Transcript From electrons to quarks – the development of Particle Physics

From electrons to quarks –1st part: the development of Particle
Physics
Outline:









What is particle physics -- why do it?
Early days – atoms, electron, proton
Models of the atom – Thomson, Rutherford,
Bohr
Cosmic rays
Detectors – scintillators, cloud chamber,
emulsion, bubble chamber, spark chamber
More particles: neutron, positron
Muon, pion
Kaon – “strange particles”
Webpages of interest





http://www-d0.fnal.gov (Fermilab homepage)
http://sg1.hep.fsu.edu/~wahl/Quarknet/index.html
(has links to many particle physics sites)
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)
What is 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;
aim is to find description in terms of the smallest
number of particles and forces (“interactions”)
at given length scale, it is useful to describe
matter in terms of specific set of constituents
which can be treated as fundamental; at shorter
length scale, these fundamental constituents may
turn out to consist of smaller parts (be
“composite”)
concept of “smallest building block” changes in
time:




in 19th century, atoms were considered smallest
building blocks,
early 20th century research: electrons,
protons, neutrons;
now evidence that nucleons have substructure quarks;
going down the size ladder: atoms -- nuclei -nucleons -- quarks – preons, strings ???... ???
WHY CAN'T WE SEE ATOMS?








“seeing an object”
 = detecting light that has been reflected off the
object's surface
light = electromagnetic wave;
“visible light”= those electromagnetic waves that our
eyes can detect
“wavelength” of e.m. wave (distance between two
successive crests) determines “color” of light
wave hardly influenced by object if size of object is
much smaller than wavelength
wavelength of visible light:
between 410-7 m (violet) and 7 10-7 m (red);
diameter of atoms: 10-10 m
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
Experimental High Energy Physics

Goal:


Why?



To understand matter and energy under extreme
conditions: at T ~ 1015 K
To understand more organized forms of matter
To understand the origin and destiny of the universe.
Basic questions:






Are there irreducible building blocks?
 Are there few or infinitely many?
 What are they?
 What are their properties?
What is mass?
What is charge?
What is flavor?
How do the building blocks interact?
Why are there 3 forces?


gravity, electroweak, strong
(or are there more?)

1869: Johann Hittorf (1824-1914) (Münster)




1870’s: William Crookes (1832-1919) (London):




determined that discharge in a vacuum tube was
accomplished by the emission of rays ( named “glow
rays” by him, later termed “cathode rays”) capable of
casting a shadow of an opaque body on the wall of the
tube.
rays seemed to travel in straight lines and produce a
fluorescent glow where they passed through the glass.
Rays deflected by magnetic field
detailed investigation of discharges;
Confirms Hittorf’s findings about deflection in
magnetic field
Concludes that rays consist of particles carrying
negative charge
1886 - 1887: Heinrich Hertz (1857-1894) (Karlsruhe)


Built apparatus to generate and detect
electromagnetic waves predicted by Maxwell’s theory
 High voltage induction coil to cause spark discharge
between two pieces of brass; once spark forms
conducting path between two brass conductors 
charge oscillated back and forth, emitting e.m.
radiation
 Circular copper wire with spark gap used as
receiver; presence of oscillating charge in receiver
signaled by spark across the spark gap
Experiment successful –


detected radiation up to 50 ft away
Established that radiation had properties
reminiscent of light: was reflected and refracted as
expected, could be polarized, speed = speed of light

1887: Heinrich Hertz:


Unexpected new observation: when receiver spark gap
is shielded from light of transmitter spark, the
maximum spark-length became smaller
Further investigation showed:
 Glass effectively shielded the spark
 Quartz did not
 Use of quartz prism to break up light into
wavelength components  find that wavelenght
which makes little spark more powerful was in the
UV
 Hertz’ conclusion: “I confine myself at present to
communicating the results obtained, without
attempting any theory respecting the manner in
which the observed phenomena are brought about”

1888: Wilhelm Hallwachs (1859-1922) (Dresden)



