Deep inside the Atom

evidence that suggests that protons and neutrons are not fundamental particles

Deep Inelastic Scattering

Location: At SLAC (Standford Linear Accelerator)

Method:

• Liquid hydrogen was kept at a very cold temperature  kept protons close together

• Electrons were accelerated using electric fields towards the hydrogen

• The scattering angle of electrons that hit the hydrogen were detected using a detector

• They knew the momentum and velocity of the scattered electrons by measuring their curvature in a magnetic field

Hypotheses:

• The electron’s wavelength is small enough, it can penetrate the proton

  • If proton has nothing inside, the electron that passes through will not lose much kinetic energy = elastic scattering will occur

  • If the proton consists of subatomic particles, the electron will collide with them and lose kinetic energy = inelastic scattering will occur

Deep inelastic scattering

Results:

• Result showed deep inelastic scattering

  • Scattering patterns were identified as being caused by particles inside the protons

• Sometimes the electrons caused the proton to shatter and hadronization to occur

  • Where quarks and gluons were emitted out of protons

  • Caused other hadrons, like pions, to be created

• Demonstrated that the proton was not a fundamental particle

Neutrons

The same process occurred for neutrons, showing that they were not fundamental particles

the existence of subatomic particles other than protons, neutrons and electrons

• Muon discovery

Discovered by Anderson and Neddermeyer while studying comic radiation

• Cosmic rays originate as primary cosmic rays

• Formed due to various astronomical processes

• Primary cosmic rays are composed of alpha particles and a small proportion of heavier nuclei (<1%)

• Primary cosmic rays decay into secondary cosmic rays.

• Secondary cosmic rays consist of sub-atomic particles

  • E.g. photons, leptons, hadrons (protons and neutrons), electrons, positrons, muons and pions.

A magnetic field was applied in the cloud chamber containing cosmic radiation.

• Field interacted with molecules in chamber

• Sub-atomic particles were produced from cosmic rays

• This released a particle with a deflection trail similar to an electron

  • Same direction (negative charge)

  • Less deflected → Radius was bigger (larger mass, assuming same charge of -1)

  • Was later confirmed muons are 200x heavier than electrons.

Were initially thought to be mesons but since they do not interact with gluons (strong force), they are leptons

the existence of subatomic particles other than protons, neutrons and electrons

• Electron neutrino discovery

Initially, it was thought that only the beta particle was emitted during beta-decay

• Due to mass difference between products and reactants, we can calculate these particles are meant to have

• The energy absorbed by the beta decay products of radium was measured

• Ejected electrons heated up water surrounding radium

  • Used change in temperature to calculate energy absorbed

  • Results showed that the energy absorbed was lower than the calculated energy using the mass defect and E=mc²

  • The total energy released could not be accounted for by the energy of the beta particle

  • This violated the law of conservation of energy

• Pauli proposed that there must be another particle emitted with the beta particle that carries the “missing” energy

  • Particle must have a small mass and not interact much with surroundings (neutral, highly penetrating)

• Existence was confirmed in 1931 in nuclear fission reactions  named the (electron) neutrino

Standard Model of Matter

describes the fundamental particles of matter + their interactions

Consists of two major types of fundamental particles, each with different flavours

• Matter particles (fermions: leptons and quarks) and force-carrier particles (bosons)

  • 6 flavours of leptons

    • no colour

  • 6 flavours of quarks, which make up hadrons (mesons and baryons)

    • Each quark could have one of three different colours (red, green and blue)

  • 5 flavours of bosons

    • Can also possess colour, except Higgs boson

• All leptons and quarks have a corresponding antiparticle, with the same mass but opposite charge

  • Antimatter = same mass but opposite charge

Note: does NOT describe gravity

Evaluation of the standard model

Quarks

combine to make other particles → cannot exist in isolation

• Matter particles

• Experience all four fundamental forces

• There are six flavours of quarks, grouped into three generations.

  • First generation: up and down (lightest, most stable, most abundant)

  • Second generation: charm and strange

  • Third generation: top and bottom (heaviest, rarest, least stable & decay into 1st gen)

• Have fractional electric charges

  • Up, charm, top: +2/3e

  • Down, strange, bottom: -1/3e

• Antiquarks are the antiparticles to quarks → same mass but the opposite charge

Hadrons

particles made from a combination of quarks

• All hadrons must be colour neutral  colour of quarks combine to make white

  • Red + green + blue = white

  • Colour + anti-colour = white (e.g. red + anti-red)

• Quarks are held together by gluons, which also have colour

• Must have integer charge

two types: baryons and mesons

Baryons

Consist of three quark/antiquark particles.

• Protons (uud)

  • Consists of two up quarks (uu) and one down quark (d)

  • Total charge = +1e

• Neutrons (udd)

  • Consists of one up quark (u) and two down quarks (dd)

  • Total charge = 0e (neutral)

• Lambda (Λ) particle (uds)

  • Up, down and strange quark

  • Total charge = 0e

  • Heavier than a proton

  • Unstable and decays into

    • A negative pion and a proton

    • A neutral pion and a neutron

Mesons

Consist of one quark and one anti-quark.

