1
Southampton IOP:Southampton IOP: The Higgs discovered at LEPThe Higgs discovered at LEP
W. J. Murray
RAL
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families,families,with with leptonsleptons
and and quarksquarks
families,families,with with leptonsleptons
and and quarksquarks
Why do we need the Higgs?Why do we need the Higgs?
FermionsFermions Gauge SymmetriesGauge Symmetries Bosons, InteractionsBosons, Interactions
families,families,with with leptonsleptons
and and quarksquarks
U(1)U(1)Y Y ::
SU(2)SU(2)LL::
SU(3)SU(3)cc::
: QED: QED
Z, WZ, W : Weak: Weak
gluons gluons : QCD: QCD
)()(2
exp)( xxYg
ix
)()(.2
exp)( xxg
ix LL
)()(2
exp)( xxg
ix qa
aS
q
g
gW
tan
solution: The Higgs Mechanismsolution: The Higgs Mechanism
RRL
L ee
,,
RRL
L dud
u,,
A mass term couples L & R and would violate SU(2)L
3
What is the Higgs What is the Higgs mechanism?mechanism?
•Doublet of SU(2)Doublet of SU(2)LL,, •Potential repects SU(2)Potential repects SU(2)LL
But Vacuum does not!But Vacuum does not!
22 2/!3
)( vV
Fermions:
Interact with Higgs field slows them down
generates mass
Bosons:
SU(2)L interact, gain mass
U(1) and SU(3)c do not, massless
3 degress of freedom 3 degress of freedom in Boson massesin Boson masses
44thth becomes fundamental scalar becomes fundamental scalar
4
What does it predict?What does it predict?
Tree level Z couplings:
axial: a=±1/2
vector: v=a(1-4|Q|sin2W)
mm = 0 = 0
mmZ Z = m= mWW/cos/cosWW
Direct consequences of the Higgs mechanism
We can test them
But the Higgs mass is not predicted
5
Loop effect sensitive to HiggsLoop effect sensitive to Higgs
%12
2
Z
t
m
m
2
2
Log4 Z
H
m
m
2
2
2
2
Log4 Z
H
Z
t
m
m
m
m
with
TTrreeee--LLeevveell CCoorrrreecctteedd
aa00 == 11// 22 aa == aa00((11++))
vv00 == aa00((11--44||QQ||ssiinn22WW)) vv == aa((11--44||QQ||ssiinn22WWeeff ff ))
ssiinn22WW == 11 –– mmWW22// mmZZ
22 ssiinn22WWeeff ff == 11 –– mmWW
22// mmZZ22((11++kk))
(m(mZ Z = m= mWW/cos/cosWW))
0.1% Precision needed!
Only sensitive to mtop
Propagator corrections Vertex corrections
6
Precision Electroweak ObservablesPrecision Electroweak Observables
LEP1
Z
LEP2
Z LineshapeZ Lineshape
WW++WW-- production productionRate of Z Rate of Z bb bb
AsymmetriesAsymmetries 2(v/a)1
2v/a
mmZ Z
sinsin22W W
mmtop top
mmWW
… … to be compared to to be compared to direct measurements ofdirect measurements of
mmW W (LEP2)(LEP2)
mmtop top (FNAL)(FNAL)
--
effeff
• Find Find =1+=1+ from LEP/SLD data; from LEP/SLD data;• Predict mPredict mtoptop and m and mWW;;• Compare with direct Compare with direct measurements;measurements;• Predict mPredict mH;H;
• Compare with direct Compare with direct measurements.measurements.
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Final state identification: ZFinal state identification: Ze) Z->-
Z Z bb: bb: Like qq events, with detached vertices, Like qq events, with detached vertices, measured in accurate vertex detectorsmeasured in accurate vertex detectors
Z Z :: Not detectable.Not detectable.
--
--
--
--
(weak and slow decays(weak and slow decaysto lighter quarks)to lighter quarks)
test.prn
Z Z qq: qq: Two jets, large particle multiplicity.Two jets, large particle multiplicity.