Performs experiment to elucidate effect observed by Hertz:
 Clean circular plate of Zn mounted on insulating stand;
plate connected by wire to gold leaf electroscope
 Electroscope charged with negative charge – stays
charged for a while; but if Zn plate illuminated with UV
light, electroscope loses charge quickly
 Electroscope charged with positive charge:
 UV light has no influence on speed of charge leakage.
But still no explanation
Calls effect “lichtelektrische Entladung” (light-electric
discharge)

1894: Hertz and Philipp Lenard (1862-1947):


Further investigations of cathode rays using discharge
tubes:
 Cathode rays penetrate through thin Al window ate
end of tube,
 Cause fluorescence over distance of few
centimeters in air
 Deflected by magnetic field
 No deflection by electric fields
(later explained due to insufficiently good
vacuum)
1895: Wilhelm Röntgen (1845-1923) (Würzburg)


Uses discharge tubes designed by Hittorf and Lenard
(but improved pump) to verify Hertz’ and Lenard’s
experiments
Discovers X-rays -- forget about cathode rays!

Röntgen and X-rays:
Hand of Anna Röntgen
From Life magazine,6
April 1896

1895: Jean Perrin (1870-1942) (Paris):



1896: Hendrik A Lorentz (1853-1928) (Leiden)



Formulates atomistic interpretation of Maxwell’s equations in
terms of electrically charged particles (called “ions” by him)
“Lorentz force” = force exerted by magnetic field on moving
charged particles
1896: Pieter A. Zeeman (1865-1943) (Amsterdam)



Modifies cathode ray tube – adds “Faraday cup” which is
connected to electrometer
Shows that cathode rays carry negative charge
Observes broadening of Na D line in magnetic field
measures broadening vs field strength
1896: Explanation of this effect by Lorentz:




based on light emitted by “ions” orbiting within Na atom
Calculates expected broadening f  (e/m)B
By comparing with measured line broadening, obtains
estimate of e/m of “ions” in Na atom:
e/m  107 emu/g  1011 C/kg
(cf modern value of 1.76x10 C11/kg)
1897: three experiments measuring e/m, all with improved
vacuum:



Emil Wiechert (1861-1928) (Königsberg)
 Measures e/m – value similar to that obtained by Lorentz
 Assuming value for charge = that of H ion, concludes that
“charge carrying entity is about 2000 times smaller than
H atom”
 Cathode rays part of atom?
 Study was his PhD thesis, published in obscure journal –
largely ignored
Walther Kaufmann (1871-1947) (Berlin)
 Obtains similar value for e/m, points out discrepancy, but
no explanation
J. J. Thomson
1897: Joseph John Thomson (1856-1940) (Cambridge)





Improves on tube built by Perrin with Faraday cup to
verify Perrin’s result of negative charge
Conclude that cathode rays are negatively charged
“corpuscles”
Then designs other tube with electric deflection plates
inside tube, for e/m measurement
Result for e/m in agreement with that obtained by
Lorentz, Wiechert, Kaufmann, Wien
Bold conclusion: “we have in the cathode rays matter in a
new state, a state in which the subdivision of
matter is carried very much further than in the ordinary
gaseous state: a state in which all matter... is of one and
the same kind; this matter being the substance from
which all the chemical elements are built up.“

1899: J.J. Thomson: studies of photoelectric effect:



Modifies cathode ray tube: make metal surface to be
exposed to light the cathode in a cathode ray tube
Finds that particles emitted due to light are the same
as cathode rays (same e/m)
1902: Philipp Lenard

Studies of photoelectric effect
 Measured variation of energy of emitted
photoelectrons with light intensity
 Use retarding potential to measure energy of
ejected electrons: photo-current stops when
retarding potential reaches Vstop
 Surprises:


Vstop does not depend on light intensity
energy of electrons does depend on color
(frequency) of light

1905: Albert Einstein (1879-1955) (Bern)

Gives explanation of observation relating to
photoelectric effect:
 Assume that incoming radiation consists of “light
quanta” of energy hf
(h = Planck’s constant, f=frequency)
  electrons will leave surface of metal with energy
E = hf – W
W = “work function” = energy necessary to
get electron out of the metal
 When cranking up retarding voltage until current
stops, the highest energy electrons must have had
energy eVstop on leaving the cathode
 Therefore
eVstop = hf – W


 Minimum light frequency for a given metal, that
for which quantum of energy is equal to work
function
1906 – 1916 Robert Millikan (1868-1963) (Chicago)
Did not accept Einstein’s explanation
 Tried to disprove it by precise measurements
 Result: confirmation of Einstein’s theory,
measurement of h with 0.5% precision


1923: Arthur Compton (1892-1962)(St.Louis):

Observes scattering of X-rays on electrons
WHAT IS INSIDE AN ATOM?