Pions are down to play, but not kaons since they are strange

• Positive pions, 𝜋+ (ud̄)

  • Consists of one up quark and one antidown quark

  • Total charge = +1e

• Negative pions, 𝜋- (ūd) → antimatter of 𝜋+

  • Consists of one antiup quark and one down quark

  • Total charge = -1e

• Neutral pions, 𝜋0 (dd̄, uū) → VERY unstable (decay into gamma rays)

  • Consist of either

    • A down and an antidown quark

    • An up and an antiup quark

    • Quantum superposition

  • Total charge = 0e

• Positive kaon, K+ (us̄)

  • Consist of up and anti-strange particle

  • Total charge = +1e

• Negative kaon, K- (ūs) → antimatter of K+

  • Consist of antiup and strange particle

  • Total charge = -1e

pions are formed during proton and antiproton collisions

Leptons

• Matter particles

• Does not experience all fundamental forces

  • Do not have colour = cannot feel strong force = cannot be held in nucleus

    • Leptons don’t make up larger particles

  • Neutrinos are neutral = cannot experience EM force

• Have three generations, dividing them into pairs

  • First generation: electron and electron neutrino (lightest)

  • Second generation: muon and muon neutrino

  • Third generation: tau and tau neutrino (heaviest, most unstable, decay into 1st gen)

• Are integer multiples of elementary charge (0 or -1)

• Obey Pauli’s Exclusion principle

• Leptons have corresponding anti-leptons

Note: antimatter for electron is positron

Bosons

Force mediating/carrier particles

• Bosons mediate the forces and interactions between quarks and leptons

• Bosons and their forces have different relative strength and range of influence

E.g. In a proton, the up quarks repel each other, but since the gluons are stronger than the photons at that small range, they remain together

Types

• gluon

• photon

• W±, Z

• Higgs (noone gaf)

Gluons

Mediates the strong force (in nucleus of atoms)

• Force between quarks

  • Glues them together to form hadrons

  • Allows interactions with different nucleons as well

• Only effective in a short range (1fm)

• Massless

Photon

Mediates the electromagnetic force

• Force between charged particles

  • Electromagnetism is due to virtual photons being transferred between charged particles → e.g. electrostatic repulsion is due to protons exchanging virtual photons

  • Virtual particles pop in and out of existence, due to Heisenberg’s uncertainty principle

• Act over an infinite range

• Massless

W and Z Bosons

Mediates the weak force

• Responsible for the flavour and charge changes of particles

  • e.g. quark/radioactive decay, neutrino oscillations, electron capture to make a proton a neutron, annihilation

  • Allows quarks and leptons to interact

• Act over a short range → these bosons are unstable and decay

• Have mass and are relatively heavy

• W+ and W- are the only bosons to have charge

W+ bosons

q=1e

• Mediate beta-plus decay → allows charge to be conserved

  • An up quark in a proton is converted into a down quark

  • Causes the proton to become a neutron

• Produces a positron and electron neutrino  conservation of lepton number

W- Bosons

q=-1e

• Mediate beta-minus decays  allows charge to be conserved

  • A down quark in a neutron is converted into an up quark

    • W- boson allows charge to be conserved

  • Causes the neutron to become a proton

• Produces an electron and electron antineutrino

Z⁰ bosons

q=0e

• Facilitating neutral current interactions → exchange energy but not change charge

• Responsible for transferring momentum

• E.g. High energy annihilation between electron and positron produces a Z boson which quickly decays to form a muon neutrino and muon antineutrino.

Higgs Boson

• They have no direction (scalar), no charge, no spin BUT DO have mass.

• Was predicted before its discovery in a particle accelerator (Large Hadron Collider) in 2013

• The Higgs field (provided by the Higgs boson) accounts for the mass of W and Z bosons

Types of Particle Accelerators

• linear

• cyclotron

• synchotron

Linear accelerators (LINAC)

uses electric fields to speed up tarticle

1. A charged particle travels through segments (tubes) alternately polarised by AC power source

2. When the particle travels through each charged tube, the polarity of the next tube changes

3. ∴ An electric field is present between adjacent tubes

4. When the particle travels through the gap of the tubes, the electric field created accelerates it

5. The length of each successive tube increases further along to accommodate for the particle’s increasing speed

   1. Want to spend the same amount of time in each tube, so that the frequency of the AC power aligns with when the particle leaves the tube