Z Z e e++ee--, , ++--:: Two charged particles (e or Two charged particles (e or .).)
Z Z ++--:: Two low multiplicity jets + missing Two low multiplicity jets + missing energy carried by the decay neutrinos.energy carried by the decay neutrinos.
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WW++WW-- q q11qq22ll:: Two hadronic jets, Two hadronic jets,
One lepton, missing energy.One lepton, missing energy.
Final state identification: WFinal state identification: W++WW--
WW++WW-- q q11qq22qq33qq44:: Four well separated jets.Four well separated jets.
---- ---WW++WW-- l l1111ll2222::
Two leptons, missing energyTwo leptons, missing energy
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Did LEP improve measurements? Did LEP improve measurements?
Errors 10
v & a before LEP and SLD v & a after LEP and SLD
sinsin22WWeffeff
10
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Compare with SM, technicolourCompare with SM, technicolour
Discovery of the Higgs
1 222
3 sincoscos WWW
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sin2W and , leptons
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Fit of mFit of mtoptop and m and mWW
mmWW and m and mtoptop direct measurementsdirect measurements
mmZZ, , and and sinsin22WW
measurementsmeasurements
Confirms Higgs MechanismConfirms Higgs Mechanism
Values agree!
12
Comparison with SM and SUSYComparison with SM and SUSY
SUSY assumes
Higgs mechanism
Light SUSY
Heavy SUSY
SM
13
Global Fit of SM: mGlobal Fit of SM: mH H PredictionPrediction
mmHH = 118 GeV/c = 118 GeV/c22- 42- 42+ 63+ 63
(+ pQCD)(+ pQCD)
(with BES)(with BES)
Satisfactory internal consistency?Satisfactory internal consistency?4% probability 4% probability
From From and m and mtoptop::
2020 500500
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Closing in on the Higgs!Closing in on the Higgs!
0100200300400500600700800900
1000
Excluded by EWfits
Allowed
Excluded byDirect Search
EW fits assume a Higgs
Search looks for one
After A. Wagner, ICHEP 2000
Bayesian
Frequentist
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Direct Searches at LEP 1Direct Searches at LEP 1
0.0 0.0 m mHH 65 GeV/c 65 GeV/c22
Excluded at 95% C.L.Excluded at 95% C.L.
EventsEventsexpectedexpectedat LEP1at LEP1
•Great effort - which I have no time to describe
•Many modes:
Stable,,ee,,,,bb
•Clean Z decays (ll, ) used
•Prior to LEP only some patchy constraints
The mass range to 0 now excluded, no holes.
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LEP Higgs production: HZLEP Higgs production: HZ
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`Higgstrahlung’
Ecms-mz
`WW fusion’
no cut-off
Total events, 4 experiments
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LEP Higgs channelsLEP Higgs channelsDecaymode
Br
bb 73.6% 7.2%Gluons 6.6%
ww 8.1%
Higgsdecay
Z decay Fraction
bb qq 51.5%
bb 14.7%Any ll 6.7%
bb 2.5% qq 5.0%
Total 80.9%
Higgs decays
Search channels
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Beam energy 99/00: From 192 to 209 GeVBeam energy 99/00: From 192 to 209 GeV
0
5
10
15
20
25
30
Accelerating field [MV/m]
Num
ber o
f cav
ities
96 GeV
100 GeV
104 GeV
192 GeV6.0 MV/m
200 GeV7.0 MV/m
204 GeV7.5 MV/m
Accelerating field (MV/m)
LEP:f=350 MHz
f = 0 Hz
f = -50 Hz
1) 1) Increase gradientIncrease gradient & & Cryogenics upgradeCryogenics upgrade
E: 192 E: 192 204 GeV 204 GeV; ; mmHH: 100 : 100 112 GeV 112 GeV
2) 2) Improve stabilityImprove stability & & Decrease security marginDecrease security margin
