Download Overview of particle physics

Overview of Particle Physics
-- the path to the Standard Model
1
Topics
 historical flashback over development of the
field
o
o
o
o
o
“prehistory” 19th century
electron, radioactivity, nucleus
cosmic rays
spectroscopy era
collider era
 standard model of particle physics
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Atoms, Nucleus
 electron: first hint that atom not indivisible
 natural radioactivity  understanding of
composition of atom, nucleus
 atom = nucleus surrounded by electrons (Geiger,
Marsden, Rutherford, 1906 -1911)
 hydrogen nucleus = proton, is component of all
nuclei (1920)
 neutron (Bothe, Becker, Joliot-Curie, Chadwick,
1930 – 1932)
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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
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Cosmic rays  new “elementary” particles
 new detectors (cloud chambers, emulsions)
exposed to cosmic rays  discovery of many new
particles
positron (anti-electron) : predicted by Dirac
(1928), discovered by Anderson 1932
 muon (μ): 1937 Nedermeyer
 pion (π) predicted by Yukawa (1935), observed
1947 (Lattes, Occhialini, Powell)
 strange particles (K, Λ, Σ,…..
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Cloud chamber
 Container filled with gas (e.g. air), plus vapor close to
its dew point (saturated)
 Passage of charged particle  ionization;
 Ions form seeds for condensation
 condensation along path of particle
 path of particle becomes visible as chain of droplets
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Positron discovery
 Positron (anti-electron)
 predicted by Dirac (1928) -- needed for relativistic quantum
mechanics
 existence of antiparticles doubled the number of known
particles!!
 track going upward
(has lower energy
after lead)
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Anderson and his cloud chamber
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Particle Zoo
 1940’s to 1960’s :
 Plethora of new particles discovered
(mainly in cosmic rays):
 e-, p, n, ν, μ-, π±, π0, Λ0, Σ+ , Σ0 , 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)?
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The Particle Zoo!
±
,
0
,
±
,
e,
±
0
0
K , K S, K L,
0
+
 , p, n,  ,
0
 , , , …
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Seeing = photon scattering experiment
 our eye is a photon detector; (photons = light “quanta” = packets
of light -- see “photoelectric effect”)
 “seeing” is performing a photon scattering experiment:
o light source provides photons
o photons “interact” with object of our interest -- some absorbed,
some scattered, reflected
o some of scattered/reflected photons make it into eye; focused onto
retina;
o photons detected by sensors in retina (photoreceptors -- rods and
cones)
o transduced into electrical signal (nerve pulse)
o amplified when needed
o transmitted to brain for processing and interpretation
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HOW TO SEE SMALL THINGS
 “seeing an object”
= detecting light that has been reflected off the
object's surface
 “visible light”= those electromagnetic waves that our eyes can
detect
 “wavelength” of e.m. wave (distance between two successive
crests) determines “color” of light
 if size of object is much smaller than wavelength, then wave is
hardly influenced by object
 wavelength of visible light: between 410-7 m (violet) and 7 10-7
m (red);
 diameter of atoms: 10-10 m  can’t see them with “ordinary”
(visible) light
 generalize meaning of seeing:
 seeing is to detect effect due to the presence of an object
 quantum theory  “particle waves”, with wavelength 1/(m v)
 use accelerated (charged) particles as probe, can “tune”
wavelength by choosing mass m and changing velocity v
 this method is used in electron microscope, as well as in
“scattering experiments” in nuclear and particle physics
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Particle physics experiments
 Particle physics experiments:
 collide particles to
o produce new particles
o reveal their internal structure and laws of their
interactions by observing regularities, measuring
cross sections,...
 colliding particles need to have high energy
o to make objects of large mass
o to resolve structure at small distances
 to study structure of small objects:
o need probe with short wavelength: use particles with
high momentum to get short wavelength
o Particles behave as if they had a wavelength  = h/p
 mass-energy equivalence (E = mc2) plays an
important role; in collisions, kinetic energy
converted into mass energy;
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About Units
 Energy - electron-volt
 1 electron-volt = kinetic energy of an electron
