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Higgs Studies at the LHC and the ILC. Albert De Roeck CERN SUSY 2005 18-23 July Durham . The Higgs Mechanism. Higgs, Englert and Brout propose to add a complex scalar field to the Lagrangian. Expect at least one new scalar particle: The (Brout-Englert-) Higgs particle.

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Higgs studies at the lhc and the ilc l.jpg

Higgs Studies at the LHC and the ILC

Albert De Roeck CERN

SUSY 2005

18-23 July Durham

The higgs mechanism l.jpg
The Higgs Mechanism

  • Higgs, Englert and Brout propose to add

  • a complex scalar field to the Lagrangian

Expect at least one new scalar particle:

The (Brout-Englert-) Higgs particle

  • SM Higgs (LEP)

    • MH>114.1 GeV @95% CL

  • MSSM neutral Higgs bosons (LEP)

    • Mh, MA>92.9, 93.3 GeV @95% CL

    • MH± >89.6 GeV @95% CL for BR(MH±→ τν) =1

    • MH± >78.6 GeV @95% CL for any BR

  • Electroweak fits to all high Q2 measurements give:

    • MH=98+52-36 GeV (old top mass)

    • MH<186 GeV @ 95% CL (“yesterdays” new top mass)

  • Tevatron searches see C. Tully’s talk

Probably the most wanted

particle in HEP

Discover … or prove that it

does not exist

High energy frontier in hep l.jpg
High Energy Frontier in HEP

Next projects on the HEP roadmap

  • Large Hadron Collider LHC at CERN: pp @ 14 TeV

    • LHC will be closed and set up for beam on 1 July 2007

    • First beam in machine: August 2007

    • First collisions expected in November 2007

    • Followed by a short pilot run

    • First physics run in 2008 (starting April/May; a few fb-1? )

  • Linear Collider (ILC) : e+e- @ 0.5-1 TeV

    • Strong world-wide effort to start construction earliest around 2009/2010, if approved and budget established

    • Turn on earliest 2015 (in the best of worlds)

    • Study groups in Europe, Americas and Asia (World Wide Study)

M. Lamont

Tev4LHC meeting

@ CERN (April)

Quest for the Higgs(*) particle is a major motivation for these new machines

(*) will discuss mostly the Standard Model Higgs in this talk

Higgs roadmap l.jpg
“Higgs Roadmap”

  • Discover the Higgs (in the range 114.4 GeV < MH < 1 TeV)

  • Determine its properties/profile

    • The mass

    • Spin and parity quantum numbers

    • How does it decay?

      • Measure Yukawa like patterns

      • Measure relations between fermion and gauge boson couplings

      • Observe rare decay modes

      • Observe unexpected decay modes? (new particles?)

      • Measure total width

  • Reconstruction of the Higgs potential by determination of the Higgs self coupling

  • Its nature: is it standard, supersymmetric, composite.

BOTH LHC and LC will be crucial in establishing Higgs Dynamics

Lhc pp collisions at 14 tev l.jpg
LHC: pp Collisions at 14 TeV

  • ~20 min bias events overlap

    at 1034cm-2 s-1

  • HZZ Z mm

    H 4 muons the cleanest

    (“golden”) signature

    This (not the H production !!) repeats every 25 ns…

Sm higgs production l.jpg
SM Higgs production

NLO Cross sections

M. Spira et al.

gg fusion

IVB fusion

Sm higgs search channels l.jpg
SM Higgs search channels

Low mass MH ≲ 200 GeV

M. pieri

Intermediate mass

(200 GeV ≲ MH ≲700 GeV)

High mass (MH ≳ 700 GeV)

  • VBF qqH →ZZ →ℓℓνν

  • VBF qqH →WW →ℓνjj

  • inclusive H → WW

  • inclusive H → ZZ

H → γγ and H → ZZ* → 4ℓ are the only channels with a very good mass resolution ~1%

Examples l.jpg

High MH > ~500 GeV/c2

Medium 130<MH<500 GeV/c2

Low MH < 140 GeV/c2

Vector boson fusion channels l.jpg
Vector Boson Fusion Channels

Dokshitzer, Khoze, Troyan;

Rainwater, Zeppenfeld et al.

ppqqH +X Higgs and two forward jets (|| ~ 3)



Tag jets to



With these new channels each experiment

can discover the Higgs with 5 with 30 fb-1

Other channels h bb l.jpg
Other Channels (Hbb)



30 fb-1

Not discovery channels but can be used to confirm/measure couplings

Diffractive higgs production l.jpg







Diffractive Higgs Production

SM Higgs:

