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12!
Review of 10 years of LEP
W. Adam
Institute of High Energy Physics
Austrian Academy of Sciences
Four Seas Conference
Thessaloniki, Greece
April 16th, 2002
A bit of history …
1976
1978
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B.Richter: e+e- machine needed to study weak interactions
at high energy; √s~200GeV at
R~6km seems feasible
1st physics study
“Blue Book”
Les Houches summer study: baseline
140GeV (200GeV with SC cavities)
at 2Rp=22.2km
What to expect? E.g. possibility of an
“invisible” Z for high Nn!
Glashow quotes 4 scenarios; considers
the assumption of a correct extrapolation of the 17 parameter model
as “arrogant”.
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A bit of history …
1979
“Pink Book”: 2Rp~30km
1983
Discovery of W and Z
LEP Design Report: 2Rp=26.7km
1984
1986
“Physics at LEP”: detailed study of the physics scenario
•Discusses effects of radiative corrections
•Treats SUSY (~neglected in first study)
•Full chapter on toponium!
1989
“Z physics at LEP1”
Start of LEP operations
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A bit of history …
Situation at LEP startup:
At LP’89 the first results from SLC
were reported:
PDG’88:
• W and Z masses known to
few %
• Limits on W and Z widths
• B0 and B± masses known,
lifetime at 10% level
• B* still disputed
• B-mixing observed
• s measured to ~10%
Altarelli (LP’89)
• Mt > 44GeV, Nn<5.9
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Experiments
The four LEP experiments: ALEPH, DELPHI, L3 and OPAL
Common features:
• 4p general purpose detectors
• Momentum measurement in
solenoidal magnetic field
• Detectors matched to LEPs
O(100kHz) collision frequency
Specialisation on different items:
• High precision muon systems
• High resolution EM calorimetry
• Particle identification
• Large acceptance central tracking
ALEPH
DELPHI
L3
OPAL
Vertex
detector
2 layer
double-sided
0.96m2
3 layer
double/single
1.37m2
2 layer
double-sided
0.52m2
2 layer
single-sided
0.53m2
Central
Tracking
(B-Field)
TPC
(1.5T)
TPC
(1.2T)
TEC
(0.5T)
Jetchamber
(0.4T)
Luminosity
W-Si
Pb/Sci+ Si
BGO
W-Si
Vital for LEPI: luminosity
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For both: b-tagging
W.Adam: Review of 10 years of LEP
Vital for LEPII: hermeticity
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Start of LEP operation
LEP pilot run: start on Aug. 13th, 1989.
A few hours later: first Z recorded in
OPAL - the others followed.
The first important LEP result was
available in winter 1990:
three generations of light neutrinos.
Nn = 3.04 ± 0.12
But for this result LEP had arrived in
second place.:
MARKII at SLC in October ‘89:
Nn = 2.8 ± 0.6
The first physics run lasted for
~3 months and provided each
experiment with 10-30k events.
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The start of several years of
competition and collaboration …
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Start of LEP operation
Already in summer ‘90 LEP had a
significant impact on the world
averages in the EW sector 
F. Dydak summarised measurements on
mass, total and partial widths of the Z,
couplings, weak mixing angle …
But no SM-eating Minotaur in view!
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Electroweak (LEP I)
Goal: extract Z-mass, widths (total and partial) and Zff couplings from measured
cross-sections and asymmetries. Common procedures developed in the LEP
EWWG.
Nine basic quantities fitted from data:
• Mass (MZ)
• Total width (GZ)
• Hadronic cross-section at pole (sh0)
• Hadronic to leptonic width (Rl=Gl/Gh)
x3
• Forward-backward asymmetry (AlFB)
Check lepton universality, then combine the
three lepton flavours.
Extensible using tau polarisation, heavy
flavour results, non-LEP measurements
(sin2q, mW).
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1990&1991:
1992&1994:
1993&1995:
7-point scans
peak
3-point scans
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Electroweak (LEP I)
MZ
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LEP beam energy
LEP beam energy
Absolute energy scale and point-to-point
spread  systematic error on MZ and GZ
dMZ ~ 0.5 d (Epeak+2+Epeak-2)
dGZ ~ 0.71 d (Epeak+2-Epeak-2)
Strategy:
•resonant depolarization as reference
(dEbeam~200keV)
•interpolate (NMR/.., flux loop, …), correcting for
systematic effects:
– temperature,…
– tidal & hydrogeological effects
– parasitic currents
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E-spread / point ~ 50MeV
Uncertainty O(MeV)
dMZLEP ~ 1.7MeV
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Electroweak (LEP I)
 lepton universality

