 or 1 or orbital angular momentum
• Can diagnose spin via angular correlations of leptons in WW, ZZ decays
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For Mh = 120 GeV
Higgs Measurements @ LHC & ILC
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•
• • Precision
Electroweak
data??
Higgs
coupling
blows up!!
Higgs
potential
collapses Higgs coupling less
than in Standard Model
•viXra Blogger’s Combination
•of Dec.13th Data
There must be New Physics
Beyond the Standard Model
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Heretical Interpretation of EW Data
Do all the data
tell the same story?
e.g., AL vs AH
What attitude towards LEP, NuTeV?
What most
of us think
Chanowitz
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Spread looks natural: no significant disagreement
Estimates of mH from different Measurements
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Higgs + Higher-Order Operators
Precision EW data suggest they are small: why?
But conspiracies
are possible: mH
could be large,
even if believe
EW data …?Do not discard possibility of heavy Higgs
Corridor to
heavy Higgs?
Barbieri, Strumia
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Elementary Higgs or Composite?
• Higgs field:
<0|H|0> ≠ 0• Quantum loop problems
• Fermion-antifermion condensate
• Just like QCD, BCS superconductivity
• Top-antitop condensate? needed mt > 200 GeV
• New technicolour force? heavy scalar resonance?• inconsistent with precision electroweak data?
• Cut-off Λ ~ 1 TeV with• Supersymmetry?
•Cutoff
•Λ = 10 TeV
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Comparison between Weakly- and Strongly-coupled Models
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Interpolating Models
• Combination of Higgs boson and vector ρ
• Two main parameters: mρ and coupling gρ
• Equivalently ratio weak/strong scale:
gρ / mρ
Grojean, Giudice, Pomarol, Rattazzi
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Generic LittleHiggs Models
Loop cancellation mechanism
SupersymmetryLittle Higgs
(Higgs as pseudo-Goldstone
boson of larger symmetry)
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Little Higgs Models
• Embed SM in larger gauge group• Higgs as pseudo-Goldstone boson• Cancel top loop
with new heavy T quark
new gauge bosons, Higgses• Higgs light, other new
physics heavyNot as complete as susy: more physics > 10 TeV
MT < 2 TeV (mh / 200 GeV)2
MW’ < 6 TeV (mh / 200 GeV)2
MH++ < 10 TeV
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What if the Higgs is not quite a Higgs?
• Tree-level Higgs couplings ~ masses– Coefficient ~ 1/v
• Couplings ~ dilaton of scale invariance• Broken by Higgs mass term –μ2, anomalies
– Cannot remove μ2 (Coleman-Weinberg)– Anomalies give couplings to γγ, gg
• Generalize to pseudo-dilaton of new (nearly) conformal strongly-interacting sector
• Couplings ~ m/V (V > v?), additions to anomalies
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A Phenomenological Profile of a Pseudo-Dilaton
• New strongly-interacting sector at scale ~ V• Pseudo-dilaton only particle with mass << V• Universal suppression of couplings to Standard
Model particles ~ v/V• Γ(gg) may be enhanced• Γ(γγ) may be suppressed• Modified self-couplings• Pseudo-baryons
as dark matter?
•Compilation
•of constraints
•Campbell, JE, Olive: arXiv:1111.4495
•Updated
•with Dec. 11
•constraints
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Higgsless Models?
• Four-dimensional versions:
Strong WW scattering @ TeV, incompatible with precision data?
• Break EW symmetry by boundary conditions in extra dimension:
delay strong WW scattering to ~ 10 TeV?
Kaluza-Klein modes: mKK > 300 GeV?
compatibility with precision data?• Warped extra dimension + brane kinetic terms?
Lightest KK mode @ few 00 GeV, strong WW @ 6-7 TeV
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Theorists getting Cold Feet
• Composite Higgs model?conflicts with precision electroweak data
• Interpretation of EW data?consistency of measurements? Discard some?
• Higgs + higher-dimensional operators?corridors to higher Higgs masses?
• Little Higgs models?extra `Top’, gauge bosons, `Higgses’
• Higgsless models?strong WW scattering, extra D?
