Physics at Electron Positron Colliders

Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 1

Physics at Electron Positron CollidersPhysics at Electron Positron Colliders

Klaus DeschUniversity of HamburgDESY Summer Student Lecture, 08/2002

• Introduction• The LEP collider and detectors• Physics highlights from LEP• The future: TESLA• Prospects for TESLA physics

A few technical but important preliminaries:A few technical but important preliminaries:

• Ask questions!

• Tell me if too fast or too slow

• Tell me if things have been shown before and are too boring for you ;-)

• There is no ‘pensum’ we have to fulfil –understanding one thing is better than not understanding many things.

Remember (most important):There are no stupid questions!!!

Particle Physics TodayParticle Physics Today

“Particle physics today is in an excellent yet curious state”(TESLA TDR)

excellent:

experimental discoveryof the fundamentalconstituents of matter:

and of the force carriers: Gravitation?

A beautiful theoretical concept to describe particles and their interactions:

•Quantum field theory

•Symmetry principle: local gauge invariance

•Perturbation theory: precise predictions at the loop-level

excellent:

Impressive agreement

between

experimental results and

thoeretical predictions:

A major part of theseResults comes from LEPand SLD!

Experiments!Experiments!

Lepton and Hadron machines are complementary!

Electron positron colliders Hadron (proton proton) colliders

pointlike particles collide composite particles collide

E(CM) = 2 E(beam) E(CM) < 2 E(beam)

clean final states superposition with spectator jets:large backgrounds

well known quantum numbers quantum numbers of hard collisionof initial state are not well known

very high energies are difficult very high energies are comparatively (synchroton radiation) easier

well suited for discovery and best suited for discoveryprecision measurements some precision measurements are possible

example: astronomy

observed: small deviations from the expected trajectories of the planets

predicted: an additional planet can explain the observation

discovery: PLUTO was found

example: Particle physics

indirect prediction for mass ofthe top quark

discovery: 1995, Tevatron (USA)at the predicted mass

Precision in PhysicsPrecision in Physics

Brief History of CollidersBrief History of Colliders

The first collding beam accelerator (electron-electron)

PRIN-STAN(Stanford)~1966

CERNCERN

The LEP collider at CERNThe LEP collider at CERN

4 x 4Number of bunches

~ 750 µACurrent per bunch

~ (~1 Z0/sec)Luminosity (at Z0)

~ (3 WW/hour)Luminosity (at LEP2)

Up to 7 MV/m (SC cavities) Accelerating gradient

4 (ALEPH,DELPHI,L3,OPAL)Interaction regions

~27 kmCircumference

< 1 MeV (at Z0)Energy calibration

92.1 GeV(LEP1) to 209 GeV(LEP 2)

Centre-of-mass energy

12301024 −−× scm12301050 −−× scm

Test phase for LEP2: 130 GeV

161-172 GeV:WW-threshold

183 GeV 1000 W-pairs

189 GeV2500 W-pairs

192-200 GeV3000 W-pairs

200-209 GeV3000 W-pairs

~91 GeV4 Million Z0’s

1990-1995LE

LEP was shut down and dismantled to make room for the LHC in Nov 2000

DetectorsDetectors

Tracking

electron chargedhadron + muon

photon neutralpion

DetectorsDetectors

Identifying eventsIdentifying events

http://www.hep.man.ac.uk/~events/home.html

e e e e+ − + −→

e e µ µ+ − + −→

e e τ τ+ − + −→

e e qq+ − →

A little quizA little quiz

Physics highlights from LEPPhysics highlights from LEP

• The basic process at LEP1

• Z-mass

• Asymmetries: electro-weak interference

• Testing the strong interaction:QCD

• LEP2: W-Bosons

• LEP2: The Higgs hunt

• Status of the SM: where is the Higgs?

Of course this is only a personal selection, many more(interesting) topics will be left out

Until today 1199 published papers by the 4 LEP experiments

The basic process at LEP1The basic process at LEP1

The total cross sectionThe total cross section

The mass of the ZThe mass of the Z--BosonBoson

Error only 2.1 MeV!Need to understand many tiny theoretical + experimental effectsImportant: Beam energy calibration

Beam energy measurementBeam energy measurement

Achieved precision: ~ 1 MeV !

