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Topics in Contemporary Physics A (very) brief history of Particle Physics Luis Roberto Flores Castillo Chinese University of Hong Kong Hong Kong SAR January.

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Presentation on theme: "Topics in Contemporary Physics A (very) brief history of Particle Physics Luis Roberto Flores Castillo Chinese University of Hong Kong Hong Kong SAR January."— Presentation transcript:

1 Topics in Contemporary Physics A (very) brief history of Particle Physics Luis Roberto Flores Castillo Chinese University of Hong Kong Hong Kong SAR January 5, 2015

2 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 … last time: Grading scale, office hours, TA’s, … Structure of the course Quick survey Overview: –Big experiments –Why particles –The SM and the Higgs boson –CERN, experiments –Distributed computing –Statistical treatment –Applications –Economic impact –The future 2

3 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 PART 1 Brief history Basic concepts Colliders & detectors From Collisions to papers The Higgs discovery BSM MVA Techniques The future 3 5σ

4 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The “Standard Model” of Particle Physics 4 Core idea: all from a small set of fundamental constituents How did we get here?

5 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Historical Overview Historic stages (following D. Griffiths, 2 nd ed.) –“Classical Era” (electron, nuclei, neutrons)( ) –Photons (quantum effects become apparent)( ) –Mesons (from Yukawa to the muon)( ) –Antiparticles (Dirac, Anderson, x-ing symm) ( ) –Neutrinos (β-decays, Pauli’s solution, 2 types)( ) –Strange Particles (new baryons and mesons)( ) –The Eightfold Way (finding structure)( ) –The Quark Model (an explanation)(1964) –The November Revolution (evidence!)( , 1995) –Intermediate Vector Bosons (1983) –The Standard Model (1978-?) 5

6 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Radioactivity Henri Becquerel (15 December 1852 – 25 August 1908) Studying phosphorescence, thought uranium salts were excited by the sun By May 1896, concluded that it was uranium itself Maria Sklodowska-Curie Discovered Polonium and Radium (both radioactive) Developed techniques to isolate them Radioactivity seemed to contradict energy conservation 6

7 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The electron J J Thomson, 1897 Cathode rays were bent by a magnetic field Adjusting strengths of E and B fields: –speed and mass-to-charge ratio Constituent of the atom, which is much heavier “Plums in pudding” 7

8 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The electron 8

9 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The nucleus Ernest Rutherford 9

10 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9,

11 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The discovery of the neutron Before the discovery of the neutron: –Nucleus assumed to have electrons and protons –Problems: impossible to confine an electron inside a space as small as the nucleus (by the uncertainty principle, too large p e ) Nitrogen nucleus: A=14, Z=7. –Had been shown to be a boson –But if this model was correct, 14p + 7e should be a fermion Completes the “classical trio” (e, n, p) 11

12 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Photon ( ) More in common with the W and Z than with p and n 1900: Max Plank –The “ultraviolet catastrophe” is avoided if radiation comes in ‘packages’ (quanta) with E = hv (E: energy, h=6.62x10-27erg s, v: frequency) –No explanation (maybe related to the emission process) 1905: Albert Einstein –Feature of the EM field itself (not only emission) –Photoelectric effect: Electron energy depends on the color, not intensity Intensity affects the number of electrons E ≤ hv – w (w: “work function” of the material) 12

13 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Photon (II) Einstein seemed to resurrect light corpuscles, so his quanta met with a hostile reception In 1916, Millikan verified it experimentally “… appears in every case to predict exactly the observed results. … Yet the semicorpuscular theory by which Einstein arrived at his equation seems at present wholly untenable” Finally settled by A. H. Compton’s experiment (1923): 13

14 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Photon (III) Change in the interpretation of field theory Action at a distance: –Classically: “mediated” by a field –Now: mediated by an exchange of particles (field “quanta”) Not merely a kinematic phenomenon In many cases (including atomic physics), the large number of photons washes out quantum effects. In elementary processes (photoelectric effect, Compton scattering, …), quantization can no longer be ignored. 14

15 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Mesons ( ) What holds the nucleus together? –A new force, stronger than EM, is needed –Not noticeable in everyday life, why? –Short range! Hideki Yukawa, 1934 –If p, n are attracted through a quantum field, what are the properties of its quanta? –Short range  heavy mediator –Yukawa estimated ~ 300 m e, “meson” (middle-weight) e : lepton (light-weight), p,n: baryons (heavy-weights) –In 1937, two groups identified “middle-weight” particles in cosmic rays, but: wrong lifetime, mass too low, inconsistent measurements. 15

