Elementary Particle Physics Status and Prospects

Mogens Dam

Niels Bohr Institute

September 4, 2002

2

Structure of Matter Atom (10-10 m)

Nucleus (10-14 m) Nucleon (10-15 m)

Electron

?

< 10-18 mQuark

?

3

Matter Particles

Lepto

ns

Quark

se-

electron

ddown

ee-neutrino

uup

I

-

muon

sstrange

-neutrino

ccharm

II

tau

bbottom

-neutrino

ttop

IIIGenerations:

Q = 0

Q = -1

Q = 2/3

Q = -1/3

Everyday world is made of 1’st generation particles: u, d, and e-

Example: proton = uud, neutron = udd

Every

particle

has a

n a

ssocia

ted

an

ti-p

article

Ferm

ions:

sp

in-1

/2

4

Fermion Masses

e-

electron

ddown

ee-neutrino

uup

-

muon

sstrange

-neutrino

ccharm

bbottom

ttop

3x10-3

5x10-3

< 10-8

5x10-4

< 0.02

1.8

175

4.5

< 0.0002

0.1

1.5

0.3

o Generations identical except for masseso Large increase in fermion masses with generation numbero Why generations anyway?

Units: GeV/c2

-

tau

neutrino

5

Fundamental Interactions

Gravitational WeakElectro-

magneticStrong

Acts on: Mass – Energy Flavour Electric Charge Colour Charge

Particles experiencing:

AllQuarks, Leptons

Electrically charged

Quarks, Gluons

Particles mediating:

Graviton(not yet observed)

W+, W-, Z0Photon

Gluons

Primary importance:

Cosmology, planetary orbits

Radioactive decays,

Stellar energy

Atomic physics

Chemistry

Hadron formation, nuclear physics

Gravity is extremely feeble (but cumulative): Negligible in particle physics interactions

6

Intermediate Vector Bosons3 fundamental interactions mediated by intermediate bosons of spin-1

e+

e-

Example reaction: e+e- +

1) Electromagnetic force:

2) Weak force:

3) Strong Force:

massless (infinite range)

G massless (in principle infinite range, but...)

W+ m = 80.4 GeV/c2W-

Z0 m = 91.2 GeV/c2

Very heavy very short range

7

Standard ModelStructure of interactions uniquely specified by symmetries! ”Local gauge symmetries”

Example Quantum ElectroDynamics (QED):Theory invariant under local U(1) transformation of fermion field

Only possible if we introduce EM field and familiar interaction: qA

(x) (x) eiq(x)

Standard Model: Nature described by the Symmetry Group

Electroweakinteractions

Strong interactionsWeak Hypercharge

Weak IsospinColour

SU(3) x SU(2) x U(1)

3 symmetry groups Interactions described by (only!) 3 coupling constants: gs, g, g’ Universal fermion-boson couplings

Problem: All boson masses must be zero! Disagrees with Z0 and W

8

Bosons acquire masses by the Higgs Mechanism

o Spontaneous breaking of SU(2) symmetry: Higgs field has minimum away from origo vacuum state:

0v/2

SU(2) doubleto Introduce complex scalar field:

o Higgs field can then be written:

0v + h(x)

ei(x) /2 3 degrees of freedom: - longitudinal modes of W and Z0

- masses !

Physical Higgs field

o and Z0 linear combinations of U(1) and SU(2) fields:

weak mixing angle: tan W = g’/g

vacuumexpectationvalue

o Boson masses: MW = gv / 2

MZ = v g2 + g’2 / 2v = 246 GeV

Standard Model – Boson Masses

9

Standard Model – Fermion Masses

Experiment: o Weak interactions are parity violating o W couple only to left-handed fermions

< 10-10 for e

⋮ 10-6 for e-

⋮ 1 for t-quark

Consequence: o Left- and right-handed components of fermion fields transform under different representations under SU(2) o Standard mass terms mf prohibited

Generate (also) fermion masses through coupling to Higgs field:

For each fermion: mf = f v / 2

Dimensionless coupling constantsValues not predicted by theory

f =

Higgs BosonThe Higgs mechanism predicts the existence of the Higgs boson

Neutral, scalar (spin-0) Mass not predicted by theory, but from consistency < 1 TeV

10

Experimental TechniquesTwo methods to explore the very small – both require very high energy:

Famous example (Rutherford 1912) 1970: substructure of proton andneutron discovered usingelectrons as projectiles

1) ”Microscope”: Bombard target with particle beam to reveal structure

2) Annihilate 2 particles and create new ones from the released energy

11

CERNEurope’s research laboratory for

particle physics in Geneva

12

LEP – Large Electron Positron Collider

Precision tests of the Standard Model including Z0 and W properties

(1989-2000)

13

Total cross sections

DORIS

PETRATRISTAN

hadronse e

PEP

CESR

e+e-

e+e-

Cross sections:Rate at which e+e-

annihilations occur

DELPHI

Precise scan of Z0 resonance Number of different neutrinos:

ee-neutrino

-neutrino

neutrino

Cross Sections

2.9841 0.0083

e+e- hadrons

14

Precision Measurements Calculation of observables is based on perturbation theory

1’st order:

e+

e-

Z0,

b

b

2’nd order: All 1-loop diagrams:

+

Example: e+e- bb

e+

e-

b

b any fermion f

e+ b

e-

b

t

w

w+ . . .

Value of observables depend on mass of particles too heavy to be directly produced (virtual particles)

This is how LEPprovides informationon e.g. the top-quarkmass

Precision measurements ⇒ Information on higher energy scales

15

Standard Model Check - Asymmetries

Parity violating effect in weakinteractions gives rise to various observable asymmetries at Z0 peak

Consistency check: measure different asymmetries and express all result in common book-keeping variable sin2W

Values agree (well, mostly...)

Through virtual effects, indirectinformation on Higgs boson mass

LEP1

16

LEP2 - W Measurements

Production cross section

Standard Model strongly favoured

Direct reconstruction of W mass in two modes

17

Global Standard Model Fit to EW Data

All electroweak data from LEP and elsewhere

Fit is of resonable quality!

Data favours lightish Higgs:mHiggs < 193 GeV @ 95% c.l.

Direct search: mHiggs > 114.4 GeV

Pre

cise to

p m

ass v

ery

importa

nt:

5 G

eV

shift

35

% sh

ift in H

iggs m

ass

18

CP violationHiggs coupling to fermion fields is non-diagonal w.r.t. weak eigenstates

Generation mixing:

= VCKM

d’s’b’

dsb

Mass Eigenstates

Weak EigenstatesMixing matrix

VCKM = Vud Vus Vub

Vsd Vss Vsb

Vtd Vts Vtb

VCKM is complex for :

CP-violation matter-antimatter asymmetry

For < 3 generations,V can be made real No CP-violation

VCKM = 1 A3 ( i) 1A2

A3(1i) A2 1

Popular parametrization (Wolfenstein):

Nearly diagonal

CP-violation measurement expressed through parameters and

19

Matter-antimatter asymmetryIn the Universe matter dominates over anti-matter:

Matter dominated Universe Symmetric Universe

Standard Model CP violation too weak to explain observed asymmetry

20

CP violation firstobserved in 1964in s-quark mesons

CP violation measurement

2000

Excellent agreement:Yet another triumphfor Standard Model

A. Sto

cchi, IC

HEP2

00

2

Three constrain on parameters and 1) CP Violation in b-mesons 2) CP Violation in s-mesons 3) Non-CP violating measurements

BaBar experiment @ SLAC, USABelle experiment @ KEK, Japan

Meassurement of CP violation inb-quark mesons

21

Neutrino OscillationsIn minimal Standard Model neutrinos are massless.In recent years indications have been found that neutrinos have mass!