Beam energy measurement: tiny effectsBeam energy measurement: tiny effects

Beam energy measurement: very tiny effectsBeam energy measurement: very tiny effects

One step further: classifying the eventsOne step further: classifying the events

SM makes precise prediction for the branching ratios of the Z0:

3F ZG M 167 MeV

12π 2ννΓ = ≈

24 84 MeVee Wxµµ ττ ννΓ = Γ = Γ = Γ ≈

232 83 1 287 MeV9 3uu cc W Wx x νν

Γ = Γ = − + Γ ≈

28 43 1 370 MeV9 3dd ss bb W Wx x νν

Γ = Γ = Γ = − + Γ ≈

for each neutrino family

2( sin )W Wx θ=(neglecting the quarks masses)

How can one measure ?ννΓ ( ) X

BR Z X Γ→ =

Counting neutrinosCounting neutrinos

prefers 3 familiesbut rather large error…

Counting neutrinos: smarter wayCounting neutrinos: smarter way

famitot ee qq liesNµµ τ νντΓ = Γ +Γ +Γ + ΓΓ +

i.e. the total width of the Z depends on the number of (light) neutrinos:

Result:

2.9841 0.0083familiesN = ±

before LEP:

5.9familiesN <

Yet another step further: angular distributionsYet another step further: angular distributions

Linear term in cos(θ) leads to a forward backward assymetry:

better method (smaller systematics) thanfit to whole distibution,since detector efficiency cancels out (only forward-backward-symmetric detector needed)

FB asymmetries allow determination of vector- and axialvector

coupling of the fermions to the Z.

Test of lepton universality!

Interpretation of the AsymmetriesInterpretation of the Asymmetries

Asymmetry measurementsCan be interpreted asMeasurement of theElectro-weak mixing angle

2sin Wθ

Too precise measurements! Too precise measurements! –– Radiative Radiative correctionscorrections

The measured observables can be predicted in the SM as a functionOf the following parameters:

Too precise measurements! Too precise measurements! –– Radiative Radiative correctionscorrections

They depend on the unknown parameters

Higgsmlinearly

only logarithmically

Lead to prediction oftop-mass before itsdiscovery:

Testing the theory of the strong interaction: QCDTesting the theory of the strong interaction: QCD

Quarks and gluons are not observed as free objects, but manifestthemselves as jets.

String effect

Process can be described in 4 Steps:1. EW creation (exactly calculable)2. Parton shower (pert. QCD treatment)3. Fragmentation of Quarks+Gluons into Hadrons (only phen. Models)4. Decay of Hadrons (mostly phenomenological)

Phases 3 and 4 implemented into Computer-Programs: inportant tools.

Quarks and Gluons show up as Jets. Need a definition of a jet!

Cluster Algorithm:

number of jets dependsupon how close one looksat them…

Measure the strong coupling constant:

But there are more fancy methods…

Summary: LEP1Summary: LEP1

• Precision study of the properties of the Z-Boson

• Triumph of the Standard Model

• Sensitivity to virtual effects from top-Quark

• Prediction of top mass before discovery

• Precise measurement of electroweak mixing angle

21 sin WW

θ− =

Precisely known from LEP1

Would like to know moreLEP2!

Increasing the energy: LEP2Increasing the energy: LEP2

Main targets:

• Precise measurement of the W-mass

• Testing the non-abelian structure of the electroweak interaction

• Hunt for the Higgs boson

• Searches for new particles

WW--Pair productionPair production

• no resonance! much smaller cross section than for Z at LEP1• only pair-production (why?)• expect a few thousand events / year• again precise SM predictions (e.g.):

- mass- branching ratios- structure of couplings (self-coupling!)

W W qqqq+ − → W W ν ν+ − → W W qqν+ − →

Three different event topologies:

Measurement of the W-mass

Error: ~40 MeVi.e. ~20x error on Z-mass

NonNon--abelian structure of EWabelian structure of EW--interactionsinteractions

First direct observationof gauge boson self-couplingin EW interactions

NonNon--abelian structure of EWabelian structure of EW--interactionsinteractionsMore precise information about this coupling from angular distributionsOf the W-Bosons and their decay products

Again too precise: Radiative corrections part 2:Again too precise: Radiative corrections part 2:Direct measurements of and can be compared to theirIndirect predictions from LEP1 measurements:

WM topM

• They agree!• They prefer a ratherLight Higgs Boson!

Sensitivity to the last SM particle: the HiggsSensitivity to the last SM particle: the Higgs

Perform a global fit to all measurements within the SMto obtain the most likely Higgs mass:

M(Higgs) <~ 200 GeV at95% C.L.

What is the yellow bar?

Hunting the Higgs BosonHunting the Higgs Boson

If not too heavy, the Higgs could be observeddirectly at LEP:

Higgs Branchiong ratios:

dominant decays:

H bb→

H τ τ+ −→

…leads together withZ decay modesto many differentfinal states:

Hunting the Higgs BosonHunting the Higgs BosonA nice candidate event (m = 115 GeV)

Are there many of these?

Hard to say from plot… statistical analysis needed:

No significant excess observed.

Final word from LEP: HiggsM >114.4 GeV@ 95%C.L.

Summary of Day1Summary of Day1

• Hadron and Lepton Colliders have complementary properties

• LEP1 provided confirmed the SM with excellent precision

-> led to prediction for top mass

• LEP2 precision study of W bosons

-> prediction for Higgs mass

• Direct Higgs search: m>114 GeV

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