Review of 10 years of LEP

Review of 10 years of LEPReview of 10 years of LEP

W. Adam

Institute of High Energy PhysicsAustrian Academy of Sciences

12!12!

Four Seas Conference Thessaloniki, Greece April 16th, 2002

W.Adam: Review of 10 years of LEP 2April 16th, 2002

A bit of history …A bit of history …

1976 B.Richter: e+e- machine needed to study weak interactionsat high energy; √s~200GeV at R~6km seems feasible

1st physics study

1978 “Blue Book”Les Houches summer study: baseline

140GeV (200GeV with SC cavities) at 2R=22.2km

What to expect? E.g. possibility of an“invisible” Z for high N!

Glashow quotes 4 scenarios; considers the assumption of a correct extra-polation of the 17 parameter model as “arrogant”.



1979 “Pink Book”: 2R~30km

1983 Discovery of W and Z

1984 LEP Design Report: 2R=26.7km

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



Situation at LEP startup:

Altarelli (LP’89)

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%

• Mt > 44GeV, N<5.9

At LP’89 the first results from SLC were reported:


ExperimentsExperiments

The four LEP experiments: ALEPH, DELPHI, L3 and OPALCommon features:

• 4 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

Vertexdetector

2 layerdouble-sided

0.96m2

3 layerdouble/single

1.37m2

2 layerdouble-sided

0.52m2

2 layersingle-sided

0.53m2

CentralTracking(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 LEPII: hermeticityVital for LEPI: luminosity For both: b-tagging


Start of LEP operationStart of LEP operation

LEP pilot run: start on Aug. 13th, 1989.

A few hours later: first Z recorded inOPAL - the others followed.

The first physics run lasted for ~3 months and provided each experiment with 10-30k events.

The first important LEP result was available in winter 1990:

three generations of light neutrinos.

N = 3.04 ± 0.12

But for this result LEP had arrived in second place.:

MARKII at SLC in October ‘89:

N = 2.8 ± 0.6

The start of several years of competition and collaboration …


Start of LEP operationStart 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!


Electroweak (LEP I)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 (Z)• Hadronic cross-section at pole (h

0)• Hadronic to leptonic width (Rl=l/h)• Forward-backward asymmetry (Al

FB)x 3

1990&1991: 7-point scans1992&1994: peak1993&1995: 3-point scans

Check lepton universality, then combine the three lepton flavours.

Extensible using tau polarisation, heavy flavour results, non-LEP measurements (sin2, mW).



MZ


LEP beam energyLEP beam energy

LEP beam energy

MZ ~ 0.5 (Epeak+2+Epeak-2)Z ~ 0.71 (Epeak+2-Epeak-2)

Absolute energy scale and point-to-point spread systematic error on MZ and Z

Strategy: •resonant depolarization as reference (Ebeam~200keV)

•interpolate (NMR/.., flux loop, …), correcting for systematic effects:

– temperature,…– tidal & hydrogeological effects

– parasitic currents

E-spread / point ~ 50MeVUncertainty O(MeV)

MZLEP ~ 1.7MeV



lepton universality

Rl



Derived quantities: partial widths had 1744.4 ± 2.0 MeV

lept 83.984 ± 0.086 MeV

inv 499.0 ± 1.5 MeV

Use inv to • Determine Nv (from inv/ll)• Set limits on other invisible

particles by comparing with SM prediction

2.9841 ± 0.0083

inv-2.7 +1.7-1.5 MeV

< 2.0MeV @ 95%CL N

Alternatives:•N from Rl and had

•N from above pole



Effective couplings:

Different definitions of sin2 used:

€

sin2ϑ l =14

(1−g Vl

g Al

) (Z−pole)

€

sin2ϑ W =1−mW

2

mZ2 (pp )

and others


Top massTop mass

An impressive demonstration of the predictive power of the SM (and of the

reliability of EW precision data)!!!

mt

CDF: evidence for …

observation of …

latest


EW heavy flavours (LEP I)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!

Impact parameter significance = d / (d)

Efficiency vs. purity for b-events(2- & 3-layer VDs)

Tagging is more challenging for charm: reconstruct charmed hadrons decays, unfold b-contribution.

More sophisticated tags developed for LEPII searches (see VRKs talk)

Combine information from all tracks (NN,prob. Methods, …)


EW heavy flavours (LEP I)EW heavy flavours (LEP I)

A seemingly small change in 1995,but …

Measurement of Rb = bb/had :sensitivity to new physics


The RThe Rbb-puzzle-puzzle

Excitement: finally a 3.7 deviation from the standard model!!

Stops? Charginos??

Lots of cross checks and a new ALEPH result:


B physicsB physics

Helpful feature of B-hadrons at LEP: they fly !

Basic (and earliest) measurements:

B lifetimes

Remark:Little was known about heavy flavours (and their lifetimes) during the early discussion about LEP.

Lucky coincidence with progress in semiconductor detectors!

Started with semi-leptonic decays, then extended to more channels.

Latest B0d - combination. Now

including Babar & Belle - butLEP is still competitive!


