quark of the getting to the - michigan state university · 1998-11-07 · 1 bottom of the top quark...

1 Bottom of the Top quark (Brock) 1/24/97 9:28 PM

Getting to the

BOTTOM of the TOP

quark

• Raymond Brock• Professor of Physics and Chairperson• Department of Physics and Astronomy• Michigan State University• [email protected]• http://www.pa.msu.edu/~brock/chips_homepage_hot.html


my goal:

...to tell the stories behind the discoveries of the fundamental constituents of matter

“stories”: physicists are folks -

some smart, some strange, some industrious...just like civilians. Often, there are “stories”...

“discoveries”: what’s that?

A “Eureka Moment”? Often, not.

“fundamental”: maybe “simple”?

maybe “indivisible”, certainly, “not complex”...a bit of a controversy

“matter”: more than just “stuff” -

really possible configurations of Energy

This is the subject matter of the subfield of physics called “Elementary Particle Physics” or “High Energy Physics”


agenda

a largely historical and descriptive journey

1900 - 1920 surprises

1920 - 1945 exploration

1945 - 1965 confusion

interlude visualizing

1970 - present confirmation


1900 - 1920:A Period of Surprises

Discovery of photoelectricity 1887 • Discovery of Xrays 1896 • Discovery of Radioactivity 1896 • Discovery of the

electronelectronelectron 1897 • Discovery of α andβ rays 1898 • • Invention of the quantum 1900 • Invention of Special

Relativity 1905 • Invention of the photoelectric effect 1905 • Discovery of the Nucleus 1911 • Invention of Quantum theory 1913 • Discovery of the periodic chart clue 1914 •

Discovery of the protonprotonproton 1917


no surprises

One century ago, if giving this talk I would describe that period also as A PERIOD OF CONFIRMATION: physical sciences were riding high

electro-magnetism

a unified theory (after much competition) of the complete theory of Maxwell, after Faraday and Hertz (Lorentz, Helmholtz)

thermodynamics

the notion of the conservation of energy was borne out and extended and a mechanical description of heat was available (Helmholtz, Meyer, Boltzmann, Lord Kelvin, Maxwell, Clausius)

mechanics

a complete mathematical theory (Hamilton, Poincare, Lagrange, Hilbert)

chemistry and engineering

terrestrial Helium, spectroscopy, radio telegraphy, the Suez Canal, the steamship, the automobile - the future looked bright


some wrinkles

some of the tidying-up required serious experimentation:– radiation from blackbodies seemed not to obey Maxwell’s

model– the periodic table seemed a confusing arrangement– there were some anomolies in the heat capacities of some

materials– battles raged on the existence or not of atoms - Mach reigned– some trouble reconciling the the 1887 experiments of

Michelson and Morley with unquestioned ether theories

fascinating glow-tubes...– Faraday first studied the application of a voltage between the

poles inside a nearly evacuated tube. – In 1858 Plucher brought a magnet nearby and saw an effect


cont.

– improvements showed increasingly interesting characteristics of the “cathode rays” - shadows showed that the source originated at the cathode

– Hertz claimed evidence that they were waves, – Crooke claimed evidence that they were electrically charged

particles (“radiant matter”)– Perrin found proof that they might be negatively charged

As the capability to produce higher potential differences and better vacuums improved, the interesting stuff started to happen...


1887: discovery of the photoelectric effect, Hertz [1857-1894]

He was the first to generate electromagnetic waves and in the course of that work...

he set up an oscillator and a spark gap, and noted sparks in a companion gap

– when the first was enclosed in a box with material between

unexpectedly, he found that when ultraviolet light is shown on one of the electrodes, that the companion glow was more intense

Further studied by many others showed that:

ejected sparks came from the cathode

the intensity depended on frequency, not intensity

a minimum frequency was required

the ejected rays are instantaneous with the light– incompatible with the classical theories of electromagnetism– studied for 20 years after Hertz’s discovery, without

explanation


Roentgen was studying the apparent penetration of cathode rays beyond the end of the tube...beyond thin aluminum walls

in the dark, with the tube covered in cardboard, he was trying to determine the thickness through which the cathode rays would penetrate - he noticed a green glow across the room!

further study showed amazing things

no fool, he flooded Europe with photographs of his hands - what he called “Xray” photographs

Reporting to the French Academie des Sciences resulted in 5 more observations in a week...

1896: discovery of Xrays, Roentgen [1845-1923]


1896: discovery of radioactivity, Becquerel [1852-1908]

Bacquerel was in the audience that day (1/20/96) and was reminded of work he had done with his father on phosphorescence - the glows seemed similar. He returned to his lab and

– put a lump of phosphorescent rock on a photographic paper to see if it would darken...no

– He tried many stones, nothing happened until a chunk of potassium uranyl sulfate which “charged” through ultraviolet light¥ it showed through the paper which wrapped the

photographic plate and he reported the Academie that phosphorescence causes Xrays...

– but the clouds decended on Paris and he put the whole mess in a drawer for a week

why did he bother to develop the stuff in the drawer after the sun came out? Lucky for him...

– He did & the plates showed the darkest blotches ever! He had been wrong about the cause - but he caused quite a stir

a material was spontaneously emitting energy!!


Thomson was the Cavendish Professor of Experimental Physics at 28

the foremost laboratory in England with 20 full-time staff and turned from the practical applications of electromagnetism to investigations of the nature of electricity

with improved techniques, Thomson and colleagues spend a decade studying cathode rays

he combined electric field and magnetic fields to deflect the rays...and then measure the deflections carefully.

by presuming that the rays are made from “corpuscles” of negative charge, he determined

– indendent of gas, cathode, voltage, etc.

1897: discovery of the electron, Thomson [1856-1940]

+ +

-cathode

grids

em

= × −1 8 10 11. Coulombs/kg


cont.

He concluded that e/m was very large

compared to hydrogen

succeeded in determining e in 1899

conclusion was clear: the carrier of electricity was tiny and 1/2000th of the mass of hydrogen– This was the first elementary “particle” and was dubbed

“electron” by Johnstone Stoney (in 1891!)

Thomson was not exactly rewarded with praise!

the strong Machian, Germanic view was that if it couldn’t be observed directly, it could not exist.

“At first there were very few who believed in the existence of these bodies smaller than atoms. I was even told long afterwards by a distinguished physicist who had been present at my lecture at the Royal Institution that he thought I had been ‘pulling their legs’ ...I myself came to this explanation with great reluctance..”


