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Particle Physics

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  1. ParticlePhysics October 2008 Yaakov (J) Stein Chief Scientist RAD Data Communications

  2. Physics ? Physics is the search for simplicity Aristotelian physics held that there were 4 terrestrial elements • earth • fire • air • water All materials under the sky are combination of several elements Aristotle (and Democritus and Epicurus) further believed that matter is not infinitely indivisible i.e. that there smallest units of matter (atoms) All Aristotelian physics was derived from pure thought (it is commonly held that Galileo invented the idea of experiments)

  3. Atoms From the quantitative study of chemistry (Lavoisier) Dalton concluded that matter is made of atoms For example - carbon and oxygen can combine in two ways In one the mass ratio was 3:4 in the other 3:8 From this he concluded that • the 2 combinations were 1:1 and 2:1 in terms of atoms • an oxygen atom is 1 1/3 times heavier than a carbon one By careful measurement he made a list of atomic weightsA (e.g. C has atomic weight 12 and O has atomic weight 16) But how many different atoms were there ?

  4. Chemistry By comparing chemical characteristics of different elements Mendeleev came up with the periodic table Here each element has a atomic number Z (serial number) For example • H has Z=1 A=1 • C has Z=6 A=12 • O has Z=8 A=16 • Cu has Z=29 A=64

  5. Complexity - not simplicity So we have a nice picture of elements made up of atoms And all materials made up of elements and thus of atoms But there are many many different kinds of atoms This is too complex ! Physics is the search for simplicity ! Perhaps the atoms themselves are made up of simpler units ? Unfortunately, the table is monotonic in atomic weight A but not linear in A so the atoms are not made up of Z smaller particles

  6. Electrons and protons The first elementary particle discovered was the electron via cathode rays (Thomson), oil drops (Millikan), and the photoelectric effect (Hertz) What was the connection between electrons and atoms ? After a series of scattering experiments Rutherford came up with the planetary atomic model the atom was mostly empty at the center was a very small nucleus electrons circulate around the nucleus since electrons are negative and the atom neutral the nucleus must be positive In later experiments Rutherford proved that the nucleus was made up of protons (nuclei of H atoms)

  7. Scattering experiments In a scattering experiment • particles are used as projectiles • other particles are targets Low energy scattering is good to measure the cross-sectional area of the target For example, Rutherford bombarded thin gold foil with alpha particles most particles go through without deflection, so nucleii are very small High energy scattering can break up the target Very high energy scattering can create new particles

  8. Sensors Weak collisions are observed by using detectors To observe new particles created in strong collisions we need a new tool In 1911 Wilson invented the cloud chamber (supercooled gas) While looking into a glass of beer in 1952 Glaser came up with the bubble chamber (superheated liquid) In both, tracks are left by all charged particles By using a magnetic field one can determine charge and mass Today there are many sophisticated sensors and many Israeli specialists in this space

  9. Bubble chamber tracks

  10. Nuclei Isotopes are the same element (same Z) but different atomic weights So there must be something in the nucleus other than the proton This also helped understand what kept the nucleus together so Rutherford invented the neutron which was found experimentally by Chadwick in 1932 Neutrons and protons experience a strong force when they are very close that overcomes the electric repulsion of the protons Beta decay changes Z without changing A and the beta particles turn out to be electrons So a neutron can change into a proton by ejecting an electron and the force responsible is called the weak force e - r / d r2

  11. Forces Let's take a short rest from matter and look into forces 4 different types of forces were known to classical physics contact gravity electric magnetic Then Maxwell unified the electric and magnetic fields Since a changing E field builds a changing B field and vice versa the field can build itself and travel far from sources the speed turns out to be the speed of light ! So the field is more fundamental than the action at a distance action at a distance

  12. Interactions Today we speak of interactions between particles There are four known interactions (in order of decreasing strength) strong (hadrons are particles that feel the strong interaction) electromagnetic (charged particles feel it) weak (hadrons and leptons feel it) gravitation (all particles feel it) Theories that further unify these are called unified field theories Everyone wants a Theory of Everything (ToE) that explains all 4 In quantum theory all interactions are mediated by bosons

  13. Antiparticles In 1932, three particles were known electron (negative, light) proton (positive, heavy) neutron (neutral, heavy) In 1928, Dirac's came up with the first relativistic quantum theory It predicted an antiparticle for each particle In 1933 Anderson discovered a positron (antielectron) in a bubble chamber picture So we need to add positron antiproton antineutron This is a nice simple picture !

