Quarks, Leptons and the Forces of Nature Paul Watts Particle Physics Masterclasses NUI Maynooth Wednesday, 19 March 2014 NUI MAYNOOTH Ollscoil na hÉireann Má Nuad Paul Watts Quarks, Leptons and the Forces of Nature
Quarks, Leptons and the Forces of Nature
Paul Watts
Particle Physics MasterclassesNUI Maynooth
Wednesday, 19 March 2014
NUI MAYNOOTH Ollscoil na hÉireann Má Nuad
Paul Watts Quarks, Leptons and the Forces of Nature
Introduction
Mankind has always known the universe was made of “stuff”. Butwhat is the “stuff” itself made of? And why do we care?
A Scientific Reason: Because we want to know what theactual nuts and bolts of the universe are, and how and whythey fasten together in the way they do.
A Philosophical Reason: By knowing what makes up the restof the universe, we learn about what we ourselves are made ofand how we might fit into the big picture.
A Practical Reason: Knowing how the universe itself is puttogether may help us create new means and technologies tobetter our lives.
A Selfish Reason: Because it’s interesting!
Paul Watts Quarks, Leptons and the Forces of Nature
Particle Physics: The Beginning
Leucippus and Democritus (c. 400 BCE)
“Matter is not infinitely divisible!”ατoµoζ – atomos, “uncuttable”
But it didn’t catch on, so the idea of fundamental bits of matterfell out of favour for nearly two millenia...
Paul Watts Quarks, Leptons and the Forces of Nature
The Return of the Atom
John Dalton (1805)
Atoms were revived as the fundamentalpieces of matter, with the added idea thatthere were a finite number of kinds classifiedby their weight.
Dmitri Mendeleev (1869)
When elements werealso grouped accordingto their chemical prop-erties, the periodic ta-ble was born.
But were these “new” atoms really indivisible?Paul Watts Quarks, Leptons and the Forces of Nature
The First Subatomic Particle
J. J. Thomson (1889)
Heating the cathode in a Crookes tube lead to the emission of raysof electrically-charged particles.
Thomson determined these rays had negative charge, and namedthem electrons.
Paul Watts Quarks, Leptons and the Forces of Nature
The Nucleus
Ernest Rutherford (1909, 1920)
The positive chargein an atom was notevenly distributed,but concentratedin the centre: thenucleus.
This charge was bound up in particles he named protons. The restof the mass of the nucleus was theorised to be due to undiscoveredneutral particles, and in 1932, neutrons were found.
Paul Watts Quarks, Leptons and the Forces of Nature
The New Physics
These new particles were very, very small and could move very,very fast. Luckily, two new theories covered both...
Special Relativity explained the phenomena that occurredwhen physics took place near the speed of light, and pointedtoward deep connections between mass, energy andmomentum.
Quantum Mechanics gave a new description of physics atextremely small scales, explaining the structures of atoms andmolecules.
Although bizarre and counterintuitive to many of the world’s bestscientists, the two theories were astoundingly successful.
Paul Watts Quarks, Leptons and the Forces of Nature
Quantum Mechanics + Relativity, Part 1: Spin
When combined, the two theories predicted a new fundamentalproperty of matter (like mass and electric charge), which comes inchunks of 1/2 and divides all matter into one of two types:
spin = 1/2, 3/2, 5/2, . . .Fermi-Dirac particles, or fermions
spin = 0, 1, 2, . . .Bose-Einstein particles, or bosons
Paul Watts Quarks, Leptons and the Forces of Nature
Quantum Mechanics + Relativity, Part 2: Antimatter
Another prediction was that every fermion had a near-exactduplicate, differing only in the sign of its electric charge. Thesenew particles were called antimatter.
The first of these antipaticles to be foundwas the counterpart to the electron, thepositron e+. The antiproton p and antineu-tron n followed in 1955 and 1956.