1898: discovery of α and β rays, Rutherford [1871-1937]

The Giant of this period was a farmer from New Zealand

With a fellowship to Cambridge in 1895, he began to work with J.J. Thomson

– after Rontgen’s discovery, they studied the ionization effects of Xrays, then after Bequerel’s discovery, the emissions from uranium

– he found two kinds of radiations:¥ one which stopped quickly in matter, he named: α¥ one which penetrated, called: β¥ soon others found that cathode rays = β rays

Moved to McGill University at Montreal to start a group¥ Chairman: ÒI think IÕd better take your classes and do the

teaching. You keep doing what you have to do.Ó


cont.

the α were puzzling..many suspected them to be particles, atoms which were charged and ejected at high-speed

after disagreements with Bequerel, Rutherford concluded that a were identical with ionized He– found that radioactive substances emitted He when heated– got the idea that radioactive substances transmuted into one

another¥ with a colleague, Frederick Soddy (a young chemist), they

concluded that each radioactive substance has a finite probability of decaying in a characteristic time


1900: invention of the quantum, Planck [1858-1947]

About the most un-revolutionary physicist in Germany in 1900 was Max Planck...

for half a decade, he studied the way in which things emit light

His approach was to presume that the source of light was little oscillators - he tried to make the oscillators infinitesimal, and then sum them together...his problem was that to make things work, they couldn’t be arbitrarily small - they had to be finite.

Troublesome was the classical “black body radiator” - traditional notions didn’t fit

He finally succumbed in an “act of desperation” to the notion that the oscillators were “quantized”He found that he got agreement with experiment by presuming that the relation between energy and frequency of light was

According to one of his students he was a “revolutionary against his will...” Planck: “We will have to live with it.”

E nh= ν h= × −6 55 10 27. erg sec


cont.

Nonetheless, for Planck the quantum features were a process of the walls of the blackbody radiator, not the light itself

It took a true revolutionary to see the situation the other way around...


1905: invention of photoelectricity, Einstein [1879-1955]

In 1905, Albert Einstein had 3 revolutionary-ideas. The first reconciled the feud between Newton and Huygens, the second revolutionized our notions of space and time, and the third demonstrated the reality of atoms.

In his 7 years at the Bern Patent Office, he used his time to become thoroughly proficient in statistical mechanics and Maxwell’s theory (only confirmed for 20 years and not fully infused into the German curriculum)

Einstein went beyond Planck and used this proficiency to investigate the behavior of light itself inside a perfectly reflecting box in a paper containing a section which proposed an explanation of the photoelectric effect.– the light itself possessed quantum behavior, independent of

Planck’s oscillators in the matter of the walls– all of the perplexing behavior seen experimentally in the

photoelectric effect were understood by this means


cont.

He wasn’t done. That same volume of Annalen der Physik contained a second paper,

where he made quantitative predictions regarding the behavior of tiny particles in a fluid under the constant bombardment of the fluid’s atoms - Brownian Motion. He proposed experiments which were eventually performed successfully.

This provided a proof of the existence of atoms.


1905: invention of special relativity, Einstein

He still wasn’t done - again, in that same journal volume, he asked simple questions about the nature of light and time.

The incompatibility of conventional notions of how objects would behave at near-light speed velocities was well-known.– Lorentz and Fitzgerald invented the contraction of objects as

they moved at velocities close to the speed of light¥ This was to save the Ether, which had not been found by the

Michelson Morley experiments in 1882¥ Lorentz concerned himself with the actual machanical

aspects of the contracting body– Poincare, in 1904, came very close to a Principle of Relativity -

stopping short, again at the need to “explain” the contraction - the presumed need of a mechanical electrodynamics

To Einstein, the ether could be shown to be “superfluous” - a direct consequence of the Principle: The laws of physics are the same for all observers moving at constant velocities. Coupled with the speed of light, c, length contraction and time dilation - the Lorentz Transformations, follow directly.


E = mc2

Additionally, the famous coupling of energy and c came as a natural consequence - a crucial ingredient to all that follows in elementary particle physics

Einstein suggested that any object with mass, when moving, gains mass increasingly as the velocity goes up.

this seems absurd, but that’s because of our notion of mass...normally, a “resisitance to acceleration”, what’s meant by inertial mass.– In a Newtonian sense, (F = ma )applying a force, results in an

acceleration– However, as one approaches the speed of light, the resistance

to acceleration by the same force becomes greater and greater and eventually cannot result in an acceleration.

– We interpret that in our Newtonian way, as an increase of the mass


cont.

Since, the amount of kinetic energy of an object depends on the mass, (for example, K.E. = 1/2 mv2) , it is not hard to extrapolate to relativistic mechanics to imagine that the energy of an object also depends on its mass...which in turn dependes on the speed.– That’s the basis behind the famous relation

E = mc2

with the definition of “rest mass”, m0, as a measurable in a lab, the relation is really a bit more complicated.

More to the point...mass and energy are the same– one is convertable to the other

m mvc

=−

02

21


1911: discovery of the nucleus, Rutherford

Back to the Giant...

Sir Arthur Schuster, holder of the chair of Physics at Manchester, decided to retire iff, Rutherford would succeed him.

with the offer, came the assistant, Hans Geiger

In 1908, Rutherford received the Nobel Prize in Chemistry(!) and renewed the study of alpha radiations by scintillation technique...

by counting, he and Geiger measured Avagadros’ number, the charge of the electron and other constants which agreed with conventional values...leading to the universal acceptance of atoms

Additionally, in 1909, a young student (from New Zealand!), Ernest Marsden, reported to his boss that alpha particles very occassionally, seemed to go at a large angle when aimed at a thin material - some backwards!

this was astounding... “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back at you.”

By 1911 Rutherford knew why


A MODEL for the atom

By this time, there were many models for the atomLorentz - electron elastically bound to a fixed centerPlanck - oscillatorsThomson - the plum pudding model...popular– none would scatter alpha particles - sometimes backwards!

various people had entertained a planetary system model... and Rutherford postulated:

A core of positive charge, surrounded by the electrons, which deflects the alpha particle– Rutherford worked out the mathematics which matched in a

statistical fashion what he measured (now called Rutherford Scattering)

This was a mechanically unstable situation- the electrons would radiate, since they are accelerating: no matter for Rutherford.

Zeα -Ze

Zeα


1913: invention of the quantum theory, Bohr [1885-1962]

In 1911, young Niels Bohr received a Carlsberg Brewery fellowship from Denmark to study in England for a year

He chose Cambridge and Thompson - the plum-pudding model had intrigued him

Not a great experience, he was notoriously shy & struggledAfter a visit of Rutherford - a raucus party - Bohr abandoned Cavendish for Manchester

The Rutherford model, not serious for it’s inventor, captured Bohr - in spite of, or because of its mechanical instability

– He had an idea on his way to his wedding, which he left with Rutherford in a memo¥ he also convinced his Danish bride that a honeymoon in

England was preferable to Norway – a true revolutionary...”we could not proceed at all in any other

way than by radical change.” - the quantum


Another MODEL for the atom

In 1885, Johann Balmer had noticed a regularity in the spectrum of hydrogen - and was able to write a formula to describe it

strictly empirical

– this was unknown to Bohr until mid February 1913... but a bolt of lightening - by March 6, he had a theory and mailed a manuscript to Rutherford¥ Rutherford: Ò...long papers have away of frightening

readers...Óhe offered to Ò...cut out any matter I consider unnecessary...Please reply.Ó

– Bohr caught the next boat to England and argued over every phrase...the paper was published as originally written, followed by 2 others.

