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TRANSCRIPT
The LEP Higgs Saga
Pippa WellsCERN
2 August 2003
LEP Higgs Saga Pippa Wells 1
Search for the Higgs Boson at LEP
• Quick reminder
– The LEP machine and detectors
– Standard Model processes
• Constraints on the Higgs mass
• Higgs production and decay
• How to tell if candidates are significant?
• What happened in 2000...?
LEP Higgs Saga Pippa Wells 2
LEP Higgs Saga Pippa Wells 3
The LEP Collider
A good fill lasted around
• 10 h at LEP1 (√
s ≈ MZ)
• 3 h at LEP2
0
20
40
60
80
100
120
-25 -22.5 -20 -17.5 -15 -12.5 -10 -7.5 -5 -2.5 0Time (hours)
E (
GeV
) o
r L
(1031
cm-2
s-1)
A day at LEP (example from 1999)EnergyLuminosity
LEP Accelerator and four detectors operated 24h a day, from Spring to
Autumn, 1989 to 2000
LEP Higgs Saga Pippa Wells 4
LEP2 Machine Performance
• Impressive machine performance - increase beam energy and
maintain high luminosity.
• Superconducting RF acceleration system pushed beyond design
gradient.
• Max. integrated luminosity per year: 65pb−1 (LEP1), 254pb−1 (LEP2)16
1
172
183
189
192
196
200
202
205
207
209
0
50
100
150
Ecm (GeV)
Inte
gra
ted
Lu
mi (
pb
-1)
1996
1997
1998
1999
2000
LEP Higgs Saga Pippa Wells 5
LEP: e+e− collisions
ALEPH, DELPHI, L3, OPAL
each recorded:
• 4.5M Z at LEP1
including off-peak data.
• 10k W-pair at LEP2
LEP1 dominated by Z
resonance
Several processes are
important at LEP2
Any sign of e+e− → HZ?
Pick out H and Z decay
products against big
backgrounds.
√s⎯
(GeV)
Cro
ss s
ectio
n (p
b)
e+e−→e+e–qq−
e+e−→qq−(γ)
e+e−→µ+µ−(γ)
e+e−→W+W−
e+e−→ZZ
e+e−→γγe+e−→W+W−γ
e+e−→HZmH = 115 GeV
L3preliminary
10-1
1
10
10 2
10 3
10 4
80 100 120 140 160 180 200LEP Higgs Saga Pippa Wells 6
Cut-away view of OPAL
θ ϕ
x
y
z
Hadron calorimetersand return yoke
Electromagneticcalorimeters Muon
detectors
Jetchamber
Vertexchamber
Microvertexdetector
Z chambers
Solenoid andpressure vessel
Time of flightdetector
Presampler
Silicon tungstenluminometer
ForwarddetectorOverall size 12×12×12 m
LEP Higgs Saga Pippa Wells 7
Z to hadrons event in ALEPH
Made on 9-Sep-1993 11:43:03 by D
RE
VE
RM
AN
N w
ith DA
LI_D
1.DALI
Run=9063 Evt=7848 ALEPH
• This example has 3 jets e+e− → qqg
• Jets include many tracks (curved in B=1.5 T field) and energy clustersLEP Higgs Saga Pippa Wells 8
e+e− → Z → e+e− event in OPAL
• Lepton pair events have
low multiplicity
• Electrons are identified
by a track in the central
detector, and a large
energy deposit in the
electromagnetic
calorimeter, E /p = 1.
Run : even t 4093 : 1150 Da t e 930527 T i me 20751
Ebeam 45 . 658 Ev i s 94 . 4 Em i s s - 3 . 1 V t x ( - 0 . 05 , 0 . 08 , 0 . 36 )
Bz=4 . 350 Th r us t =0 . 9979 Ap l an=0 . 0000 Ob l a t =0 . 0039 Sphe r =0 . 0001
C t r k ( N= 2 Sump= 92 . 4 ) Eca l ( N= 9 SumE= 90 . 5 ) Hca l ( N= 0 SumE= 0 . 0 )
Muon ( N= 0 ) Sec V t x ( N= 0 ) Fde t ( N= 1 SumE= 0 . 0 )
Y
XZ
200 . cm.
Cen t r e o f s c r een i s ( 0 . 0000 , 0 . 0000 , 0 . 0000 )
50 GeV2010 5
LEP Higgs Saga Pippa Wells 9
e+e− → Z → µ+µ− event in L3
• Muons penetrate entire
detector. Little energy in
the calorimeters.