E: 204 E: 204 205.5 GeV 205.5 GeV; ; mmHH: 112 : 112 113 GeV 113 GeV
E: 205.5 E: 205.5 207 GeV 207 GeV; ; mmHH: 113 : 113 114 GeV 114 GeV
• Two- to one-klystron margin (1h30):Two- to one-klystron margin (1h30):
• Mini-ramp to no margin at all (15 mins):Mini-ramp to no margin at all (15 mins):
3) 3) Re-install 8 Cu cavitiesRe-install 8 Cu cavities
E: 207 E: 207 207.4 GeV 207.4 GeV; ; mmHH: 114 : 114 114.25 GeV 114.25 GeV
4) 4) Use orbit correctors as magnetic dipolesUse orbit correctors as magnetic dipoles
E: 207.4 E: 207.4 207.8 GeV 207.8 GeV; ; mmHH: 114.25 : 114.25 114.5 GeV 114.5 GeV
5) 5) Decrease the RF frequencyDecrease the RF frequency
E: 207.8 E: 207.8 209.2 GeV209.2 GeV; ; mmHH: 114.5 : 114.5 115.1 GeV 115.1 GeV
More dipolar More dipolar magnetic fieldmagnetic field
seen in theseen in thequadrupoles! quadrupoles!
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LEP machine in 2000LEP machine in 2000
Results exclude last week or two of data
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`Higgs discovery mode’
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First pbFirst pb-1-1’s above 206 GeV:’s above 206 GeV: First Candidate Event First Candidate Event
(14-Jun-2000, 206.7 GeV)(14-Jun-2000, 206.7 GeV)
• Mass 114.3 GeV/c2;Mass 114.3 GeV/c2;• Good HZ fit;Good HZ fit;• Poor WW and ZZ fits;Poor WW and ZZ fits;• P(Background) : 2%P(Background) : 2% • s/b(115) = 4.7s/b(115) = 4.7
The purest candidate event ever!The purest candidate event ever!
b-tagging (0 = light quarks, 1 = b quarks)
• Higgs jets: 0.99 and 0.99;
• Z jets: 0.14 and 0.01.
e+e- bbqq_ _
MissingMomentum
High pT muon
21
Some candidate events at 115 GeV/cSome candidate events at 115 GeV/c22
31-Jul-200031-Jul-2000Mass: 112 GeVMass: 112 GeVs/bs/b115115 = 2.0 = 2.0
21-Aug-200021-Aug-2000Mass: 110 Mass: 110 GeVGeVs/bs/b115115 = 0.9 = 0.9
21-Jul-200021-Jul-2000Mass: 114 Mass: 114 GeVGeVs/bs/b115115 = 0.4 = 0.4
e+e- bb__
DELPHIDELPHI
L3L314-Oct-200014-Oct-2000Mass: 114 Mass: 114 GeVGeVs/bs/b115115 = 2.0 = 2.0
27-Jun-200027-Jun-2000Mass: 113 Mass: 113 GeVGeVs/bs/b115115 = 0.52 = 0.52
ALEPHALEPH
22
The 14 Most Significant EventsThe 14 Most Significant Events s/b
Rec. mass(GeV/c2)
Channel Expt
4.7 114 Hqq ALEPH
2.3 112 Hqq ALEPH
2.0 114 H L3
0.90 110 Hqq ALEPH
0.60 118 Hee ALEPH
0.52 113 Hqq OPAL
0.50 111 Hqq OPAL
0.50 115 H ALEPH
0.50 115 Hqq ALEPH
0.49 114 H L3
0.47 115 Hqq L3
0.45 97 Hqq DELPHI
0.40 114 Hqq DELPHI
0.32 104 H OPAL
Expected: 7Expected: 7
Observed: 14Observed: 14
In ALEPH: 6In ALEPH: 6
In L3: 3In L3: 3
In OPAL: 3In OPAL: 3
In DELPHI: 2In DELPHI: 2
In Hqq: 9 In Hqq: 9
(70%)(70%)
In HIn H: 3 (20%): 3 (20%)
In HlIn Hl++ll--: 1 (7%): 1 (7%)
In HIn H: 1 (3%): 1 (3%)
s/b > 0.3: Expected signal-to-noise ratio of s/b > 0.3: Expected signal-to-noise ratio of ~1~1
Number of events Number of events compatible with s+bcompatible with s+b
Number of eventsNumber of eventsin each experiment in each experiment
compatible with beingcompatible with beingdemocraticdemocratic
(~1.6 bkg expected)(~1.6 bkg expected)
Number of eventsNumber of eventsin each Z decay in each Z decay compatible with compatible with HZ predictionsHZ predictions
Values still PRELIMINARYValues still PRELIMINARY
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LEP mass distributionsLEP mass distributionsLittle visible with loose
selections
Something appears with medium ones
Tight cuts give reasonable agreement with 115GeV Higgs
24
