From electrons to quarks –- 1st part: the development of Particle

Physics

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)

Outline:

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 410-7 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: To understand matter and energy under extreme

conditions: at T ~ 1015 K Why?

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) 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 1870’s: William Crookes (1832-1919) (London):

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:

From Life magazine,6 April 1896

Hand of Anna Röntgen

1895: Jean Perrin (1870-1942) (Paris): Modifies cathode ray tube – adds “Faraday cup” which is

connected to electrometer Shows that cathode rays carry negative charge

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) 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

energyE = 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”) 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.

problem with Rutherford atom: 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

decay changes a neutron into a proton Only observed the electron and the recoiling

nucleus “non-conservation” of energy

Pauli predicted a light, neutral, feebly interacting particle (1930)

the neutrino Although accepted since it “fit” so well, not

actually observed initiating interactions until 1956-1958 (Cowan and Reines)

decay n p + e + e

_

e+

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

K0 production and decayin a bubble chamber

KL 0

Not seen, but should be common

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 : E = K + mc2 = (pc)2 + (mc2)c2

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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 quark production: e+e- tt,

qq tt, gg tt, …

Shoot particle beam on a target (“fixed target”): Ecom = 2Emc2 ~ 20 GeV for E = 100 GeV,

m = 1 GeV/c2

Collide two particle beams (“collider : Ecom = 2E ~ 200 GeV for E = 100 GeV

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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)

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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

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Luminosity and cross section

Luminosity is a measure of the beam intensity (particles per area per second) ( L~1031/cm2/s )

“integrated luminosity” is a measure of the amount of data collected (e.g. ~100 pb-1)

cross section is measure of effective interaction area, proportional to the probability that a given process will

occur. 1 barn = 10-24 cm2

1 pb = 10-12 b = 10-36 cm2 = 10- 40 m2

interaction rate:

LdtnLdtdn /

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, cont’d

Scintillator: 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, cont’d

multi wire proportional chamber: 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, cont’d

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: “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.

Note: many of the detectors and techniques developed for particle and nuclear physics are now being used in medicine, mostly diagnosis, but also for therapy.

Particle detectors, cont’d

Scintillator: 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, cont’d

multi wire proportional chamber: 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, cont’d

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: “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.

Note: many of the detectors and techniques developed for particle and nuclear physics are now being used in medicine, mostly diagnosis, but also for therapy.

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 up, down, charm,strange, top bottom

leptons electron, muon, tau, neutrinos

“Force particles” Gauge Bosons

(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 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: 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” (Super-Kamiokande experiment)

2000: direct evidence for tau neutrino () at Fermilab (DONUT experiment)

2003: Higgs particle observed at Fermilab (?????)

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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

portions, suggesting the idea that there is a smallest unit of electricity.

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 cathode-ray 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,

both will be proven correct.

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.