The Bohr modelPaschen

Lyman

Balmer

electron orbits are fixed as are transitions between them

v

mm = × −

3 289 10 1

4115

2.


1914: discovery of the clue to the periodic chart, Moseley [1887-1915]

One of the first applications of Bohr’s model was to Xray spectra

when electrons in a material are ejected near the nucleus, electrons from higher shells will transition down, emitting radiation.

typical Xray wavelengths are 1/2 - 3/2 A...corresponding to transitions among n=1-3,4...called K,L,M,N etc.

Bohr’s model predicted an unambiguous relation between Z2 and λ of the radiation

In 1913, a breakthrough in tube design made measurements much easier - at a fixed voltage, a heated cathode was introduced, allowing control of the cathode ray emission

H.G.J. Moseley, another Rutherford protege, carefully measured this relationship and got a perfect fit for Bohr’s model and discovered that the atomic number increased by 1 from one element to the next in the periodic chart.– missing elements were identified– alpha emission of -2 units of charge was understood...the

entire radioactive chain was then ripe for understanding


1919: discovery of the proton, Rutherford

By 1917, Rutherford was one of the few left at Manchester...he undertook a variety of studies on his own with alpha and gamma radiation (known to be high energy photons from the nucleus)

following some ideas of a former student, he studied what happened when nitrogen gas was bombarded by alpha particles

fragments appeared: “From the results so far obtained it is difficult to avoid the conclusion that the long-range atoms resulting from collisions of a particles with nitrogen atoms are not nitrogen atoms but probably atoms of hydrogen...We conclude that the nitrogen atom is disintigrated under the intense forces developed in a close collision with a swift alpha...[if high energy projectiles were used] we might expect to break down the nucleus structure of many of the lighter atoms.”

In modern language, what he had done was

he discovered, and named, the proton (“the first one”).

He also speculated on a neutral manifestation of a proton, one in which the electron had fallen into the proton to neutralize it’s charge

since, atomic number, A ≅ 2Zand Mnucleus ≅ Amp

7N14 + 2He4 → 8O17 + 1H1


units

Too many zeros is problematic: mp=1.67×10-24 g

The electron-volt is the accepted unit for energy (which can be quickly related to mass or momentum with Einstein’s relation)

1 electron volt = amount of energy gained by a charged particle when it is accelerated through a potential difference of 1 Volt

keV = 103 eVMeV = 106 eVGeV = 109 eV

TeV = 1012 eVso mp = 938.3 MeV


status:

Where are we in 1920?

electron discovered

quanta established

relativity established

nucleus discovered

proton discovered

Elementary Particle Physics had begun.

particle mass(MeV)

Q birthday

DISCOVEREDelectron, e 0.511 -1 1897proton, p 938.3 +1 1917

PREDICTED

photon, γ 0 0ANTICIPATED

neutron, n 0


1920 - 1945:A Period of Exploration

Discovery of the photonphotonphoton 1922 • Invention of the wave-particle duality 1932 • Invention of quantum mechanics

1926 • Discovery of β decay problems 1927 • Invention of antimatter 1928 • Invention of the neutrino 1930 •

Discovery of the neutron 1932 • Discovery of the positron positron positron 1932 • MODEL for β decay 1933 • Invention of the pion

1935 • Discovery of the pion -NOT 1937


1922: Discovery of the photon, Compton [1892-1962]

Where were the quanta?

The photoelectric effect was suggestive

Holly Compton wanted to study the scattering of light from free electrons

numbers are very smallhe used Xrays, which have a high enough energy that the bound electrons appear “free” λ Xrays ≈ 1Α, whereas λATOMIC ≈ 1000A

he could predict the frequency of the scattered Xray, as well as the energy and angle of the scattered electron - succesfully

Now there was no doubt - photons were the corpuscules of light

which is sometimes a wave, and sometimes a particle!

photon, ν

photon, ν ’< ν

electronelectron


1925: Invention of wave-particle duality, deBroglie [1892-1987]

If photons had this dual existence, what about electrons?

Prince de Broglie had an unusual scientific career:

student of history at the Sorbonneradio operator in WWI..became interested in physicswrote a doctoral thesis, extending Bohr’s theory

¥ called Òthe French ComedyÓ by many physicists

He found that if he chose to relate the momentum of an electron to a wavelength,

he could fit 1 cycle into the first Bohr orbit, 2 cycles into the second, and so on...

If these “de Broglie waves” are integral to the electron, then they should accompany a free electron

free electrons should exhibit diffractionthey do!

λ = h

mv


1926: Invention of quantum mechanics, Heisenberg [1901-1976] and Schroedinger [1887-1961]

The real mathematical theory of quantum phenomena was written down nearly simultaneously by two people:

Schroedinger:

wrote a theory which required a complex (meaning imaginary) mathematical object, called the wave function, which when squared behaved like the intensity of a wave.– using the obscure mathematical techniques of partial

differential equations, he could reproduce the Bohr theory, with a breakdown of a “picture” for what was physically happening

Heisenberg

wrote a theory which involved only measurable quantities– using the obscure mathematical techniques of matrices and

non-commuting algebra, he could reproduce the Bohr atomic selection rules, with a breakdown of the notion of “measurement”

Dirac (and Schroedinger and Heisenberg) showed that the two formulations were mathematically identical


uncertainty

The Heisenberg Uncertainty Principle is the least understood, and unambiguously true feature of the new “quantum mechanics”

one’s ability to measure to arbitrary precision for all quantities is limited.

the granularity of the quantum, embodied in h, makes our ability to measure granular as well.– one can measure energy with arbitrary precision, but at the

expense of any information about time, and visa versa– one can measure position with arbitrary precision, but at the

expense of any information about momentum, and visa versa

∆ ∆

∆ ∆

p x h

E t h

⋅ ≥

⋅ ≥

2

2

π

π


cont.

¥ Imagine a 1mg object, the size of an atom (10-8 cm) - how well can its velocity be determined?

¥ no problem...what about an electron (m=10-27 gm)? Now, ∆v = 108 cm/s, corresponding to a kinetic energy of 10-11

erg...which is comparable to the binding energy!

One cannot speak of a definate electron orbit - the electron is “everywhere”

Also, we can accomodate violations of the conservation of energy which are too quick to measure!

hmmm....

∆ ∆v h

xmerg s cm s≅ ≈ × −

× ×=

−− −

−2

6 102 10 10

1027

8 316

π π/


1928: Invention of antimatter, Dirac [1902-1984]

In order to accomodate quantum mechanics and relativity, something had to be done with negative-energy solutions to the equations...

Dirac proposed that the groundstate of the universe was composed of the positive energy particle world, and a negative energy antiparticle world

the former is largely empty => lots of states availablethe latter is largely full, in the sense of the Exclusion principle

E>0

E<0

photon

photon → electron + hole in negative sea a hole in a region which is negatively charged looks like a relatively positive charge...

these holes are antiparticles...