Carl Anderson(1932)
Paul Watts Quarks, Leptons and the Forces of Nature
A Mysterious New Particle
When neutron decay – into a proton and electron – was observedin 1934, the energy didn’t add up: the total energy of the finalparticles was less than that of a neutron. The proposed solution?A third particle with zero mass and charge was being produced; a“little neutral one”, or in Italian, neutrino.
The very hard-to-detect neu-trino wasn’t found until over20 years later!
Clyde Cowan and Frederick Reines (1956)
Paul Watts Quarks, Leptons and the Forces of Nature
The Particle “Zoo”: Things Get Messy...
As detectors and accelerators got more sophisticated in the ’30s,’40s and ’50s, more and more particles were discovered...
Baryons (large masses, spin 1/2, 3/2, . . .):n, p, Λ, Σ, Ω, ∆, Ξ, . . .
Mesons (medium masses, spin 0, 1, . . .):π, ρ, K , K , η, φ, ω, . . .
Leptons (small masses, spin 1/2):e−, νe , µ−, νµ, . . .
I. I. Rabi, on the muon: “Who ordered that?”
Paul Watts Quarks, Leptons and the Forces of Nature
The Zoo Explained: Quarks and Leptons
Murray Gell-Mann and George Zweig (1964)
The fundamental particles of matter are allfermions: the well-known leptons (electrons,neutrinos, etc.) plus a new type, quarks,where
baryons are made of three quarks;
mesons are made of a quark and anantiquark.
Paul Watts Quarks, Leptons and the Forces of Nature
New “Periodic Tables”
The three most common quarks – the up, down and strange – cancombine according to these rules to group the most commonbaryons and mesons into natural families:
The less common quarks – the charm, bottom and top – onlyappear in very rare particles.
Paul Watts Quarks, Leptons and the Forces of Nature
What is a Force?
Newton’s Second Law (1687)
”The alteration of motion is ever propor-tional to the motive force impress’d; andis made in the direction of the right linein which that force is impress’d.”
In other words,
~F = m~a
A force causes a mass to change speed and direction. How doesthis work at the subatomic level?
Paul Watts Quarks, Leptons and the Forces of Nature
Forces as Particle Exchange
By a process of photon emissionand absorption, two electrons canrepel each other, as illustrated bya Feynman diagram.
All forces are due to an exchange of special particles called gaugebosons; the photon is the gauge boson which “carries” theelectromagnetic force.
So forces are now described as interactions that determine whichparticles can absorb, emit, combine with or decay into each other.
Paul Watts Quarks, Leptons and the Forces of Nature
Conservation Laws
Allowed interactions are restricted by conserved quantities, whosetotal sum after the interaction must be the same as before. Apartfrom energy and momentum, there are other “quantum numbers”:
Electric charge Q
Baryon number B
electron number Le , muon number Lµ and tau number Lτ
These are conserved in every physical process that we know of.
Paul Watts Quarks, Leptons and the Forces of Nature
Quantum Numbers
leptons
Q B Le Lµ Lτ
e− -1 0 +1 0 0
νe 0 0 +1 0 0
µ− -1 0 0 +1 0
νµ 0 0 0 +1 0
τ− -1 0 0 0 +1
ντ 0 0 0 0 +1
quarks
Q B Le Lµ Lτ
u +2/3 +1/3 0 0 0
d −1/3 +1/3 0 0 0
c +2/3 +1/3 0 0 0
s −1/3 +1/3 0 0 0
t +2/3 +1/3 0 0 0
b −1/3 +1/3 0 0 0
Antileptons and antiquarks have the opposite sign for all quantumnumbers, and Q = B = Le = Lµ = Lτ = 0 for all bosons.
