Particle Physics - FSU Physics Department

Particle Physics
Particle physics – what is it? – why do it?
Standard model
• Quantum field theory
• Constituents, forces
• Milestones of particle physics
Particle physics experiments
shortcomings of standard model
Summary
Webpages of interest
• http://www.fnal.gov (Fermilab homepage)
• http://www-d0.fnal.gov (DØ homepage)
• http://www.hep.fsu.edu/~wahl/Quarknet/links.html
(has links to many particle physics sites and other sites of interest)
• http://www.fnal.gov/pub/tour.html (Fermilab particle physics tour)
• http://ParticleAdventure.org/ (Lawrence Berkeley Lab.)
• http://www.cern.ch (CERN -- European Laboratory for Particle Physics)
1

Topics
• what is particle physics, goals and issues
• historical flashback over development of the
field
o cosmic rays
o particle discoveries
o forces
o new theories
• the standard model of particle physics
2

About Units
Energy - electron-volt
• 1 electron-volt = kinetic energy of an electron when
moving through potential difference of 1 Volt;
o 1 eV = 1.6 × 10 -19 Joules = 2.1 × 10 -6 W•s
o 1 kW•hr = 3.6 × 10 6 Joules = 2.25 × 10 25 eV
o 1 MeV = 10 6 eV, 1 GeV = 10 9 eV, 1 TeV = 10 12 eV
mass - eV/c 2
o 1 eV/c 2 = 1.78 × 10 -36 kg
o electron mass = 0.511 MeV/c 2
o proton mass = 938 MeV/c 2 = 0.938 GeV/ c 2
o neutron mass = 939.6 MeV/c 2
momentum - eV/c:
o 1 eV/c = 5.3 × 10 -28 kg m/s
o momentum of baseball at 80 mi/hr ≈ 5.29 kgm/s ≈ 9.9 × 10 27
eV/c
Distance
o 1 femtometer (“Fermi”) = 10 -15 m
3

Outline
what is particle physics?
Origins of particle physics
• Atom (p, e - ), radioactivity, discovery of neutron (n) (1895-
1932)
• Cosmic rays: positron (e + ), muon (μ - ), pion (π), Kaon (K ±, K 0 )
(1932 – 1959)
the advent of accelerators:
• more and more particles discovered, patterns emerge (1960’s
and on):
o leptons and hadrons
o Electromagnetic, weak, strong interactions
present scenario: the Standard Model of electroweak and strong
interactions
• Formulation and discovery (1960’s to 1980’s)
• Precision experimental tests (from 1990’s)
quest for new physics (beyond the standard model)
• Open questions, possible strategies
• Present and future experiments, facilities
• Search for Higgs particle
outlook
4

Sizes and
distance scales
Molecule 10 -9 m
Atom 10 -10 m
nucleus 10 -14 m
virus 10 -7 m
nucleon 10 -15 m
Quark

The Building Blocks of a Dew Drop
A dew drop is made up of 10 21
molecules of water.
Each molecule = one oxygen
atom and two hydrogen atoms
(H 2 O).
Each atom consists of a
nucleus surrounded by
electrons.
Electrons are leptons that
are bound to the nucleus by
photons, which are bosons.
The nucleus of a hydrogen
atom is just a single proton.
Protons consist of three
quarks. In the proton, gluons
hold the quarks together just
as photons hold the electron
to the nucleus in the atom
6

Contemporary
Physics
Education
7
Project

Goals of particle physics
particle physics or high energy physics
• is looking for the smallest constituents of matter (the
“ultimate building blocks”) and for the fundamental forces
between them (“interactions”);
• aim is to find description in terms of the smallest number of
particles and forces
• Try to describe matter in terms of specific set of
constituents which can be treated as fundamental;
• With deeper probing (at shorter length scale), these
fundamental constituents may turn out to consist of smaller
parts (be “composite”).
• “Smallest constituents” vs time:
o in 19th century, atoms were considered smallest building
blocks,
o early 20th century research: electrons, protons, neutrons;
o now evidence that nucleons have substructure - quarks;
o going down the size ladder: atoms -- nuclei -- nucleons -- quarks
– ???... ??? (preons, toohoos, voohoos,….????)
8