1932: Discovery of the positron, Anderson [1905-]

Cosmic rays had been known since the beginning of the century

terrestrial cosmic rays are abundant, but difficult to study. To understand the components of this “rain” of particles

need to know when a cosmic ray has passed your detectorfind the chargeestablish the mass

Cloud chambers were used extensively

Carl Anderson was photographing cosmic rays passing through his chamber inside of a magnetic field which contained lead platesAugust 2,1932 he saw an “electron” with the opposite sign - 6 months after the neutron

Soon everyone saw them - Dirac’s speculation had been correct - this was the positron.

droplets indicate thepassage

piston


1932: Discovery of the neutron, Chadwick [1891-1974]

Many people were colliding alpha particles with materials

this industry was assisted by the steady accumulation of newly discovered radioactive nuclei - Polonium was one such choice

people observed strange instances of a penetrating radiation which had no electric charge.

Rutherford urged his protege, James Chadwick, to look into this – he combined Polonium with Beryllium and aimed them at an

ionization chamber filled with nitrogen, hydrogen, and helium

by moving the plate and varying the gas, Chadwick could estimate the range and the mass of the neutral radiation to be about the same as the proton...he called it the neutron

He reported on February 17,1932

Po Be

neutral radiationα

chamber with gas

recoiling proton

collectingplate


1927: Discovery of β decay problems

Studies of the beta decay of various nuclei took a turn toward absurdity...

The reaction in general was determined to be

ZNA → Z+1NA + e-

If the energy of the initial nucleus was E0, and the energy of the daughter nucleus was ED, then the energy of the beta particle had to be E0 - ED...

When precise techiniques for measuring electron energies were developed, it was found that IT WASN’T...this was a catastrophie

– all experiments agreed– all searches for missing radiation, like gamma rays failed– conservation of energy was seriously called into question by

Bohr and others


1930: Invention of the neutrino, Pauli [1900-1958]

Wolfgang Pauli was a phenomenon himself...

He was responsible for understanding the nature of the electron and proton as having a characteristic which could be described as a quantized mechanical angular momentum - “spin”.

The mathematics makes + or - 1/2 as the value most natural

responsible for the “Pauli Exclusion Principle” which states that no two identical electrons can occupy the same state (two electrons with opposite spins, are not identical) - this explained the atomic structure and behavior in magnetic fields (the Zeeman spectroscopic splitting)

Distressed over the crisis in beta decay - and unwilling to part with the conservation of energy, he postulated the existence of a new (unobservable!) particle

– “I have hit on a desperate remedy to save the...energy theorem. Namely, [there is] the possibility that there could exist in the nuclei electrically neutral particles that I wish to call neutrons...I don’t feel secure enough to publish anything about this idea...”

the idea was discussed for a couple of years...


1933: A MODEL for β decay, Fermi [1901-1954]

Fermi heard of Pauli’s idea for a “neutron” in 1931, but it took the discovery of the Chadwick neutron to give him the tools to develop a theory. (He renamed Pauli’s particle the “neutrino”.)

He was able to combine the new relativistic quantum mechanics, the family relationship between the neutron and proton into a striking idea -

– the neutron itself is unstable, and transmutes into a proton, an electron, and the neutrino

a new force of nature is responsible for this decay - called Weak:– electromagnetism’s strength is 1/137 ≈ 10-2

– the weak force’s strength is 10-13

Fermi could calculate the energy spectrum of the beta, under the assumption of a massless neutrino - his paper was rejected by Nature and so he published in Italy.

After careful experiments, it was clear that his model worked.

np

eν


1934: Invention of the pion, Yukawa [1907-1981]

What was the nature of the force which binds protons to protons and neutrons in a nucleus?

It has to be strong, to overcome the electric replusion...

Fermi’s model seemed a guide, following a suggestion of Heisenberg’s of a local exchange force:

this had problems, but once the neutron was found the idea still had merit:

pp e(n)

pe

p

p p e (n)


cont.

Yukawa proposed that just as the photon is related to the electromagnetic field and its forces - so should there be a quantum associated with the strong force.

active only over the distance across a proton or neutron...from the Uncertainty Principle, he reasoned

Cthe Yukawa particle with a mass of about 100MeV, or 200xmelectron - it should interact fiercely with protons and neutrons and cause protons to stick together

pn

pπp

p n

∆ ∆E mc ht

hr c

m

≡ ≅ ≈

= ××

= ×

= × = × ≈

−

−

−

2

27

13 104

4 25

2 26 67 10

2 1 6 10 102 10

2 10 2 2 10 0 1

π π

π

/.

( . ) /. .

/c gcm/s

/c g m

2 2

2proton


1937: Discovery of the pion NOT,

Anderson and M.L. Stevenson (at Pike’s Peak) and indepedently, J.C. Street and R.B. Brode found more unusual things in cosmic rays

not protons - tracks too thinnot electrons or positrons - too penetratingmass estimates were about 200 times that of the electron!

They concluded that they had found a new particle - the “mesotron”...”meson”

•WWII hit Europe and Japan and led to the evacuation of many of the leading scientists and the isolation of Yukawa


cont.

•In Italy, M. Conversi, O. Piccioni worked in basements with black-market materials - fearing for their lives after the Nazi occupation

they invented and pasted together a circuit which could measure very short time differences

they determined that the lifetime of the “mesotron” was about 2 microseconds and that the negative mesons actually were captured in orbit around the nucleus before decaying - this couldn’t be Yukawa’s particle

Yukawa’s particle would not live so long.

In Japan, a theory was worked out which made the meson the decay products of Yukawa’s particle– but this wasn’t published until after the war...by then the

Americans had the story.


status:


photon discovered

quantum mechanics established

beta decay understood

neutrino established

neutron discovered

positron discovered

“mesotron” discovered

particle mass(MeV)

Q birthday

DISCOVEREDelectron, e 0.511 -1 1897photon, γ 0 0 1922proton, p 938.3 +1 1917neutron, n 939.6 0 1932positron, e+ 0.511 +1 1932mesotron 100 ±1 1937PREDICTEDYukawa 100 ±1,0 1935ANTICIPATED

neutrino, ν 0 0 1930


1940 - 1965:Years of Confusion

Synchrocyclotron produces α 1946 • Discovery of the pionpionpion 1947 • Discovery of the muonmuonmuon (1937) 1945 • Discovery of the first strange strange strange particles particles particles 1946-7 • Discovery of the ∆∆∆ resonance 1952 • Discovery of antiprotonsantiprotonsantiprotons 1955 • Discovery of the neutrinoneutrinoneutrino 1956 • Invention

of parity-violation 1956 • Discovery of parity-violation 1957 • Discovery of meson spectroscopy 1961 • Invention of quarks 1962

• Discovery of the Ω- 1964 • Discovery of partons 1969


1947: Discovery of the pion

Before WWI, Kinoshita, working in Rutherford’s lab noticed that alpha particles passed through a photographic emulsion leaving grains of developable emulsion

Cecil Powell, a student of Rutherford’s engineered a group with chemists and industry to produce a dense emulsion

They were taken to a French observatory, 3000m altitude– when developed a slow proton looked like a “solid rod of

silver” - under a microscope– stopping particles of mass near the mesotron were seen

almost immediately– Robert Marshak conjectured that Yukawa’s particle (dubbed

“pion”) decayed high up in the atmosphere into the “mesotron” (dubbed “muon”)

Yukawa → ”mesotron” → electron + unseen:

24 sheets, 2 cm2,50 microns thick

π µ νν ν

- → +→ + +

−

−e


1946/7: Discovery of the strange particles

Cosmic rays continued to be a fruitful source of surprises when coupled to cloud chambers and emulsion stacks

1946 - In Manchester, pictures showed the presence of “V”’s...the apparent production of a neutral particle which decayed into a pair

the chain appeared to be: neutral → 2 pionscalled the Kaon, or K0 with mass = 0.496 MeV. Another V was found which decayed into a pion and a nucleon, the Λ.

emulsions showed charged tracks which decayed into 3 pions, K ±

these particles were long-lived again, similar to the lifetimes of the neutron and the muon...early speculation began to relate them together as all members of the weak interaction of Fermi

Collectively, they were dubbed “Strange” and that name stuck

Cosmic rays had just about used up their usefulness.


