Historical Overview • The field of Elementary Particle has developed in a natural progression since the turn of the last century • This is an attempt to provide the historical context and examples of the experiments that shape our current understanding of the most fundamental principles of nature. • What were some of the major historical developments over the last century? • Interplay between: – Accelerators – Experiments – Theory Radioactivity

In 1896 accidentally exposed photographic plates to uranium. In 1898 Marie and isolated polonium and radium (much stronger sources).

Henri Becquerel Pierre Curie The In 1897 J.J. Thomson studied “cathode rays” emitted by a hot electrode. Measured deflection by B and E fields. 2 FE = q E, FB = qv x B qvB = m v /rcurv B x r Computed velocity v = E/B using crossed fields, then curv q/m = v/(B rcurv) using B alone. Found very large q/m.

Inferred small me. Thomson’s atomic model:

• Electron is point-like • At least smaller than 10-17 cm • Like charges repel • Hard to keep in a small pack ATOMS ARE NOT ELEMENTARY! Quantum Theory

• Nils Bohr described atomic • extends the laws of structure using early concepts classical mechanics to describe of Quantum Mechanics. velocities that approach the speed of light. All matter should obey the laws of quantum mechanics and . Quantum Theory

• Energy and matter are related – Energy can be transformed to mass and vice versa • Conservation of mass-energy • Measured energy of the electron is only 0.5 MeV – Can explain a size of 10-10 to 10-13 cm – Cannot explain < 10-17 cm as measured • Need lots of energy to pack charge tightly inside the electron – Breakdown of theory of electromagnetism • Uncertainty Principle: You can violate energy conservation but only for a short time

Werner Heisenberg The Photon

Light is a particle: In 1905 Einstein interpreted the photoelectric effect as electron emission due to absorption of a quantum of light: Ee = hν –�. Increasing the light intensity does not increase Ee. Confirmed by Millikan (1916), Compton (1923). Photon energy proportional to frequency: E = h n glass tube under vacuum

Current measurement The Nucleus In 1909 Rutherford scattered alpha particles (He nuclei) off a gold foil. One in 8000 scattered at a large angle.

Ernest Rutherford “as if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you” Evidence that the positive charge in an atom is in a tiny core. Scattering a – atom scattering at low energies is dominated by Coulomb interaction a - particle

Atom: spherical distribution impact of electric charges parameter b Geiger & Marsden a – particles with impact parameter = b “see” only electric charge within sphere of radius = b a, b source (Gauss theorem for forces proportional to r-2 )

E. Rutherford Zinc Sulphide Screen 1927, Rutherford, as President of the Royal Society, expressed a wish for a supply of "atoms and which have an individual energy far transcending that of the alpha and beta particles from radioactive bodies..." The Neutron Why did helium have twice the charge of hydrogen, but four times the mass? He = 4 p + 2 e was proposed. In 1928, W. Bothe and H. Becker found that particles from Po cause Be to emit a penetrating radiation, first thought to be photons (γ rays). In 1932 studied ejection of from various elements, showed that the neutral particle (n) had a mass very close to mp. Neutron source in Chadwick’s experiments: a 210Po radioactive source (5 MeV a – particles ) mixed with Beryllium powder Þ emission of electrically neutral radiation capable of traversing several centimeters of Pb: 4 9 12 He2 + Be4 ® C6 + neutron ­ James Chadwick a - particle Penetrating Power of Particles

a b g

Neutron Paper sheet

Aluminum Lead Paraffin The Birth of

• In 1896, Thomson showed • In 1905, Einstein • In 1907, Rutherford showed that the mass of that electrons were argued that photons particles, not a fluid. an atom was behave like particles. concentrated in a nucleus. Particles that should obey the laws of quantum mechanics and relativity. Nuclear Physics

• α, β, ɣ emission • Properties of neutrons • Fission of heavy elements • Nuclear “chemistry” • Nuclear forces • • postulated • Theories of beta decay Particle Detectors

• In 1912, Wilson develops the • Photographic emulsions exposed for seeing the by the passage of charged paths of fundamental particles. particles. Cloud Chamber

• Supersaturated Gas • Cloud formation • Used until 1950’s • Condensation started around the ions generated by passing charged particles (ionization), and the resulting droplets were photographed. The electron spin