• L3 emphasizes lepton
and photon id with a
precise BGO crystal
ECAL, and large muon
spectrometer.
• Tracking volume
relatively small (r=1m)
• ALL detectors inside
r=6m solenoid, B=0.5T.
HCAL
BGO
Tracking
MuonChambers
LEP Higgs Saga Pippa Wells 10
e+e− → Z → τ+τ− event in DELPHI
• Tau lepton decays
dominated by 1 and 3
charged tracks, with or
without neutrals, missing
neutrino(s), back-to-back
very narrow “jets”.
• DELPHI has extra
particle ID detectors,
RICH.
DELPHI Interactive AnalysisRun: 23438Evt: 581
Beam: 45.6 GeV
Proc: 8-Mar-1992
DAS : 18-Jun-199103:22:19
Scan: 29-Apr-1992
TD TE TS TK TV ST PA
Act
Deact
44
( 44)
0
( 0)
45
( 48)
0
( 4)
0
( 0)
0
( 0)
8
( 9)
0
( 6)
0
( 9)
0
( 5)
0
( 0)
0
( 0)
0
( 0)
0
( 0)
X
Y
Z
Barrel RICH
LEP Higgs Saga Pippa Wells 11
Tagging heavy quarks - leptons
Heavy hadrons have weak decays, sometimes final state leptons, long
lifetimes, characteristic masses and event shapes.
b and c hadrons have ≈ 20% of
decays to leptons with high p and
for b hadrons with high pT
Electrons: ionisation in tracking
chambers dE /dx , E /p, shower
shape
Muons: Match between central track
and muon chambers
Leptons also give charge of the
decaying hadron
Example shows e+e− → Z → bbevent in L3
L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3L 3
µ+p = 17 GeVp = 1.7 GeV
⊥
e+ E = 14 GeVp = 2.7 GeV
⊥
LEP Higgs Saga Pippa Wells 12
Tagging heavy quarks - lifetimes
Heavy hadrons have long
lifetime and large boost
d0 and L are signed
quantities. A badly
measured track may
intercept the “wrong-side” of
the beam spot.
Rely on silicon microvertex
detectors for resolution.
Use several variables
together.
Impact parameter, d0
Decay length, L
Primary vertex
Secondary vertex
B
B
Fragmentationtrack
LEP Higgs Saga Pippa Wells 13
W pair events at LEP2
WW → qqqq (four jets) WW → qq�ν (two jets, lepton and
missing energy)
LEP Higgs Saga Pippa Wells 14
WW and ZZ production at LEP23 diagrams “CC03” 2 diagrams “NC02”
e-
e+
νe
f
ff
f
W
W
e-
e+
γ
f
ff
f
W
W
e-
e+
Z
f
ff
f
W
W
e-
e+
e
f
ff
f
Z
Z
e-
e+
e
f
ff
f
Z
Z
√s (GeV)
σ WW
(pb
)
YFSWW and RacoonWW
LEP PRELIMINARY
11/07/2003
0
10
20
160 180 200
16
17
18
190 195 200 205 0
0.5
1
180 190 200
√s (GeV)
σ ZZ (
pb)
ZZTO and YFSZZLEP PRELIMINARY
11/07/2003
ZZ production is an irreducible background to ZH production.LEP Higgs Saga Pippa Wells 15
Global electroweak fits and Higgs Mass
Fit to data from LEP,
SLD, Tevatron...
Electroweak variables
depend on m2t and
log mH through radiative
corrections
Consistency between
predicted top and W
mass (Z pole) and direct
measurements
Preference for low Higgs
mass.80.2
80.3
80.4
80.5
80.6
130 150 170 190 210
mH [GeV]114 300 1000
mt [GeV]
mW
[G
eV]
Preliminary
68% CL
∆α
LEP1, SLD Data
LEP2, pp− Data
LEP Higgs Saga Pippa Wells 16
Constraints on Higgs mass
Electroweak fits
mH < 237 GeV (95% CL)
Theory: self consistency
of SM to GUT scale
≈ 1016 GeV
130 < mH < 190 GeV.
