0

5

10

15

20

25

0 20 40 60 80 100 120

mHrec (GeV/c2)

Eve

nts

/ 3 G

eV/c

2

LEP Loose√s– = 200-209 GeV

Data

Background

Signal (115 GeV/c2)

Data 119

Backgd 116.5

Signal 10

all > 109 GeV/c2

17

15.8

7.1

) 2

(GeV/cHHiggs Mass m100 105 110 115 120 125 130 135 140

)-1

Int.

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SUSY/Higgs Workshop(’98-’99)

Higgs Sensitivity Study (’03)statistical power only(no systematics)

Discoveryσ5 Evidenceσ3

95% CL Exclusion

1

10

10 2

102

103

mH (GeV)

Sig

nal s

igni

fica

nce

H → γ γ + WH, ttH (H → γ γ ) ttH (H → bb) H → ZZ(*) → 4 l

H → ZZ → llνν H → WW → lνjj

H → WW(*) → lνlν

Total significance

5 σ

∫ L dt = 100 fb-1

(no K-factors)

Electroweak Symmetry Breaking:Experimental Status Report

Peter RatoffLancaster University

TevatronLHC

NLC

Outline

• Introduction• Precise EW limits (very brief - see Bob Clare’s talk)

• LEP Higgs Search – the (recent) past

• Tevatron Higgs Potential – the ‘present’

• LHC Higgs Prospects – the (near) future

• e+e- Linear Collider Higgs Prospects – the (far) future

• Summary/Conclusions

~ almost no theory – see Sally Dawson’s talk

Electroweak Symmetry Breaking – The Higgs Mechanism

The Higgs Boson

(see Sally Dawson’s talk)

The Waldergrave¶ Challenge

¶ William Waldergrave, UK Science Minister, late 1980’s

… to UK particle physicists: “Explain the Higgs boson to the general public”

Higgs enters the broader culture …1

2

3

Higgs field generates all masses …

mf = resistance to movementν = density of the crowd (“Vev”)

mf = gHff . ν

…with a prize winning explanation!

Higgs generates its own mass …(but its value is unknown)

1 2

MH² = 2 λν²

V = µ² |Φ²| + λ (|Φ²|)² (µ<0)

… and the cultural connection broadens!

… into contemporaryMusic

Higgs Boson -28 year old BritishJazz Fusion Piano/Keyboardartist

www.higgsboson.fsnet.co.uk

A brief history of Higgs searchesMany reported ‘sightings’, but none confirmed … !

1981-2001 ...Two decades of increasingly tighter limits, spurious signals, moments of great excitement, but many disappointments ...

L.Okun’s Summary Talk: LP’81

…. described scalar particles as the #1 problem in particle physics!

“Scalars are at the epicentre of particle physics. The theoretical seaquake,the eruption and tumbling of numerous theoretical models, heralds the birth of a new physical continent.”

“Painstaking search for light scalars should be considered as the highestpriority for the existing machines such as CESR, PETRA, PEP and theCERN SPS Collider, and even more for the next generation of accelerators, such as LEP, Tevatron, UNK, and HERA. Especiallypromising is the project of a very high energy electron-positron linearcollider. The future of theoretical physics depends on the energy andluminosity of these machines.”

n.b. LHC/SSC not even on the horizon!

Lepton Photon Symposium, Bonn, 1981

Technipion search at PETRA (1981)

Reported at LP’81 (Bonn)

Dynamical symmetry breaking by bound statesof new types of quarks and leptons with super-strong interactions called technicolour

Quartet of scalar colour singlet Goldstone bosons (technipions) predicted in a minimally extended technicolour scheme=> technipion masses ~ 10-20 GeV ?

Search for e+ e- → P+ P- → τ + ν + hadrons

=> 2 acoplanar jets, one consistent with τ decay

n.b. this search also sensitive to charged Higgs bosons in SUSY models; they have similar properties to the charged technipions

First (?) search for new scalar particles ...