• In 1927, P.A.M. Dirac invented a relativistic wave equation for the electron, containing solutions with electron E = +(p2c2 + m2c4)1/2 and E = -(p2c2 + m2c4)1/2. magnetic dipole moment µe e! -5 • Relativistic Quantum Mechanics µ = » 5.79´10 [eV/T] e 2m – Schrödinger equation e • Not relativistic (space2 but time1) • He interpreted the latter as “filled states”, the “”. • Excitations would have opposite charge but the same 2 mass as electrons. Eg = 2 mec = 1.2 MeV in this case. • Predicted • You can create more massive objects than you have energy - but they are virtual - i.e., they disappear promptly and rematerialize in particle states that conserve mass-energy • Vacuum is full of quantum bubbles! Paul Adrian Maurice Dirac The Positron

• Just as the equation x2=4 can have two possible solutions (x=2 or x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. 2 2 2 2 2 E – p c = (m0c ) “relativistic invariant” (same value in all reference frames)

• Dirac interpreted this to mean that for every particle that exists there is a corresponding , exactly matching the particle but with opposite charge. For the electron, for instance, there should be an “anti-electron” called the positron identical in every way but with a positive electric charge. The Positron

Carl David Anderson discovered the positive electron (positron) in 1932, for which he won the for Physics in 1936

Direction of deflection inside a + - magnetic field: it is a positively g rays ® e +e charged particle

positron track

Equal amounts of matter and antimatter are produced if energy is converted to matter. The Neutrino 1930 Pauli’s neutrino: Energy conservation in beta (b) decay requires the existence of a light, neutral particle n ® p+ + e- + n (e- = b) observed in 1956

210 Radium E: Bi83 (a radioactive isotope produced in the 1 1 0 __0 of 238U) n0®p1+e-1+u0 • Large variations in the emission velocities of the b particle seemed to indicate that both energy and momentum were not conserved. • This led to the proposal by Wolfgang Pauli of another particle, the neutrino, being emitted in b decay to carry away the missing mass and momentum. • must be present to account for conservation energy & momentum Theory of β-Decay b- decay: n ® p + e- + n + + 14 14 + b decay: p ® n + e + n (e.g. O8 ® N7 + e + n) ν : the particle proposed by Pauli (named “neutrino” by Fermi)

ν : its antiparticle (antineutrino)

Fermi’s theory: a point interaction among four spin ½ particles, using the mathematical formalism of creation and operators invented by Jordan. Particles emitted in b – decay need not exist before emission; they are “created” at the instant of decay

Prediction of b – decay rates and electron energy spectra as a function of only one parameter: Fermi GF (determined from experiments) Discoveries in Cosmic Rays

• Era of using cosmic rays for fundamental discoveries begins • Penetrating radiation observed at high altitudes; observation of increase of radioactivity with altitude during a balloon flight • Solutions to Dirac’s equations interpreted as “positive electrons” • Yukawa proposed a “” to explain the strong nuclear force • Anderson observed in 1932 and in 1936 • In 1912, Viktor Hess investigated • Perkins discovered terrestrial radioactivity in balloon experiments. photographic emulsions in 1947 Cosmic Rays Until the late 1940’s, the only existing source of high-energy particles 1932: Discovery of the antiparticle of the electron, the positron (Anderson). Confirmed the existence and prediction that anti-matter does exist (Dirac). 1937: Discovery of the (Street and Stevenson). It’s very much like a “heavy electron”. 1947: Discovery of the (Powell). Hadron lighter that the .

Carl Anderson 1936 Nobel Prize Anderson’s experiment: cloud chamber (discovery of the positron) C.D. Anderson, Physical Review 43, 491 (1933) The Nucleus

• What is responsible for strong binding between nucleons? • In 1934 H. Yukawa proposed a new light particle, a meson, to carry the strong force between neutrons and protons. • Range of force led to prediction of mmeson = 200 me, “middleweight” between me and mp = 1837 me • New potential which is large at short distances and decreases rapidly at distances larger than about 2 fm. • Treated the problem in a relativistic quantum theory • He clearly showed that in the relativistic quantum world particles interact by exchanging virtual quanta which mediate the force • He predicted the mass of pions Relation between interaction radius & −g2 e−r /a Yukawa Potential, U(r) = s meson mass m: 4π r ! mc2 » 200 MeV R = -13 int mc for Rint » 10 cm Which Meson is it?

In 1937 cosmic rays with about the right mass were found. But in 1946 it was shown that the bulk of these particles interact too weakly with nuclei to be Yukawa’s meson.

These are muons (μ). They only feel the electromagnetic_ and weak interactions, and decay via μ ® e + νe + νμ -6 With lifetime tµ = 2 x 10 sec.

