Particles and Forces Higher Physics

Orders of Magnitude Higher Physics

We and all things around us are made of atoms Human Hair ~ 50 mm = 50 10-6 m = 0.000050 m Atom ~ 10-10 m = 0.0000000001 m Magritte

Atoms Atoms are all similarly made of: - protons and neutrons in the nucleus - electrons orbiting around proton The electron was the first elementary particle to be discovered (JJ Thomson 1897) Protons, neutrons are made up of quarks electron neutron

From the atom to the quark How small are the smallest constituents of matter? <10-18 m <10-1 8 m ~ 10-14 m ~ 10-10 m ~ 10-15 m Atoms and sub-atomic particles are much smaller than visible light wave-length Therefore, we cannot really “see” them (all graphics are artist’s impressions) To learn about the sub-atomic structure we need particle accelerators

The Standard Model of Particles

Some history of Particle Physics 1895 Discovery of X-rays (W. Roentgen) 1896 Discovery of radioactivity (H. Becquerel) 1897 Discovery of electron (J.J. Thomson) 1898 Isolation of radium (M. Curie and P. Curie) 1905 Special theory of relativity (A. Einstein) 1909 Alpha particle shown to be helium nucleus (Rutherford and Royds) 1911 Discovery of nucleus (E. Rutherford) 1912 Discovery of cosmic radiation (Victor Hess) 1913 Planetary atomic model (N. Bohr) 1915 General theory of relativity (final form) (A. Einstein) 1919 Eddington observes deflection of light by Sun in total eclipse 1926 Quantum mechanics (E. Schrodinger) 1927 Dirac equation and prediction of antiparticles (P. Dirac)

1928 Theory of - radioactivity (Gamow, Gurney, Condon) 1930 Hubble discovers expansion of universe 1930 Neutrino hypothesis (W. Pauli) 1930 Invention of cyclotron (E.O. Lawrence) 1932 Discovery of positron in cosmic rays (Anderson) 1932 Discovery of neutron (Chadwick) 1934 Theory of -radioactivity (E. Fermi) 1935 Meson hypothesis (Yukawa) 1937 Discovery of muon in cosmic rays (Neddermeyer, Anderson) 1947 Discovery of pion in cosmic rays (Powell) 1947 Discovery of V-particles in cosmic rays (strange meson - kaon) (Rochester and Butler) 1950 Discovery of more V-particles (strange baryon - ) (Anderson)

1952 More strange particles (, ) discovered in cosmic rays. 1955 Discovery of antiproton at Berkeley Bevatron (Chamberlain and Segre) 1956 Discovery of antineutron at Berkeley Bevatron 1956 Experimental detection of neutrino (Reines and Cowan) 1974 Discovery of J/ resonance (Charm quark) (Richter and Ting) 1975 Discovery of -lepton (Perl) 1977 Discovery of Bottom quark 1983 Discovery of W and Z bosons (Rubbia and Van der Meer) 1995 Discovery of Top quark (D0 and CDF) 2000 Discovery of tau-neutrino (DONUT) 1995- Discovery of neutrino mass and oscillations (solar and atmospheric 2001 neutrino) (Homestake, GALLEX, SAGE, Super-K, SNO)

Particles discovered 1898 - 1964: The particle zoo !

Particles discovered since 1964:

The Universal Building Blocks Seem to be a bewildering array: e.g. earth, air, fire and water Despite appearances, there are just 4 distinct pieces which make up everything around us

FOUR PARTICLES Leptons Quarks electron up neutrino down

The particles of ordinary matter ne e- u d -1/3 +2/3 charge Leptons: n = neutrino e = electron Quarks: u = up d = down -1 All stable matter around us can be described using electrons, neutrinos, u and d “quarks”

In fact, experts tell us that there are four more quarks! Confession…….. In fact, experts tell us that there are four more quarks! up strange bottom down charm top Necessary for making wierd, short-lived particles, but not normal matter - like you or me.

Confession II ….. Particles u d e n Anti-particles u d e+ n positron

Where are the protons and neutrons ?