   2. allows particle to accelerate

   3. Increased velocity = increased distance for same time

Examples: SLAC

Van de Graaf accelerator

1. A belt rubs off electrons from the surrounding conductor

   1. As it moves along, it becomes neutralised by the spray supply, so that it can keep removing electrons

2. This makes conductor increasingly positive

3. This pushes positive ions down the accelerator tube

Cyclotrons

1. Two hollow D shaped structures called the ‘dees’

2. Dees are separated by a gap

3. An electric field is created in the gap by a potential difference

   1. When the charged particle travels across the gap from one dee to another, electric field accelerates it

   2. Polarity changes to allow the particle to accelerate to the other dee when it turns around

4. Magnetic fields are present in the two dees

   1. Particles undergo UCM in magnetic field

5. The particle follows an outwards spiral path

   1. Electric field accelerates it = increased velocity

   2. Increased velocity = increased radius when in magnetic field (r=mv/qB)

Synchrotrons

1. Contain a LINAC = accelerates the charged particle from rest

2. When the charge starts moving, it enters the booster ring

3. Electric and magnetic fields that accelerate the particle further in a circular motion

   1. Electric fields = increase speed

   2. Magnetic fields concentrated to specific areas = changes its direction to follow a circular path (not a spiral)

      1. Synchronised with particle speed to keep its radius of UCM constant

      2. Also accounts for increased relativistic momentum to keep radius the same

4. After a specific speed reached = particle enters the storage ring

   1. Acceleration produces EM radiation (synchrotron light)

Example: The Hadron Collider

Summary of particle accelerator types

Role of Particle Accelerators

By accelerating particles to very high speeds, various concepts in physics can be tested, verified and predictions can be made.

• standard model

  • detect and investigate fundamental particles

  • create new particles

• big bang

  • model conditions

• special relativity

  • Time Dilation

  • Length Contraction

  • Relativistic momentum and mass

Evidence for the standard model

• Detecting and investigating the nature of fundamental particles

JJ Thomson’s Charge to Mass Ratio Experiment (electrons)

• Cathode ray tube containing both electric fields accelerate electrons into a region of perpendicular magnetic and electric fields

• He calculated their q/m by changing the strength of these fields so that the electron would not be deflected

• FE = FB = FB ∴ q/m = E/B2r

• Proved the existence of the electron

  • A type of lepton, Spin of ½

  • Charge of -1e as determined by Millikan

• Showed they were in all atoms since there were emitted by cathodes of all elements

High energy electron-proton Collisions (quarks)

• Electron is accelerated into a proton, causing it to emit quarks and for hadronization to occur

• Called deep inelastic scattering

• Shows that protons were not fundamental particles but a type of baryon (hadron to be specific)

• Made up of two up quarks and a down quark

  • Spin of ½, Charge of +1e

Electron-positron collisions allowing us to prove the existence of gluons

• Used particle accelerators to accelerate electrons and positrons at each other

• Annihilation would occasionally generate a gluon and a quark-antiquark pair

• Allow us to observe the nature of force carrier particles and their interactions

  • Particle accelerators allowed physicists to test nature of gluons

  • Discover it mediates the strong force and keeps quarks together to form hadrons

Evidence for the standard model

• The creation of new particles (particle zoo)

• Particle accelerators use electric fields to accelerate charged particles

  • The Large Hadron Collider (synchrotron)  also uses magnetic fields for the synchrotron/cyclotron to confine the particle

  • The SLAC (linear accelerator)

• Increases their kinetic energy

• Allows new, heavier particles to be created via E=mc2

• New particles (e.g. Pions, Kaons) to be observed  makes up the particle zoo we know today

  • Their properties can only be explained using the standard model

  • Mesons – consist of a quark and antiquark

  • Baryons – consist of three quarks held together by gluons

Particle accelerators Evidence for the Big Bang

Used to model early conditions of the Big Bang

• Early periods (e.g. inflation and post inflation) had extremely high temperatures which we can model using particle accelerators

• Can simulate the high energy situation and cause matter and antimatter to be created from energy → via E=mc²

  • After expansion of singularity (inflation period)

  • Initially only unstable fundamental particles like quarks and leptons were made (post inflation)

  • Over time = more cooling as universe expands further → allows quarks to join using gluons to form protons and neutron

  • More time: nuclei created due to residual strong force → neutral atoms created via EM force mediated by photons → matter decouples from energy and releases IR radiation →over time atoms gather together due to gravity to form heavier atoms, then stars, planets and galaxies

• Reveals how matter was created after the singularity and how that evolved into the universe we see today

Note: This method is not successful in showing why the symmetry between matter and anti-matter production was broken during early stages of the Universe

Evidence for special relativity

Time Dilation

• We can accelerate unstable particles (e.g. muons)

• They last a longer time in the Earth’s FOR than they normally would (we are experiencing the dilated time)

E.g. muons are a second generation of lepton with a lifespan of 2.2 microseconds

• Too short for us to test them

• By accelerating them to near light speeds using particle accelerators, relativistic effects take place due to einsteins second postulate of special relativity

• For speed of light to be kept constant, length and time are relative

• Increases lifespan of muon in FOR of earth = means that the length of path travelled is contracted for the muon

• Allows us to determine its properties like its charge (-1e), mass (heavier than an electron) and its interactions with other particles (e.g. muon, anti-muon annihilation)

Length Contraction

• In the particle’s FOR, its lifetime is still short

• Length contracts so that they can travel through space further than the normally could without relativistic effects

Relativistic momentum and mass

• When it is travelling at high speeds, the momentum of the particle increases due to its observed mass increasing

• Makes it harder to accelerate since the work done goes in to increasing the relativistic mass