Signal vs BackgroundSignal vs Backgroundee++ee-- HZ HZs = 0.1 pbs = 0.1 pb
ee++ee-- ZZ ZZs ~ 2 pbs ~ 2 pb
ee++ee-- WW++WW--
s ~ 20 pbs ~ 20 pbee++ee-- qqqq
s ~ 100 pbs ~ 100 pb
-
• Reconstructed Higgs boson mass;Reconstructed Higgs boson mass;
• Other kinematic variables;Other kinematic variables;
• b-tagging (lifetime, leptons, …);b-tagging (lifetime, leptons, …); Backg
roun
d
Backg
roun
d
Backg
roun
d
Backg
roun
d++++
Backg
roun
d
Backg
roun
d++
Signa
l
Signa
l
Must evaluate Must evaluate the “signal-ness”,the “signal-ness”, s/bs/b, of the candidate events, of the candidate events
Zoom of Zoom of 1cm around 1cm around the interaction pointthe interaction point
25
How is significance assessed?How is significance assessed?•Maximum likelihood fit to observed distribution
Most channels work in 2D
•Each bin needs signal and background estimates, from simulation, dependent upon Ecms, channel etc.
i
ii
i b
bsSL ln
•This is a weighted sum of events.
•L is compared with distributions expected for background and signal to quantify probabilities
`Qi’
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LEP signal to backgroundLEP signal to background
Most events buried under background
Only 3 events have s/b greater
than 1 for MH=115
Was 4 for MH=114: cross-section dropped
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LEP log-likelihoodLEP log-likelihood-2*ll~2
minimum at 115GeV/c2
Probability of fluctuation:
4 in 1000
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Rate median for signal
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Results by experimentResults by experimentTitle:wjm_showpos4.epsCreator:HIGZ Version 1.26/04Preview:This EPS picture was not savedwith a preview included in it.Comment:This EPS picture will print to aPostScript printer, but not toother types of printers. Experiments
scatter reasonably in
the signal distribution
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Results by Physics channelResults by Physics channel
Distribution by decay
mode very good!
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More consistency checks:More consistency checks:Title:wjm_showpos3.ps (Portrait A 4)Creator:HIGZ Version 1.26/04Preview:This EPS picture was not savedwith a preview included in it.Comment:This EPS picture will print to aPostScript printer, but not toother types of printers.
Yes!
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Yes!
New data Tail at low mass
31
What if you remove best results?What if you remove best results?
Yes, consistent,
but not strong
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ALEPH removed
4-jets removed
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The roadmap of 2000The roadmap of 2000
At Sept 5th LEP seminar, we predicted 70pb-1 would give a 3 evidence for a MH=115
Actually, 55pb-1 gave 2.9
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What did 2001 bring?What did 2001 bring?•Six more months in 2001;Six more months in 2001;
• gave an integrated luminosity of 200 pbgave an integrated luminosity of 200 pb-1-1;;
• With an energy of 208.5 With an energy of 208.5 210 GeV; 210 GeV; (made possible with add’l cavities and a few tricks)(made possible with add’l cavities and a few tricks)
The 3The 3 evidence was turned into evidence was turned into
A 5.5A 5.5discoverydiscovery
Background subtracted:Background subtracted:
Expected mass spectrum:Expected mass spectrum:
~28 signal events~28 signal events
34
What did 2001 REALLY bring?