Cross section ~3fb (Khoze et al)

MSSM: s ~ x10 larger (tan)


100 fb

  • Exclusive production:

  •  Jz=0 suppression of ggbb bkg

  • Higgs mass via missing mass

  • CP structure of the Higgs from angular distribution of the protons

  • Of course, need Roman potsFP420 project


Kaidalov et al.,


M = O(1.0 - 2.0) GeV


Also HWW*

Lhc reach for a higgs discovery l.jpg
LHC Reach for a Higgs Discovery

Total sensitivity

Different channels

30 fb-1 2-3 years

LHC can cover the whole region of interest with 10 fb-1

Mass and width resolution l.jpg
Mass and Width Resolution




MSSM HiggsDm/m (%) 300 fb-1

h, A, H  gg0.1-0.4

H  4  0.1-0.4

H/A  mm 0.1-1.5

h  bb 1-2

H  hh  bb gg 1-2

A  Zh  bb  1-2

H/A  tt 1-10

Analysis of indirect widthsfor mass range below 200 GeV:

10-20% precision

Branching ratios and couplings l.jpg
Branching Ratios and Couplings

Precision on BR

Ratios of couplings

With “mild” theoretical


Cannot determine total Higgs cross section

No absolute meas. of partial dec. widths

Duhrssen et al., hep-ph/0406323

Precision 10-40 (20)%


(within 5%)

Also measurement of H

Dominated by luminosity


Precision 10-40%

Spin and cp quantum numbers h zz 4l l.jpg

Higgs rest frame

Spin and CP-quantum Numbers: H  ZZ4l


100 fb-1

  •  MH>250 GeV: distinguish between S=0,1 and CP even.odd

  •  MH<250 GeV: only see difference between SM-Higgs and S=0, CP=-1

  •  , less powerful

Heavy mssm higgs search l.jpg
Heavy MSSM Higgs Search

  • H  

  • H  tb

  • A/H  

  • A/H  

  • A/H bb/  in bb H/A

MSSM 5 Higgses: h,H,A,H

At low tan , we may exploit

the sparticle decay modes:

Contours for 5  discovery

MHMAX scenario

New: includes VBF channels

 A, H 20 20  4l+ ETmiss

 A, H in cascade decays of sparticles

Cp violating scenario l.jpg
CP Violating Scenario

M. Schumacher

  • CP eigenstates h, A, H mix to mass eigenstates H1, H2, H3

  • maximise effect  CPX scenario(Carena et al., Phys.Lett B495 155(2000))

  • arg(At)=arg(Ab)=arg(Mgluino)=900

Small area remains uncovered

Could be covered by MH1 < 70 GeV

(not studied yet)

Significant dependence on the

top mass (now 172.7±2.9 GeV)

Higgs studies at an e e linear collider l.jpg
Higgs Studies at an e+e- Linear Collider

  •  L > 1034cm-2s-1 80% electron polarization

  •  Energy flexibility between √s = 90-500 GeV

  • Future: possibility of γγ, e-e-, e+ polarization, Giga –Z

Can detect the

Higgs via the recoil

to the Z

e.g. Desch



 Fully simulated+reconstructed HZ event

 Backgrounds low  Robust signal: if (eeH+x) 100 times lower, still observable

Observation of the Higgs independent of decay modes

Higgs production at an e e linear collider l.jpg
Higgs Production at an e+e- Linear Collider

Dominant production processes at ILC:



 ~ln(s)

Example: s=350 GeV

mH = 120 GeV

L= 500 fb-1 (~2-4 years)

~90 K Higgs events produced

 ~1/s

Higgs mass measurement l.jpg
Higgs Mass Measurement

Garcia-Abia, et al., hep-ex/0505096

s= 350 GeV 500 fb-1

Beam systematics included

Determine the Higgs mass to

about 40-70 MeV

How much can theory handle/does theory want?

Higgs branching ratios l.jpg
Higgs Branching Ratios

Tim Barklow, LCWS04

  • Model independent

  • Absolute branching ratios! Normalized to absolute HZ cross section

  •  Precise measurements: few % to 10%.