Issues of High Energy Physics
Issues of High Energy Physics
Basic questions:
• Are there irreducible building blocks?
o How many?
o What are their properties?
• mass? charge? flavor?
• What is mass?
• What is charge ?
• How do the building blocks interact?
o forces?
o Differences? similarities?
• Why more matter than antimatter?
• why is our universe the way it is?
o Coincidence?
o Theoretical necessity
o Design?
Why do we want to know?
• Curiosity
• Understanding constituents may help in understanding composites
• Implications for origin and destiny of Universe
9

Cosmic rays
• Discovered by Victor Hess (1912)
• Observations on mountains and in balloon: intensity of cosmic
radiation increases with height above surface of Earth – must come
from “outer space”
• Much of cosmic radiation from sun (rather low energy protons)
• Very high energy radiation from outside solar system, but probably
from within galaxy
10

Positron
Positron (anti-electron)
• Predicted by Dirac (1928) -- needed for relativistic
quantum mechanics
• existence of antiparticles doubled the number of
known particles!!!
• Positron track going
upward through lead
plate
o Photographed by Carl
Anderson (Aug. 2, 1932)
o Particle moving upward,
as determined by
increase in curvature
of the top half of the
track after it passed
through lead plate,
o and curving to the left,
meaning its charge is
positive
12

Anderson and his cloud chamber
13

Neutron
Bothe + Becker (1930):
• Some light elements (e.g. Be), when bombarded with alpha
particles, emit neutral radiation, “penetrating”– gamma?
Curie-Joliot and Joliot (1932):
• This radiation from Be and B able to eject protons from
material containing hydrogen
Chadwick (1932)
• Doubts interpretation of this radiation as gamma
• Performs new experiments; protons ejected not only from
hydrogen, but also from other light elements;
• measures energy of ejected protons (by measuring their
range),
• results not compatible with assumption that unknown
radiation consists of gamma radiation (contradiction with
energy-momentum conservation), but are compatible with
assumption of neutral particles with mass approximately
equal to that of proton – calls it “neutron”
• Neutron assumed to be “proton and electron in close
association”
14

Chadwick’s s experiment
15

Nuclear force – field quantum
photon carries the electromagnetic force.
Analogy: postulate particle as carrier of nuclear
force (Hideki Yukawa, 1935) with mass intermediate
between the electron and the proton
This particle also to be responsible for beta-decay.
potential energy between the nucleon field quanta
has the form
U
2
g ⎛ R
− exp⎜
−
4π
R ⎝ c
/ mc
=
2
⎞
⎟
⎠
m = mass of the exchanged quantum;
from observed range of nuclear force: mass of the
exchanged particle ≈ 200MeV
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More particles: Muon
1937: “mesotron” is observed
in cosmic rays (Carl Anderson,
Seth Neddermeyer) – first
mistaken for Yukawa’s particle
However it was shown
in 1941 that mesotrons didn’t
interact strongly with matter.
17

Discovery of pion
in 1947, Lattes, Occhialini and Powell from Bristol
observed decay of a new particle into two particles
one of the decay products is the muon (discovered by
Neddermeyer), and the other is invisible (Pauli's
neutrino).
muon in turn also decays in
18

Kaons
First observation of
Kaons:
• Experiment
by Clifford
Butler and
George
Rochester at
Manchester
• Cloud chamber exposed to cosmic rays
• Left picture: neutral Kaon decay (1946)
• Right picture: charged Kaon decay into muon
and neutrino
• Kaons first called “V” particles
• Called “strange” because they behaved
differently from others
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“Strange particles”
Kaon: discovered 1946; first called “V” particles
K 0 production and decay
in a bubble chamber
20

Bubble chamber
-
p p
-
→ p n
-
K 0 K - π+ π- π 0
n
-
+ p → 3 pions
π 0 → γγ, γ → e+ e-
K 0 → π+ π-
21