DELPHI 2001DELPHI 2001
LEP had run its LEP had run its
coursecourse
LHC is sure to LHC is sure to
find a Higgs...find a Higgs...
35
Next-in-Line:Next-in-Line: The TEVATRONThe TEVATRONThe Tevatron:The Tevatron:
• A A pp colliderpp collider, near Chicago;, near Chicago;
• Center-of-mass energy: Center-of-mass energy: 2 TeV2 TeV;;
• Luminosity in Run I: Luminosity in Run I: 0.1 fb0.1 fb-1-1;;
• TwoTwo multi-purpose multi-purpose detectorsdetectors..
--
CDF:CDF:D0:D0:
Run II is starting Run II is starting NOW:NOW:
• 2 fb2 fb-1-1 in 2002; in 2002;
• 5 fb5 fb-1-1 in 2004; in 2004;
• 15 fb15 fb-1-1 in 2007. in 2007.
s = 2 TeVs = 2 TeV
pp colliderpp collider--
36
Tevatron Higgs Potential Tevatron Higgs Potential Same process as at LEP:Same process as at LEP:
Produced: 50 events / fbProduced: 50 events / fb-1-1
WH bbl
mH=120 GeV
--
10 fb10 fb-1-1
33 in 2004 in 2004
55 in 2007 in 2007
22 in 2002 in 2002
• Should confirm LEP hints;Should confirm LEP hints;
• Much more difficult Much more difficult environment;environment;
• Somewhat later;Somewhat later; (3(3from 2004 to 2007);from 2004 to 2007);
• No detailed studies of the No detailed studies of the Higgs mechanism…Higgs mechanism…
• Difficult above 115 GeV/cDifficult above 115 GeV/c22..
37
LHC Discovery PotentialLHC Discovery Potential
CMS +ATLASCMS +ATLAS
ATLASATLAS ATLASATLASCMSCMSH H H H ZZ ZZ eeee eeee H H ZZ ZZ
10 fb-1: 2007?
100 fb-1: 2010?
• 115 GeV not too 115 GeV not too good;good;
• 2007 may be very 2007 may be very
hot!hot! But…But…
• LHC will cover the LHC will cover the whole mass rangewhole mass range in a year (5in a year (5) or in ) or in a month (95% a month (95% C.L.)C.L.)
38
b
b
Discovery PotentialDiscovery Potential for mfor mHH = 115 = 115
GeV/cGeV/c22
IVB Fusion
ttH ttbb:
ATLAS
mH=120 GeV/c2
ATLAS
mH=120 GeV/c2
bb--
• 50 events / fb50 events / fb-1-1;;
• Branching Ratio 10Branching Ratio 10-3-3..
100 fb-1100 fb-1
gg H ---- --
• s/b ~ 1/10 - 1/100: No detailed study of EWSB;s/b ~ 1/10 - 1/100: No detailed study of EWSB;
• H H does not really test the Higgs mechanism; does not really test the Higgs mechanism; EquallyEquallyPromising:Promising:
ATLAS and CMSATLAS and CMS
Minutes of 56th LEPC, 3rd November 2000
`The committee noted that there is unfortunately no single channel that is background-free.’
39
LHC Potential: SupersymmetryLHC Potential: SupersymmetryTitle:../figures/lhc10_maxmix_all.epsCreator:HIGZ Version 1.26/02Preview:This EPS picture was not savedwith a preview included in it.Comment:This EPS picture will print to aPostScript printer, but not toother types of printers.