  14. Photon In 1923 Einstein predicted that electromagnetic fields were made up of photons Later relativistic quantum theories showed him to be correct The photon was the first boson discovered Photons have no mass, and thus travel at the speed of light Photons have no charge and are their own antiparticles But photons do have energy The frequency of EM radiation is related to the photon energy through the fundamental relation E = h u

  15. Quantum numbers According to quantum theory all elementary particles have certain characteristics These include its mass, charge, and spin Later new quantum numbers needed to be added In interactions, characteristics are ruled by conservation laws Table of particles we know so far :

  16. Fermions and Bosons Classical particles obey Maxwell-Boltzmann statistics but quantum particles are indistinguishable In quantum mechanics particles are described by a field  The probability of finding a particle is ||2 Indistinguishability means |(1) F(2)|2 = | F(1) (2)|2 which can either mean • (1) F(2)= F(1) (2) Bose-Einstein statistics (bosons) • (1) F(2)= - F(1) (2) Fermi-Dirac statistics (fermions) Note that two Fermions can't be in the same state (Pauli principle) Spin-statistics theorem - • fermions have half integral spin • bosons have integral spin

  17. Neutrinos Enrico Fermi observed that in beta decay not all the expected energy was in the emitted electron It was later more directly observed He concluded that some other particle took some of the energy and called it the neutrino (small neutral particle) The neutrino is almost massless and only reacts via the weak interaction And we also need an antineutrino ! Later it was discovered that there are different types of neutrino

  18. Muons While observing byproducts of cosmic radiation in 1936 Anderson observed a very heavy electron (mass about 100 MeV) Since its mass was between • the light electron (lepton = light) and • the proton (baryon = heavy) he called it a meson But today that name is used for other particles and we call this negatively charge particle the muon or more precisely the mu minus and the muon is known to be a lepton not a meson Its antiparticle is the mu plus

  19. Pions Yukawa's theory of the strong force predicts a boson with intermediate mass - the meson At first the muon was thought to be that particle but it turned out to be a fermion and not to participate in the strong force In 1947 the pi meson (or simply pion) was discovered with mass about 140 MeV There are three types - pi zero, pi plus, and pi minus Later other mesons were predicted and discovered - K and eta

  20. So what Fermions do we have ? Leptons : • electron, positron • electron neutrino, electron antineutrino • mu minus, mu plus • muon neutrino, muon antineutrino • tau minus, tau plus • tau neutrino, tau antineutrino Mesons : • pi zero, pi plus, pi minus • kay zero, antikay zero, kay plus, kay minus • eta Baryons : • proton, antiproton • neutron, antineutron • lambda, antilambda • sigma zero, sigma plus, sigma minus and their three anti-s • xi zero, antixi zero, xi minus, antixi plus • omega minus, antiomega plus

  21. So what Bosons do we have ? Gauge bosons : • photon (charge 0) - electromagnetic interaction • gluon (g) (charge 0) - strong interaction • W (charge -1) and antiW (charge +1) - weak interaction • Z (charge 0) - weak interaction • graviton (?) - gravity Higgs boson- in electroweak theory creates mass And many more are unconfirmed as yet … • X • Y • W-prime, Z-prime, … grand unified theories

  22. The eight-fold way The Fermion picture is no longer simple In the early 1960s, Gellmann and Neeman (independently) observed new symmetries that connected baryons/mesons

  23. Quarks This observation led to a new picture, called the standard model In the standard model, baryons and mesons are composite Quarks com in 6 flavors - up, down, charm, strange, top, and bottom There are thus 6 particles and 6 antiparticles (all are spin ½) Due to colorconfinement, quarks never exist as free particles Instead, they form hadrons - particles that feel the strong interaction • baryons are made up of 3 quarks • mesons are made of one quark and one antiquark

  24. Color confinement Quarks can be either red, green, or blue Antiquarks can be either antired, antigreen, or antiblue Only combinations with resulting color white attract Hadrons are made up of quarks such that the resulting color is zero and the resulting charge is always an integer The model explains all the properties of the baryons and mesons For example, • proton = u u d (charge +1) • neutron = u d d (charge 0) • lambda = u d s (charge 0) • pi-plus = u anti-d (charge +1) • kay zero = d anti-s (charge 0)

  25. A simple picture again ! 6 quark types (u d c s t b) 6 lepton type (e e-neutrino mu mu-neutrino tau tau-neutrino) 4 gauge boson types (photon gluon Z W) and maybe one Higgs ! Detector from the LHC (Geneva)