Particle Zoo
1940’s to 1960’s :
• Plethora of new particles discovered
(mainly in cosmic rays):
• e-, p, n, ν, μ - , π ± , π 0 , Λ 0 , Σ + , Σ 0 , Ξ,….
question:
• Can nature be so messy?
• are all these particles really intrinsically
different?
• or can we recognize patterns or
symmetries in their nature (charge, mass,
flavor) or the way they behave (decays)?
22

Particle spectroscopy era
1950’s – 1960’s: accelerators, better detectors
even more new particles are found, many of them extremely
short-lived (decay after 10 -21 sec)
“particle spectroscopy era”
• Bubble chamber allows detailed study of reactions,
reconstruction of all particles created in the reactions
• find that often observed particles actually originate from
decay of very short-lived particles (“resonances”)
1962: “eightfold way”, “flavor SU(3)” symmetry (Gell-Mann,
Ne’eman)
• allows classification of particles into “multiplets”
• Mass formula relating masses of particles in same multiplet
• Allows prediction of new particle Ω - , with all of its
properties (mass, spin, expected decay modes,..)
• subsequent observation of Ω - with expected properties at
BNL (1964)
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Particle nomenclature
by mass:
• baryons – heavy particles .. p, n, Λ, Σ, Ω - , Δ ++ ….
o nucleons and their excited states: p, n, N * , Δ ++ , ……
o hyperons: Λ, Σ, Ξ, Ω - , and their excited states
• mesons – medium-heavy particles … π, K, K * , ρ, ω …
• leptons – light particles e, μ, ν e , ν μ …
by spin:
• fermions: spin = odd-integer multiple of ½:
½, 3/2, 5/2,……
o leptons and baryons
• bosons: integer spin 0, 1, 2, …. – mesons are bosons
by interaction:
• hadrons: partake in strong interactions
• leptons: no strong interaction
by lifetime:
• stable particles: lifetime > 10 -17 sec
(decay by weak or electromagnetic interaction)
• unstable particles (“resonances”) lifetime

Spin and statistics
Fermions obey “Fermi-Dirac” statistics:
• multi-fermion states are antisymmetric with
respect to exchange of any two identical fermions
(wave function changes sign)
|f 1 , f 2 , f 3 > = - |f 2 , f 1 , f 3 >
• Pauli exclusion principle is special case of this
Bosons obey “Bose-Einstein” statistics:
• multi-boson states are symmetric with respect to
exchange of any two identical fermions (wave
function stays the same)
|b 1 , b 2 , b 3 > = |b 2 , b 1 , b 3 >
• consequence: bosons like to “stick together” (e.g.
Bose-Einstein condensate)
25

Towards the standard model
Quark Model (Gell-Mann, Zweig, 1964)
• observed SU(3) symmetry can be explained by
assuming that all hadrons are made of “quarks”
• There are 3 quarks: u (up), d (down), s (strange);
• quarks have non-integer charge:
o u 2/3, d -1/3, s -1/3
• baryons are made of 3 quarks:
o p = uud, n = udd, Λ = uds, Σ = uus Ξ = uss,
Ω - = sss
• mesons are made of quark-antiquark pairs:
_ _
_ _
o π + = ud, π - = u d, π 0 = u u + d d, ……..
26

The 8-fold 8
way
mesons
qq
K 0
ds
us
K +
π - π 0 ηφ π +
du
ud
uu,dd,ss
Δ -
ddd
baryons qqq
Δ 0
udd
n
uud
p
Δ +
Σ - Σ 0 Λ Σ +
dds
uus
uds
dss
uss
Ξ - Ξ 0
Δ ++
uuu
su
sd
K - K 0
Ω -
sss
27

color charge
problem with quark model
• Quarks have spin ½, i.e. are fermions ⇒ must obey Pauli
principle
• Ω - = sss, has spin 3/2; spins of 3 s quarks must be
aligned, i.e. Ω - has 3 quarks in identical state ---
forbidden
• similarly for Δ ++ = uuu, spin 3/2
way out: quarks have additional hidden property – “color
charge”
• 3 colors: green, red, blue
• each quark can carry one of three colors
o red blue green
• antiquarks carry anticolor
o anti-red anti-blue anti-green
• observed particles are “color-neutral”;
• only colorless (“white”) combinations of
quarks and antiquarks can form particles:
• qqq
• qq
28