16 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Mesons ( ) 1947: puzzle resolved: –Two different ‘middle-weight’ particles –Named pion ( π ) and muon ( μ ) –The true Yukawa meson is the π Abundant in the upper atmosphere, but very short lived –The μ is lighter and lives longer Eventually recognized as a simple “heavy electron”, with no role in the nuclear interaction. Isidor Isaac Rabi: “Who ordered that?!” 16

17 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Antiparticles ( ) In 1927, Dirac combined QM and Special Relativity to describe free electrons The Dirac Equation had a big problem: for every solution there was one with negative energy. If correct, electrons would –always be moving towards ever more negative energy states –radiate an infinite amount of energy in the process To solve this, Dirac postulated that negative-energy states were already filled by an infinite ‘sea’ of electrons –Always there, and perfectly uniform, so no net force –By Pauli exclusion, observed e’s cannot occupy those states 17

18 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Antiparticles ( ) Imparting enough energy to an e from the “sea”, it would jump to a positive energy state How would the ‘hole’ left look like? –Missing negative charge: positive charge –Missing negative energy: positive energy –i.e., as an ordinary particle with positive charge Was it the proton? (as Dirac hoped) –No (the mass needed to be the correct one) –No such particle (m=m e, q=-q e ) known at the time 18

19 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Antiparticles ( ) : Carl David Anderson discovers the positron 19 Cloud Chamber photograph; a 6mm lead plate separates the two halves. From 63 MeV to 23 MeV, at least ten times larger than a proton path of this curvature. Dimitri Skobeltsyn had observed it in 1929 Chung-Yao Chao, a Chinese grad student at Caltech, had indications, but they were inconclusive and not pursued Frédéric and Irène Joliot- Curie had dismissed them as protons

20 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Antiparticles ( ) Dirac’s equation signaled a profound and universal symmetry: for every kind of particle, an antiparticle (same mass, opposite charge) –Electron ( e - )  positron ( e + ) –Muon ( μ - )  antimuon ( μ + ) –Proton ( p )  antiproton (pbar) (Berkeley, 1955) –Neutron ( n )  antineutron (nbar) (idem, 1956) –Photon ( γ )  Photon!! Which one is matter and which is ‘anti’-matter is arbitrary… to some extent 20

21 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Antiparticles ( ) 21

22 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Crossing symmetry If the following reaction is seen: Then the following will also be allowed: 22 (as long as energy is conserved)

23 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Neutrinos ( ) Beta decays: A  B + e - Problem: the e - seemed to violate energy conservation –In the rest frame of the ‘parent’ nucleus, the electron energy should be –i.e. it should be fixed by the masses of the nuclei A and B. –… but the experiment showed significant variations… and the equation above represented only the maximum e - energy. –What to do? Abandon E conservation? (as Bohr suggested) 23

24 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Neutrinos ( ) Wolfgang Pauli suggested an invisible particle –It would carry off the missing energy –Should have q=0 For charge conservation, and Because it hadn’t been detected Pauli wanted to call it “neutron”, but in 1932 Chadwick used that name for the particle he discovered Many skeptical about Pauli’s proposal, but… 1933: Enrico Fermi –included Pauli’s particle (‘neutrino’) into a new theory of beta decay –The theory was so successful that the neutrino was taken seriously 24

25 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Neutrinos ( ) Beta decay: 25 Pions decaying into unseen particles:

26 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Neutrinos ( ) Still, by 1950, no direct evidence –Where they real, or just math? Detection: from beta decay: one can infer the ‘inverse’ beta decay: But neutrinos interact extremely rarely. –Very intense source: Savannah River nuclear reactor, SC 5x10 13 v / s /cm 2 –Large detector: ~ 200 litters of water –And then? 26

27 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Neutrinos ( ) 27 Expected rate: 2 or 3 events per hour. Cowan-Reines experiment:

28 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Neutrinos ( ) Other experiments showed that Neutrinos are not their own antiparticles ( γ and π are) How to tell which reactions can occur? Lepton number –e, μ, ν : L=+1. Antiparticles: L=-1 –“Lepton number conservation” 28