mH higher - theory
non-perturbative,
mH lower - vacuum
unstable. 0
1
2
3
4
5
6
10020 400
mH [GeV]
∆χ2
Excluded Preliminary
∆αhad =∆α(5)
0.02761±0.00036
0.02747±0.00012
incl. low Q2 data
Theory uncertainty
LEP Higgs Saga Pippa Wells 17
Higgs production cross-section
e-
e+
Zf
f
H
Z*
10-3
10-2
10-1
1
10
10 2
10 3
10 4
60 80 100 120 140 160 180 200
√s– (GeV)
σ (p
b)
10-3
10-2
10-1
1
10
10 2
10 3
10 4
60 80 100 120 140 160 180 200
e+e- → hadrons
e+e- → HZ
ZZ
WW
60
mh = 70 GeV 90 110 115
e-
e+
Z*
f
f
H
Z
With a luminosity of about 100pb−1 and reasonable detection efficiency,
sensitive to a cross section of O(0.1) pb.
Need LEP2 to produce mH >∼ 65 GeV. Reach mH <∼√
s − MZ
Must take into account many background processes
LEP Higgs Saga Pippa Wells 18
Higgs decay branching ratios
“Higgs couples to mass”
τ+τ-
cc_
gg
W*W*Z*Z*
mH
[GeV]
10-2
10-1
1
60 70 80 90 100 110 120
bb_
Hig
gs B
ranc
hing
Rat
io
BR(%) Higgs Z boson115 GeV
qq 70
bb 74 15cc 4 12gg 6 0�+�− 10τ+τ− 7 3νν 20W∗W∗ 8Z∗Z∗ 1
LEP Higgs Saga Pippa Wells 19
HZ decays
q
q
b
b
−
−ν
b
b
ν
−
−
e+ or µ+
b
b
e− or µ−
−
τ+
b
b
τ−
−
Four jets, 60%
H → bb , Z → qq
Missing energy, 18%
H → bb , Z → νν
Leptonic, 6%
H → bb , Z → �+�−
Tau channels, 9%
H → bb(τ+τ−) , Z → τ+τ−(qq)LEP Higgs Saga Pippa Wells 20
Got some candidates? Are they significant?
bi expected number of background
si(mH) expected signal, function of “test
mass” mH
Count these in bins of reconstructed Higgs
mass mrecH and a global discriminating vari-
able G
si (MH),bi
MHrec
G
Discriminant G takes into account b-tagging, τ-id, kinematic variables that
distinguish signal and background.
“How Higgs-like is the event?”
Expectations account for luminosity, Ecm, resolution, efficiency...
Compare likelihoods of “s + b” and “b only”.
LEP Higgs Saga Pippa Wells 21
Likelihood comparison
Likelihoods of s + b and b from Poisson probabilities of observing
ni data events in each bin:
Q(mH) =Ls+b
Lb=
∏i
(si + bi)ni e−(si +bi )/ni
bnii e−bi /ni
More convenient to work with log likelihood:
−2 ln Q(mH) = 2stot − 2∑
i
ni ln(
1 +si (mH)
bi
)
Sum is over all bins, channels (four jet, missing energy...), and experiments.
LEP Higgs Saga Pippa Wells 22
Likelihood ratio, −2 ln Q−2 ln Q vs. test mass mH.
Find expected (median)
curves and statistical spread
from a set of ficticious MC
samples of the same
luminosity/Ecm mix as the
data.
Compare with the
observation - here’s an
example of what you might
hope to see in the real data!
Taking slices at different test
masses - separation of b and
s+b decreases as mass
increases.
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
105 110 115
"b"
"s+b"
"Observed"
MH (GeV)
-2ln
(Q)
1σ
2σ
} Expected
LEP Higgs Saga Pippa Wells 23
Confidence levels
This is a slice through the previous
plot at one test mass, showing the
distributions for MC experiments,
and the data observation.
Separation of b and s+b curves
indicates sensitivity of analysis.
For EACH test mass, mH, define
confidence levels
b1−CLCL
−20 −15
s+b
−10
Pro
b. D
ensi
ty
−5 0 5 10 15
−2 ln Q
"s+b"
"b"
observed
1 − CLb Measure of inconsistency with “b”
CLs+b Measure of inconsistency with “s + b”
CLs = CLs+b/CLb Lower bound on Higgs mass
LEP Higgs Saga Pippa Wells 24
5 September 2000 LEPC
LEPC - The CERN Committee in charge of the LEP physics programme
One of a planned series of presentations of results from the four
experiments during 2000 in case something new came up during the last
year of LEP running at higher energy than ever before...