“Elementary” particles?
“leptons” (electron, muon and their
neutrinos) are fundamental, interact
electromagnetically and “weakly”
“hadrons” (p, n, Λ, Σ, Ω - , Δ ++ , π, K, K * ,
ρ, ω,…) are not fundamental particles –
are made of quarks, interact
electromagnetically, “weakly”, and
“strongly”
29

Standard Model
A theoretical model of
interactions of elementary
particles, based on quantum field
theory
Symmetry:
• SU(3) x SU(2) x U(1)
“Matter particles”
• Quarks: up, down,
charm,strange, top, bottom
• Leptons: electron, muon, tau,
neutrinos
“Force particles”
• Gauge Bosons
o
o
o
γ (electromagnetic force)
W ± , Z (weak, electromagnetic)
g gluons (strong force)
Higgs boson
• spontaneous symmetry
breaking of SU(2)
• mass
30

Matter and forces
Fundamental forces (mediated by “force particles”)
• strong interaction between quarks, mediated by gluons
(which themselves feel the force) (QCD)
o leads to all sorts of interesting behavior, like the
existence of hadrons (proton, neutron) and the failure to
find free quarks
• Electroweak interaction between quarks and leptons,
mediated by photons (electromagnetism) and W and Z
bosons (weak force)
Fundamental constituent particles
• Leptons q = quarks q =
-1 e μτ 2/3 u c t
0 ν e ν μ ν τ –1/3 d s b
Role of symmetry:
• Symmetry (invariance under certain transformations)
governs behavior of physical systems:
o Invariance ⇒ “conservation laws” (Noether)
o Invariance under “local gauge transformations”
⇒ interactions (forces)
31

From Contemporary Physics Education Project
http://www.cpepweb.org/particles.html
33

From Contemporary Physics Education Project
http://www.cpepweb.org/particles.html
34

From Contemporary Physics Education Project
http://www.cpepweb.org/particles.html
36

Strong quark interactions
quarks carry “color charge” (red, blue,
green) and interact exchanging gluons,
the carriers of the strong force
theory of strong interaction is “gauge
theory”; form of interaction governed
by invariance under local SU(3) c (“color
SU(3)”)
8 gluons carry color charge ⇒ interact
with each other
37

Electroweak interactions
leptons (and also quarks)
carry a “weak charge”
(in addition to usual
electric charge)
they interact exchanging
• neutral EW force
carriers: photon γ, Z 0
• charged EW force
carriers: W ±
theory describing EW
interaction is gauge
theory; gauge symmetry
group
SU(2)xU(1)
38

Some milestones
Quantum electrodynamics (QED) (1950’s) (Feynman,
Schwinger, Tomonaga)
electroweak unification – the standard model (1960s)
(Glashow, Weinberg, Salam)
deep inelastic scattering experiments – partons
(SLAC/MIT) (1956 – 1973)
Quark Model (1964) (Gell-Mann, Zweig)
Quantum Chromodynamics (1970s) (Gross, Wilczek,
Politzer)
neutral weak current (1973) (Gargamelle, CERN)
Charm discovery (1974) (S. Ting, B. Richter)
Bottom quark discovery (1977) (L. Lederman)
gluon observation (1979) (DESY)
W,Z observation (1983) (UA1, UA2, C. Rubbia, CERN)
top quark (1995) (DØ, CDF, Fermilab)
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Brief History of the Standard Model
Late 1920’s - early 1930’s: Dirac, Heisenberg, Pauli, & others
extend Maxwell’s theory of EM to include Special Relativity & QM
(QED) - but it only works to lowest order!
1933: Enrico Fermi introduces 1st theory of weak interactions,
analogous to QED, to explain β decay.
1935: Hideki Yukawa predicts the pion as carrier of a new, strong
force to explain recently observed hadronic resonances.
1937: muon is observed in cosmic rays – first mistaken for
Yukawa’s particle
1938: heavy W as mediator of weak interactions? (Klein)
1947: pion is observed in cosmic rays
1949: Dyson, Feynman, Schwinger, and Tomonaga introduce
renormalization into QED - most accurate theory to date!
1954: Yang and Mills develop Gauge Theories
1950’s - early 1960’s: more than 100 hadronic “resonances” have
been observed !
1962 two neutrinos!
1964: Gell-Mann & Zweig propose a scheme whereby resonances
are interpreted as composites of 3 “quarks”. (up, down, strange)
40