29 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Neutrinos ( ) Other experiments showed that Neutrinos are not their own antiparticles ( γ and π are) How to tell which reactions can occur? Lepton number –e, μ, ν : L=+1. Antiparticles: L=-1 –“Lepton number conservation” –It was then found that μ does not decay into e+γ –Two separate conservation laws: One for the “muon number”, L μ, One for the “electron number”, L e 29

30 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Neutrinos ( ) Other experiments showed that Neutrinos are not their own antiparticles ( γ and π are) How to tell which reactions can occur? Lepton number –e, μ, ν : L=+1. Antiparticles: L=-1 –“Lepton number conservation” –It was then found that μ does not decay into e+γ –Two separate conservation laws: One for the “muon number”, L μ, One for the “electron number”, L e 30

31 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Strange Particles ( ) In 1947, things seemed under control: –Yukawa’s pion identified, neutrino generally accepted –The muon was unexpected (“who ordered that?”) December 1947: G.D. Rochester and C.C. Butler –Cloud chamber. Cosmic ray shower  π + π cm of lead Neutral m > 2m π “ K 0 ”

32 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Strange Particles ( ) 1949: Brown et al.: 32 K 0 : originally V 0, then θ 0 K+: originally τ + Included in the “meson” family Other mesons were discovered: η, ρ, ω, φ, …

33 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Strange Particles ( ) 1950: Anderson’s group discovers another neutral “V” Much heavier than the proton 33 (not the discovery picture)

34 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Strange Particles ( ) Before the Λ, the need for a “conservation of baryon number” was recognized (for p and n). The Λ had to be a baryon Then several more were discovered: Σ, Ξ, Δ, … Btw: there is no “conservation of meson number” 1952: Brookhaven Cosmotron began operation… many more were discovered. 34

35 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Strange Particles ( ) “Strange” particles were specially intriguing because –They are produced on a time scale of seconds –They decay in a much slower scale (~ seconds) This hinted that maybe production and decay were through different fundamental forces 35 When the Nobel Prizes were first awarded in 1901, physicists knew […] of only two […] “elementary particles”: the electron and the proton. A deluge of other “elementary” particles appeared after 1930: […]. I have heard it said that “the finder of a new elementary particle used to be rewarded by a Nobel Prize, but such a discovery now ought to be punished by a $10,000 fine” Willis Lamb, 1995 Nobel Prize acceptance speech

36 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Strange Particles ( ) Abraham Pais developed a model for these decays, with it happening always in pairs (“associated production”) 1953: Gell-Mann and Nishijima built on Pais’s idea: –New property to each particle: “strangeness” –Should be conserved in strong interactions –Not conserved in weak interactions 36

37 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Strange Particles ( ) Kaons have S=+1, Σ and Λ : -1, ordinary ones ( π,p,n ): 0 Always in pairs, so as to keep ΔS=0: When particles decay, strangeness is not conserved: 37

38 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Where are we so far? From O(100) “elements”, to –Three particles –Then four (adding the muon/pion) –Then five (muon≠pion) –… then many more Three types of leptons, each their own neutrino Mesons, baryons, leptons, strange particles Odd “conservation laws” for new properties Maybe two types of interaction 38

39 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Eightfold Way ( ) 1961: Murray Gell-Mann introduced the “Eightfold Way”. Baryons: 39 “Baryon octet”

40 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Eightfold Way ( ) Mesons: 40 Pseudo-scalar meson octet (eight lightest mesons)

41 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Eightfold Way ( ) Heavier baryons: 41 Baryon decuplet Current notation: Σ(1385), Ξ(1530) instead of Σ* and Ξ* Gell-Mann predicted Q=-1, S=-3 Described its production Calculated its mass and lifetime Found in 1964 Gell-Mann predicted Q=-1, S=-3 Described its production Calculated its mass and lifetime Found in 1964

42 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Eightfold Way ( ) 42

43 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Eightfold Way ( ) New hadrons found their place in these supermultiplets For baryons, there is also an ‘antibaryon supermultiplet’ For mesons, antiparticles are in the same supermultiplet 43