THOMSON'S MODEL OF ATOM


(“RAISIN CAKE MODEL”):
 atom = sphere of positive charge
(diameter 10-10 m),
 with electrons embedded in it, evenly
distributed (like raisins in cake)
Geiger & Marsden’s SCATTERING EXPERIMENT:





(Geiger, Marsden, 1906 - 1911) (interpreted by
Rutherford, 1911)
get particles from radioactive source
make “beam” of particles using “collimators” (lead
plates with holes in them, holes aligned in straight
line)
bombard foils of gold, silver, copper with beam
measure scattering angles of particles with
scintillating screen (ZnS) .
Geiger, Marsden, Rutherford expt.

result:
 most particles only slightly deflected (i.e. by
small angles), but some by large angles - even
backward
 measured angular distribution of scattered
particles did not agree with expectations from
Thomson model (only small angles expected),
 but did agree with that expected from
scattering on small, dense positively charged
nucleus with diameter < 10-14 m, surrounded by
electrons at 10-10 m
Rutherford model

RUTHERFORD MODEL OF ATOM:
(“planetary model of atom”)




problem with Rutherford atom:





positive charge concentrated in nucleus (<10-14 m);
negative electrons in orbit around nucleus at
distance 10-10 m;
electrons bound to nucleus by Coulomb force.
electron in orbit around nucleus is accelerated
(centripetal acceleration to change direction of
velocity);
according to theory of electromagnetism
(Maxwell's equations), accelerated electron emits
electromagnetic radiation (frequency = revolution
frequency);
electron loses energy by radiation  orbit decays,
changing revolution frequency  continuous
emission spectrum (no line spectra), and atoms
would be unstable (lifetime  10-10 s )
 we would not exist to think about this!!
Beta decay
b decay n p + e- + ne


b decay changes a neutron into a_proton
Only observed the electron and the recoiling
nucleus


Pauli predicted a light, neutral, feebly interacting
particle (1930)


“non-conservation” of energy
the neutrino
Although accepted since it “fit” so well, not
actually observed initiating interactions until
1956-1958 (Cowan and Reines)
e+
g
e-

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
takes place along path of particle  path of
particle becomes visible as chain of droplets
Positron

Positron (anti-electron)




Predicted by Dirac (1928) -- needed for relativistic
quantum mechanics
Anderson + Neddermeyer discovered it (1932) in a
cloud chamber
existence of antiparticles doubled the number of
known particles!!!
Positron track going upward through lead plate
Experimentalists ...

Strange quark

kaons discovered 1947
0
KL -+
Not seen, but should be com
K0 production and decay
in a bubble chamber
Experimental
High Energy Physics

Method



Subject matter to extreme temperatures and
densities.
 Energy ~ 2 trillion eV
 Temperature ~ 24,000 trillion K
 Density ~ 2000 x nuclear density
Accelerate sub-atomic particles, to closer than 100
millionth the speed of light, and arrange for them
to collide head on.
Study the debris of particles that emerges from
the collisions.
Particle physics experiments

Particle physics experiments:




collide particles to
 produce new particles
 reveal their internal structure and laws of
their interactions by observing regularities,
measuring cross sections,...
colliding particles need to have high energy
 to make objects of large mass
 to resolve structure at small distances
to study structure of small objects:
 need probe with short wavelength: use
particles with high momentum to get short
wavelength
 remember de Broglie wavelength of a particle
 = h/p
in particle physics, mass-energy equivalence plays an
important role; in collisions, kinetic energy
converted into mass energy;

relation between kinetic energy K, total energy
E and momentum p :
2 + (mc2)c2
E = K + mc2 = (pc)
___________
About Units

