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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.
Lu
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ty p
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fb
1
10
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
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mHrec (GeV/c2)
Eve
nts
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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
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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
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0 20 40 60 80 100 120 1400
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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
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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
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SUSY/Higgs Workshop(’98-’99)
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.)
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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.)
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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
Int.
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SUSY/Higgs Workshop(’98-’99)
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)
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