Transcript Particle Physics what do we know?

Particle Physics
what do we know?
Ulrich Heintz
Boston University
8/5/2002
Ulrich Heintz - Quarknet 2002
1
Particle Physics
• What associations does the word particle
physics bring to your mind?
8/5/2002
Ulrich Heintz - Quarknet 2002
2
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?
8/5/2002
Ulrich Heintz - Quarknet 2002
3
Particle Physics
it’s fun and fascinating
8/5/2002
Ulrich Heintz - Quarknet 2002
4
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
8/5/2002
Ulrich Heintz - Quarknet 2002
5
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
8/5/2002
Ulrich Heintz - Quarknet 2002
6
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?”
8/5/2002
Ulrich Heintz - Quarknet 2002
7
The ascent of accelerators
• previous discoveries used
– cosmic rays
– “natural accelerators” (radioactivity)
• after WWII
– accelerators
8/5/2002
Ulrich Heintz - Quarknet 2002
8
The particle “Zoo”
• 1947: strange particles
– K0+ -, K++ + – p+ – , 
– long lifetime  ¼ 10-10 s
• more particles...
– p,
– 
– short lifetime  ¼ 10-24 s
8/5/2002
Ulrich Heintz - Quarknet 2002
9
The quark model
• 1964 Gell-Mann, Zweig
– there are three quarks and their antiparticles
Quark
Charge
Up
+2/3
Down
-1/3
Strange
-1/3
– each quark can carry one of three colors
• red blue green
– antiquarks carry anticolor
• anti-red anti-blue anti-green
8/5/2002
Ulrich Heintz - Quarknet 2002
10
The quark model
– only colorless (“white”) combinations of quarks
and antiquarks can form particles
• qqq
• qq
• no others observed
8/5/2002
Ulrich Heintz - Quarknet 2002
11
The 8-fold way
baryons qqq
mesons
qq
K0
K+
ds
us
-
0
+
++
ddd
udd
uud
uuu
n
p
0 
dds
-
ud
uu,dd,ss
uds
dss
uss
-
su
sd
K-
K0
8/5/2002
uus
+
0  
du
+
0
sss
Ulrich Heintz - Quarknet 2002
12
Quark confinement
• What holds quarks/antiquarks together?
– strong force
– acts between all “colored” objects
– short range
– independent of distance
8/5/2002
Ulrich Heintz - Quarknet 2002
13
So what is the world made of?
The Standard Model
e
e
u
d
0.511 MeV
0
a few MeV
a few MeV
spin = ½
(fermions)
8/5/2002


c
s
106 MeV
0
1100 MeV
150 MeV


t
b
1.8 GeV
0
175 GeV
4.2 GeV
leptons
quarks
Ulrich Heintz - Quarknet 2002
14
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
8/5/2002
Ulrich Heintz - Quarknet 2002
15
How do particles interact?
• particles attract or repel each other by
exchanging “messenger” particles (field
quanta)
e
e


8/5/2002
Ulrich Heintz - Quarknet 2002
Feynman
diagram

16
What holds the world together?
strong
electromagnetic
weak
gravity
quarks
charged
particles
all
particles
all
particles
relative strength
10
10-2
10-13
10-42
field quantum
g

W§ Z0
G
force
acts between
spin = 1 (bosons)
8/5/2002
Ulrich Heintz - Quarknet 2002
17
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
8/5/2002
Ulrich Heintz - Quarknet 2002
18
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
electromagnetism
electroweak force
Strong force
GUTs
Gravity
8/5/2002
string theory...
Ulrich Heintz - Quarknet 2002
19
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
8/5/2002
Ulrich Heintz - Quarknet 2002
20
What might we find?
• Super Symmetry
– fermions  bosons
electron
selectron
neutrino
sneutrino
quark
squark
photon
photino
gluon
gluino
W
Wino
Z
Zino
– we have already found half the particles....
8/5/2002
Ulrich Heintz - Quarknet 2002
21