Energy - electron-volt


1 electron-volt = kinetic energy of an electron when
moving through potential difference of 1 Volt;
 1 eV = 1.6 × 10-19 Joules = 2.1 × 10-6 W•s
 1 kW•hr = 3.6 × 106 Joules = 2.25 × 1025 eV
mass - eV/c2





1 eV/c2 = 1.78 × 10-36 kg
electron mass = 0.511 MeV/c2
proton mass = 938 MeV/c2
professor’s mass (80 kg)  4.5 × 1037 eV/c2
momentum - eV/c:


1 eV/c = 5.3 × 10-28 kg m/s
momentum of baseball at 80 mi/hr
 5.29 kgm/s  9.9 × 1027 eV/c
How to do a particle physics experiment

Outline of experiment:






get particles (e.g. protons, antiprotons,…)
accelerate them
throw them against each other
observe and record what happens
analyse and interpret the data
ingredients needed:







particle source
accelerator and aiming device
detector
trigger (decide what to record)
recording device
many people to:
 design, build, test, operate accelerator
 design, build, test, calibrate, operate, and
understand detector
 analyze data
lots of money to pay for all of this
How to get high energy -collisions



Need Ecom to be large enough to
 allow high momentum transfer (probe small
distances)
 produce heavy objects (top quarks, Higgs
boson)
_
 e.g. top
e+e- tt,
_ quark production:
_
-  tt, gg  tt, …
qq
Shoot particle beam
on a target (“fixed target”):
_____
 Ecom = 2Emc2 ~ 20 GeV for E = 100 GeV,
m = 1 GeV/c2
Collide two particle beams (“collider :
 Ecom = 2E ~ 200 GeV for E = 100 GeV
How to make qq collisions, cont’d




However, quarks are not found free in nature!
But (anti)quarks are elements of (anti)protons.
So, if we collide protons_and anti-protons we should get
some qq collisions.
-
Proton structure functions give the probability that a
single quark (or gluon) carries a fraction x of the proton
momentum (which is 900 GeV/c at the Tevatron)
Accelerator

accelerators:




use electric fields to accelerate particles,
magnetic fields to steer and focus the beams
synchrotron:
particle beams kept in circular orbit by
magnetic field; at every turn, particles “kicked”
by electric field in accelerating station;
fixed target operation: particle beam
extracted from synchrotron, steered onto a
target
collider operation:
accelerate bunches of protons and antiprotons
moving in opposite direction in same ring; make
them collide at certain places where detectors
are installed
Fermilab accelerator complex
Central Scintillator
Forward Scintillator
+ New Electronics, Trig, DAQ
New Solenoid, Tracking System
Si, SciFi,Preshowers
Shielding
Forward Mini-drift
chambers
D Upgrade
Luminosity and cross section



Luminosity is a measure of the beam intensity
(particles per area per second)
( L~1031/cm2/s )
“integrated luminosity”
measure of the amount of data
~100 pb-1)
cross section s is measure of effective
interaction area, proportional to the probability
that a given process will
occur.



is a
collected (e.g.
1 barn = 10-24 cm2
1 pb = 10-12 b = 10-36 cm2 = 10- 40 m2
interaction rate:
dn / dt  L  s

n  s  Ldt
Examples of particle detectors

photomultiplier:






photomultiplier tubes convert small light signal
(even single photon) into detectable charge (current
pulse)
photons liberate electrons from photocathode,
electrons “multiplied” in several (6 to 14) stages by
ionization and acceleration in high electric field
between “dynodes”, with gain  104 to 1010
photocathode and dynodes made from material
with low ionization energy;
photocathodes: thin layer of semiconductor made
e.g. from Sb (antimony) plus one or more alkali
metals, deposited on glass or quartz;
dynodes: alkali or alkaline earth metal oxide
deposited on metal, e.g. BeO on Cu (gives high
secondary emission);
Examples of particle detectors