Crystal Ball’s Zeta(8.3)

Wilczek mechanism (Heavy Quarkonium radiative decay): HQQ +→γBR(J/ψ→ γ + H) ~ 6 x 10-5 BR(ϒ→ γ + H) ~ 2.5 x 10-4

LP’85

4.1 σ signalBR~0.5%

n.b. CESR(CUSB) previously set 90% CL limit of ~ 3 x 10-3 (LP’83)

e+e- → ϒ(1S) → γ + X DORIS(DESY)

12 Year Higgs Search at LEPAugust 1989 – November 2000

LEP SM Higgs Search Published (final paper!)

The global EWWG fit

NEW: MW(Aleph) lower, small shiftsin heavy flavors, atomic PV close to SM(new Mt D0 Run I and CDF Run II not included)

OVERALL, SM fares wellexcept for NuTeV

(more details in Bob Clare’s talk)

MH=96 GeV, MH<219 GeV at 95%CLχ 2/dof=25.4/15 4.5% prob

MH=91 GeV, MH<202 GeV at 95%CLχ 2/dof=16.8/14 26.5% prob

without NuTeV

The global EWWG fit

low x

high x

ss∫ =− !0)]()([ dxxsxs

Kretzer, Olness, Pumplin, Stump,Tung et al.

Impact on RPW in NuTeV setup estimated w.r.t. to CTEQ s=sbar fit:

0.0012 < δs2w < 0.0037→ reduce discrepancy below 2σ ?

NuTeV – Strange Sea Asymmetry?

Higgs Production at LEP

“Higgs-strahlung”

Vector Boson Fusion

Higgs Search Channels

4 Jet ALEPH Higgs Candidate (double b-tag)

Reconstructed MH

0

5

10

15

20

25

0 20 40 60 80 100 120

mHrec (GeV/c2)

Eve

nts

/ 3 G

eV/c

2

LEP Loose√s– = 200-209 GeV

Data

Background

Signal (115 GeV/c2)

Data 119

Backgd 116.5

Signal 10

all > 109 GeV/c2

17

15.8

7.1

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120

mHrec (GeV/c2)

Eve

nts

/ 3 G

eV/c

2

LEP Tight√s– = 200-209 GeV

Data

Background

Signal (115 GeV/c2)

Data 18

Backgd 14

Signal 2.9

all > 109 GeV/c2

4

1.2

2.2

main background: e+e- → ZZ

low purity selection high purity selection

0 +-

Higgs Discovery ?

10-3

10-2

10-1

1

80 85 90 95 100 105 110 115 120

mH(GeV/c2)

1-C

Lb

3σ

2σ

LEP

ObservedExpected for signal plus backgroundExpected for background

1 – CLb = prob. to obtain more ‘s+b like’ event config. than observed

statisticalfluctuation!

S~0.1 SM rate

compatible with SM Higgs boson and with backgroundhypothesis

LEP Higgs Mass Lower Bound

10-6

10-5

10-4

10-3

10-2

10-1

1

100 102 104 106 108 110 112 114 116 118 120

mH(GeV/c2)

CL

s

114.4115.3

LEP

Observed

Expected forbackground

CLs = CLs+b/CLb

When CLs(MH) < 5% a Higgs boson with a mass MH is excludedat > 95% CL

LEP excludes a 114.4 GeV Higgs boson

(Expected 115.3 GeV at the 95% CL)

MSSM Higgs

1

10

0 20 40 60 80 100 120 140

1

10

LEP 88-209 GeV Preliminary

mh° (GeV/c2)

tanβ

Excludedby LEP

TheoreticallyInaccessible

mh°-max

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 1400

20

40

60

80

100

120

140

160 LEP 88-209 GeV Preliminary

mh° (GeV/c2)

mA

° (G

eV/c

2 )

Excludedby LEP

TheoreticallyInaccessible

mh°-max

“Mh – max” benchmark (mSUGRA)corresponds to most conservative range of excluded tanβ for fixed Mt and MSUSY

MSSM Higgs

1

10

0 100 200 300 400 500

1

10

LEP 88-209 GeV Preliminary

mA° (GeV/c2)

tanβ

Excludedby LEP

No Mixing

MSUSY=1 TeVM2=200 GeVµ=-200 GeVmgluino=800 GeVNo stop mixing: Xt=0

“no mixing” benchmark (mSUGRA)no mixing between scalar partnres of L and R top quark