B lifetime resultsB lifetime results

(Bd)

±/0

(Bs)

(baryon)

Alldominated

byLEP!

1.542±0.016ps

1.083±0.017

1.464±0.057ps

1.208±0.051ps


B physicsB 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 Exclusive decays

Semileptonic branching ratio and“wrong signed” charm decays B- D0D*-


B physicsB physics

Determination of |Vub| from BR(bXul) and b:

Determination of |Vcb| from

• Exclusive D*l or

• (bXcl)


B physicsB physics

Bd oscillations Bs oscillations

LEP results still significant, but now hunting ground for Belle &

Babar!

ms > 14.9 ps-1 (sensitivity: 19.3 ps-1!!!)

Will have to wait for Tevatron RunII results …


B-physicsB-physics

These results can be used to constrain the unitarity triangle:

(0,0) (1,0)

€

(ρ ,η )

€

(1−λ2

2 )Vub

λVcb

€

Vtd

λVts

€

∝Δmd

Δms

1=3=

2=

(Belle: 0.82±0.12±0.05Babar: 0.75±0.09±0.04)

Equivalent to • sin2 = 0.696 ± 0.068 (with k)• sin2 = 0.676 +0.078-0.096


QCD at LEPQCD at LEP

Even if “specialised” on weak interactions - abundant results on QCD

s from Rl 0.1224 ±0.0038 +0.0033-0.0000

from sin2lep 0.1180 ±0.0030 +0.0026-0.0000

Fit LEP I & II 0.1199 ±0.0030

From event shapesand jet rates

Strong coupling constant from EW fits

s from EW(only exp. errors)



Renormalisation scale in NLO (event shapes & jet rates):

No consistency using fixed scale try to fit scale with s !Consistency achieved. Conclusion?

Fixed scale Q=MZ2

Optimised scale Q=x MZ

2



Running of mb


UnificationUnification

Early LEP data show: need new physics to achieve unification

Significance increased from 2 (1987) to 8 (1991).

A possible solution: SUSY

Amaldi et al.,Phys.Lett.B281(1992)374


Towards higher energiesTowards 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:

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.


Electroweak (LEP II)Electroweak (LEP II)

MZ and sin2 measured at LEPI need MW for better constraint.

Method 1: cross section at threshold

EWWG (1996)

Little sensitivity for √s >> 161GeV

ZWW vertex exists!



Method 2: direct reconstruction

Concern: mass bias due to final state interactions in qqqq (Bose-Einstein, colour reconnection)?

measure FSIs (need model!)Indication:M(4q-qql) = 9±44MeV



mW

€

pp LEP

Moriond ‘02

LEP uncertainties: stat (±26MeV)syst (±21MeV)FSI (±13MeV)LEP (±17MeV)



Also the width can be obtained from the direct W-mass reconstruction:



“Single W” ZZ


TGCsTGCs

Non-abelian structure of SU(2)U(1) gauge boson self interactions

Charged triple boson couplings: WW, WWZ

SM tree level (see above)

General:• Operators >= dim. 6• Lorentz & EM gauge invariance• C, P, CP conservation• Low energy results

g1

Z, , (all 0 in SM)

Inputs:• WW: tot & helicity info• Single W, : tot & differential

distributions

EWWG (2000)

No neutral TGCs in SM (tree level).All measurements compatible with 0.


QGCsQGCs

EWWG (2001)

Charged quartic gauge couplings:negligible in the SM

Neutral quartic gauge couplings:none in SM at tree level

Anomalous contributions to couplings:• a0

V/2, acV/2 (VV)

• an/2 (WWZ)All a = 0 at tree level

WW, , qq, …:


SearchesSearches

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

inv 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, …


SUSYSUSY

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!

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


SfermionsSfermions

L-R mixing! Lower mass states are candidates for NLSP.

Search for pair production, with decay in fermion + LSP

Lower mass reach than Tevatron, but sensitivity to amaller Evis!


SUSYSUSY

mSUGRA:Parameters: tan, sign(), m0, m1/2, A0. Limits use also Z and Higgs search

chargino Higgs LEP1 sfermion

Translated into LSP mass limits:

MLSP>60.1GeV (>0) MLSP>59.6GeV (<0)for mt=175GeV


GMSBGMSB

Light gravitino = LSPRadiative decays of neutralinos

Lifetime of sparticles depends on gravitino mass: dedicated searches for long lifetimes

“standard”

Large impact parameters and kinks

Direct detection

Acoplanar two-photon

events

GMSB inter-pretation as neutralinopair production


Other searchesOther searches

Single top production via FCNC:anomalous couplings Zt(u,c),

Summer ‘01

Limits depend strongly on mt!

Low scale gravity:Graviton propagates in 4+n dim.

M2Planck=(MD)2+nRn

Modifies 2-boson and 2-fermion d/d:LEP(): Ms>0.97TeV (=+1)

Ms>0.94TeV (=-1)


Summary & ConclusionsSummary & 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.


Summary & ConclusionsSummary & 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 thosewho 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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Review of 10 years of LEP

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