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Particle Spectrum in Simplest Model with Extra Dimensions
Lowest-lying states have flat wave functions (n = 0)
Excitations (Kaluza-Klein) have nodes (n > 0):
Mass ~ n/R (R = radius of circle)
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‘Fold’ Circle: Orbifold
• Identify two halves of circle: up to a minus sign
• ‘Even’ particles include massless: odd ones all massive• A way to give masses to particles that are asymmetric
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Mechanism to break Gauge Symmetry
• Identify two halves up to a group transformation U
• Unbroken part of gauge group commutes with U• Masses for asymmetric particles:
– e.g., SU(2) × U(1) U(1)
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Search for Vector Resonance in Higgsless Model
Vector resonance structure in WZ scattering
Simulation of resonance structure in mWZ @ LHC
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Theoretical Constraints on Higgs Mass
• Large Mh → large self-coupling → blow up at low-energy scale Λ due to renormalization
• Small: renormalization due to t quark drives quartic coupling < 0at some scale Λ→ vacuum unstable
• Vacuum could be stabilized by supersymmetry•Espinosa, JE, Giudice, Hoecker, Riotto, arXiv0906.0954
•LHC 95%
•exclusion
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Vacuum Stability vs Metastability
• Dependence on scale up to which Standard Model remains
– Stable
– Metastable at non-zero temperature
– Metastable at zero temperature
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What is the probable fate of the SM?Confidence Levels (CL) for different fatesConfidence Levels (CL)
without/with Tevatron exclusion
Blow-up excludedat 99.2% CL
Espinosa, JE, Giudice, Hoecker, Riotto
CL asfunction ofinstability
scale
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The LHC will Tell the Fate of the SM
Espinosa, JE, Giudice, Hoecker, Riotto
Examples with LHC measurement of mH = 120 or 115 GeV
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How to Stabilize a Light Higgs Boson?
• Top quark destabilizes potential: introduce introduce stop-like scalar:
• Can delay collapse of potential:• But new coupling must be
fine-tuned to avoid blow-up:• Stabilize with new fermions:
– just like Higgsinos
• Very like Supersymmetry!
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•Favoured values of Mh ~ 119 GeV:
•Range consistent with evidence from LHC !
•Higgs mass
•χ2 price to pay if Mh = 125 GeV is < 2
•Buchmueller, JE et al: arXiv:1112.3564
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The Stakes in the Higgs Search
• How is gauge symmetry broken?• Is there any elementary scalar field?• Would have caused phase transition in the Universe when
it was about 10-12 seconds old• May have generated then the matter in the Universe:
electroweak baryogenesis• A related inflaton might have expanded the Universe
when it was about 10-35 seconds old • Contributes to today’s dark energy: 1060 too much!
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Intermediate Models
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Effects on Higgs Decays
• Dependences on of Higgs branching ratios
• Standard Model recovered in limit 0
•Grojean, Giudice, Pomarol, Rattazzi
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At what Energy is the New Physics?
A lot accessibleto the LHC
Some accessible only indirectly:Astrophysics & cosmology?
Dark matter
Origin of mass
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Indications on the Higgs Mass
Sample observable:W mass @ LEP & Tevatron
Combined informationon Higgs mass
Summer 2008
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The State of the Higgs: November 2009
• Direct search limit from LEP:
mH > 114.4 GeV
• Electroweak fit sensitive to mt
(Now mt = 173.1 ± 1.3 GeV)
• Best-fit value for Higgs mass:
mH = 89+35–26 GeV
• 95% confidence-level upper limit:
mH < 157 GeV, or 186 GeV including direct limit
• Tevatron exclusion:
mH < 162 GeV or > 166 GeV
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Higgs Search @ Tevatron
Tevatron excludes Higgs between 162 & 166 GeV
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Higgs Search @ Tevatron
Tevatron excludes Higgs between 162 & 166 GeV
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Combining the Higgs Information
mH = 116.4+ 15.6-1.3 GeV
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Progress of Tevatron
Higgs Search
Possibility of
future evidence
Past improvements
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Theoretical Constraints on Higgs Mass
• Large → large self-coupling → blow up at low energy scale Λ due to renormalization
• Small: renormalization due to t quark drives quartic coupling < 0at some scale Λ→ vacuum unstable
• Bounds on Higgs mass depend on Λ
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The Large Hadron Collider (LHC)
Proton- Proton Collider
7 TeV + 7 TeV
1,000,000,000 collisions/second
Primary targets: •Origin of mass•Nature of Dark Matter•Primordial Plasma•Matter vs Antimatter
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Event rates in ATLAS or CMS at L = 1033 cm-2 s-1
Huge Statistics thanks to High Energy and Luminosity
LHC is a factory for anything: top, W/Z, Higgs, SUSY, etc…. mass reach for discovery of new particles up to m ~ 5 TeV
Process Events/s Events per year Total statistics collected at previous machines by 2007
W e 15 108 104 LEP / 107 Tevatron
Z ee 1.5 107 107 LEP
1 107 104 Tevatron
106 1012 – 1013 109 Belle/BaBar ?