The Standard Model Particle families

The Standard Model. Particles Matter particles Force mediating   Force mediating particles   Matter particles Fermions Bosons Example – photon, gluon,Higgs Hadrons Leptons consist of quarks do not consist of quarks Example - proton Example - electron

Fundamental matter particles Composite particles Fundamental force particles Quarks and Composite particles can be fermions or bosons Hadrons – formed by quarks Mesons – 1 quark + 1 antiquark – is a boson Baryons - 3 quarks – is a fermion Bosons can have mass (W,Z) or mass-less (photon, gluon)

Fundamental Particles The Standard Model Fundamental Particles

Electron Mass 1/2000 of proton Charge -1 <10-18m in size, tiny mass. -1 e in atomic nomenclature ( Atomic Mass ) (Atomic number) - Discovered by J.J. Thomson A fundamental particle, one of the generation 1 fermions, and a lepton.

Discovery of the electron Thomson, in 1897, was the first to suggest that the fundamental unit was more than 1,000 times smaller than an atom, suggesting the subatomic particle now known as the electron. Thomson discovered this through his explorations on the properties of cathode rays. He concluded that the rays were composed of very light, negatively charged particles which were a universal building block of atoms. He called the particles "corpuscles", but later scientists preferred the name electron. He estimated the mass of cathode rays by measuring the heat generated when the rays hit a thermal junction and comparing this with the magnetic deflection of the rays. His experiments suggested not only that cathode rays were over 1,000 times lighter than the hydrogen atom, but also that their mass was the same in whichever type of atom they came from. A month after Thomson's announcement of the corpuscle he found that he could reliably deflect the rays by an electric field if he evacuated the discharge tube to a very low pressure. By comparing the deflection of a beam of cathode rays by electric and magnetic fields he obtained more robust measurements of the mass to charge ratio that confirmed his previous estimates. This became the classic means of measuring the charge and mass of the electron.

Quarks There are 6 types, or flavour, of quark: up and down make up protons and neutrons.

3 Families (or Generations) 1st generation 2nd generation 3rd generation ne e- u d -1/3 +2/3 +2/3 +2/3 nm m- c s nt t- t b -1 -1 -1/3 -1 -1/3 Ordinary matter Accelerators Cosmic rays 3 generations in everything similar but the mass

Is the whole Universe made only of quarks and electrons? No! There are also neutrinos! Electron, proton and neutrons are rarities! For each of them in the Universe there is 1 billion neutrinos Neutrinos are the most abundant matter-particles in the Universe! n n n n n nnnnnnnn nn n n nn n nnnnnnn n nn nn nn n n n n n nn n n nn n nnnnnn n nn nn nn nnnnnnn nn nn n nnn nnnnnnn Within each cm3 of space: ~300 neutrinos from Big Bang 1 cm 1 cm Neutrinos are everywhere! in the outer space, on Earth, in our bodies..

Puzzling neutrinos Almost no interactions (only weak) Can cross light-years of material without being affected Can travel from the most remote corners of the Universe bringing information from the origin of space and time Neutrinos do matter to us: If there were no neutrinos the sun would not shine!

Neutrinos get under your skin! per second from Sun are zipping through you n Every cm2 of Earth surface is crossed every second by more than 10 billion (1010) neutrinos produced in the Sun Within your body at any instant: roughly 30 million neutrinos from the Big Bang No worries! Neutrinos do not harm us. Our bodies are transparent to neutrinos

Electron neutrino ν in atomic nomenclature Mass Charge Spin Electron neutrino ν in atomic nomenclature Size difficult to estimate, they travel at near the speed of light. No charge and only a tiny mass. Theorised in 1930 but not detected until 1956. Pass through us all the time. A fundamental particle, one of the generation 1 fermions, and a lepton.

Discovery of the neutrino

The Standard Model Composite Particles

Hadrons Hadros is Greek for ‘massive’. These are particles that are made up of quarks. There are two types of hadron. Baryons – these are made up of three quarks. Mesons – these are made up of a quark and anti-quark pair.