Muons (μ±) capable of traversing as much as 1 m of Pb without interacting; tracks observed in cloud chambers in the 1930’s.

Determination of the mass by simultaneous measurement of momentum p = mv(1 – v2/c2)-½ (track curvature in magnetic field) and velocity v (ionization): 2 mµ = 105.66 MeV/c » 207 me Who ordered the Muon? • In 1937 a new particle of mass 105.7 MeV was discovered – However, it interacted with matter very weakly, a heavy – Created in upper reaches of the atmosphere many of them were able to reach the ground level traversing a large amount of matter – Muon - I. Rabi asked, “Who ordered the muon?” – Yukawa’s (pions) were eventually discovered in 1947 the muon is not Yukawa’s meson The Muon

The discovery of the muon was published in "New Evidence for the Existence of a Particle Intermediate Between the Proton and Electron", Phys. Rev. 52, 1003 (1937). Before this point the fundamental particles were presumed to be electrons, protons and the (then) newly discovered neutron. The discovery brought attention to the prediction by Yukawa in 1935 that an intermediate mass "meson" might be responsible for the nuclear strong force. Yukawa had predicted a mass of about 100 MeV and the muon had a mass very close to that. Moreover, the mesons decayed emitting electrons, and Yukawa's nuclear quanta were expected to be responsible for b-radioactivity by disintegrating into electrons and undetectable neutrinos. From 1941 and through the difficult years of World War II, three young Italian , Piccioni, Conversi, and Pancini, carried out a series of observations of mesons stopped in matter, which seemed at the beginning to support Yukawa’s predictions. At the end of 1946, they reported that the rates of absorption of mesons in light materials were in strong disagreement with the theory. The experiment was based on the magnetic separation of positive and negative muons. Negative muons decayed when at rest in Carbon, rather than being absorbed by the nucleus (as they should do being the quanta of the ). muon stopping Muon Decay in a cloud chamber and Decay electron decaying to an electron momentum distribution µ± ® e± + n + n µ Muon spin = ½

- 6 Muon lifetime at rest: τμ = 2.197 x 10 s º 2.197 μs Muon decay mean free path in flight: decay electron track vτ µ pτ µ p λdecay = = = τ c p : muon momentum 2 µ mµ mµ c 1-(v / c) τμ c » 0.66 km Muons can reach the Earth’s surface after a path ³ 10 km because the decay mean free path is stretched by the relativistic time expansion Discovery of the Pion Cecil Powell and colleagues at Bristol used nuclear emulsions to see charged tracks in the upper atmosphere. In 1947, they announced the discovery of a particle called -8 the p-meson or pion (p) for short (tp = 3 x 10 sec). The p was Yukawa’s meson! Cecil Powell Muon (µ) 1950 Nobel Prize Pion (p) One neutrino is also comes to rest comes to rest produced but escapes here, and then Two more neutrinos here, and then undetected. decays: are also produced but decays: µ ® e + n + n escape undetected. p ® µ + n µ e

p

In 1950 the neutral pion (p0) was discovered via p0 ® γγ Strange Particles

Later in 1947, C. Butler and G. Rochester (Manchester) used cloud chamber to discover two new hadrons (strongly interacting particles) in cosmic rays, kaons: 0 + - + + KS ® p p and K ® µ nµ The Particle Zoo

• 1947: strange particles – K0 ® p+ p-, K+ ® p+ p+ p- – L ® p+ p- – S, X – long lifetime t ~ 10-10 s • more particles... – D ® pp, – r ® pp – short lifetime t ~10-24 s

Cosmic rays 1st, followed by accelerator Known Particles in 1950 Particle Accelerators Because one has no control over cosmic rays (energy, types of particles, location, etc.), scientists focused their efforts on accelerating particles in the lab and smashing them together. Generically people refer to them as particle accelerators. Efficient production of particles with higher masses is going to required high energy: Before 1950’s E=mc2 was still just a theory…

Circa 1950, these particle accelerators began to uncover many new particles. Most of these particles are unstable and decay very quickly, and hence had not been seen in cosmic rays. Notice the discovery of the proton’s antiparticle, the , in 1955: more antimatter. Particle Accelerators

• In 1932, Cockroft and Walton • From 1930-1939, Lawrence built accelerated protons to 600 keV, bigger and bigger cyclotrons, produced the reaction accelerating protons to higher and higher energies: 80 keV ➝ and verified E=mc2. 100 MeV. New Accelerators: Synchrotrons