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 1400

20

40

60

80

100

120

140

160 LEP 88-209 GeV Preliminary

mh° (GeV/c2)

mA

° (G

eV/c

2 )

Excludedby LEP

TheoreticallyInaccessible

No Mixing

MSSM Charged Higgs

0

0.2

0.4

0.6

0.8

1

60 65 70 75 80 85 90 95

charged Higgs mass (GeV/c2)

Br(

H→

τν)

LEP 189-209 GeV

H+ → cs and H+ → τυ

observed

expected

Invisible Higgs

0

0.05

0.1

0.15

0.2

0.25

0.3

90 95 100 105 110 115 120

MH, GeV/c2

cros

s se

ctio

n, p

b

Observed

Median Background

SM rate

Higgs decay to neutralinos or majorons

no significant excess of events observed- limit set assuming 100% invisible BR

MH > 114.4 GeV (95% CL)

Fermiophobic Higgs

10-1

1

10

20 40 60 80 100 120

Mγγ (GeV)

Eve

nts/

2 G

eV Photonic Higgs SearchLEP Combined263 obs / 289.6 expected data

SM Background

m=110 GeV signal

10-3

10-2

10-1

20 30 40 50 60 70 80 90 100 110 12010

-3

10-2

10-1

20 30 40 50 60 70 80 90 100 110 120

LEP Combined

Excluded Region

Mh0 (GeV)

Upp

er L

imit

on

B(h

0 →γγ

)σ(e

+ e- → h

0 Z)/

σ(SM

)

Fermiophobic BR

Limit = 109.7 GeVExpected Limit = 109.4 GeV

SM BR(H→γγ) too small for observation at LEPin some 2 Higgs doublet models, Higgs coupling to fermions is very small and

Higgs decays preferentially to bosons(Akeroyd 1996, Brucher/Santos 2000)

MH > 108.2 GeV (95% CL)

The LEP Legacy

Claus Grupen (2000)

Higgs at the Tevatron ?

Main Injector& Recycler

Tevatron

Chicago↓

⎯p source

BoosterCDF

DØ

⎯p

p

p ⎯p

1.96 TeV

CDFDØ

Higgs Production Cross-Sections

g

g t

t-

Ho

Gluon fusion

q

q–

W*W+

H0

Associated productionWH or ZH

Gluon fusion dominates but WH/ZH production more accessible ...

Higgs Decay Branching Ratios

• For MH ≤ 135 GeV– H0 → dominates … but

rate falling rapidly– QCD background precludes

gg →

• For MH ≥ 135 GeV– Gauge boson decays dominate

( H0 → WW* )– exploit large gg cross-section

bb

bb

Tevatron: low mass Higgs searches

q

q–

W*W+

H0q

q–

Z*Z0

H0

For MH ≤ 135 GeV: use the same basic strategy as LEP …… study associated production of ZH and WH

To the standard leptonic HZ channels add W → l ν with H → bb ...

µ

b

b-

ν

e,

… the qqbb channel is very difficult as the QCD backgrounds are severe

Low mass Higgs sensitivity depends on• the integrated luminosity collected • b-quark jet tagging performance• mass resolution of reconstructed bb jets

SM Higgs searches in Run I

1

10

10 2

70 80 90 100 110 120 130 140

Higgs Mass (GeV/c2)

σβ (p

b)σ(pp

− → VH0)β CDF 95% C.L.

CDF PRELIMINARY

VH0 → qq− bb

−

VH0 → lν− bb

−

combined (lν−, qq

−)

VH0 → νν− bb

−

combined VH0 limits :VH0 → l+l- / νν

− bb

−

standard model

CDF SM low mass Higgs searches ...