after moving through potential difference of 1
Volt;
o 1 eV = 1.6 × 10-19 Joules = 1.6 × 10-19 W•s
o 1 kW•hr = 3.6 × 106 Joules = 2.25 × 1025 eV
 mass - eV/c2
o 1 eV/c2 = 1.78 × 10-36 kg
o electron mass = 0.511 MeV/c2
o proton mass = 938.27 MeV/c2
 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 × 1027 eV/c
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ACCELERATORS
 are devices to increase the energy of charged particles;
 magnetic fields to shape (focus and bend) the trajectory of the
particles;
 electric fields for acceleration.
 types of accelerators:
 electrostatic (DC) accelerators
o Cockcroft-Walton accelerator (protons up to 2 MeV)
o Van de Graaff accelerator (protons up to 10 MeV)
o Tandem Van de Graaff accelerator (protons up to 20 MeV)
 resonance accelerators
o cyclotron (protons up to 25 MeV)
o linear accelerators
 electron linac: 100 MeV to 50 GeV
 proton linac: up to 70 MeV
 synchronous accelerators
o synchrocyclotron (protons up to 750 MeV)
o proton synchrotron (protons up to TeV)
o electron synchrotron (electrons from 50 MeV to 90 GeV)
 storage ring accelerators (colliders)
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Van de Graaff accelerator
 use powersupply to deposit charges on belt; pick charges
off at other end of belt and deposit on “terminal”
 now rubber belt replaced by “pellet” chain – “pelletron”
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Cyclotron
 two hollow metal chambers (“dees”)
with “gap” between them
 dees connected to AC voltage
source - one dee positive when
other negative  electric field in
gap between dees, but no electric
field inside the dees;
 source of protons in center,
everything in vacuum chamber;
 whole apparatus in magnetic field
perpendicular to plane of dees;
 frequency of AC voltage such that
particles always accelerated when
reaching the gap between the dees;
 in magnetic field, particles are
deflected: p = qBR
p = momentum, q = charge,
B = magnetic field strength,
R = radius of curvature
 radius of path increases as
momentum of proton increases
time for passage always the same
as long as momentum proportional
to velocity;
this is not true when velocity
becomes too big (relativistic
effects)
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Synchrotron
 synchrotron
 Magnetic field B (to keep particles on circle) synchronized
with electric field (for acceleration);
magnetic field increases during acceleration,
radius of orbit fixed.
 synchrotron is most common accelerator used in particle
physics
 first synchrotrons:
 Cosmotron (Brookhaven), 3.3 GeV, 1953
 Bevatron (Berkeley): 6.2 GeV, 1954
 PS (CERN) 26 -> 28 GeV, 1959
 AGS (Brookhaven) 30 -> 33 GeV, 1960
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Bubble chamber
 Operating principle:
 Vessel, filled with (e.g.) liquid hydrogen at a
temperature above the normal boiling point but held
under a pressure of about 10 atmospheres by a large
piston to prevent boiling.
 After passage of particles, move piston to reduce
pressure  boiling point lowered  boiling starts
along particle tracks  bubbles develop.
 Let bubbles grow (about 3 milliseconds), then tracks
are photographed (flash); provide stereo views of
tracks by use of several cameras .
 Then move piston back  recompress the liquid 
collapse bubbles before boiling all over.
 Invented by Glaser in 1952 (when he was drinking beer)
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 pbar p 
p nbar K0 K- + - 0
 nbar + p  3 pions
 0  ,   e+ e K0  + -
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“Strange particles”
 Kaon: discovered 1947; first called “V” particles
K0 production and decay
in a bubble chamber
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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)
 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
 quark model – three different kinds of quarks
(u, d, s)
 Allows prediction of new particle Ω- , with all of its
properties (mass, spin, expected decay modes,..)
 subsequent observation of Ω- with expected
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properties at BNL (1964)
ΩBNL
1964
http://www.bnl.gov/bnlweb/history/Omega-minus.asp
 eight-fold way  quark model – particles
made up of three different “quarks” – u, d, s
 p = uud, n = udd,… Ω- = sss
 refinement of these ideas, more quarks,
“color”, gauge field theory
 Standard Model
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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  (electromagnetic force)
o W, Z (weak, electromagnetic)
o g gluons (strong force)
 Higgs boson
 spontaneous symmetry
breaking of SU(2)
 mass
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Contemporary
Physics
Education
Project26
“every-day” matter
Proton
Neutron
d
u
u
u
d
Photon
d

