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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
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 2
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!!!
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 3
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?
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 4
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
Particle Physics TodayParticle Physics Today
excellent:
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 5
Particle Physics TodayParticle Physics Today
excellent:
Impressive agreement
between
experimental results and
thoeretical predictions:
A major part of theseResults comes from LEPand SLD!
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 6
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
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 7
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
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 8
Brief History of CollidersBrief History of Colliders
The first collding beam accelerator (electron-electron)
PRIN-STAN(Stanford)~1966
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 9
Brief History of CollidersBrief History of Colliders
TESLA
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 10
Brief History of CollidersBrief History of Colliders
TESLA
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 11
CERNCERN
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 12
The LEP collider at CERNThe LEP collider at CERN
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 13
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
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 14
The LEP collider at CERNThe LEP collider at CERN
Test phase for LEP2: 130 GeV
1995
161-172 GeV:WW-threshold
1996
183 GeV 1000 W-pairs
1997
189 GeV2500 W-pairs
1998
192-200 GeV3000 W-pairs
1999
200-209 GeV3000 W-pairs
2000
~91 GeV4 Million Z0’s
1990-1995LE
P 1
LEP 2
LEP was shut down and dismantled to make room for the LHC in Nov 2000
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 15
DetectorsDetectors
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 16
DetectorsDetectors
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 17
VTX
Tracking
ECAL
HCAL
Muon
electron chargedhadron + muon
photon neutralpion
??
DetectorsDetectors
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 18
Identifying eventsIdentifying events
http://www.hep.man.ac.uk/~events/home.html
e e e e+ − + −→
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 19
Identifying eventsIdentifying events
e e µ µ+ − + −→
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 20
Identifying eventsIdentifying events
e e τ τ+ − + −→
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 21
Identifying eventsIdentifying events
e e qq+ − →
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 22
A little quizA little quiz
???
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 23
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
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 24
The basic process at LEP1The basic process at LEP1
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 25
The total cross sectionThe total cross section
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 26
The total cross sectionThe total cross section
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 27
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
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 28
Beam energy measurementBeam energy measurement
Achieved precision: ~ 1 MeV !
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 29
Beam energy measurement: tiny effectsBeam energy measurement: tiny effects
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 30
Beam energy measurement: very tiny effectsBeam energy measurement: very tiny effects
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 31
Beam energy measurement: very tiny effectsBeam energy measurement: very tiny effects
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 32
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
tot
BR Z X Γ→ =
Γ
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 33
Counting neutrinosCounting neutrinos
prefers 3 familiesbut rather large error…
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 34
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 <
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 35
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)
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 36
Yet another step further: angular distributionsYet another step further: angular distributions
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 37
Yet another step further: angular distributionsYet another step further: angular distributions
FB asymmetries allow determination of vector- and axialvector
coupling of the fermions to the Z.
Test of lepton universality!
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 38
Interpretation of the AsymmetriesInterpretation of the Asymmetries
Asymmetry measurementsCan be interpreted asMeasurement of theElectro-weak mixing angle
2sin Wθ
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 39
Too precise measurements! Too precise measurements! –– Radiative Radiative correctionscorrections
The measured observables can be predicted in the SM as a functionOf the following parameters:
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 40
Too precise measurements! Too precise measurements! –– Radiative Radiative correctionscorrections
They depend on the unknown parameters
topm
Higgsmlinearly
only logarithmically
Lead to prediction oftop-mass before itsdiscovery:
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 41
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
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 42
Testing the theory of the strong interaction: QCDTesting the theory of the strong interaction: QCD
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.
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 43
Testing the theory of the strong interaction: QCDTesting the theory of the strong interaction: QCD
Quarks and Gluons show up as Jets. Need a definition of a jet!
Cluster Algorithm:
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 44
Testing the theory of the strong interaction: QCDTesting the theory of the strong interaction: QCD
number of jets dependsupon how close one looksat them…
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 45
Testing the theory of the strong interaction: QCDTesting the theory of the strong interaction: QCD
Measure the strong coupling constant:
But there are more fancy methods…
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 46
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
Z
MM
θ− =
Precisely known from LEP1
Would like to know moreLEP2!
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 47
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
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 48
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!)
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 49
WW--Pair productionPair production
W W qqqq+ − → W W ν ν+ − → W W qqν+ − →
Three different event topologies:
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 50
WW--Pair productionPair production
Measurement of the W-mass
Error: ~40 MeVi.e. ~20x error on Z-mass
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 51
NonNon--abelian structure of EWabelian structure of EW--interactionsinteractions
First direct observationof gauge boson self-couplingin EW interactions
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 52
NonNon--abelian structure of EWabelian structure of EW--interactionsinteractionsMore precise information about this coupling from angular distributionsOf the W-Bosons and their decay products
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 53
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!
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 54
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?
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 55
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→
and
H τ τ+ −→
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 56
Hunting the Higgs BosonHunting the Higgs Boson
…leads together withZ decay modesto many differentfinal states:
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 57
Hunting the Higgs BosonHunting the Higgs BosonA nice candidate event (m = 115 GeV)
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 58
Hunting the Higgs BosonHunting the Higgs Boson
Are there many of these?
Hard to say from plot… statistical analysis needed:
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 59
Hunting the Higgs BosonHunting the Higgs Boson
No significant excess observed.
Final word from LEP: HiggsM >114.4 GeV@ 95%C.L.
Klaus Desch, Physics at e+e- Colliders, DESY Summer Student Lecture 08/2002 Page 60
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