1952: Brookhaven 3 GeV “Cosmotron” 1954: Berkeley 6 GeV “Bevatron” New Detectors: Bubble Chambers

The Berkeley 72 inch liquid hydrogen bubble chamber Bubble Chamber

Interaction of an antiproton in a bubble chamber: 8 pions are produced. One of them (positive) decays into a muon and a muon-neutrino More and More Particles

Late 1950’s – early 1960’s: Discovery of many strongly interacting particles at the high energy proton accelerators (Berkeley Bevatron, BNL AGS, CERN PS), all with very short mean life times (10–20 – 10–23 s, typical of strong decays) Þ catalog of > 100 strongly interacting particles (collectively named “hadrons”)

Fermilab: Bubble Chamber Photo

Too many to be elementary! Known Particles in 1957 Strongly Interacting Particles: 1961 Strongly Interacting Particles: 1963 Conserved Quantum Numbers Why is the free proton stable? Possible proton decay modes (allowed by all known conservation laws: energy – momentum, electric charge, angular momentum): p ® p0 + e+ p ® p0 + µ+ p ® p+ + n . . . . . No proton decay ever observed – the proton is stable 25 Limit on the proton mean life: tp > 1.6 x 10 years

Invent a new quantum number: “Baryonic Number” B B = 1 for proton, neutron B = -1 for antiproton, B = 0 for e±, µ±, neutrinos, mesons, photons Require conservation of baryonic number in all particle processes: B = B å i å f i f ( i : initial state particle ; f : final state particle) Symmetries and Quantum Numbers

• Strong interactions seem to be independent of nucleon flavor (proton or neutron) • This symmetry for strong interaction implies a conserved current or quantum number (Noether’s theorem) –

– Proton = +1/2 Iz, Neutron = -1/2 Iz – Isospin follows spin angular momentum algebra 1 1 1 0 2 + 2 = + – Pions are isospin 1 states, there should be three of them with about the same mass as was observed.

• Weak interactions do not conserve isospin

• Neutron beta decay, n ® p e ne Strange Particles

In the late 1940’s a variety of heavier mesons (K – mesons) and (“”) were discovered. In 1952 Brookhaven Cosmotron produces strange particles in pairs. Also strange baryons (decay into p or n), such as L0 ® p+ p-.

Examples of mass values: Mesons (spin = 0): m(K±) = 493.68 MeV/c2; m(K°) = 497.67 MeV/c2 Hyperons (spin = ½): m(L) = 1115.7 MeV/c2; m(S±) = 1189.4 MeV/c2 m(X0) = 1314.8 MeV/c2; m(X – ) = 1321.3 MeV/c2 Properties: • Abundant production in proton – nucleus and p – nucleus collisions • Production cross-section typical of strong interactions (s > 10-27 cm2) • Long lifetimes: Decaying to lighter particles with mean life values 10–8 – 10–10 s (as expected for a weak decay) • Production in pairs (example: p– + p ® K° + L ; K– + p ® X – + K+ ) • Heavy Examples of decay modes: K± ® p± p° ; K± ® p± p+p– ; K± ® p± p° p° ; K°® p+p– ; K°® p° p° ; . . . L ® p p– ; L ® n p°; S+ ® p p°; S+ ® n p+ ; S+ ® n p– ; . . . X – ® L p– ; X° ® L p° Strange Particles p - + p ® L0 + K 0 Example of a K– stopping in liquid hydrogen: K – + p ® L + p° (strangeness conserving) followed by the decay L ® p + p – (strangeness violation)

p° ® e+ e– g (a rare decay)

L – is produced in A p and decays in B

p K– Strangeness Invention of a new, additive quantum number “Strangeness” (S) (Gell-Mann, Nakano, Nishijima, 1953)

Notion of strangeness conservation – like electric charge conservation, but violated by the weak interactions.

S = S • conserved in strong interaction processes: ∑ i ∑ f • i f not conserved in weak decays: Si − S f =1 + ∑ S = +1: K , K° f S = –1: Λ, Σ ±, Σ° S = –2: Ξ°, Ξ– S = 0: all other particles (and opposite strangeness –S for the corresponding ) 0 + - L ® p p mL = 1110 MeV > mp,n = 940 MeV p- p+ ® K0 L0 p- p+ ® K+ S- p- p+ ® K0 S0 p- p+ ® K0 n p- p+ ® p+ S- Parity Violation

Initially the K+ was thought to be 2 particles q+ ® p+ p0 (P = +1) and t+ ® p+ p0 p0 (P = -1)

