The DELPHI experimentat

the LEP accelerator

at

the CERN laboratory

Part 1. The LEP accelerator

Part 2. The DELPHI experiment

Part 3. Particle physics research at LEP

2

The LEP accelerator

The study of collisions between electrons and positrons.

LEP 1 : The collision energy = 91 GeV = Z

LEP 2: The collision energy = 209 GeV > 2W

3

The largest accelerators in the world

LEP

HERA

TEVATRON

(CERN)

(DESY)

(FERMILAB)

Length: 27 km (4184 magnets)

Length: 6 km (1650 magnets)

Length: 6 km (990 magnets)

Experiments: DELPHI, OPAL, ALEPH, L3

Experiments: H1, ZEUS

Experiments: CDF, D0

e

ee

ee-

+

ee+

46 GeV104 GeV

46 GeV104 GeV

hadrons

hadrons

Z0q

q-

30 GeV

γ

ee+q

q

qg

g

ghadrons

p

920 GeV

e

q

q

qg

g

g

hadrons

p

1000 GeV

e

q

q

qg

g

gp1000 GeV

hadrons

g

4

5

6

7

8

9

10

LEP 1 LEP 2In order to increase the collision energy onehad to build 240 superconducting radio-frequencycavities.

The energy lost due to synchrotron radiation is 2.3 GeV / turn

Radio-frequency accelerating voltage has to be 2.3 GV / turn

Accelerating gradient = 6 MV / mFrequency = 352 MHz

Collision energy: 91 136 174 184 189 204 209Year: 1989-94 1995 1996 1997 1998 1999 2000

(GeV)

11

12

The collision energy

At LEP the collision energy could be determined with a very high accuracy:

91.187 GeV with an error of 0.002 GeV

Things which affected the energy of LEP:

1. The level of the water in the lake !2. The moon !3. The trains to Paris !

13

Days

∆E (

MeV

)

Energy calibrations

Horizontal Orbit (XARC)

Lake level fit

-20

-10

0

120 140 160 180 200 220 240

Geological shifts

During 1993 the LEP energy was observed tochange with time.

Part of the change was due to the water level inlake Geneva which caused small geologicalshifts of the accelerator.

Rainfalls and the water table in the Juramountains also affected the LEP energy.

14

∆ E

[M

eV]

November 11th, 1992

∆ E

[M

eV]

August 29th, 1993

Daytime

October 11th, 1993

-5

0

5

23:00 3:00 7:00 11:00 15:00 19:00 23:00 3:00

-5

0

5

11:0

0

13:0

0

15:0

0

17:0

0

19:0

0

21:0

0

23:0

0

18:0

0

20:0

0

22:0

0

24:0

0

2:00

4:00

6:00

8:00

Tides

Earth tides caused by the moon willproduce small distortions of the earth´scrust.

This can affect the accelerator sothat the electrons orbit change.

An orbit change of 1 mm will changethe energy with about 10 MeV.

15

Vacuum Chamber Current Correlation

LEP

SPS

CERN

airpor

t

Belleg

arde

Lau

sann

e

La Versoix

Lac de

Geneve

Meyrin

Zimeysa

Cornavin

IP1

IP2

IP3

IP4IP5

IP6

IP7

IP8

1 km

Vol

tage

on

rails

[V]

RAIL

TGV

Gen

eve

Mey

rin

Zim

eysa

17.11.1995

Vol

tage

on

beam

pipe

[V]

LEP beam pipe

Time

Ben

ding

B f

ield

[Gau

ss]

LEP NMR

-7

0

-0.024

-0.02

-0.016

-0.012

746.28

746.30

746.32

746.34

746.36

16:50 16:55

Beampipe current

LEP Lake

Trains

River

Geneva

The trains from Geneva to Francecaused parasitic currents on the LEPbeampipe.

These currents (1 A) affected the magnetic field in the LEP magnetsand this changed the energy.