Particle Accelerators

First, radioactive sources, then cosmic rays - both difficult, rare, and uncontrolled as “beams”Rely on electromagnetism to accelerate charged particles and to bend them where they are to go...

electric fields accelerate

magnetic fields bend

a television set is a little particle accelerator

Artificial beams were first produced in the late 1940’s in the form of cylotrons magentic field

beam

now, these accelerators are used for nuclear physics research

The best example in the world is the National Superconducting Cyclotron Laboratory here on campus


electric fieldcavity accelerates particlesin the beampipe

magnets all around the ring keep the beam goingin a circle

a detector sits insidethe tunnel where thebeams are forced to collidehead-on

protons orpositrons

antiprotons orelectrons

cont.

Higher energies and particle fluxes required a different approach, the synchrotron

a cartoon of a collidingbeam accelerator


Accelerators in the world

proton accelerators

in operation

Fermi National Accelerator Laboratory, Batavia, IL– proton-antiproton, 1000 GeV x 1000 GeV, few x 1031 cm-2 s-1

planned

Fermilab TeV33 2003?– proton-antiproton, 1000 GeV x 1000 GeV, 1033 cm-2 s-1

Large Hadron Collider, CERN, Geneva, Swizerland 2005?– proton-proton, 7,000 GeV x 7,000 GeV, 1034 cm-2 s-1

electron accelerators (plus upgrades)Large electron-positron (LEP), CERN– electron-positron, 55 GeV x 55 GeV, 1031 cm-2 s-1

Stanford Linear Accelerator Center (SLAC), Stanford Univ., CA– electron-positron, 50 GeV x 50 GeV, 4 x 1030 cm-2 s-1

Cornell Electron Storage Ring (CESR), Cornell Univ., NY– electron-positron, 6 GeV x 6 GeV, 3 x 1032 cm-2 s-1


1952: Discovery of ∆++, Fermi

Once accelerators were available, then the surprises began with every increase of available energy

In 1952 a group led by Fermi began taking data with artificially produced pion beams at Chicago - they were rewarded immediately with a surprise when plotting the number of scatterings for pions scattered from protons:

there seems to be an attraction for pions and protons at 180MeV kinetic energy of the pion.– Mathematically, this occurs for an intermediate state

of 1236 MeV -– the pion and nucleon “resonate” at this mass.

¥ This is a particle which lives a very short time - the ∆++

Resonance

pion K.E. MeV150 200 250

number of scatterings


1955: other resonances, strange particles, and antinucleons

quickly, other resonances appeared

The delta family had 4 members with similar masses: ∆++, ∆+, ∆0, ∆- . Analysis of these showed that they had spins which were 3/2!

The width of the resonance can be related to the lifetime through the uncertainty relation - 100MeV in the ∆, suggests a lifetime of 10-23 s

If the final state was restricted to 2 pions and a neutron or proton, then other resonances appeared - this time, with spins of 1, the “rho” and “omega”, ρ+, ρ-, ρ0, and the ω0

There were 3 pion resonances...etc.

there were new strange particles related to the “V” events, with lifetimes which were “long”..the Σ+, Σ0, Σ-. They decayed to a pion and a proton or neutron

The last lucky cosmic ray event was doubly strange: Ξ → Λ + π, strange decaying in a cascade to another strange particle, hence the name “cascade” - Ξ− and Ξ+

Finally, the antiproton and antineutron were found. - then it really got weird.


1956: Discovery of the neutrino, Cowan [1919-1974] and Reines[1918-]

While nobody doubted the existence of the neutrino, there was only evidence that something with its properties left many weak interactions!

Because of its rare interaction probability, it would take billions to create an interaction in matter. Reactors make large fluxes of neutrons...

n → p + e- + νe

νe + p → e+ + n

The experiment was very difficult and brilliantly conceived...to get 1 interaction every 20 minutes

Once that was accomplished, a detailed problem in muon decay was studied which showed that there were two kinds of neutrinos

those always associated with electrons and those always associated with muonscalled νe and νµ


1956 & 57: The downfall of parity

There were some anomalies in the patterns of strange particle decays that led two young theorists, C.N. Yang and T.D. Lee, to ask whether there had ever been any test of the conservation of PARITY in the weak interactions

This was a very odd notion and they only pointed out what tests might be done All such tests involve finding some aspect of a process which pick out a direction in space...and then attempt to do the experiment in a mirror– Co60 is an isotope which beta decays and as a nucleus has a

spin. (Define the spin direction by a Right Hand rule: fingers in the rotational direction, thumb is the spin direction.)

If parity is conserved, R = L for electrons up...

R is preferred! The mirror image is different!!

spin

spin

e e

mirror

L R


some jargon

“fermions” are any particles which have half-integer spins

“bosons” are any particles with integer spins

leptons are those particles like the electron and the muon which don’t have anything but electromagnetic or weak interactions

– they have spin 1/2, (hence, “fermions”) and can be massless– they come in pairs, lepton and its neutrino– when interacting Weakly, Parity is always violated...neutrinos

are inherently non-conserving of parity, they are “Left Handed”. Electrons and Muons are both, unless in a weak interaction...then they are LH also!

hadrons are those particles which also have a strong interaction...there are 2 kinds, both of which have mass

– mesons, which have spin 0 or 1 (hence, “bosons”) – baryons, which have spin 1/2, 3/2, 5/2, etc. (hence, “fermions”) – strange baryon and meson decays are Weak

finally, there is the photon– which has spin 1 (hence, a “boson”) and no mass


family relationships

Things are out of hand! By late 1960’s there were 100’s of “fundamental “ particles! There were clues, however, and unraveling them involved a similar kind of organizing as what led to Mendeleev-like classifications

We could classify particles by:

type of interactions - – no strong interaction: leptons – participates in strong interaction: hadrons

decay products– Does the final state contain protons or neutrons?

¥ a baryon → a baryon ...-always-

– Does the final state include leptons, mesons, and/or photons?¥ a meson → leptons and photons, always¥ a lepton → leptons and/or mesons, always


cont.

speed of decay

strange particles¥ easy to produce in strong interaction...very slow to decay¥ is there some quality which is nearly conserved?