Paul Watts Quarks, Leptons and the Forces of Nature
The Electromagnetic Interaction
Felt by all particles with electric charge
Responsible for atomic structure and chemical properties
Manifests as visible light, radio waves, microwaves and X-rays
Carried by the photon γ, a massless neutral spin-1 boson
Example: electron-positron annihilation into two photons,e− + e+ → γ + γ
Paul Watts Quarks, Leptons and the Forces of Nature
The Strong Interaction
Felt by all hadrons, namely, particles composed of quarks andantiquarks
Responsible for nuclear structure
Becomes stronger as the separation between quarks increases
Carried by the gluon g , a massless neutral spin-1 boson.
Example: creation of a neutral pion from a proton-proton collision,p + p → p + p + π0
Paul Watts Quarks, Leptons and the Forces of Nature
The Weak Interaction
Felt by all leptons, quarks and their antiparticles
Responsible for neutron decay and β-radiation
The shortest-ranged of the four forces; negligible at distanceslarger than a nucleus
Carried by three heavy spin-1 bosons, the neutral Z and thecharged W+ and W−
Example: neutron decay into a proton, electron andelectron-antineutrino, n→ p + e− + νe
Paul Watts Quarks, Leptons and the Forces of Nature
The Gravitational Interaction
The most familiar of the four forces; felt by all known particles
Responsible for planets, stars and galaxies
At short distances, the weakest of the four forces; utterlynegligible at subatomic scales
The only force which has not been successfully explainedusing quantum mechanics
Hypothesised to be carried by a massless, neutral spin-2boson, the graviton
Example: none known yet at a subatomic level, but many knownat planetary scales and larger
Paul Watts Quarks, Leptons and the Forces of Nature
The Question of Mass
Peter Higgs and Francois Englert (1964)
Higgs and Englert proposed a processby which particles interact with a bo-son which “slows them down” as theytravel through space, giving them aneffective mass: the Higgs mechanism.
The discovery of a heavy, neutral spin-0 particle which seems toplay this role was announced by CERN last year, and has beententatively identified with this elusive “Higgs boson”, H.
Paul Watts Quarks, Leptons and the Forces of Nature
The Standard Model
The most successful and comprehensive theory of fundamentalparticles and forces that physics has yet thought of...
ElectromagnetismStrong ForceWeak ForceHiggs Mechanism
The Particles The Interactions
Paul Watts Quarks, Leptons and the Forces of Nature
The Order of the Day: W -Physics
We will spend the afternoon looking at some of the reactionspredicted by the Standard Model that involve W -bosons:
W+ decay
W+ → e+ + νe or W+ → µ+ + νµ
W− decay
W− → e− + νe or W− → µ− + νµ
Higgs production from gluons, then decay into leptons via twoW s
g + g → H → W+ + W− → `+ + ν` + `− + ν`
(Note that all of these conserve Q, B, Le , Lµ and Lτ !)
Paul Watts Quarks, Leptons and the Forces of Nature
A Few Pluses and Minuses of the Standard Model
Good:
In terms of explaining past observations and making testablepredictions, one of the most successful scientific theories ever
Explains why electromagnetism and the weak force seem to beintimately related to one another: the electroweak interaction
Less Good:
Many of the parameters describing the interactions seemarbitrary and have to be put in by hand
Requires neutrinos to be massless, despite experimentalevidence that they may not be
Paul Watts Quarks, Leptons and the Forces of Nature
Current Questions in Particle Physics
Can a theory which includes both quantum mechanics and thegravitational force be found? String theory and loop quantumgravity claim so, but no evidence for either yet.
The evolution of the universe points toward the existence of“dark matter” and “dark energy”. What are they? New formsof matter and energy, or old ones in disguise?
Electromagnetism and the weak force seem to be differentaspects of the same interaction; is the same true of the strongforce? Is there a “grand unified theory” that combines themall?
Why is there so much more matter than antimatter in theuniverse? What is the theoretical reason for this profoundasymmetry in the makeup of the universe?
And finally...
Paul Watts Quarks, Leptons and the Forces of Nature
...The Biggest One of All
When will Ireland joinCERN?
Thank you!
Paul Watts Quarks, Leptons and the Forces of Nature