Electron
e
Electron Neutrino
e
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Forces (interactions)
 Strong interaction
1
 Binds protons and neutrons to form
nuclei
 Electromagnetic interaction 10-2
 Binds electrons and nuclei to form
atoms
 Binds atoms to form molecules etc.
 Weak interaction
 10-10
 changes “flavors” (e.g.  decay)
 important in stars’ energy “production”
 Gravitational interaction
10-39
 Binds matter on large scales
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Electromagnetic interaction
Proton
q1
Photon
q1q2
F k 2
r
Electron
q2
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The Strong Force
d
u
g
u
Strong force caused by
the exchange of gluons
d
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Weak interaction Beta decay
Neutron
u
d
Mean lifetime of a free
neutron ~ 10.3 minutes
Proton
d
u
d
Mean lifetime of a free
proton > 1031 years!
u
W-
Anti-electron Neutrino
Electron
e
e
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Testing the Standard Model
 want to probe small structures, create massive
particles
 need more powerful accelerators – colliders
 more sophisticated detectors
 resources concentrated in large laboratories,
effort international in scope
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Fermilab
 Fermi National Accelerator Laboratory
(http://www.fnal.gov/)
 Founded 1972
 One of the top laboratories for high energy physics
 Near Batavia, Illinois (45 mi West of Chicago)
 presently (still – just barely) world’s highest energy
accelerator: Tevatron = proton synchrotron,
Emax=980GeV
 Operated as collider: proton – antiproton collisions
at Ecm = 1.96 TeV
 Physics Program
 Collider experiments CDF, DØ, CMS
 neutrino physics: Minos, Mini-Boone
 Astrophysics: Auger Observatory, Sloan Sky Survey
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 ………….
Fermi
National
Accelerator
Laboratory
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The TeVatron Collider
 Tevatron collider
 Colliding bunches of protons and anti-protons;
bunches meet each other every 396 ns in the
center of two detectors (DØ and CDF)
(steered apart at other places)
 Each particle has ~ 980 GeV of energy,
so the total energy in the center of mass
is 1960 GeV = 1.96 TeV
 About 2,500,000 collisions per second
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 peak luminosity 1032 cm-1s-1 (5X1032 cm-1s-1 )
 energy in c.m.s. 1.9 TeV, bunch crossing time 396 ns
 expect integrated luminosity 5fb-1
Turn-on March 1, 2001

 First collisions April 3, 2001
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Fermilab aerial view
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Fermilab TeVatron tunnel
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Modern particle physics detectors
 today’s particle physics detectors:
 combine many detection techniques
 “Russian doll” like structure -- many layers
surrounding interaction region
 “general purpose detectors” – detect, identify
and measure as many different kinds of particles
as possible, (nearly) complete coverage of
interaction region (“hermetic”)
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Identifying particles
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DØ detector
Muon System
1.9T magnetized Fe,
Prop. drift tubes
40,000 channels
Central Tracking
Calorimeter
Uranium-liquid Argon
60,000 channels
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the new DØ detector
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DØ Detector in hall January 2001
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The Discovery of Top Quark
1977 – 1992
Many null results
1992 – 1993
A few interesting
events show up
1994, CDF
First evidence
mt ~ 170 GeV/c2
1995 – CDF, DØ
Discovery!
1994, DØ
mt > 131 GeV/c2
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Creating Top Anti-Top Quark pairs
b
P
t