… similar results from DØ for lνbb

MSSM Higgs searches in Run ILarge bb cross-section at the Tevatron can be used to exploit enhanced Yukawa couplings e.g. Abb coupling ∝ tanβ ⇒ cross-section ∝ tan2β

∴ search for bbφ production where φ = h, H, A

b

q

q−

b

b−

Φ

b

b−

g

gb

b−

Φ

b

b−

Trilepton final states~ low bgnds but small rateGolden Modes: like-sign,like-flavour leptons

Like sign dileptons + jets~ many SM bgnds(VVV, Vtt, VVjj, tt, Vjjj)

Dileptons + ET

~ large SM bgnds(VV, tt, ττ, tW)

Tevatron: high mass Higgs searches

gg fusion Assoc. prod

H →VV(V=W,Z)

MH > 135 GeV

High mass Higgs

−+− µe,

+−+− µ

+

e,

ν

+− +−µe,

µe,

−µe,ν

ν

−+− µe,

+−+− µe,

+

q

-q

trileptons like sign dileptons + jets

dileptons + missing ET

standard new physicssearch topology- many backgrounds

very low SM rate- sensitive to new physics!

use kinematic andangular info to suppressbackgrounds

High mass Higgs discovery potential

Low mass region

Results now superseded by new

study in 2003

… based on ‘old’ CDF/D0 study in 1999/2000generic detector simulation, old cross-section calculations, …

For 10 inv fb: 95% exclusion for 140-185 GeV, but little discovery potential

Low mass Higgs revisited (2003)

) 2

(GeV/cHHiggs Mass m100 105 110 115 120 125 130 135 140

)-1

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Higgs Sensitivity Study (’03)statistical power only(no systematics)

Discoveryσ5 Evidenceσ3

95% CL Exclusion

Higgs background processes at the TevatronNeed to understand:-

• W/Z boson production (+jets)• VV • V + 2 jets • VVV• VV+2 jets, V+3 jets

• top production• tt pairs • single top• Vtt

• Drell-Yan pairs (qq → γ* → ee, µµ,ττ)• QCD jets• ...

Can investigate thesebackground processes:

• theoretically• experimentally• both (ideally)

Low and highHiggs masses

What would we like from theory ?Bruce Knuteson’s wish-list from the Run 2 Monte Carlo workshop

…all at NLO

W+jets production

Di-jet Mass ∆R between di-jetsW(eν)+jets W(eν)+jets

• Reconstructed di-jet mass and ∆R(= ∆φ2 + ∆η2 ) between jets– MC reproduces jet distributions well– First step towards study of W(→leptons)H(→ bb) decay process

QCD BKGQCD BKG

Mjj (GeV) ∆Rjj

: Data: MC

: Data: MC

Z+jets production• Number of jets in Z + jets final states• Reconstructed di-jet mass and ∆R(= ∆φ2 + ∆η2 ) between jets

– MC describes jet distributions well– First step towards Z(→leptons)H(→ bb) study

Di-jet Mass ∆R between di-jets#jets in Z+jets

Combined Z(ee)+jets and Z(µµ)+jets

Dibosons: WW→llνν

CDF Run II Preliminary

Event selectionTwo isolated high pT e or µ with opposite charge

FakesET>25 GeV∆φ(ET,l/j)>200

or ET>50 GeVDrellYan,Z→ττ

Z vetoJet veto tt

//

2 Candidates in ~ 2 Candidates in ~ 72 pb72 pb--11

-

Important background for Higgs Search

Single Top: W → tb, g*W → tb

Wb

b

u

t

g

d

Wt b

u

tg

d

u

d

b

t

W(a) (b)

ET (jet3) + 5 × ET (jet4) [GeV]

No.

of

Eve

nts

DØ DataBackgroundtttqbtb

10-3

10-2

10-1

1

10

102

5 47 89 131 173 215

Keep Reject

SM process not yet observed experimentally!

Run 1

Important background to W/Z H

(a) s-channel annihilation (b) t-channel W-gluon fusion

Tevatron Higgs Sensitivity Study (2003)

Signal and background:WH→lνbb (CDF)

(GeV)Hm50 100 150 200

Nu

mb

er o

f E

ven

ts/4

GeV

0

10

20

30

Signal(115)WZ/ZZWbb/Zbbtttbqtb

MH = 115 GeV(10 inv fb, single expt.)

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

120

140

160

180

200

mbb (GeV)

Eve

nts/

10 G

eV

WH mH=115 GeV

WZ

Wbb

top

Pseudoexperiment

WH Channels (NN)10 fb-1

Signal and background: ZH→ννbb (D0)

(GeV)Hm50 100 150 200

Nu

mb

er o

f E

ven

ts/4

GeV

0

10

20

30Signal(115)

WZ/ZZWbb/Zbbtttb/qtbQCD

MH = 115 GeV

ZH→llbb contribution added by scaling ZH→ννbb by 1.33

(10 inv fb, single expt.)