150pb−1 per experiment with Ecm > 200 GeV
of which 75pb−1 per experiment with Ecm > 206 GeV
Here are some slides shown in that meeting...
LEP Higgs Saga Pippa Wells 25
���� ����� � ��� ���� �� ����
�������� �� ��� ��
�� ������� �� ��� ���������
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
100 105 110 115
Background Only
Background+Signal
Observed
MH (GeV)
-2ln
(Q)
L3
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
100 105 110 115
Background Only
Background+Signal
Observed
MH (GeV)
-2ln
(Q)
ALEPH
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
100 105 110 115
Background Only
Background+Signal
Observed
MH (GeV)
-2ln
(Q)
DELPHI
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
100 105 110 115
Background Only
Background+Signal
Observed
MH (GeV)
-2ln
(Q)
OPAL
���� ������ � ��� ���� ��� ��� � � � �����
LEP Higgs Saga Pippa Wells 26
���� ����� � ��� ���� �� ����
�������� �� ��� ��
�������� � ��� ��
-10
-7.5
-5
-2.5
0
2.5
5
7.5
10
105 110 115
Background Only
Background+Signal
Observed
MH (GeV)
-2ln
(Q)
ADLO
10-5
10-4
10-3
10-2
10-1
105 110 115MH (GeV)
1-C
Lb
3σ
ADLO
������� ������
� �� �� ���
�� ��� ������
� ����
����������
LEP Higgs Saga Pippa Wells 27
ALEPH Events - four jets with b tags
Zoom right inside
the beam pipe:
Made on 4-O
ct-2000 09:39:52 by DR
EV
ER
MA
NN
with D
AL
I_F1.Filenam
e: DC
054698_004881_001004_0937.PS_H1_X
Y_X
Y_T
U
0 −0.3cm 0.2cmX’
0 −1cm
1cm
Y’
µ
Made on 15-Sep-2000 13:12:13 by D
RE
VE
RM
AN
N w
ith DA
LI_F1.
Filename: D
C054698_004881_000915_1311.PS_H
1_XY
_WW
DALI
Run=54698 Evt=4881 ALEPH
End of detectorEnd of tracks
ECM = 206.7
LEP Higgs Saga Pippa Wells 28
5 September Decision
Approve 1 month extension of LEP running from scheduled stop on
1 October to 2 November 2000.
Hope that this will allow time to double the luminosity above 206 GeV (add
75pb−1 per experiment)
(Big end-of-LEP celebration on 11 October had to go ahead!)
Slides from the 3 November meeting...
LEP Higgs Saga Pippa Wells 29
�
�
�
�
Data Sets
• REFERENCE data set ... where it all begun ...
data set combined for the Sept 5 LEP seminar ...
Revisited ... changes within the experiments⇒ Recalibration of data
⇒ Revision of procedures (corrections)
⇒ Improvements ... better sensitivity
• DELTA set ... data collected since “REF”
(... until the “cutoff date” ... Oct 18-25)
• TOTAL = REF + DELTA
Integrated luminosities ... A+D+L+O = “ADLO”(contributions from single experiments ... within ±5%)
Not included ... latest data ... ≈30 pb−1
L (pb−1) REF DELTA TOTAL
Ecm > 200 GeV 596.6 213.7 810.3
Ecm > 206 GeV 303.5 184.5 488.0
P. Igo-Kemenes - LEP Seminar - Nov. 3, 2000 Page 5
LEP Higgs Saga Pippa Wells 30
�
�
�
�
−2 ln(Q) ... REF, DELTA, TOTAL
-10
-5
0
5
10
15
20
25
100 102 104 106 108 110 112 114 116 118 120
mH(GeV/c2)
-2 ln
(Q)
Observed
Expected background
Expected signal + background
LEP REF
-10
-5
0
5
10
15
20
25
100 102 104 106 108 110 112 114 116 118 120
mH(GeV/c2)
-2 ln
(Q)
Observed
Expected background
Expected signal + background
LEP DELTA
-10
-5
0
5
10
15
20
25
100 102 104 106 108 110 112 114 116 118 120
mH(GeV/c2)
-2 ln
(Q)
Observed
Expected background
Expected signal + background
LEP TOTAL⇐Minimum @mH ≈ 115 GeV
Agreement with SM Higgs cross-sect. for
mH = 115.0+1.3−0.9 GeV