  •  Special options to improve further e.g. BR(H) ~ 2% at photon collider

Extraction of higgs couplings l.jpg
Extraction of Higgs Couplings

  • Use measured branching ratios to extract Higgs couplings to fermions

  • and bosons

  • Global fit to all observables (cross sections and branching ratios)

    & take into account correlations

  • The precise determination of the effective couplings opens a window

  • of the sensitivity to the nature of the Higgs Boson

TESLA-TDR values

Rare higgs decay modes l.jpg
Rare Higgs Decay Modes

  • Rare Higgs decay modes become accessible eg

  • Hbb at higher masses (Yukawa couplings)

  • H

  • HZ

gH/gH ~15%

for 1 ab-1


gHbb/gHbb ~17%

for 1 ab-1

H a search at a photon collider l.jpg
H,A Search at a Photon Collider

J. Gunion et al.

M. Krawczyk et al.

 Extent discovery range to close to kinematic range= 0.8Ecms(e+e-)

Measurement of / to10-20% with 1 year of data

Invisible higgs decays l.jpg
Invisible Higgs Decays

Invisible Higgs decays –Higgs

decay in undetected particles-

can be observed directly in

ZH events

 Observe a peak in the

recoil mass of ZH events

Sum of width

Branching ratio can be determined

with good precision:

Better than 5% for large enough

branching ratios


Spin and cp quantum numbers l.jpg
Spin and CP Quantum Numbers

 At threshold: determine J from the  dependence of ZH

 At continuum: use angular distributions to determine CP composition

HZ production

+ also H

Top higgs yukawa coupling l.jpg
Top-Higgs Yukawa coupling

  • The top-Higgs Yukawa coupling is very large (gttH ~ 0.7 while gbbH ~ 0.02). Precise measurements important since could could show largest deviations to new physics

  • Needs 0.8-1.0 TeV collider and large luminosity

  • If mH<2mt e+e- ttH

  • If mH>2mtmeasure BR(Htt)

Lhc lc data top yukawa coupling l.jpg
LHCLC data: Top Yukawa coupling

Dawson, Desch, Juste, Rainwater, Reina, Schumacher, Wackeroth

Assume a light Higgs < 2mt

Production processes

LC: e+e-  ttH No precise measurement at 350-500 GeV LC

LHC: gg  ttH measures •BR (ttbb,ttWW)

depends on g2ttH g2bbH and g2ttH g2WWH

g2bbH and g2WWH can be measured precisely in a

model independent way at the ILC (few %)

  can determine g2ttH without any model assumptions

LHC alone~ 0.3

(and model



350 GeV 500 fb-1

Measuring the higgs potential l.jpg
Measuring the Higgs Potential

 Measure the Higgs self-coupling: HH production

Larger precision at higher energies

Eg CLIC: a 3 to 5 TeV LC

MH = 240 GeV

180 GeV

140 GeV

120 GeV

LHC: gHHH (3000 fb-1) for 150<MH<200 GeV

Slide30 l.jpg

~3 years

~1 year

5  discovery

mH > 114.4

Summary: Higgs at the LHC and LC

  • Higgs can be discovered over full allowed mass range in 1 year of (good)

  • LHC operation

  •  final word about SM Higgs mechanism

  • However: it will take time to understand and calibrate ATLAS and CMS

  • If Higgs found, mass can be measured to 0.1% up to mH~ 500 GeV

  • A LC will provide precision measurements on absolute couplings ~%, quantum

  • numbers (spin, CP…), rare decays of the Higgs, and the Higgs potential

  • A LC aims for a full validation of the Higgs Mechanism

Lhc higgs summary l.jpg
LHC Higgs Summary

  • LHC will discover the SM Higgs in the full region up to 1 TeV or exclude its existence. If no Higgs, other new phenomena in the WW should be observed around 1 TeV

  • The LHC will measure with full luminosity (300 fb-1)

  • The Higgs mass with 0.1-1% precision

  • The Higgs width, for mH> 200 GeV, with ~5-8% precision

  • Cross sections x branching ratios with 6-20% precision

  • Ratios of couplings with 10-40% precision

  • Absolute couplings only with additional assumptions

  • Spin information in the ZZ channel for mH>200 GeV

Ilc higgs summary l.jpg
ILC Higgs Summary

  • The Higgs cannot escape the ILC, if within its kinematical range

  • The Higgs mass can be measured down to 40-70 MeV

  • Absolute branching ratios can be determined to the % level

  • Couplings can be determined to the % level

  • Note new phenomena such as heavy vector bosons or Higgs triplets give contributions to the Higgs couplings of O(5%)

  • Rare decay modes can be studied

  • Invisible decay modes can be detected (to some level also at the LHC)

  • Spin and CP quantum numbers can be determined

  • The Higgs potential can be measured (particulalry with a multi-TeV LC)

  • LHC+ILC(500) combined data give the best top-yukawa coupling measurement