Oscillations between neutrino flavours (e.g. e will occur, if 1) different neutrino flavours have different mass; 2) the weak interactions is non-diagonal in the mass basis (like for quarks)

)ELΔm.(θ)νP(ν eμ

222 271sin2sin

Two-flavour transition probability:

Mass difference squared

Mixing angleDistancetravelled

Energy

22

Solar Neutrinos

Longstanding problem: Measured flux of e smaller than anticipated from know solar energy production

SNO experiment measures 1) flux of e (low)

2) total neutrino flux (as expected)

SNO(2000

1000 tonnes of D20

Neutrino flavour change:Consistent with neutrino oscillations

Homestake

23

Atmospheric Neutrinos

Super Kamiokande

Neutrinos produced by cosmic rays in atmosphere.Expect ratio e ~ 2.

-e as expected- fewer than expected

- Energy dependence

disappear (?) Oscillations? Masses?

Obs

erve

d

25

Neutrino Summary

Experimental situation: Solar ’s: Flavour conversion from e to or

Atmospheric ’s: Disappearance of most likely to

Interpretation of data within Standard Model: in both cases m2 is of the order 10-4 - 10-3 eV2

(but that does not give us the masses) mixing angles are generally larger than in quark sector

Rich experimental programme: Numerous observatories being set up to look for solar/atmospheric/cosmic/Super Nova ’s Long Base Line experiments terrestial oscillations ”Neutrino factories”: very high rate neutrino sources Improved neutrino mass measurements

27

Standard Model StatusThe Standard Model is incredibly successful: fits all (?) current data

However, many unanswered questions Why 3 generations of fermions ? Is there any underlying pattern in the fermion masses ? Quarks: 0.005 -- 175 GeV Leptons: 10-11 -- 1.8 GeV What is the origin of the Spontaneous Symmetry Breaking

Is the Higgs fundamental ? Where does gravity fit in ?

Questions relate to the concept of mass and thus to the Electroweak Symmetry Breaking sector

Look for an underlying more fundamental structure

Answers. . .

28

How to Progress1. Find the Higgs (or exclude it: should be lighter than ~ 1 TeV in SM)

If Higgs: measure its properties... Is it Standard Model ? If no Higgs: Standard Model in trouble. Probably find something

else

2. Search for signatures of a more global theory. Many candidates SUSY Compositeness TeV-scale gravity ...

There are good reasons to believe in new physics at the 1 TeV scale

Higgs: Something else at EW scale responsible for symmetry breaking

Higgs: Hierarchy problem... Stability of the Higgs mass

29

Future Experimental Programmes

High luminosity Tevatron

2 TeV proton-proton May find Higgs if light

LHC – Large Hadron Collider (2007) Vast discovery potential (Higgs, SUSY, other new physics)

Linear Collider (201x) Precision measurements of Z0, t-quark, Higgs, and new

particles

LEP2 searches EW fits

30

The LHC Projectpp collider in the LEP tunnel14 GeV centre of mass energyStartup in 2007

31

LHC General Purpose Detectors

Large discovery potential: Higgs SUSY TeV Scale Gravity etc.

Precision measurements: b and t quarks W boson Higgs mass SUSY particles

32

LHC Potential: HiggsDiscovery modes

Significance > 5 up to 1 TeV

Mass measurement

33

LHC Potential: SUSY

Clear signature

Mass measurements via position of edges in cascade decays

34

e+e- Linear ColliderWhereas pp machines are great fordiscovery, e+e- colliders are more powerful for precision measurement.

The next e+e- will be linear to avoidsynchrotron energy loss.

Proof of principle: The SLC at SLAC

One of the suggested 500 GeV linearcolliders is based on superconductingacceleration cavities: ”Tesla”

Possible location

35

Linear Collider Physics

Higgs branching fractions

SUSY spectroscopy

500 GeV study

If no new physicsobserved, extremely good limits on new models

Higgs mass

36

Summary and Outlook The last decade has seen rapid progress in particle physics

Precise EW (and QCD) results from LEP and elsewhere CP violation measurement in the b-system Exciting new results on neutrino properties . . .

Standard Model is incredibly successful in describing the data It is however not believed to be a fundamental theory:

(too) many free parameters especially in the Higgs sector no explanation for fermion generations and masses

Exciting experimental programme ahead New neutrino experiments The LHC programme A linear e+e- collider (hopefully) . . .

Lets explore the Standard Model...... and beyond !!!