Brief History of the Standard Model - 2
1970: Glashow, Iliopoulos, Maiani: 4th quark (charm) explains
suppression of K decay into μμ
1964-1967: spontaneous symmetry breaking (Higgs, Kibble)
1967: Weinberg & Salam propose a unified Gauge Theory of electroweak
interactions, introducing the W ± ,Z as force carriers and the Higgs field
to provide the symmetry breaking mechanism.
1967: deep inelastic scattering shows “Bjorken scaling”
1969: “parton” picture (Feynman, Bjorken)
1971-1972: Gauge theories are renormalizable (even when symmetry is
spontaneoulsy broken) (t’Hooft, Veltman, Lee, Zinn-Justin..)
1972: high p t
pions observed at the CERN ISR
1973: Quantum Chromodynamics (Gross, Wilczek, Politzer, Gell-Mann &
Fritzsch) : quarks are held together by a Gauge-Field whose quanta,
gluons, mediate the strong force
1973: “neutral currents” observed (Gargamelle bubble chamber at CERN)
41

Brief History of the Standard Model - 3
1974: J/Ψ discovered at BNL/SLAC;
1975: J/Ψ interpreted as cc
-
bound state (“charmonium”)
1976: τ lepton discovered at SLAC
-
1977: Υ discovered at Fermilab in 1977, interpreted as bb bound
state (“bottomonium”) ⇒ 3rd generation
-
1979: gluon – jets observed at DESY
-
1982: direct evidence for jets in hadron hadron interactions at
CERN (pp-
collider)
1983: W ± , Z observed at CERN (pp-
collider built for that purpose)
1995: top quark found at Fermilab (DØ, CDF)
1999: indications for “neutrino oscillations” (Super-Kamiokande
experiment)
2000: direct evidence for tau neutrino (ν τ
) at Fermilab (DONUT
experiment)
2005: clear evidence for neutrino oscillations (Kamiokande, SNO)
42

Feynman diagrams
Feynman Diagrams
• In our current understanding, all interactions
arise from the exchange of quanta
• The mathematics describing such interactions
can be represented by a diagram, called a
Feynman diagram
e
e
γ
Feynman
diagram
μ
μ
43

Present scenario
Most of what’s around us is made of very few
particles: electrons, protons, neutrons (e, u, d)
this is because our world lives at very low
energy
all other particles were created at high
energies during very early stages of our
universe
can recreate some of them (albeit for very
short time) in our laboratories (high energy
accelerators and colliders)
this allows us to study their nature, test the
standard model, and discover direct or indirect
signals for new physics
44

Study of high energy interactions -- going back in time
45

Homework set 5
HW 5.1
• go to Particle Data Group website:
http://pdg.lbl.gov
• find masses (in MeV or GeV) , principal
decay modes and lifetimes of the
following particles: π ± , π 0 , K 0 , J/Ψ, p, n,
Λ 0 , Σ + , Σ 0 , Ξ, Ω -
• give the quark composition of π ± , π 0 ,
K 0 , J/Ψ, p, n, Λ 0 , Ω -
46

Summary
Particle physics was born during last century, grew out of
atomic and nuclear physics huge progress in understanding
over last 50 years, due to revolutionary ideas in both
theory and experiment
intense dialog between experimenters and theorists
precision tests of standard model ongoing, looking for
hints of new physics
next:
• symmetries
• tests of standard model, experiments, accelerators
• problems and shortcomings of standard model
• new projects, outlook
47