Spark chamber







gas volume with metal plates (electrodes); filled
with gas (noble gas, e.g. argon)
charged particle in gas  ionization  electrons
liberated;
 string of
electron - ion pairs along particle path
passage of particle through “trigger counters”
(scintillation counters) triggers HV
HV between electrodes  strong electric field;
electrons accelerated in electric field  can
liberate other electrons by ionization which in turn
are accelerated and ionize  “avalanche of
electrons”, eventually formation of plasma between
electrodes along particle path;
gas conductive along particle path

electric breakdown  discharge  spark
HV turned off to avoid discharge in whole gas
volume
Examples of particle detectors, contd

Scintillation counter:




energy liberated in de-excitation and capture of
ionization electrons emitted as light - “scintillation
light”
light channeled to photomultiplier in light guide (e.g.
piece of lucite or optical fibers);
scintillating materials: certain crystals (e.g. NaI),
transparent plastics with doping (fluors and
wavelength shifters)
Geiger-Müller counter:




metallic tube with thin wire in center, filled with
gas, HV between wall (-, “cathode”) and central wire
(+,”anode”);  strong electric field near wire;
charged particle in gas  ionization  electrons
liberated;
electrons accelerated in electric field  liberate
other electrons by ionization which in turn are
accelerated and ionize  “avalanche of electrons”;
avalanche becomes so big that all of gas ionized 
plasma formation  discharge
gas is usually noble gas (e.g. argon), with some
additives e.g. carbon dioxide, methane, isobutane,..)
as “quenchers”;
Particle detectors,

Scintillator:




cont’d
energy liberated in de-excitation and capture of
ionization electrons emitted as light - ``scintillation
light'’
light channeled to photomultiplier in light guide (e.g.
optical fibers);
scintillating materials: certain crystals (e.g. NaI),
transparent plastics with doping (fluors and
wavelength shifters)
proportional tube:




metallic tube with thin wire in center, filled with
gas, HV between wall (-, “cathode”) and central wire
(+,”anode”);  strong electric field near wire;
charged particle in gas  ionization  electrons
liberated;
electrons accelerated in electric field  can
liberate other electrons by ionization which in turn
are accelerated and ionize  “avalanche of
electrons” moves to wire  current pulse; current
pulse amplified  electronic signal:
gas is usually noble gas (e.g. argon), with some
additives e.g. carbon dioxide, methane, isobutane,..)
as “quenchers”;
Particle detectors,

multi wire proportional chamber:




cont’d
contains many parallel anode wires between two
cathode planes (array of prop.tubes with separating
walls taken out)
operation similar to proportional tube;
cathodes can be metal strips or wires  get additional
position information from cathode signals.
drift chamber:



field shaping wires and electrodes on wall to create
very uniform electric field, and divide chamber volume
into “drift cells”, each containing one anode wire;
within drift cell, electrons liberated by passage of
particle move to anode wire, with avalanche
multiplication near anode wire;
arrival time of pulse gives information about distance
of particle from anode wire; ratio of pulses at two ends
of anode wire gives position along anode wire;
Particle detectors,

Cherenkov detector:


measure Cherenkov light (amount and/or angle)
emitted by particle going through counter volume
filled with transparent gas liquid, aerogel, or
solid  get information about speed of particle.
calorimeter:




cont’d
“destructive” method of measuring a particle's
energy: put enough material into particle's way to
force formation of electromagnetic or hadronic
shower (depending on kind of particle)
eventually particle loses all of its energy in
calorimeter;
energy deposit gives measure of original particle
energy.
many of the
detectors and techniques developed for particle
and nuclear physics are now being used in
medicine, mostly diagnosis, but also for therapy.
Note:
Particle detectors,

Scintillator:




cont’d
energy liberated in de-excitation and capture of
ionization electrons emitted as light – “scintillation
light'’
light channeled to photomultiplier in light guide (e.g.
optical fibers);
scintillating materials: certain crystals (e.g. NaI),
transparent plastics with doping (fluors and
wavelength shifters)
proportional tube:




metallic tube with thin wire in center, filled with
gas, HV between wall (-, “cathode”) and central wire
(+,”anode”);  strong electric field near wire;
charged particle in gas  ionization  electrons
liberated;
electrons accelerated in electric field  can
liberate other electrons by ionization which in turn
are accelerated and ionize  “avalanche of
electrons” moves to wire  current pulse; current
pulse amplified  electronic signal:
gas is usually noble gas (e.g. argon), with some
additives e.g. carbon dioxide, methane, isobutane,..)
as “quenchers”;
Particle detectors,