STRANGENESS, S

internal dynamics

spin, Jelectric charge, descibed in language akin to spin...Isospin, I – can have quantized values, for whole families, which are

distinguished by the “I-3 component” of isospin which ranges from -I to +I in steps of one unit.¥ for example¥ the proton and neutron form a family of I=1/2...

¥ proton: I3 = 1/2¥ neutron: I3 = -1/2


classification

the labels in 1965 looked like thisparticle mass J S I I3 P B

photon γ 1 0

leptons e+,- 0.511 1/2 0 0µ +,- 105 1/2 0 0

ν 0 1/2 0 0mesons π +,-,0 140 0 0 1 ±1,0 - 0

K- 494 0 -1 1/2 ±1/2 - 0K0 498 0 -1 1/2 ±1/2 - 0

ρ +,-,0 770 1 0 1 ±1,0 - 0

η 548 0 0 0 0 - 0

ω 782 1 0 0 0 - 0baryons p, n 938 1/2 0 1/2 ±1/2 + 1

Λ0 1115 1/2 -1 0 0 + 1

Σ+,-,0 1190 1/2 -1 1 ±1,0 + 1

Ξ0,− 1318 1/2 -2 1/2 ±1/2 + 1

∆++,+,−,0 1232 3/2 0 3/2 ±3/2,±1/2

+ 1

Σ(1385) 1385 3/2 -1 1 ±1,0 + 1


1962: Invention of quarks and the discovery of the Ω, Gell-Mann [1929-]

the patterns among the hadrons were understood in within the realm of mathematics which deals with patterns, called Group Theory

Murray Gell-Mann and independently, Yuval Ne’eman found relations which linked things... for example

I3 = Q - (B + S)/2

1

-1

-2

-1/2 1/2 1 3/2-3/2 -1

pn

Σ0 Σ+

Σ0Ξ-

Σ-

Λ0I3

B + S

the “baryon octect”

plus related masses of one particle as compared to another...called the 8-fold Way by Gell-Mann because of the relevence of octets of objects


the quark model, 1964

Gell-Mann’s idea (also simultaneously with George Zweig) was to imagine the hadrons built up from pieces, called quarks

baryons would have three quarks

mesons would have a quark and an anti-quark

the quarks have quantum numbers, designed to work...not real!particle mass

MeV/c 2J S I I3 Q P B

photon γ 1 0 0

leptons e+,- 0. 511 1/2 0 ±1 0µ +,- 105 1/2 0 ±1 0

νe 0 1/2 0 0 0

νµ 0 1/2 0 0 0

π +,-,0 140 0 0 1 ±1,0 - 0quarks up, u (300)* 1/2 0 1/2 +1/2 +2/3 + 1/3

down, d (300)* 1/2 0 1/2 -1/2 -1/3 + 1/3strange, s (500)* 1/2 -1 0 0 -1/3 + 1/3

* so-called “constituent mass - a difficult point, actually


hypothetical building blocks of hadrons

baryons:

mesons:

u d

d

proton [uud] u

neutron [udd]

∆++ [uuu] uu u

Σ+ → p + π 0

d suu du

d du

π + [u anti-d] u

u u

spin


history repeats: periodic chart: remember scandium and germanium?

The missing particle’s quantum numbers were known, (quark content of sss) so its decay’s could be anticipated ... plus Gell-Mann predicted its mass to be 1680MeV

it was found very quickly, as predicted, with a mass of 1672MeV

1

-1

-2

-1/2 1/2 1 3/2-3/2 -1

B + S

I3

∆+(1232)∆0(1232)

Σ+(1385)

∆-(1232)

Σ0(1385)Σ-(1385)

Ξ-(1532)

∆++(1232)

Ξ0(1532)

Ω−

the “baryon decuplet” looked like this to Gell-Mann


1969: Discovery of partons, Kendall, Freedman, and Taylor

electron scattering at Stanford and Cornell had uncovered much about nuclear sizes

the electron was presumed to emit a photon with a wavelength inversely proportional to its energy

– the nuclear size measurements had photons which matched nuclear sizes

individual protons and neutrons were broken up, and Rutherford scattering emerged!– something inside the protons was much, much smaller than

the proton, and the higher energy photons– called PARTONS by Feynman

Could they be the quarks? “Yes” & “No” were the answers....

ee

γ

nucleus

as the energies of the electrons were increased - an amazing thing happened


Status

Where are we in 1965? ...a mess

anti proton and neutron found

neutrino found

Parity gone!

100’s of “elementary particles” established?

clearly 4 forces of nature:

electromagneticweakstronggravitational

a cute bookkeeping scheme called quarks

granularity within the proton?


Interlude: A MODEL for weak interactions, and a way to visualize


Richard Feynman [1918-1988]

Calculations involving relativistic quantum field theory are complicated

actually, one swims around in a particle-antiparticle world which directly involves a future and past blending

anti particles moving forward in time are mathematically equivalent to particles moving backwards in time

Feynman figured out a “game” for dealing with the details of these calculations involving drawing pictures according to rules

they function as a mneumonic for setting up the calculation correctly andthey function as a nice visualization for what’s happeningCalled Feynman Diagrams

subset of rules for us:

fermion lines are continuous and have arrowsarrows entering a diagram in the initial state are particlesarrows leaving a diagram in the inital state are antiparticlesand visa versa


examples of Feynman diagrams for processes we have seen...

electron-proton scattering

in quark language

Compton scattering

∆++ productionas a resonance

e

p p

γ

e e

u u

γ

e

e

e

γ

d

uu

u

d

proton

π +

π +

proton

∆ ++


Remember Uncertainty?

Recall that the Uncertainty Principle allows for a violation of the conservation of energy, as long as it happens quicker than a measurement could detect it...

This has been interpreted, with much evidence, that little “vacuum fluctuations” occur all the time - called “Virtual Particles”

this has a measurable effect on the charge and magnetic moment of the electron, measured very precisely...

typically, these are most relevent as fermion-antifermion pairs which come into and go out of existence

sometimes it results in virtual particles which manifest themselves as real particles

d

uu

u

d

proton

π +

π +

proton

π 0d

d

e +

e -


Connection with electrodynamics

The idea that quantum electrodynamics relied on a local interaction, mediated by a spin-1 boson (the photon) had certain appeal

recall Heisenberg’s notion of an exchange force and Yukawa’s implementation

In the late 1950’s, Feynman and Gell-Mann put this idea on a mathematical footing for the Weak Interactions

They anticipated that the weak force also was propagated by a spin-1 boson but it was different from the photon.

¥ It had to be:

– electrically charged– very massive (the weak force is propagated over short

distances)– capable of changing isospin...ie, it can change one particle into

another within isospin families (within other conservation requirements)

dubbed the “W Boson”


Other processes we have seen

With the W, reactions we have discussed look like this...

np

eW -

ν

d u

eW -

ν

d

u

neutronproton

neutron beta decay

quark language

eW -

ν

u

d W -π + µ +

νν

pion decay chain


1965 - present:Years of Confirmation...and surprises?