Rl
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Electroweak (LEP I)
Derived quantities: partial widths
Ghad
1744.4 ± 2.0 MeV
Glept
83.984 ± 0.086 MeV
Ginv
499.0 ± 1.5 MeV
n
2.9841 ± 0.0083
Ginv
-2.7 +1.7-1.5 MeV
< 2.0MeV @ 95%CL
Nn
Alternatives:
•Nn from Rl and shad
•Nn from nng above pole
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Use Ginv to
• Determine Nv (from Ginv/Gll)
• Set limits on other invisible
particles by comparing with
SM prediction
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Electroweak (LEP I)
Effective couplings:
Different definitions of sin2q used:
1
g
sin 2 l  (1 Vl ) (Z  pole)
4
g Al
2
m
sin 2  W  1 W2
(pp)
mZ
and others
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Top mass
mt
latest
CDF: evidence for …
observation of …
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An impressive demonstration of the
predictive power of the SM (and of the
reliability of EW precision data)!!!
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EW heavy flavours (LEP I)
Now adding info from heavy flavours: partial widths and asymmetry for
c’s and b’s (quark mass sector: 10 of 17 SM parameters!).
How? Use of (more & more sophisticated) vertex detectors!
Combine information
from all tracks (NN,
prob. Methods, …)
Efficiency vs. purity for b-events
(2- & 3-layer VDs)
Impact parameter significance
= d / s(d)
More sophisticated tags developed
for LEPII searches (see VRKs talk)
Tagging is more challenging for charm:
reconstruct charmed hadrons decays, unfold b-contribution.
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EW heavy flavours (LEP I)
Measurement of Rb = Gbb/Ghad :
sensitivity to new physics
A seemingly small change in 1995,
but …
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The Rb-puzzle
Excitement: finally a 3.7s deviation
from the standard model!!
Lots of cross checks and a
new ALEPH result:
Stops? Charginos??
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B physics
Helpful feature of B-hadrons at LEP:
they fly !

Basic (and earliest) measurements:
B lifetimes
Started with semi-leptonic decays,
then extended to more channels.
Remark:

Little was known about heavy flavours
(and their lifetimes) during the early
discussion about LEP.
Lucky coincidence with progress in
semiconductor detectors!
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Latest B0d - combination. Now
including Babar & Belle - but
LEP is still competitive!
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B lifetime results
(Bs)
(Bd)
1.464±0.057ps
1.542±0.016ps
±/0
1.083±0.017
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All
dominated
by
LEP!
W.Adam: Review of 10 years of LEP
(baryon)
1.208±0.051ps
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B physics
Based on the techniques developed for the lifetimes the whole field of
B-physics was explored: about 1/3 of all LEP publications!
Comparison with HQET
Semileptonic branching ratio and
“wrong signed” charm decays
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Exclusive decays
B-  D0D*20
B physics
Determination of |Vub| from
BR(bXuln) and b:
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Determination of |Vcb| from
• Exclusive D*ln or
• G(bXcln)
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B physics
Bd oscillations
Bs oscillations
LEP results still significant, but
now hunting ground for Belle &
Babar!
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ms > 14.9 ps-1
(sensitivity: 19.3 ps-1!!!)
Will have to wait for Tevatron
RunII results …
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B-physics
These results can be used to constrain
the unitarity triangle:
(1
2 )V
2
V td
m d

 Vts
m s
ub
 Vcb
(, )
F2=
F3=g
F1=b
(0,0)
(Belle: 0.82±0.12±0.05
Babar: 0.75±0.09±0.04)
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(1,0)
Equivalent to
• sin2b = 0.696 ± 0.068 (with ek)
• sin2b = 0.676 +0.078-0.096
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QCD at LEP
Even if “specialised” on weak interactions - abundant results on QCD
Strong coupling constant from EW fits
s from Rl
0.1224
±0.0038
from sin2qlep
0.1180
±0.0030
Fit LEP I & II
0.1199
±0.0030
+0.0033
-0.0000
+0.0026
-0.0000
s from EW
(only exp. errors)
From event shapes
and jet rates
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QCD at LEP
Renormalisation scale in NLO (event shapes & jet rates):
Optimised scale
Q=xm MZ2
Fixed scale Q=MZ2
No consistency using fixed scale  try to fit scale with s !
Consistency achieved. Conclusion?
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QCD at LEP
Running of mb
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Unification
Early LEP data show: need new physics to achieve unification
Amaldi et al.,
Phys.Lett.B281(1992)374
Significance increased from
2s (1987) to 8s (1991).
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A possible solution:
SUSY
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Towards higher energies
In 1995 the LEPI era ended with the
final scan of the Z-resonance. From
then on the focus shifted to the
extension of the search range and to
precision measurements above the
WW threshold.
The 1995 run was ended with ~6pb-1
/expt at √s=130–140GeV (LEP1.5).
In 1996 the WW threshold was passed:
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In the same year the production of
superconducting RF cavities (the key
to high energy LEP running) was
stopped. 
With the successive installation of all
288 SC cavities, the reuse of old copper
cavities and a lot of work of the
accelerator physicists the energy was
increased, up to the 210GeV reached
in 2000.
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Electroweak (LEP II)
MZ and sin2q measured at LEPI  need MW for better constraint.
Method 1: cross section at threshold
Little sensitivity for √s >> 161GeV
EWWG (1996)
ZWW vertex exists!
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Electroweak (LEP II)
Method 2: direct reconstruction
Concern: mass bias due to final state
interactions in qqqq (Bose-Einstein,
colour reconnection)?
April 16th, 2002
measure FSIs (need model!)
Indication:
M(4q-qqln) = 9±44MeV
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Electroweak (LEP II)
pp
LEP
mW
Moriond ‘02
LEP uncertainties:
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stat (±26MeV)
syst (±21MeV)
FSI (±13MeV)
LEP (±17MeV)
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Electroweak (LEP II)
Also the width can be obtained from
the direct W-mass reconstruction:
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Electroweak (LEP II)
“Single W”
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ZZ
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TGCs
Non-abelian structure of SU(2)U(1)  gauge boson self interactions
Charged triple boson couplings:
WWg, WWZ
SM tree level (see above)
General:
• Operators >= dim. 6
• Lorentz & EM gauge invariance
• C, P, CP conservation
• Low energy results