In 1956 T.D. Lee and C.N. Yang resolved the t-q puzzle by proposing that parity is violated in the weak interactions. Quickly confirmed by C.S. Wu using e- direction in beta decay of polarized 60Co. Antiproton Discovery Discovered 1955 by Emilio Segrè and Threshold energy for antiproton ( p ) production in proton – proton collisions ~6 GeV number conservation Þ simultaneous production of p and p (or p and n) Owen Chamberlain Emilio Segrè Example: p + p ® p + p + p + p § build a beam line for 1.19 GeV/c momentum § select negatively charged particles (mostly p– ) § reject fast p– by Čerenkov effect: light emission in transparent medium if particle velocity v > c / n (n: refraction index) – have v < c / n Þ no Čerenkov light § measure time of flight between counters S1 and S2 (12 m path): 40 ns for p– , 51 ns for antiprotons

For fixed momentum, time of flight gives particle velocity, hence particle mass “Bevatron”: 6 GeV proton synchrotron at Berkeley The Antiproton

Example of antiproton annihilation at rest in a liquid hydrogen bubble chamber A Theory of Electromagnetism By 1950, a Quantum Theory of Electromagnetism – (QED) – was developed by , and Sin-Itiro Tomonaga. Charge particles interact via the exchange of a photon (γ).

Richard Feynman Julian Schwinger Sin-Itiro Tomonaga Many Mesons and Baryons • Pions (p0, p±) Example of mesons: – Strong binding is independent of p+ º ud ; p- º ud ; p0 º (dd -uu)/ 2 proton/neutron numbers K - º su ; K 0 º sd ; K + º su ; K 0 º sd • Isospin symmetry implied Example of baryons: three I=1 Yukawa pions proton º uud ; neutron º udd – Angular excitations, vector mesons (r, w, …) + 0 − ± Σ ≡ suu ; Σ ≡ sud ; Σ ≡ sdd • Kaons (KS, KL, K ) 0 - – Strange particles produced in pairs X º ssu ; X º ssd – Strong and EM interactions conserve strangeness a new quantum number • But weak interactions violate strangeness • Kaons decay to pions and • Organizing the mesons and baryons – Flavor (softly broken) symmetries – Gell-Mann’s eight fold way Þ substructure () • SU(3) symmetry invoked to explain octet of pseudo scalar mesons • Predicted missing member of decuplet of baryons, which was discovered – However, predicted fractionally charged quarks were not observed The Eightfold Way In 1961 M. Gell-Mann introduces “The Eightfold Way” to organize the hadrons. Predicts the W- (S = -3).

mesons baryons baryons

D- D0 D+ D++ ddd udd uud uuu n p

S- S0 L S+ dds uds uus

dss uss X- X0

W- sss Organizing the Data Mass

“Strangeness” Electric charge

Spin The Model In 1964 Gell-Mann and Zweig introduce quarks (aces) to explain success of “The Eightfold Way”. Hadron classification into “families”; observation that all hadrons “ ” could be built from three spin _ ½ building blocks d s u u d s Electric charge +2/3 -1/3 -1/3 Baryonic number 1/3 1/3 1/3 Strangeness 0 0 -1 _ _ s u d and three antiquarks (u, d, s ) with opposite antiquarks electric charge and opposite baryonic quarks number and strangeness

Qu = 2/3, Qd = Qs = -1/3 Su = Sd = 0, Ss = -1 _ Mesons are qq pairs; baryons are qqq triplets.

Are quarks real, or a mathematical mnemonic? Quarks In 1964, Murray Gell-Mann and George Zweig (independently) came up with the idea that one could account for the entire “Zoo of Particles”, if there existed objects called Quarks. Murray Gell-Mann George Zweig

Murray Gell-Mann had just been reading Finnegan's Wake by James Joyce which contains the phrase "three quarks for Muster Mark"

When the quark model was proposed, it was just considered to be a convenient description of all these particles. A mathematical convenience to account for all these new particles…

After all, fractionally charged particles An excerpt from Gell-Mann’s 1964 paper: “A search for stable quarks of charge –1/3 or +2/3 and/or stable di-quarks of charge –2/3 or +1/3 or +4/3 at the highest energy accelerators would help to reassure us of the non-existence of real quarks”. 1964: Quarks?