Weinberg’s “standard model” 1967 • Discovery of charmed quark charmed quark charmed quark 1974 • Discovery of neutral currents 1974

• Discovery of the tau tau tau 1974 • Discovery of the bottom bottom bottom quark quark quark 1977 • Discovery of the gluongluongluon 1977 • Discovery of

the WWW and ZZZ 1983 • Discovery of the top quark top quark top quark 1995


similarities between weak and electromagnetic interactions

some of them are subtle and technical, however

both are tractable theories with the common notion of a spin 1 mediating boson (photon and W )

there is a bit of magic

W - W -

γ γ

e

e

eν

p

p

np

neutrons and protons are “fuzzy” messy objects with an infinity of virtual particles

the photon couples to themessy proton with the same strength as to the bare electron

the W couples to themessy proton with the same strength as to the bare electron

technical


1967: Standard Model, Weinberg [1933- ]

This was enough for Stephen Weinberg to suggest that the weak and electromagnetic interactions are the same

probably in the early universe - which underwent a phase transition to the current epoch where we see them as being very different

This required the postulation of another spin 1 boson and another kind of weak interaction– called the Z 0

– with a mass which is directly related to the mass of the W– and a set of predictable new interactions due to the coupling of

the Z 0 to other fermions¥ Prediction: it couples to the same particles as the photon!

Anywhere there is an electromagnetic interaction, mixed in will be this much weaker Weak interaction

Great idea...he published this model in a 3 page paper in 1967, all predictions, the whole thing.

totally ignored (3 citations in all physics literature before 1970)

until...


1973: Discovery of neutral currents

The Z will also couple to neutrinos, alone.

“neutral current interactionswere found in 1973

ν

e e

ν

Z

Weinberg’s modelincluded the following

ν νµµ γe

eud s

W Z h

±, , 0

obviously, there is a hole in the pattern...there was a suggestionin 1970, but it was notorious for the whimsical manner in whichit was proposed

This lent credibility to Weinberg’s model...and led to scutiny

(h is called the Higgs particle...see below)


1974: Discovery of Charmed mesons, Richter[1931- ] and Ting [1936- ]

So much for whimsy:

In 1974, simultaneously at Stanford and Brookhaven on Long Island events were seen which clearly suggested that a new quark state had been found -

called “Charm”killed any other model in particle physics which did not include quarks as real entities

Now the situation was:

ν νµµ γe

eudcs

W Z h

±, , 0

The masses of the quarks were approximatelymu 300 MeVmd 300 MeVms 500 MeVmc 1500 MeV

this lasted a year...


1974: Discovery of heavy leptons, Perl [1927- ]

when at Stanford

a new state hidden inside the charm data was shown to be another charged lepton!

called τ (“tau”)– we understand about electrons– the muon is nothing more than a “heavy electron”, 200 times

more massive - that was troubling - why?– this was an even MORE massive electron!

¥ mass of the tau is 3500 times the mass of the electron, 1.8GeV!

now the situation was:

– the tau neutrino has still not been seen directly...only indirectly

ν νµ

ντ

µ τ γee

udcs

W Z h

±[ ] , , 0

this lasted 3 years


1978: Discovery of Bottom mesons, Lederman [1922- ]

when at Fermilab, in an experiment similar to that of the charm discovery approach, a new state was found...

It had to be another quark

called BOTTOM - it is very heavy...

now the situation was:

The masses of the quarks and leptons were then approximatelymu 300 MeV mν e 0 MeVmd 300 MeV me 1/2 MeVms 500 MeV mν µ 0 MeVmc 1500 MeV mµ 107 MeVmb 5000 MeV mν τ 0 MeV

mτ 1800 MeV

ν νµ

ντ

µ τ γee

udcs b

W Z h

±[ ] , , 0

see the pattern? this lasted for 8 years


1983: Discovery of the W and Z: Rubbia [1934- ]

There were other models which accomodated the data in the late 1970’s

The predictions of Weinberg depended on two things

The existence of the ZThe interactions of the ZThe relation between the masses of the W and the Z– connected through a single parameter, typically expressed as

an angle which was measurable in many different reactions

This became a world-wide project - to increase the precision of the “Weinberg Angle” determination and pin down the Z mass.

By 1981, we knew that MW had to be around 80GeV and that the Weinberg angle was around 39º which suggested that MZ had to be in the neighborhood of 90.5 GeV.

cos22

2ϑWMMW

Z

=


cont.

By 1983 the new accelerator under the Jura mountains at CERN in Geneva, Switzerland was operating well and two experiments designed to detect W and Z’s were taking data

January, 1983 the laboratory announced that both had been discovered through their characteristic decays signatures

the masses were right on...

but what about the hole? the partner of the bottom quark? Uncertainty provided another clue -

W -e

ν

Ze

e

Ze

e

Zfermion

antifermion

“fermion” is any known or unknown particle! Thereare calculable effects for massivefermions


cont.2

The effects would be found in very, very precise measurements of a variety of quantities - most notably the mass of the W and the mass of the Z.

For 5 years the mass of the W has been pursued to a precision of 0.3% by my colleagues and I at Fermilab.

The mass of the Z has been known to a precision of about 0.1% by work at CERN...

In the first days of this effort, many thought that the mass of the top quark might be as much as 40 GeV...then the precision measurements began to emerge– firm lower limit of 80GeV by 1989– the direct searches began to raise that lower limit a bit at a

time, until we knew that it had to be larger than 130GeV!

This is very exciting - the Standard Model goes right to the heart of what it means for an elementary particle to have mass. The W and Z are crucial to this argument..

and here’s a crummy quark with a mass signficantly larger than them!!


Higgs

The Higgs particle is central to this...and relevent to the old questions of the ether - here’s a poor analogy to what the mathematics suggest -

suppose you are viewing someone walking up the stadium steps from far off

they are proceeding briskly, but suddenly they slow down and I ask you to tell me from what you remotely observe whether– a. The steps abruptly got sticky or– b. The climber suddenly found her pockets full of lead (forget

how for the sake of argument!)

An increase of the climber’s mass could be indistiguishable from having to slog through a sticky, viscous environment

The Standard Model presumes an early universe in which the stairs are clean (or the pockets, empty) - all massless particles

However, something happens and the medium through which all particles move, becomes viscous– We see this as an apparent being given to the particles - W and

Z in particular


Unraveling the vacuum

Our environment is The Vacuum.

The model presumes that it is not empty

Rather, like the old days, that it is full - of a special particle called the Higgs particle

The Higgs comes about, so the story goes, because everything in the universe underwent a phase transition (like Ferromagnetism, Superconductivity, Superfluidity, etc.) long ago

Before this, the weak and electrmagnetic interactions were identical and the ancestors of the W and Z had no massAfter this, the two interactions separate into the distinct forces of nature that we measure in our currently cold universe

The model makes predictions regarding the relationships among the masses of the W, the Z, the Higgs, and all quarks -

especially the heaviest ones, bottom and top

The apparently large top mass plus the ability to measured these correlations form the most important experimental problems today.


particle detectors

A quick introduction to detecting elementary particles

Rely on basically two interactions: electromagnetic & strong

electromagnetic interaction– tendency for all electrically charged particles to ionize media

through which they travel¥ pick the right medium for specific purposes

want to only slightly perturb the path? stop the particle in a burst of ionization? depends on the jobat hand.

