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Particle Physicswhat do we know?

Ulrich Heintz

Boston University

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Particle Physics• What associations does the word particle

physics bring to your mind?

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Particle Physics• What are the fundamental building blocks of

the universe?

• What are the interactions between them?

• How can we explain the universe?– its history– its present form– its future

• Is there a theory of everything?

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Particle Physics

it’s fun and fascinating

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What is a particle?• a small piece of matter...• characterized by

– charge– mass– lifetime– spin

• particles can scatter off each other like billiard balls

• unlike billiard balls, most particles are unstable and decay

• particles can be produced by colliding other particles

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What was the world made of in 1932?

• electrons (1897)– orbit atomic nucleus

• proton (1911)– nucleus of lightest atom

• neutron (1932)– neutral constituent of the nucleus

• photon (1905)– quantum of the electromagnetic field

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and...• 1927 Dirac’s relativistic quantum mechanics• 1931 the positive electron (positron)

– antiparticles: for every particle there exists an antiparticle with same mass, lifetime, spin, but opposite charge

• 1930 Pauli’s neutrino– energy conservation in beta decay requires the

existence of a light, neutral particle– n p+ + e- + – observed in 1956

• 1936-1947 the muon and the pions (+,0,-)– Rabi: “who ordered that?”

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The ascent of accelerators• previous discoveries used

– cosmic rays – “natural accelerators” (radioactivity)

• after WWII – accelerators

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The particle “Zoo”• 1947: strange particles

– K0+ -, K++ + -

p+ -

, – long lifetime ¼ 10-10 s

• more particles... p, – short lifetime ¼ 10-24 s

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The quark model• 1964 Gell-Mann, Zweig

– there are three quarks and their antiparticles

– each quark can carry one of three colors• red blue green

– antiquarks carry anticolor• anti-red anti-blue anti-green

Quark Up Down Strange

Charge +2/3 -1/3 -1/3

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The quark model– only colorless (“white”) combinations of quarks

and antiquarks can form particles

• qqq

• qq

• no others observed

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The 8-fold way

K0

-

K+

+0

K- K0

sd

ud

su

du

ds us

uu,dd,ss

0

-

+

+0

- 0

uss

uus

dss

dds

udd uud

uds

-

ddd++

uuu

-

sss

n p

mesonsqq

baryons qqq

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Quark confinement• What holds quarks/antiquarks together?

– strong force– acts between all “colored” objects– short range– independent of distance

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So what is the world made of?

e e u d0.511 MeV 0 a few MeV a few MeV

c s106 MeV 0 1100 MeV 150 MeV

t b1.8 GeV 0 175 GeV 4.2 GeV

leptons quarksspin = ½ (fermions)

The Standard Model

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Are these fundamental?• As far as we know....

– we can measure structure as small as 10-18 m

• Accelerators are like huge microscopes– To measure smaller distances– go to higher energies

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How do particles interact?• particles attract or repel each other by

exchanging “messenger” particles (field quanta)

ee

Feynmandiagram

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What holds the world together?

force

acts between

relative strength

field quantum

strong

quarks

10

g

electro-magnetic

charged particles

10-2

weak

all particles

10-13

W§ Z0

gravity

all particles

10-42

G

spin = 1 (bosons)

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The Higgs boson• the standard model requires the existence

of one more particle

• Higgs boson– uncharged– unknown mass (>115 GeV)– spin = 0

• required to be able to describe massive fermions and bosons

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Is this the theory of everything?• NO

– Standard Model doesn’t work at all energies– Standard Model does not include gravity– we haven’t found the Higgs yet...

• unification

Electricity

Magnetism

Weak force

Strong force

Gravity string theory...

electromagnetism

electroweak force

GUTs

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Accelerators• 1983: CERN pp collider

– E = 540 GeV W§ (80 GeV), Z0 (91 GeV)

• 1995: Fermilab Tevatron pp collider– E=1.8 TeV top quark (175 GeV)

• ¼ 2008: CERN LHC pp collider– E=14 TeV discover Higgs?

• ????: Linear e+e- Collider– E=1-2 TeV study Higgs in detail

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What might we find?• Super Symmetry

– fermions bosons

– we have already found half the particles....

electron selectron

neutrino sneutrino

quark squark

photon photino

gluon gluino

W Wino

Z Zino