multi wire proportional chamber:




cont’d
contains many parallel anode wires between two
cathode planes (array of prop.tubes with separating
walls taken out)
operation similar to proportional tube;
cathodes can be metal strips or wires  get additional
position information from cathode signals.
drift chamber:



field shaping wires and electrodes on wall to create
very uniform electric field, and divide chamber volume
into “drift cells”, each containing one anode wire;
within drift cell, electrons liberated by passage of
particle move to anode wire, with avalanche
multiplication near anode wire;
arrival time of pulse gives information about distance
of particle from anode wire; ratio of pulses at two ends
of anode wire gives position along anode wire;
Particle detectors,

Cherenkov detector:


measure Cherenkov light (amount and/or angle)
emitted by particle going through counter volume
filled with transparent gas liquid, aerogel, or
solid  get information about speed of particle.
calorimeter:




cont’d
“destructive” method of measuring a particle's
energy: put enough material into particle's way to
force formation of electromagnetic or hadronic
shower (depending on kind of particle)
eventually particle loses all of its energy in
calorimeter;
energy deposit gives measure of original particle
energy.
many of the
detectors and techniques developed for particle
and nuclear physics are now being used in
medicine, mostly diagnosis, but also for therapy.
Note:
Detectors

Detectors



use characteristic effects from interaction of
particle with matter to detect, identify and/or
measure properties of particle; has “transducer” to
translate direct effect into observable/recordable
(e.g. electrical) signal
example: our eye is a photon detector; (photons =
light “quanta” = packets of light)
“seeing” is performing a photon scattering
experiment:







light source provides photons
photons hit object of our interest -- some
absorbed, some scattered, reflected
some of scattered/reflected photons make it
into eye; focused onto retina;
photons detected by sensors in retina
(photoreceptors -- rods and cones)
transduced into electrical signal (nerve pulse)
amplified when needed
transmitted to brain for processing and
interpretation
Standard Model


A theoretical model of interactions of
elementary particles
Symmetry:


SU(3) x SU(2) x U(1)
“Matter particles”

quarks


leptons


electron, muon, tau, neutrinos
“Force particles”

Gauge Bosons




up, down, charm,strange, top bottom
g (electromagnetic force)
W, Z (weak, elctromagnetic)
g gluons (strong force)
Higgs boson


spontaneous symmetry breaking of SU(2)
mass
Standard Model
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: Fermi introduces 1st theory of weak
interactions, analogous to QED, to explain b decay.
1935: 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)
Brief History of the Standard Model
(continued)









1970: Glashow, Iliopoulos, Maiani: 4th quark
(charm) explains suppression of K decay into 
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
(t’Hooft, Veltman, Lee, Zinn-Justin..)
1972: high pt pions observed at the CERN ISR
1973: Gell-Mann & Fritzsch propose that quarks
are held together by a Gauge-Field whose quanta,
gluons, mediate the strong force Quantum
Chromodynamics
1973: “neutral currents” observed (Gargamelle
bubble chamber at CERN)
Brief History of the Standard Model
(continued)

1975: J/ interpreted as cc bound state
(“charmonium”)

1974: J/ discovered at BNL/SLAC;

1976: t lepton discovered at SLAC








1977:  discovered at Fermilab in 1977, interpreted as
bb bound state (“bottomonium”)  3rd generation
1979: gluon “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 (D0, CDF)
1999: indications for “neutrino oscillations” (SuperKamiokande experiment)
2000: direct evidence for tau neutrino (nt) at
Fermilab (DONUT experiment)
2003: Higgs particle observed at Fermilab (?????)
Cathode ray history









1855 German inventor Heinrich Geissler develops
mercury pump - produces first good vacuum tubes,
these tubes, as
modified by Sir William Crookes, become the first to
produce cathode rays, leading eventually to the
discovery of the
electron (and a bit farther down the road to television).
1858 Julius Plücker shows that cathode rays bend
under the influence of a magnet suggesting that they
are connected in some way; this leads in 1897 to
discovery that cathode rays are composed of electrons.
1865 H. Sprengel improves the Geissler vacuum pump.
Plücker uses Geissler tubes to show that at lower
pressure,
the Faraday dark space grows larger. He also finds that
there is an extended glow on the walls of the tube and
that
this glow is affected by an external magnetic field.
1869 J.W. Hittorf finds that a solid body put in front
of the cathode cuts off the glow from the walls of the
tube.
Establishes that "rays" from the cathode travel in
straight lines.