¥ need to collect that ionization and detect it as a small current

want to amplify the ionization before detection? or detect small amount, and amplify the detected signal? depends on job at hand and tradeoffs.

in bulk material,electrons will produce“electromagnetic showers”


cont.

strong interaction– tendency for hadrons to interact with nuclei of materials, which

in turn interact strongly, and electromagnetically...eventually resulting in ionization, which can be detected.

quarks will “dressthemselves intoparticles like pionsbefore they can bedetected

in turn, pions (or protons, etc) willinitiate “hadronic showers” in densematerial

quarks initate “jets”


position determination

a single tube ionizationcounter

signal

charged particle

layers can be usedto measure position - a modern experimentwill have thousands of such wires and electricalchannels


calorimetery

by totally absorbing a particle, causing it to deposit all of its energy of motion into ionization, and measuring that ionization...the energy of the particle can be determined

given the name “calorimetry”

by counting the hitcells, or more usually,by measuring the total amount of signal one has a measure of the energy of the passing particle...when allit has stopped and it’s secondaryparticles have been absorbed.

absorber,which causesmore interactions

a modern detector would have tons of absorber and thousands of readout channels


collider detectors

proton beam

antiproton beam

magnetizediron

muon detectioncounters

hadron calorimeter (determinationof quark energies)

electromagneticcalorimeter (determinationof electron energies

trackingchambers


Hints:

quarks produce “jets”, which induce broad hadron showers in material

electrons shower into electromagnetic showers in high Z material

neutrinos escape without interacting

leave telltale signature - they take away momentum which should be conserved in the direction perpendicular to the beams– called “missing Et” for transverse energy

muons are very penetrating and go through everything leaving only minor ionization along their paths -

often momentum analyzed by forcing them through magnetic fields and measuring the bend


signatures for various final states

p + pbar → Z + very little elseZ → e + e 2 electromagnetic showers

p + pbar → W + very little elseW → e + ν 1 electromagnetic shower

missing Et

p + pbar → quark +antiquark + very little else2 hadron jets


Signposts for Top

How would Top manifest itself?The reaction is p + p → t + t produced in pairs

First of all, that top can be produced at all is an striking example of Einstein’s famous equation -

It is as if we took two 2lb bowling balls, rolled them together and produced two 400lb wrecking balls! An enourmous energy of motion is required to transform into the large top quark masses.

The top decay chain is distinctive

it actually decays faster than it has time to dress itself into a ‘real particle’...like a pion– It always decays into a bottom quark and a W (if it is the

“standard model top”)– The decay string looks like muon decay, except that it

produces a real W, which itself decays -

t → b + W → e + ν or → µ + ν or → quark + antiquark

→ c + e + ν or → c + µ + ν


top signatures

p + pbar → top + antitopt → W + b with W → e + ν em sh & missing Et

b → c + µ + ν jet & muon

t → W + b with W → µ + ν muon & missing Et

b → c + (e) + ν jet– called the dilepton mode: dileptons + missing Et plus 2 jets

¥ 5% of the decays, clean

p + pbar → top + antitopt → W + b with W → e + ν em sh & missing Et

b → c + (µ) + ν jet

t → W + b with W → q + qbar 2 jets

b → c + (e) + ν jet– called the lepton+jet mode: high energy lepton+missing Et plus

4 jets¥ 30% of the decays, can be mimiced by other processes

( ) means “missed”


1995: Discovery of the top quark

The two experiments at the Fermilab collider frantically searched

We collected data for three straight years of running, 24 hours a day, 7 days a week.

A superhuman analysis effort actually kept up with the data-collection...including another huge effort of simulating the experiment in computer codes which actually take longer to run than the data take to collect!

must simulate all scenarios, true ones, as well as possible fake ones

D0, our experiment found a single event and published it in 1992...suspicious, but one event is not definative

CDF, the other experiment, thought they had “evidence” in spring of 1994

published it, cautiously - the rate of production was too high, according to expectations by x2


discovery

In the winter of 1994-95 we began to think we had it

we broke into two groups, one led by me and the other led by another MSU physicist, Harry Weerts– usually, these kinds of groups review work done before

publication– these two groups were different, we were to push and evaluate

the work as it was being done

we thought we were on to something and wanted to be first, but we needed to be sure– that meant 2 months of furious argment, calculations, writing,

and yelling– “Was it background?” “How signficant was the signal?”– Things fell apart, and came together over and over.

The laboratory management was monitoring both experiments’ progress and managed to let enough information leak in both directions that the pace to deciding was similar.


finally

On Friday, February 24, 1995, after a week of shared information, the discovery was announced and two papers were submitted to the Physical Review Letters.

The earlier CDF “evidence” was partially correct, but there was a mistake which made the rate too high

The D0 first event, indeed, turned out to be Top - a near-perfect event.

The mass of the top, is huge.

CDF: mt = 176+-8+-10 GeV/c2

27 events over background of 7

D0: mt = 199+-20+-22 GeV/c2

17 events over background of 3.8

Currently, but experiments have more data and better analysis, but neither is yet quoting a new mass, although we now think that

mt = 170+-15+-10 GeV/c2


status

Here is the zoology of our world, right now...

particle massMeV/c 2

J S I I3 Q P B

forcecarriers

γ 0 1 0 0

W 80,330 1 ±1 0Z 91,188.4 1 0 0gluon, g 0 1 0 0

leptons e 0. 511 1/2 ±1νe 0 1/2 0

µ 105 1/2 ±1

νµ 0 1/2 0

τ 1800 1/2 ±1

ντ 0? 1/2 0quarks up, u (300)* 1/2 0 1/2 +1/2 +2/3 + 1/3

down, d (300)* 1/2 0 1/2 -1/2 -1/3 + 1/3strange, s (500)* 1/2 -1 0 0 -1/3 + 1/3charm, c 1500 1/2 0 0 +2/3 + 1/3bottom, b 5000 1/2 0 0 -1/3 + 1/3top, t 170,000 1/2 0 0 +2/3 + 1/3


status, cont.


quark-partons established

weak force = electromagnetic force in the early universe

the top quark is found

a number of other successes have established that Weinberg’s first, simple model is still consistent with all data.

It raises huge questions....

MASS - what is the “cause” of it? Why are there such differences?

Where is all of the antimatter in the universe?

Why are there different spins? Is there a similar unification of bosons with fermions? “Supersymmetry” is a theory that does that.

Does the neutrino have mass?

Is there a Higgs particle...is that the right mechanism?

More accelerators are coming...MSU is deeply involved in the physics of this generation as well as the next generation.

quark of the getting to the - michigan state university · 1998-11-07 · 1 bottom of the top quark...

Documents