t
b
e   uc

-1/ 3
2 / 3
W



 e    d s
P
-2 / 3
1/ 3
e   uc
-
W
-
-
-
 e    d s
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-
Artist’s impression of a top event
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What do we actually “see”
_
t t e jets
Muon
Jet-1
Jet-2
Missing energy
Electron
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“event display” of a DØ
top event
t t  e   jets
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Ωb (http://www.fnal.gov/pub/presspass/images/DZero-Omega-discovery.html
 2008 DØ experiment
at Fermilab:
 discover brother
of Ω- , the Ωb
 Ω- = sss,
Ωb = ssb,
 theory predicts
properties, decay
modes, ..
 confirmed by
experiment
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Particles of Standard Model
Leptons
-1/3
-1
0
u
u
u
d
d
d
e
e
c
c
c
s
s
s


t
t
t
b
b
b


g
g
g
g
g
g
g
g
I
II
III

Z
W±
Bosons Fermions
+2/3
Quarks
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Summary
 we’ve come a long way ……
 technical breakthroughs in accelerators and detectors
allowed new discoveries and new understanding
 Standard Model (theory of particle interactions) works
embarrassingly well!
 Has been tested by many hundreds of precision
measurements over last three decades – very few
measurements differ by more than 1 or 2 standard
deviations
 Even some amount of frustration – always hope to see
experimental result in disagreement with theory
 But there are some open questions …………………
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Summary
 many different types of accelerators have been developed
for nuclear and particle physics research
 different techniques suitable for different particles and
energy regimes
 most accelerators in large research laboratories use
several of these techniques in a chain of accelerators
 active research going on to develop new accelerating
techniques for future applications
 many types of accelerators have found applications in
fields other than nuclear and particle physics (e.g. medicine,
ion implantation for electronics chips, condensed matter
research, biology,….)
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Summary
 Particle detection is based on interaction of
particles with material in the detector; detectors
usually have some “amplification” mechanism to render
result of this interaction observable
 Many detection techniques have been developed over
the last century
 breakthrough in detection techniques often led to
breakthrough discoveries
 many of the detectors and/or techniques that were
originally developed for basic research in nuclear or
particle physics are now used in other fields; they
often have led to advances in medical diagnosis (e.g.
MRI, PET,….)
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A Century of Particle Physics
J.J Thomson
Top quark
1995
Electron – 1897
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Sizes and
distance scales
 visible light:
wavelength
≈5∙10-7m
 virus 10-7m
 molecule 10-9m
 atom 10-10m
 nucleus 10-14m
 nucleon 10-15m
 quark <10-18m
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The Building Blocks of a Dew Drop
 dew drop: 1021 molecules
of water.
 Each molecule = one
oxygen atom and two
hydrogen atoms (H2O).
 Atom: nucleus
surrounded by electrons.
 Electrons bound to the
nucleus by photons
 nucleus of a hydrogen
atom = single proton.
 Proton: three quarks,
held together by gluons
just as photons hold the
electron to the nucleus
in the atom
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Very early era (19th century)
 chemistry, electromagnetism
 discharge tubes, “canal rays”, “cathode rays”
 photoelectric effect (Hertz, 1887)
 radioactivity (Becquerel, 1895)
 X-rays (Röntgen, 1895)

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What holds the world together?
interaction
strong
electromagnetic
weak
gravity
participants
quarks
charged
particles
all
particles
all
particles
1
10-2
10-10
10-39
g
gluon

relative strength
field quantum
(boson)
photon
W±
Z0
G
graviton
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The CMS Detector
HF
HE
HB
HO
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Transverse slice through CMS detector
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