1871 C.F. Varley is first to publish suggestion that
cathode rays are composed of particles. Crookes
proposes that
they are molecules that have picked up a negative
charge from the cathode and are repelled by it.
1874 George Johnstone Stoney estimates the charge
of the then unknown electron to be about 10-20
coulomb, close to
the modern value of 1.6021892 x 10-19 coulomb. (He
used the Faraday constant (total electric charge per
mole of
univalent atoms) divided by Avogadro's Number. James
Clerk Maxwell had recognized this method soon after
Faraday had published, but he did not accept the idea
that electricity is composed of particles.) Stoney also
proposes
the name "electrine" for the unit of charge on a
hydrogen ion. In 1891, he changes the name to
"electron."
1876 Eugen Goldstein shows that the radiation in a
vacuum tube produced when an electric current is
forced through
the tube starts at the cathode; Goldstein introduces
the term cathode ray to describe the light emitted.
1881 Herman Ludwig von Helmholtz shows that the
electrical charges in atoms are divided into definite
integral








1883 Heinrich Hertz shows that cathode rays are
not deflected by electrically charged metal
plates, which would
seem to indicate (incorrectly) that cathode rays
cannot be charged particles.
1886 Eugen Goldstein observes that a cathoderay tube produces, in addition to the cathode ray,
radiation that travels
in the opposite direction - away from the anode;
these rays are called canal rays because of holes
(canals) bored in
the cathode; later these will be found to be ions
that have had electrons stripped in producing the
cathode ray.
1890 Arthur Schuster calculates the ratio of
charge to mass of the particles making up cathode
rays (today known as
electrons) by measuring the magnetic deflection
of cathode rays. Joseph John (J.J.) Thomson
first becomes interested
in the discharge of electricity through a gas a low
pressure, that is to say, cathode rays.











1892 Heinrich Hertz who has concluded (incorrectly)
that cathode rays must be some form of wave, shows
that the
rays can penetrate thin foils of metal, which he takes
to support the wave hypothesis. Philipp von Lenard
develops a
cathode-ray tube with a thin aluminum window that
permits the rays to escape, allowing the rays to be
studied in the
open air.
1894 J.J. Thomson announces that he has found that
the velocity of cathode rays is much lower than that of
light. He
obtained the value of 1.9 x 107 cm/sec, as compared to
the value 3.0 x 1010 cm/sec for light. This was in
response to
the prediction by Lenard that cathode rays would move
with the velocity of light. However, by 1897, he
distrusts this
measurement.
Special Note: At this time there was great rivalry
between German and British researchers. As
concerning the nature
of the cathode ray, the Germans tended to the
explanation that cathode rays were a wave (like light),
whereas the
British tended to believe that the cathode ray was a
particle. As events unfold over the next few decades,










In fact, J.J. Thomson will be awarded the Nobel Prize
in Physics in 1906 for proving the electron is a particle
and his
son, George Paget Thomson, will be awarded the Nobel
Prize in Physics in 1937 for showing that the electron is
a
wave.
1895 Jean-Baptiste Perrin shows that cathode rays
deposit a negative electric charge where they impact,
refuting
Hertz's concept of cathode rays as waves and showing
they are particles.
1896 Pieter P. Zeeman discovers that spectral lines of
gases placed in a magnetic field are split, a phenomenon
call
the Zeeman effect; Hendrik Antoon Lorentz explains
this effect by assuming that light is produced by the
motion of
charged particles in the atom. Lorentz uses Zeeman's
observations of the behavior of light in magnetic field
to
calculate the charge to mass ratio of the electron in an
atom, a year before electrons are discovered and 15
years
before it is known that electron are constituents of
atoms.