16

DELPHIVertex Detector

Inner Detector

Time Projection Chamber

Small Angle Tile Calorimeter

Very Small Angle Tagger

Beam Pipe

Quadrupole

Barrel RICH

Outer Detector

High Density Projection Chamber

Superconducting Coil

Scintillators

Barrel Hadron Calorimeter

Barrel Muon ChambersForward Chamber A

Forward RICH

Forward Chamber B

Forward EM Calorimeter

Forward Hadron Calorimeter

Forward Hodoscope

Forward Muon Chambers

Surround Muon Chambers

The DELPHI experiment

STIC

Electromagneticcalorimeter

Hadroniccalorimeter

Muon detectorTrackingdetector

17

18

e

e-

+

e+

e Zo-

µ

µ

-

+

e+

e Zo

-

τ

τ

-

+

e+

e Zo-

Cross-section

Collision energy

19

DELPHI Interactive AnalysisRun: 26154Evt: 2958

Beam: 45.6 GeV

Proc: 1-Oct-1991

DAS : 25-Aug-199121:46:38

Scan: 4-Dec-1992

TD TE TS TK TV ST PA

Act

Deact

1

( 37)

0

( 0)

35

( 35)

0

( 1)

0

( 0)

0

( 0)

2

( 4)

0

( 3)

0

( 0)

0

( 0)

0

( 0)

0

( 0)

0

( 0)

0

( 0)

X

Y

Z

e

e-

+

e+

e Zo-

Hadroniccalorimeter

Muonspectrometer

Electromag.calorimeter

Magnet

Cherenkov

TPC

20

DELPHI Interactive AnalysisRun: 26154Evt: 1417

Beam: 45.6 GeV

Proc: 1-Oct-1991

DAS : 25-Aug-199121:36:22

Scan: 19-Feb-1992

TD TE TS TK TV ST PA

Act

Deact

26

( 28)

0

( 0)

29

( 29)

0

( 2)

0

( 0)

0

( 0)

2

( 2)

0

( 2)

0

( 3)

0

( 0)

0

( 0)

0

( 0)

0

( 0)

0

( 0)

X

Y

Z

µ

µ

-

+

e+

e Zo-

Hadroniccalorimeter

Muonspectrometer

Electromag.calorimeter

Magne

t

Cherenkov

TPC

21

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

τ

τ

-

+

e+

e Zo-

Hadroniccalorimeter

MuonspectrometerElectromag.

calorimeter

Magnet

Cherenkov

TPC

22

DELPHI Interactive AnalysisRun: 26154Evt: 3018

Beam: 45.6 GeV

Proc: 1-Oct-1991

DAS : 25-Aug-199121:47:02

Scan: 19-Feb-1992

TD TE TS TK TV ST PA

Act

Deact

14

( 93)

0

( 0)

72

(133)

0

( 13)

0

( 0)

0

( 0)

17

( 23)

0

( 23)

0

( 18)

0

( 12)

0

( 0)

0

( 0)

0

( 0)

0

( 0)

X

Y

Z

e+

e Zo-

q

q

Hadroniccalorimeter

Muonspectrometer

Electromag.calorimeter

Magnet

Cherenkov

TPC

23

What does a particle physicist do ?

• The building of detectors Examples:

- The TPC (Time Projection Chamber)- The STIC (Electromagnetic calorimeter)

• Analysis of the data from the experiment

Examples at LEP 1:

- Luminosity- Studies of the Z-boson

Examples at LEP 2:

- Studies of the W-boson- Search for new particles

24

The Time Projection Chamber

e+

e-

A charged particle ionize the gas in the cylinderand the electrons drift in an electrical field to thedetectors at the ends of the cylinder.

Three-dimensional tracks can be reconstructedfrom the signals from the detectors.