g1Z, kg, g (all 0 in SM)
Inputs:
• WW: stot & helicity info
• Single W, nng: stot & differential
distributions
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EWWG (2000)
No neutral TGCs in SM (tree level).
All measurements compatible with 0.
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QGCs
Charged quartic gauge couplings:
negligible in the SM
WWg, nngg, qqgg, …:
Neutral quartic gauge couplings:
none in SM at tree level
Anomalous contributions to couplings:
• a0V/L2, acV/L2 (VVgg)
• an/L2 (WWZg)
All a = 0 at tree level
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EWWG (2001)
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Searches
The situation after LEP1 (with a grain of salt):
• whatever couples to the Z is excluded to mZ/2 (double production)
or mZ (single production) …
• Severe constraints for invisible states from the comparison of
Ginv with SM expectations
Now every year LEP2 opened new search windows!
Change in the way collaborations analysed data:
• Installation of “hot lines”
• Continuous update of analyses results during a run
The unexpected could be just around the corner!
List of topics (for direct and indirect searches) would fill pages:
Higgs, technicolour, SUSY (in different breaking models), contact
interactions, compositeness, extra dimensions, anomalous couplings, …
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SUSY

Well motivated & elegant

Low energy effective theory adds
some 100 parameters to model:
• Masses
• Soft breaking terms

Need SUSY breaking models.
Investigated at LEP:
• SUGRA
• GMSB
• AMSB
Models predict production rates,
decay channels, …
 Use topological approach!
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Topological search

Cross section limits
for specific channel

Mass limits /
limits in model parameter space
Two fundamentally different scenarios
(at least experimentally):
•R-parity conservation:
»Pair production
»Emiss signature due to LSP
•R-parity violation:
»Single production possible
»Multitude of decay chains
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Sfermions
L-R mixing! Lower mass
states are candidates for
NLSP.
Lower mass reach
than Tevatron, but
sensitivity to
amaller Evis!
Search for pair production,
with decay in fermion + LSP
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SUSY
mSUGRA:
Parameters: tanb, sign(m), m0, m1/2, A0. Limits use also GZ and Higgs search
Translated into LSP mass limits:
MLSP>60.1GeV (m>0)
MLSP>59.6GeV (m<0)
chargino
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Higgs
LEP1
sfermion
for mt=175GeV
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GMSB
Light gravitino = LSP
Radiative decays of neutralinos
Acoplanar
two-photon
events
Lifetime of sparticles depends on gravitino
mass: dedicated searches for long lifetimes
“standard”
Direct detection
Large impact parameters
and kinks
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GMSB interpretation as
neutralino
pair production
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Other searches
Single top production via FCNC:
anomalous couplings Zt(u,c),
g
Low scale gravity:
Graviton propagates in 4+n dim.
M2Planck=
(MD)2+nRn
Summer ‘01
Limits depend strongly on mt!
April 16th, 2002
Modifies 2-boson and 2-fermion ds/dq:
LEP(gg): Ms>0.97TeV (=+1)
Ms>0.94TeV (=-1)
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Summary & Conclusions
It’s difficult to summarize one decade of
LEP experiments. Let’s try to start with a
result:
LEP experiments have verified the SM with
precision measurements, achieving
accuracies at the level of 10-5. This often
exceeded expectations by an order of
magnitude.
How?
• Excellent performance of the accelerator:
luminosity, peak energy, systematics, …
• Improvements in detector technology at
the beginning & during operation.
• Analyses: redundancy and minimisation
of the (inevitable) dependence on MC.
• Close contact between collaborations and
support from theory.
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Summary & Conclusions
Acknowledgments: thanks a lot to
• The machine people who pushed LEP beyond the 200GeV.
• The technical staff who contributed to the experiments.
• Theorists and phenomenologists who showed us where to look, and to
interpret what we found there.
• The hundreds of people who contributed to the analyses and to those
who are still working on the finalisation and combination.
In it’s last few months LEP might have opened a door
• to another decade of SM, but more likely
• to the physics beyond.
Whether the excitement was justified or not: LEP has created the solid basis
which is indispensable for the exploration of new physics at future colliders.
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