• Murray Gell-Mann: • George Zweig: Physical meson states are Hadrons are composed of more representations of the SU(3) elementary objects: symmetry group:

Physical baryon states are representations of the SU(3) symmetry group: 1964: Observation of the Ω-

Observed in the 80 inch bubble chamber at Brookhaven in 1964. W–: the bound state of three s – quarks with the lowest mass with total angular momentum = 3/ 2 Þ Pauli’s exclusion principle requires that the three quarks cannot be identical Quarks and Color • Overwhelming evidence for nucleon and meson substructure in terms of quarks – Quarks are spin-1/2 and fractionally charged • However, quarks were never observed directly • Some thing confines them into mesons (qq) and baryons (qqq) – Baryons should have antisymmetric wave function • Proton, p = uud, neutron, n=udd are OK • How about, D++=uuu? – Solve both problems • Invent a new quantum number color • All particles are color less: q, qq, … cannot exist ++ • Overcome statistics problem by choosing p=uRuGdB, D =uRuGuB • This seemingly contrived solution is actually the scheme chosen by nature! Rather than Yukawa’s theory, the color dynamics works Quarks are real

In late 1960s SLAC repeats the Rutherford scattering experiment at ~10000 times the original energy.

Sees “Bjorken scaling” in “deep inelastic scattering”: Quarks are “asymptotically free” when probed at very short distances, even though they are bound tightly at long distances (Gross, Politzer, Wilczek, 1973).

electron electron

u d proton u Elastic Scattering

Electron Proton

Used to measure the size of the proton. Inelastic Scattering

Electron Proton Deep Inelastic Scattering

“Partons”

Electron Proton

Angular distribution consistent with scattering from point-like spin ½ particles inside the proton

Exactly the same as the Rutherford scattering experiment 1968: Deep Inelastic Scattering

2 mile long, 30 GeV electron accelerator

Hydrogen target People Analyzing magnets Detector

F (|Q2| ) = 1 for a point-like | 2| F( Q ) particle Þ the proton is not a point- like particle ds = F( Q2 )s dW M |Q2| (GeV2) CP Violation 1956: Suggestion (by T.D. Lee and C.N. Yang) Weak interactions are NOT INVARIANT under Parity The weak interactions were known to violate P and C (charge conjugation). But the product CP was thought to be a good symmetry. The two neutral kaons, KL and KS, were assigned + - + - 0 opposite CP, based on KS -> p p KL -> p p p p+ ® µ+ + n decay Parity invariance requires that the two states + + n µ n µ p+ p+ n spin µ spin n spin µ spin A B must be produced with equal probabilities Þ the emitted µ+ is not polarized

In 1964 Christenson, Cronin, Fitch and Turlay used + - a KL beam at Brookhaven to discover KL -> p p hence CP violation. Mystery for many years. Electroweak Theory 1962-66: Formulation of a Unified Electroweak Theory 4 intermediate spin 1 interaction carriers (“”): § the photon (g) responsible for all electromagnetic processes § three weak, heavy bosons W+ , W– , Z Sheldon Glashow W± responsible for processes with electric charge transfer = ±1 ( processes) Examples: n ® p e– n : n ® p + W– followed by W– ® e– n + + + + + + µ ® e ne nµ : µ ® nµ + W followed by W ® e ne Z responsible for weak processes with no electric charge transfer ( processes) PROCESSES NEVER OBSERVED BEFORE Require neutrino beams to search for these processes, to remove the much larger electromagnetic effects expected with charged particle beams

Steven Weinberg Weak Neutral Currents

In the 1960s theories were developed by Glashow, Weinberg and Salam which unified the weak and electromagnetic interactions, but predicted a new, neutral, weak , the Z0.

First experimental tests came in neutrino scattering experiments at CERN in 1973 (X = “anything”) + - W exchange: nµ p ® µ X 0 Z exchange: nµ p ® nµ X

neutrino beam

no muon Observation of Neutral Currents

• Observed in 1973 at CERN in a liquid freon bubble chamber.

• Masses of the W± and Z0 predicted to be of order 100 GeV/c2 1974: The November Revolution

The SPEAR synchtrotron: 8 GeV electron- positron collider at SLACSimultaneously observed at Brookhaven where it was called the “J”. To this day it is called the ...The Mark-I detector tracked charged particles using spark chambers.The charm quark was heavy and non-relativistic. Charmonium behaved like a hydrogen atom made of quarks. The November Revolution

Era of fundamental discoveries with colliding beams begins. New, heavy hadron (m = 3.1 GeV) discovered simultaneously at Brookhaven (J) and SLAC _ (Ψ). cc bound state. Others soon followed. σ (e+e− → hadrons) R = revealed a sharp peak at about 3 GeV mass σ (e+e− → µ+µ − )

+ - p + Be -> J + X -> e+e-X e e -> y -> X The Third Generation

By late 1974, electrons, muons, neutrinos and quarks could be fit into a nice pattern, with 2 “generations”.