EB

E

Chargedparticle

25

26

The calorimetric processes

An electromagnetic shower

Air Lead

Air Lead

A hadronic shower

Bremsstrahlung

Photon -conversions

Hadron- NucleonInteractions

e+

γ

γ

e

ππ

π

πππ

π

KK

p

15 cm

2 cm

20 cm

20 cm 100 cm

27

DELPHISTIC side C

STIC side A

TPC

Microvertex

ID Jet Chamber

ID Trigger Layers

STIC

STIC was two electromagnetic calorimeters builtby Lund in collaboration with physicists from 12other institutes in 6 countries.

28

29

30

31

32

33

34

DELPHISTIC side C

STIC side A

TPC

Microvertex

ID Jet Chamber

ID Trigger Layers

Luminosity

What is luminosity ?The number of events per s = cross section x Luminosity

How is the luminosity measured ?By counting the number of events from a process for whichthe cross section can be calculated.

Which process was used at LEP ?

γ γ

e

e

e

e

e

e

e

e

BhabhaScattering

e+

e-

35

5

10

15

20

25

30

35

88 89 90 91 92 93 94 95

N=2

N=3

N=4

DELPHI

Energy, GeV

The hadronic lineshape of the

The width of the Z-boson peak depends on thenumber of light neutrino species (N).

Z-boson

σ =e+

e Zo-

q

qLuminosityZ

36

WW-events

e+

e-

e+

e Zo-

νe

W+

W-

/γ

W+

W-

qq qq lνlνqq lν46% 44% 11%

37

0

50

100

150

200

250

50 60 70 80 90 100

DATA

WW

(Mw = 80.35)

ZZ

QCD

W mass [ GeV/c2 ]

even

ts /

2 G

eV/c

2The mass of the W-bosonqq qq

46%

ϕϕ

ΜW

2 = ( )PP +q q_

_ _

ΜW

2 = 2 E E ( 1 - cos )q q_

if m = 0q

( 4-vectors )2

38

0

2.5

5

7.5

10

12.5

15

17.5

20

160 170 180 190 200 210

√s (GeV)

WW

Cro

ss S

ectio

n (p

b)

GENTLE 2.0 (±2%)RacoonWW, YFSWW

DELPHI(preliminary)

The W-cross section

σ = Luminositye+

e-

e+

e Zo-

νe

W+

W-

/γ

W+

W-

W

σW

0

50

100

150

0

25

50

75

100

0

50

100

0 0.2 0.4 0.6 0.8 1

γGBackg.γνν

_

Data

xγ

√s¬ = 180-190 GeV534 events obs.

549 events exp.

√s¬ = 190-200 GeV429 events obs.

452 events exp.

√s¬ = 200-210 GeV563 events obs.

568 events exp.

Eve

nts/

0.05

Eve

nts/

0.05

Eve

nts/

0.05

The search for new particlesExtra dimensions

Basic idea: Unification of gravity with other interactions by introducing new compact dimensions of space in

which only gravity propagates.

Cross sec.: γe+e- )(σ depends on :Grav. +n - the number of extra dimensionsMD - the fundamental mass scale in the theory

e+

e-

γ

Zoν

ν

Standard Model:

e+e γG

MD = 0.75 TeV n = 2

Eγ Ebeam⁄

40

The search for Higgs eventse+

e

H0

Zo

bb ττ WW gg cc

qq νν ee µµ ττ

74% 8% 8% 6% 4%

70% 20% 3% 3% 3%

-

bb

qq

bb

νν

bb

eeµµ

bb

ττ

41

14

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120

M (GeV/c2)

even

ts/3

GeV

/c2

Data 16MC 23.5signal 3.25mH=114 GeV/c2

DELPHIEcm=200-209 GeVL=224 pb-1

The mass spectrum of Higgs candidates

ZZ and WW events that have been wronglyidentified as Higgs events

Expected signal from Higgs decays

Mass (GeV)

42

Summary

The LEP accelerator was the largest acceleratorthe world has ever seen.

DELPHI was one of the huge experiments thatstudied the collisions between electrons andpositrons.

DELPHI has contributed much to our presentunderstanding of the standard model.

The accelerator and the experiments have beendismantled but the physicists continue to analyzethe data that was collected.