P. Igo-Kemenes - LEP Seminar - Nov. 3, 2000 Page 8LEP Higgs Saga Pippa Wells 31
�
�
�
�
Distributions of Reconstructed Mass
Sequence: “Loose”, “Medium” and “Tight” selection (∗)
0
10
20
30
40
Even
ts /
3 G
eV/c
2
√s– = 200-210 GeV
LEP S/B=0.3backgroundhZ Signal(mh=115 GeV)
allcnd= 200bgd= 201.75sgl= 10.26
> 109 GeV2720.416.11
0
5
10
15
Even
ts /
3 G
eV/c
2
√s– = 200-210 GeV
LEP S/B=1.0backgroundhZ Signal(mh=115 GeV)
allcnd= 59bgd= 55.26sgl= 4.66
> 109 GeV63.562.94
0
2
4
6
8
0 20 40 60 80 100 120Reconstructed Mass mH [GeV/c2]
Even
ts /
3 G
eV/c
2
√s– = 200-210 GeV
LEP S/B=2.0backgroundhZ Signal(mh=115 GeV)
allcnd= 24bgd= 22.79sgl= 2.74
> 109 GeV41.131.78
(∗) Special selection ... not biasing the mass distribution
P. Igo-Kemenes - LEP Seminar - Nov. 3, 2000 Page 17
LEP Higgs Saga Pippa Wells 32
�
�
�
�
Results by Experiments
for TOTAL data set, @ mH = 115 GeV
-20 -10 0 -20 -10 0
-20 -10 0 -20 -10 0
ALEPH DELPHI
L3
-2 ln Q
Prob
. Den
sity
OPAL
ObservedBackgroundSignal
P. Igo-Kemenes - LEP Seminar - Nov. 3, 2000 Page 12
LEP Higgs Saga Pippa Wells 33
�
�
�
�
SUMMARY
REFERENCE ⇒ TOTAL
2.2σ ⇒ 2.9σ
One expt “s+b”-like ⇒ Three expt “s+b”-like
4-jet “s+b”-like ⇒ 4-jet, E-miss “s+b”-like
Perfect compatibility with SM Higgs cross section
for
mH = 115.0+1.3−0.9 GeV
! ALL THIS IS VERY EXCITING !
Current bound on Higgs boson mass
mH > 113.5 GeV @95% c.l.
for 115.3 GeV expected
P. Igo-Kemenes - LEP Seminar - Nov. 3, 2000 Page 18
LEP Higgs Saga Pippa Wells 34
Run LEP in 2001?
Evidence was consistent with a hint of Higgs production at 115 GeV
• 3/4 experiments somewhat more “s+b” than “b”
• Two channels more “s+b” than “b”
• Spread of s/b and mrecH for significant candidates consistent with Higgs
BUT
• Evidence still weak (< 3σ - a “discovery” is usually considered to be
5σ . Fluctuations happen.)
• No guarantee that extra running would confirm a discovery
• Big impact on LHC schedule and resources (civil engineering directly
delayed by LEP extension)
• LHC could see this Higgs boson, and if it’s a light SUSY Higgs could
simultaneously investigate other SUSY particles...
LEP SHUTDOWN DEFINITIVELY AT THE END OF 2000
LEP Higgs Saga Pippa Wells 35
The final word on the SM Higgs (April 2003)
Full dataset, calibration updates, some improvements to analyses.
-30
-20
-10
0
10
20
30
40
50
106 108 110 112 114 116 118 120
mH(GeV/c2)
-2 ln
(Q)
ObservedExpected for backgroundExpected for signal plus background
LEP
LEP Higgs Saga Pippa Wells 36
The final word on the SM Higgs (April 2003)
0
0.02
0.04
0.06
0.08
0.1
0.12
-15 -10 -5 0 5 10 15
-2 ln(Q)
Pro
babi
lity
dens
ity
ObservedExpected for backgroundExpected for signalplus background
LEPmH = 115 GeV/c2
(a)
Higgs boson excluded up to 114.4 GeV at 95% CL
LEP Higgs Saga Pippa Wells 37
The End
The LEP experiements have published more than 1000 papers on many
topics.
High precision tests of the Standard Model have been made, and are
sensitive to radiative corrections.
The electroweak data prefer a light Higgs boson.
The SM Higgs boson search gives a limit at 114.4 GeV, with an
inconclusive hint of a signal at around 115 GeV.
Pass the baton to the Tevatron (Run II - CDF, D0 in progress) and the LHC
(ATLAS, CMS first data in 2007).
LEP Higgs Saga Pippa Wells 38