µ However, in 1975, at the æn e ö æn ö ænt ö ç ÷ ç ÷ same detector where ç ÷ ç ÷ ç ÷ èe ø èµ ø the y was discovered, èt ø a new lepton appeared, æu ö æcö the t. First member æt ö ç ÷ ç ÷ of 3rd generation. ç ÷ ç ÷ ç ÷ è d ø è sø èbø Bottom Quarks

• Discovered in 1977 at a 400 GeV fixed target experiment, Fermilab E-288. • Studied in detail with the ARGUS detector at Hamburg and CLEO at Cornell in the 80’s and 90’s. • B-factories at SLAC (BaBar) and in Japan (Belle) • Detailed studies of how the weak force interacts with quarks. Belle The CERN SPS

• Produce W± and Z0 directly by colliding quarks and anti-quarks: UA1 and UA2 Experiments Search for W± ® e± + n (UA1, UA2) ; W± ® µ± + n (UA1) Z ® e+e– (UA1, UA2) ; Z ® µ+ µ– (UA1) (1981 – 1990)

UA2: non-magnetic, UA1: magnetic volume with trackers, calorimetric detector surrounded by “hermetic” calorimeter with inner tracker and muon detectors Weak Bosons

Masses of W and Z bosons could be estimated at 80, 91 GeV, from weak neutral current data. Far heavier than any particle produced to date. _ _ CERN builds the SppS, p (270 GeV) + p (270 GeV), UA1, UA2 detectors observe W, Z decays to leptons. 1983: Observation of W and Z Bosons

+ + WZ0 ààμμ+ µν-µ 1987: Stanford Linear Collider

SLD detector Large Electron Positron Collider

Swiss Alps Geneva Airport

LEP tunnel ALPEH, DELPHI, L3, OPAL Z0 Production at LEP Physics from LEP

• Only 3 generations of quarks and leptons. 1997: LEP II – W+W- Production

Wen

WWg

WWZ

All Feynman diagrams are needed to explain the observed W+W- production cross section. Discovery of the

The top quark was discovered in 1995 by the CDF and DØ experiments at Fermilab at a mass of 174 GeV/c2.

At Fermilab Tevatron

Top

quark antiquark (proton) (antiproton)

Anti-top

Completed the 3rd generation. Fundamental Particles of Matter

?

?

• In 1994 the top quark was discovered by the CDF and DØ experiments at Fermilab • In 2000 the tau neutrino was observed by the DONUT experiment at Fermilab • The top quark is very heavy (174 GeV/c2) and it decays directly via The

• Direct searches at LEP did not find it.

• Although not directly observed, it should influence precision measurements: Neutrino Masses

1998 and 2001: Evidence for neutrino oscillations and neutrino mass from Super-Kamiokande in Japan and the Sudbury Neutrino Observatory in Canada. The Large Hadron Collider • Replace LEP with a proton-proton collider • Seven-fold increase in energy – 13 TeV • Turned on in 2008 (7, 8, 13 TeV) CMS and ATLAS

High energy collisions and high intensity beams require complex detectors.

People The Higgs Discovery July 4, 2012: Discovery of Higgs boson, Mass = 125 GeV

Main goal of LHC: clarify mechanism of EW

1 What we still don’t understand • Why is the Higgs mass finite?

• Supersymmetry would fix this problem but would introduce hundreds of new particles. • Neutrinos have mass! That breaks the . • Why are there only three generations of quarks and leptons? • Are there only 4 space-time dimensions? • No easy way to incorporate gravity... Summary A Brief History of Particle Physics

1930s The known 'Elementary Particles' were: • electron • proton • neutron (inside the nucleus) • neutrino (now anti-neutrino) in beta decay • photon (γ – the quantum of the electromagnetic field) 1932 The positive electron (positron) discovered by Carl Anderson

Carl Anderson C.D. Anderson, Physical Review 43, 491 (1933) The Neutron

1932 Neutron discovered by James Chadwick

James Chadwick

1933 Fermi theory of beta decay (weak interactions) n → p + e– + ν

Enrico Fermi Pions and Muons

1935 Yukawa's meson hypothesis – nuclear force due to exchange of particles with mass (mesons) 1937 μ lepton (muon) discovered by Carl Anderson and . Initially assumed to be Yukawa's meson but it was too penetrating Hideki Yukawa 1946 Charged π meson (pion) discovered by Cecil Powell. The previous produced from p decays via π → µ + ν

Cecil Powell 1950 Neutral pion (π0) discovered via π0 → γ + γ g Strange Particles

1947 Discovery of the kaon (K meson). 'Strange' long lived particles discovered in cosmic ray events by Clifford Butler and George Rochester. Gave rise to a new quantum number 'strangeness'. Further 'V' events discovered at Brookhaven, New York in 1952/53.