Hadrons - Protons and neutrons in the quark model Quarks have fractional electric charge up quark electric charge + 2/3 down electric charge -1/3 But quarks are nor found on their own So we know now that protons and neutrons are made of quarks. u d

u u u d d d u electric charge + 2/3 d electric charge -1/3 proton (charge +1) neutron (charge 0) u u u d d d So we know now that protons and neutrons are made of quarks.

Proton p in atomic nomenclature <10-15m in size Has around 2000 times the mass of an electron, but no charge. Discovered in 1919 by Rutherford Not a fundamental particle, a proton is a composite particle of two up quarks and 1 down quark (p=uud). It is a hadron (composed of quarks) and a baryon (3 quarks) Free protons have a life of 1029 yrs

Discovery of the proton Rutherford: atoms are not elementary particles! In 1911 Rutherford found a nucleus in the atom by firing alpha particles at gold and observing them bounce back Precursor of modern scattering experiments at accelerator

Neutron n in atomic nomenclature <10-15m in size n in atomic nomenclature <10-15m in size Has around 200 times the mass of an electron, but no charge. Discovered in 1931 by Chadwick Not a fundamental particle, a neutron is a composite particle of two down quarks and 1 up quark (n=ddu). It is a hadron (composed of quarks) and a baryon (3 quarks) Free neutrons will decay with a half life of 15 mins into proton, electron and antineutrino (beta decay)

Discovery of the neutron Bothe and Becker, and Curie and Joliot (1932) - experiment which involves irradiation of beryllium by alpha particles from polonium source. At that time alpha particles were already known (Rutherford) to be doubly ionised helium atoms. They observed neutral penetrating radiation that they thought was X-rays. In fact, they observed the reaction: Curie and Joliot showed that this radiation was able to knock protons out of paraffin. But they misinterpreted the phenomenon as scattering of gamma rays on protons (a process similar to the Compton effect - scattering of gamma rays on electrons).

Discovery of the neutron: Chadwick’s experiment Chadwick used ionisation chamber in which he could measure ionisation produced by a charged particle and the length of the track. He also used alpha particles from polonium source and beryllium as a target for alpha particles. He put several additional target materials (hydrogen, helium, lithium, beryllium, carbon, air and argon) on the way of neutral radiation from beryllium. Particles ejected from hydrogen behaved like protons. (what else can we expect to be ejected from hydrogen?) with speeds up to 3.2109 cm/s. The particles ejected from heavier targets had larger ionising power and were in each case recoil ions of the element. If the ejection of a proton is due to the scattering of photon on nucleus, then to speed up proton up to 3.2109 cm/s, a 52 MeV photon is needed. This exceeded all known energies of photons, emitted by nuclei. Similar process on nitrogen with 52 MeV photons would produce 400 keV nitrogen recoils with ionisation yield and track length much less than observed in Chadwick’s experiment. These particles were neutral particles with the mass equal to that of proton. Chadwick called it the neutron in a letter to Nature in February 17, 1932. 1935 - Chadwick received the Nobel Prize.

Quarks and colour – not needed for Higher Physics – for interest only All quark flavours come in 3 versions, called “colours” u d +2/3 -1/3 up down Quarks combine together to form colourless particles Baryons (three quarks: red+ green + blue = white) Mesons (quark-antiquark pair) such as red+anti-red u-ubar state Strong forces “glue” quarks together in bound states proton p u pion u p

Quarks detected within protons Freeway 280 End Station A experimental area 2 miles long accelerator Stanford (SLAC), California, late 1960s Fire electrons at proton: big deflections seen!