44 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Quark Model (1964) So far, discovery and classification, but … why these patterns? Gell-Mann and George Zweig proposed that hadrons are built of more basic objects. Gell-Mann called them ‘quarks’ Composition rules: –Every baryon is composed of 3 quarks (antibaryon: antiquarks) –Every meson is composed of a quark and an antiquark. 44 George Zweig

45 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Quark Model (1964) All supermultiplets emerge from the quark model The same combination may have excited states Forbidden: meson with Q=+2 or S=-3 45

46 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Problems (at the time) for the Quark Model Lack of detection –In accelerators, it should be possible to extract one from a p –Having fractional charges, they would be easy to identify –At least the lightest quark should be stable Yet no one had ever seen a quark… “Quark confinement” ?? Some evidence of “three lumps” in protons, but inconclusive Violation of Pauli exclusion? – Quarks must carry spin ½,  fermions –The Δ ++, should be uuu, all in the same state –O.W. Greenberg suggested quarks come in three “colors” A baryon would then have one of each color 46

47 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Problems (at the time) for the Quark Model All naturally occurring particles are colorless either total amount of each color is zero, or all three colors are present in equal amounts These would “explain” why only combinations of 2 and 3 quarks: the only colorless combinations are “The last gasp of the quark model”? What rescued the quark model? 47

48 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The “November revolution” 1974: The discovery of the J/ψ Samuel Ting’s group observed it in the summer of 1974; kept it secret until Nov (BNL: J ) Discovered independently by Burton Richter’s group (SLAC: ψ ) Exceptional particle: ~ 3m p Extremely long lifetime ( vs s of other hadrons) A new quark: c 48

49 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Towards a revolution Leptons: e (-1), v e (0), μ (-1), v μ (0) Quarks: d (-1/3), u (2/3), s (-1/3), Shouldn’t there be a fourth quark? When the J/Psi was discovered, the quark model was ready with an explanation and its implications. Having c and cbar, total charm = 0, “hidden”; needed to find ‘naked’ or ‘bare’ charm. –First charmed baryons in 1975 (first with u, d, then with s) –First charmed mesons in 1976 (same pattern) 49

50 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9,

51 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The aftermath In 1975, a new lepton was discovered (the tau) and its own neutrino. Four quarks and six leptons… In 1977 a new meson was found, and identified as having a fifth quark: ‘bottom’ or ‘beauty’ (so experimentalists started to look for naked beauty and bare bottom) –Λ b = udb in the 1980’s –Σ b – uub in 2006 –Ξ b = dsb in 2007 (FNAL) Not hard to predict then a sixth quark –The top quark was discovered in 1995 at Fermilab –174 GeV/c 2 !! (~ 40 times the mass of the b) –Decays too fast! No bound states 51

52 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Intermediate Vector Bosons Fermi’s beta decay theory did not use a mediating particle Excellent approximations A theory w/ particle mediator was expected to replace it EW theory from Glashow, Weinberg and Salam: –M W = 82 ± 2 GeV/c 2 –M Z = 92 ± 2 GeV/c 2 CERN began construction of ppbar machine (late 1970s) Discoveries: –January 1983: W ( ± GeV/c 2 ) –June 1983: Z ( ± GeV/c 2 ) “Relief, not shock or surprise”. 52

53 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Standard Model Three interactions, force mediators for all. Strong force mediator: pion? Eta? Rho? All of them are composite; we should rather look at the mediator between quarks: the gluon Eight gluons (photon:1, W+W-Z:3, gluons:8) They carry color; as quarks, only colorless combs exist Detectable only within hadrons or in colorless combs with other gluons (“glueballs”) 53

54 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The Standard Model 12 leptons, 36 quarks, 12 mediators, 1 Higgs: elementary particles; Too many? –Maybe … maybe not (a lot of structure) 54

55 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 Some BSM thoughts Why three generations? –At least one reason: predominance of matter over antimatter Why only three? –Seems like a good question, … but ~ 1988 SLAC and CERN closed the possibility of more: Z bosons decay into any q/qbar or l/lbar pair May decay into other particles (if below half the Z mass) –Number of light neutrinos: masses, three angles and a phase, Weinberg angle (EW mixing). In total over 20 arbitrary parameters. Acceptable in a ‘final’ theory? 55

56 L. R. Flores CastilloTopics in Contemporary Physics CUHK January 9, 2015 The future Experiment Neutrino oscillations CP violation Higgs particle properties Theory GUTs SUSY String theory 56


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