Charged K± decay

Neutral K0 decay

Robin Marshall, University of Manchester Antimatter

1955 Discovery of the anti-proton (p) by Owen Chamberlain and Emilio Segrè

Owen Chamberlain Emilio Segrè The Particle Zoo

1960s/70s Hundreds of 'elementary particles' discovered: r, w , K*, D , S , … … a real mess!

All these particles explained by combinations of more fundamental 'quarks', u, d, s and their anti-quarks The Omega Minus

1964 Discovery of the Omega Minus (Ω-). New quark theory predicted as yet unseen particle with 3 strange quarks. Its discovery at Brookhaven was a great triumph for the new theory and eventually lead to its wide acceptance. Theoretical Advances

1970s Theory of Strong Interactions – , QCD, - quarks interact via exchange of ''

Improved understanding of the – combined with electromagnetism to give 'Electroweak' theory – predicts exchange particles W+, W– and Z0 as carriers of the weak force

Sheldon Glashow Abdus Salam New Quarks and Leptons 1974 New fourth quark called 'charm' (c) discovered at Stanford and Brookhaven, USA

1975 Burt Richter Sam Ting Third charged lepton tau (τ ) discovered at Stanford, USA

Martin Perl 1978 Fifth quark called 'bottom' (b) discovered at Fermilab, USA Six Quarks

1990 Number of neutrinos limited to 3 by measurements at LEP, CERN. Implies a total of 6 quarks

1995 Sixth quark 'top' (t) discovered at Fermilab, USA Force Carriers

1979 The , carrier of the Strong Interaction discovered at DESY Hamburg

1983 The W± and Z0, carriers of the discovered at CERN, Geneva

Carlo Rubbia The Higgs Boson

4th July 2012 Evidence for a new particle in the search for the Higgs Boson from ATLAS and CMS at the Large Hadron Collider at CERN

Peter Higgs Early Discoveries

• Electron (1897) J.J. Thomson • Cloud Chamber (1912) C.T.R. Wilson • Cosmic Rays (1913) V.F. Hess & C.D. Anderson • Discovery of Proton (1919) E. Rutherford • Compton Scattering ɣe ® ɣe (1923) A.H. Compton • Waves nature of electron (1927) C. Davisson & L. Germer 1932 – 1947

• Neutron (1932) J. Chadwick • Triggered Cloud Chamber (1932) P. Blackett & G.P.S. Occialini • Positron (1932) C.D. Anderson • Muon (1936) C.D. Anderson & S.H. Neddermeyer Street & Stevenson • Muon Decay (1939) B. Rossi & D.B. Hall, Williams • Kaon (1944) L. Leprince-Ringuet • Pion (1947) C. Powell & D.H. Perkins 1947 – 1953

• Scintillation Counters (1947) F. Marshall • Pion decay (1947) C. Lattes • Unstable V’s (1947) G.D. Rochester • Semi-Conductor Detectors (1949) K.G. McKay • Spark Chambers (1949) J.W. Keuffel • Photographic Nuclear Emulsions C. Powell, M. Blau • K Meson decays (1951) R. Armenteros 1953 – 1968

• Neutrino (1953) F. Reines • Bubble Chamber (1953) D.A. Glaser • K+ Lifetime (1955) L.W. Alvarez • Antiproton (1955) E. Segre & O. Chamberlain • Flash Tubes (1955) M. Conversi • Spark Chamber (1959) S. Fukui • Streamer Chambers (1964) B.A. Dolgoshein • MWPC (1968) G. Charpak 1968 – 2012

• J/y (charm) (1974) B. Richter, S. Ting • t lepton (1975) M. Perl • B-mesons (1981) CLEO • W, Z (1983) UA1 • number of n (1991) LEP • t-quark (1994) CDF • Neutrino Mass (1998) SuperK, Sudbury • Tau Neutrino (2000) DONUT • Higgs Boson (2012) ATLAS, CMS