8/5/2002ulrich heintz - quarknet 20021 particle physics what do we know? ulrich heintz boston...
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8/5/2002 Ulrich Heintz - Quarknet 2002 1
Particle Physicswhat do we know?
Ulrich Heintz
Boston University
8/5/2002 Ulrich Heintz - Quarknet 2002 2
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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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
• 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
8/5/2002 Ulrich Heintz - Quarknet 2002 20
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