0

25

50

75

100

125

150

175

200

225

250

0 20 40 60 80 100 120 140 160 180 2000

25

50

75

100

125

150

175

200

225

250

mbb (GeV)

Eve

nts/

10 G

eV

Higgs mH=115 GeV

WZ,ZZ

Wbb,Zbb

QCD+top

Pseudoexperiment

ZH (ννbb, llbb)10 fb-1

CDF/D0 combination results

) 2

(GeV/cHHiggs Mass m100 105 110 115 120 125 130 135 140

)-1

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Higgs Sensitivity Study (’03)statistical power only(no systematics)

Discoveryσ5 Evidenceσ3

95% CL Exclusion

Assumes 10% dijet mass resolution

Uses Run IIB Silicon (Beware!)

Width of HSS bands determined by method uncertainty

Width of SHW bands given by 30%uncertainty

SHW included H → WW (contributesat high MH)

Tevatron Higgs Sensitivity Group June 2003 Update

• Many believed the SHW study (1999) was over optimistic!• Complete studies of realistic detector performance place results on firmer ground

• b-tagging, trigger, di-jet mass resolution, QCD background• HSS exceed the expectations of SHW!• Should get better (new ideas, optimisation, lots of time, smart people)• But, Run IIB silicon upgrade cancellation is big setback (double b-tag eff. reduced)

Tevatron: SUSY Higgs potential?

Exclusion and discoveryfor maximal stop mixing,sparticle masses = 1 TeV

100 150 200 250 300 350 400

5

10

15

20

25

30

35

40

45

50

95% CL Exclusion, Maximal Mixing Scenario 5 fb-1 10 fb-1

MA (GeV)

tanβ

100 150 200 250 300 350 400

5

10

15

20

25

30

35

40

45

50

5σ Discovery, Maximal Mixing Scenario15 fb-1 20 fb-1 30 fb-1

MA (GeV)

tanβ

40

45

50

95% CL Exclusion, Suppressed Vφ→V bb−

5 fb-1 10 fb-1

40

45

50

5σ Discovery, Suppressed Vφ→V bb−

15 fb-1 20 fb-1 30 fb-1

95% exclusion 5σ discovery

5 fb-1 15 fb-1 20 fb-1

Luminosity per experiment, CDF + DØ combined

tan β = 35

bb(h/A) → 4b

170 GeV

oneexpt

σ ~ 1 pb for tanβ = 30 and mh = 130 GeV

old estimates- need updating!

Doubly-Charged Higgs BosonsD-Zero Run II (preliminary)

Future Machines

• LHC• Linear Collider• Muon Collider (σH = 4x104 σH

LC !!!)• VLHC • ??

On the ‘road map’

Speculative

ATLAS H0 → 4 charged leptons

Same mass resolution Same mass resolution for for eeee, , emuemu and and mumumumuassumedassumed

LowLow--Luminosity setupLuminosity setup

MMhh=130 =130 GeVGeV Athena 6.6.0Athena 6.6.0

Higgs at CMS

H→ZZ→eeqq

ttH Production

ATLAS30 fb-1

MSSM Higgs Scenarios at the LHC

Little Higgs Model

Peter Higgs+ grandchild

Higgs Scenariosinspired by P.Grannisat LCWS 2000

Summary and Conclusions• The Higgs mechanism is still the favoured EWSB scheme (SM, MSSM, …)• But after > 20 years of searching, the Higgs boson remains elusive!

• Tantalising hints of a discovery from LEP in 2000 (MH > 114.4 GeV at 95% CL)• Precise EW data favour a light Higgs (MH < 202 GeV at 95% CL)

• The Tevatron has the field to itself until 2007/8 but luminosity is big problem and b-tagging capability will not be optimal …

• The LHC is very likley to find Higgs bosons (if they exist!) in most scenarios but may not (a) find all of them and (b) will not measure detailed properties

• The 500+ GeV LC is the precision tool for making detailed studies of the Higgs boson(s) and exploring the properties

• In a decade from now we might observe the following …

Peter Higgs,Nobel Prize inPhysics (2013)