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10fb-1 (End 2007) covers most of plane
Some of it only by observing OTHER higgses
40
Precision:Precision: ee++ee-- Linear Colliders Linear Colliders
Channel B/B (Stat) B/B (Syst)
H bb 2.4% 1.5%
H cc 8.3% 14%
H gg 5.5% 11%
H 5.0% 5.5%
H WW 5.1% 7.2%
H 19%
HHH 22%
Similar to LEP, but Similar to LEP, but s ~ 350-500 GeV, and L s ~ 350-500 GeV, and L 1000 1000
1) Fast Discovery (or confirmation)1) Fast Discovery (or confirmation) 2) Precision Measurements of all BR’s2) Precision Measurements of all BR’s
5 confirmation in half-a-day !
At 115 GeV:At 115 GeV:
A weekA week
See D. MillerSee D. Miller
Precision Precision Tests of theTests of the
Higgs MechanismHiggs MechanismPredictionsPredictions
41
Precision Higgs PhysicsPrecision Higgs Physics Collider Collider
Observable With 100 pb-1 With 2.5 f b-1
Mass 0.1 MeV/ c2 0.05 MeV/ c2
Width 0.5 MeV 0.1 MeV
peak 1 pb 0.2 pb
Statistics limited !
, W, W++
, W, W--
--
Standard ModelStandard ModelH H = 3.2 MeV= 3.2 MeV
mmAA = 300 GeV = 300 GeVH H = 4.7 MeV= 4.7 MeV
mmAA = 400 GeV = 400 GeVH H = 4.0 MeV= 4.0 MeV
MUONS MUONS are heavyare heavy Large couplings to Higgs bosonsLarge couplings to Higgs bosons; ; Very good energy resolutionVery good energy resolution..
Higgs LineshapeHiggs LineshapeMeasurements!Measurements!
4 MW4 MWs ~ ms ~ mHH
4%4%
1%1%
42
Scan the H and A ResonancesScan the H and A Resonancestantan = 10 = 10
tantan = 8 = 8
tantan = 6 = 6 mmAA = 400 GeV/c = 400 GeV/c22 mmhh = 115 GeV/c = 115 GeV/c22
mmSUSYSUSY = 1 TeV/c = 1 TeV/c22..
One week of runningOne week of running
E/E = 0
E/E = 3 10-4
• Determine mDetermine mHH, m, mAA, and tan, and tan to an excellent accuracy; to an excellent accuracy;
• Fit for e.g., stop masses (mFit for e.g., stop masses (mSUSYSUSY) and mixing (A) and mixing (Att, , ). ).
• Start precision tests of SUSY breaking through rad. corr. Start precision tests of SUSY breaking through rad. corr.
to masses and widths; to masses and widths; ( LEP for standard model and EWSB)
• Tests of CP violation with muon polarizationTests of CP violation with muon polarization
H,A
b
bp
b-
Background level
H, AH, A
--
s ~ ms ~ mAA, m, mHH
43
The Higgs discovered at LEPThe Higgs discovered at LEP
• Precision electroweak measurements Precision electroweak measurements DEMANDDEMAND a a Higgs:Higgs:•MMWW agrees with Higgs predictions to 1 per mille, agrees with Higgs predictions to 1 per mille, •MMtoptop agreement to 10% agreement to 10%
• Direct Searches (~3Direct Searches (~3 effect) effect)
mmHH = 118 GeV/c = 118 GeV/c22- 42- 42
+ 63+ 63
mmHH = 115.0 GeV/c = 115.0 GeV/c22- 0.3- 0.3
+ 0.7+ 0.7Just a statistical fluke?Just a statistical fluke?
After 12 years of outstanding Physics:After 12 years of outstanding Physics:
In about 5 years, Tevatron/LHC should decideIn about 5 years, Tevatron/LHC should decide
More More PrecisionPrecision Measurements with Measurements with
Lepton Colliders will follow after 2010Lepton Colliders will follow after 2010
No Need for Higgs Boson in the data?No Need for Higgs Boson in the data?mmWW = m = mZ Z coscosWW is just a coincidence? is just a coincidence?
Support your local linear Collider