Mesons These are bosons (and hadrons) made up of a quark and anti-quark pair. Because of this they are very unstable. A pion (π+) is made of an up quark and a down anti- quark. It has charge ⅔ + ⅓ = 1 P pion u u

The Standard Model Antimatter

Anti-matter For every fundamental particle of matter there is an anti-particle with same mass and properties but opposite charge Matter Anti-Matter ne e- u d -1/3 +2/3 e+ +1/3 -2/3 +1 -1 Bar on top to indicate anti-particle positron Correspondent anti-particles exist for all three families Anti-matter can be produced using accelerators

Positron - e+ in atomic nomenclature 1 e+ in atomic nomenclature Antimatter particle of electron (anti-electron) Same mass as electron with opposite charge (+1) When a low-energy positron collides with a low-energy electron, annihilation occurs, resulting in the production of two or more gamma ray photons. Can be created by high energy photons hitting an atom. Gamma rays, emitted indirectly by a positron-emitting radionuclide (tracer), are detected in positron emission tomography (PET) scanners used in hospitals. PET scanners create detailed three-dimensional images of metabolic activity within the human body.

Discovery of the positron A cloud chamber was normally used at that time to detect tracks of charged particles. It contained a supersaturated vapour. When a charged particle enters the chamber, it collides with air or alcohol vapour atoms, producing free ions (ionisation process). Vapour in the chamber condenses around these free ions, forming droplets. The droplets are what form the trail. Expansion type cloud chamber Original Wilson chamber

Matter-antimatter pair creation Electron-positron pair created out of photons hitting the bubble-chamber liquid Example of conversion of photon energy into matter and anti-matter Matter and anti-matter spiral in opposite directions in the magnetic field due to the opposite charge Energy and momentum is conserved

Discovery of the positron by Anderson in 1933 Greater curvature of the track in the upper part of the chamber indicates that the particle entered the chamber from below. This determines the positive charge of the particle. Anderson concluded that the positive charge of the particle is probably exactly equal to that of proton (electron) and the mass is less than twenty times the electron mass. The first known particle created in an interaction of another particle with matter (not emitted from atom or nucleus).

Why has all the anti-matter gone? Puff Good thing for us that there is no antimatter around! matter Anti-matter Antiparticles look and behave just like their corresponding matter particles, except they have opposite charges. E.g. a proton is positive whereas an antiproton is negative. Gravity affects matter and antimatter the same way and a matter particle has the same mass as its antiparticle. When a matter particle and antimatter particle meet, they annihilate into pure energy! The development of the Universe containing matter and no antimatter requires that matter and antimatter behave differently

The Higgs boson was the last particle of the Standard Model to be discovered. It is a critical component of the Standard Model. Its discovery helps confirm the mechanism by which fundamental particles get mass.

Discovery dates The electron was discovered first in 1897 by Thomson using a cathode ray tube. The proton was discovered in 1919 by radiating nitrogen with alpha-particles. The atom was completed in 1932 with the discovery of neutron by irradiating beryllium by alpha particles. The positron was discovered in 1933 using a cloud chamber. As particle accelerators became more powerful many more particles were discovered.

Scattering experiment and particle asccelerators The Standard Model Scattering experiment and particle asccelerators

Precursor of modern scattering experiments at accelerator

Rutherford Concluded Since most particles pass straight through Since some of the positive alpha particles were deflected away or scattered back towards the emitter Since the alpha particles were very fast moving, with a large momentum. The atom is mainly empty space They must have been repelled by a positive nucleus The positive nucleus of the gold atom must have a large mass to be able to stop some of the alpha particles from moving forward and then repel them back again.

Linear Accelerator Circular Accelerator – e.g. CERN LHC

Particle accelerators, whether linear or circular, accelerate particles to a very high speed (and energy) to create collisions that can split other particles to create new particles. They have the following basic parts: Particle source - provides the particles that will be accelerated Copper tube - the particle beam travels in a vacuum inside this tube Electromagnets (conventional, superconducting) - keep the particles confined to a narrow beam while they are travelling in the vacuum, and also steer the beam when necessary Targets - what the accelerated particles collide with Detectors - devices that look at the pieces and radiation thrown out from the collision Plus, many cooling, monitoring and safety devices

The Standard Model Beta Decay

In beta decay, it was found that the electron always had less energy than expected (from E=mc2), and instead of all electrons having the same energy, there was a continuous distribution. Pauli proposed the existence of a small neutral particle. Fermi incorporated this into his theory of beta decay, naming the particle the neutrino (small neutral one).