H m=130 GeV 0.02 105 ?
m= 1 TeV 0.001 104 ---
Black holes 0.0001 103 ---m > 3 TeV (MD=3 TeV, n=4)
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The LHC Physics Haystack(s)
Interesting cross sections
Higgs
Susy
• Cross sections for heavy particles
~ 1 /(1 TeV)2
• Most have small couplings ~ α2
• Compare with total cross section
~ 1/(100 MeV)2
• Fraction ~ 1/1,000,000,000,000
• Need ~ 1,000 events for signal
• Compare needle
~ 1/100,000,000 m3
• Haystack ~ 100 m3
• Must look in ~ 100,000 haystacks
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The LHC’s Most Anticipated Discovery
a Higgs boson
a Higgs boson
1 fb-1
…
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The LHC Roulette WheelStandard Model
Higgs boson
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A Simulated Higgs Event in CMS
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Higgs Production at the LHC
A la recherche du
Higgs perdu …
… not far away?
Combining direct,
Indirect information
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Some Sample Higgs SignalsA la recherche du Higgs perdu …
γγ
ZZ* -> 4 leptonsττ
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Some Sample Higgs Signals
ttH
bbH
γγZZ* -> llll
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Measuring Higgs Self-Coupling
Heavier Higgs possible @ SLHCLight Higgs @ low-energy LC
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When will ATLAS discover the Higgs boson?
Duehrssen, La Thuile 2010
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When will the LHC discover the Higgs boson?
1 ‘year’ @ 1033
‘month’ @ 1033
‘month’ @ 1032
Blaising, JE et al: 2006
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The Stakes in the Higgs Search
• How is gauge symmetry broken?• Is there any elementary scalar field?• Would have caused phase transition in the Universe when
it was about 10-12 seconds old• May have generated then the matter in the Universe:
electroweak baryogenesis• A related inflaton might have expanded the Universe
when it was about 10-35 seconds old • Contributes to today’s dark energy: 1060 too much!
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International Linear Collider
• e+e- collisionsup to Ecm = 1 TeV
• Option for next collider
• Now subject of Global Design Effort
• Hope for decisionby 2012?
• To be constructed by2020?
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Tasks for the TeV ILC
• Measure mt to < 100 MeV
• If there is a light Higgs of any kind, pin it down:
Does it have standard model couplings?
What is its precise mass?
• If there are extra light particles: Measure mass and properties
• If LHC sees nothing new below ~ 500 GeV:
Look for indirect signatures
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Measuring Properties of Light Higgs
Measuring top-Higgs couplings
Other studies …LC capabilities
bb, ττ, gg, cc, WW, γγ
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For Mh = 120 GeV
For Mh = 140 GeV
Higgs Measurements @ ILC & LHC
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Sensitivity to Strong WW scattering
@ LHC @ 800 GeV LC
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Measuring a WW Resonance
Form factor measurements@ 500 GeV LC
Resonance parameters@ LHC
Resonance parameters @ 500 GeV LC
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After LHC @ CERN - CLIC?
Electron-Positron
collisions up to 3 TeV
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Can measure rare decay modes …
Large Cross Section @ CLIC
H bb
Δg/g = 4% Δg/g = 2%
mH = 120 GeV mH = 180 GeV
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If the Higgs boson is heavier …
Can establish its existencebeyond any doubt if < 1 TeV:
ee H ee
Find resonance in strongWW scattering if > 1 TeV:
ee H νν
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Measuring Effective Higgs Potential @ Higher-Energy LC
Large cross section
for HH pair production
Accuracy in measurement of HHH coupling
MH = 240 GeV
180 GeV
140 GeV
120 GeV
11%
9%