Neutron beta decay Stage 1: The neutron is made up of an up quark (u) and two down quarks (dd). Stage 2: One of the down quarks is transformed into an up quark. The down quark has a charge of -1/3 and the up quark has a charge of 2/3. The transformation is mediated by a W- particle, which carries away a -1 charge. Stage 3: The new up quark rebounds away from the emitted W- particle. The neutron now has become a proton.

Neutron beta decay Stage 4: An electron (e-) and an antineutrino (ve) emerge from the W- particle. Stage 5: The proton, electron, and the antineutrino move away from one another.

Neutron versus proton beta decay β+ Decay is from a proton, and produces a positron and neutrino β- Decay is from a neutron and produces electron and anti-neutrino In Atomic nomenclature for neutron decay X A Z Y Z + 1 + b -1

The Standard Model Forces

The 4 forces of Nature Weak Electromagnetic weak charge Electric Beta-decay fusion Electromagnetic TV, PCs Magnets e- e+ creation weak charge Electric charge Strong Quark binding Gravity Responsible of Keeping us well-planted on earth strong charge mass

FOUR FORCES u d u d e n e n u d u d e n e n

Gravity u d Universal attractive force between all particles e n Familiar to us - pulls us downwards (towards Earth) Decreases with distance (not familiar) Increases with mass (familiar) We can defy gravity - just jump! Small animals / bacteria don’t know much about it!

Electromagnetism Charge +2/3 u d Charge -1/3 e n Charge -1 No charge u 2/3 + 2/3 + (-1/3) = +1 (proton) e + (-1) = 0 (hydrogen atom)

Electromagnetic force The repulsive force that two approaching electrons “feel” e- e- Photon is the particle associated to the electromagnetic force “smallest bundle” of force Photon

Electromagnetism - the force like charges ( same sign ) repel unlike charges attract force decreases with distance important for holding matter together not important in Universal terms - pretty neutral place!

Electromagnetism - how strong? Compared to gravity, for example MUCH STRONGER 10-14-10-15 m u d

Colour (Strong) Force u d e n Not familiar to us - involves bonding of quarks Quarks have a property called colour (it’s a property just like charge is – not a real colour)

Colour Force is Strong u d 10-14-10-15 m u d

Changes the flavour of quarks – Beta Decay Weak Force u d e n Changes the flavour of quarks – Beta Decay u d e n + neutron proton beta radiation Without such weak interactions the Sun would shut down.

Force Particles Particles interact and/or decay thanks to forces Forces are also responsible of binding particles together Strong: gluons Only quarks (because of their colour charge) Weak: W+, W-, Z0 Leptons and quarks (only force for neutrinos) Electromagnetic: photons Quarks and charged leptons (no neutrinos) Gravity: graviton? Still to be discovered Negligible effects on particles

Further points on nuclear reactions Pb 214 82 Bi 83 + b -1 We will return to this later in the unit. For now, it is important to remember that the following are conserved during any interaction between particles: Charge (hence the lower numbers add up) Baryon number. See below Energy Momentum

Baryon number A quantum number equal to the number of baryons in a system of subatomic particles minus the number of antibaryons. Baryons have a baryon number of +1, while antibaryons have a baryon number of -1. Quarks and antiquarks have baryon numbers of +1/3 and -1/3 , respectively (baryons consists of three quarks).

Examples Decay 1 Consider baryon number p + n→ p + μ+ Decay 2 Consider baryon number and then charge p+n = p+n+p+p

Examples Decay 1 Considering baryon number p + n→ p + μ+ 1 + 1 ≠1 + 0 so not possible Decay 2 Considering baryon number, consider charge p+n = p+n+p+p 1 + 1 = 1 + 1 + 1 + −1 possible for baryon 1 + 0 = 1 + 0 + 1 + −1 possible also for baryon

Summary A limited number of forces and matter particles describe all the Universe we know about; A theory of the interactions of matter with forces called the Standard Model describes successfully the phenomena of the subatomic world; There is lot more that we do not know about such as the missing anti-matter, dark matter, puzzling neutrino properties, and the Standard Model key-vault ..the Higgs