Introduction to Particle Physics

1. Introduction to Particle Physics

2. Particle Physics • This is an introduction to the • Phenomena (particles & forces) • Theoretical Background (symmetry) • Experimental Methods (accelerators & detectors) of modern particle physics • That is, it is not a “real” introduction to particle theory (there are other modules!) • Rather, it will attempt to give you the information and tools needed to understand and appreciate the history and new results in the field

3. Particle Physics • Elementary particle physics is concerned with the basic forces of nature • Combines the insights of our deepest physical theories • Special Relativity • Quantum Mechanics • Matter, at its deepest level, interacts by the exchange of particles

4. Hierarchies of Nature • Animal Life • Biology • Chemistry • Atomic Physics • Nuclear Physics • Subatomic physics • Particle physics does not and will not explain everything in nature. • It does provide strong constraints on what nature can do

5. What is a particle? • Not an easy question! • Is a speck of dust a particle? • Is an atom a particle? • Is a nucleus a particle? • Is a proton a particle? • Is an electron a particle? • At different times, each of these were considered to be particles • No substructure seen – need to break it • No excited states seen – watch it decay • How does one probe smaller and smaller sizes?

6. Probing structure • We see with our eyes by • Light scattered from objects • Light emitted from objects • The size of the objects we can see are limited by the wavelength of visible light • How do we see smaller structure?

7. Accelerators and Detectors • Accelerators provide a consistent source of charged particles traveling at speeds near that of light • The energy of the accelerated particles dictates the kind of physics you are probing • Atomic scale – 10’s of eV (Hydrogen) • Nuclear physics – 10’s of MeV (Binding energy) • Particle physics – 100’s of MeV (exciting proton structure)  100’s of GeV (Electroweak unification) • At the lower scales, particles are really particles since you do not perceive their substructure or excited states

9. Conserved Quantities: Mechanics • Noether’s theorem • For every continuous symmetry of the laws of physics, there must exist a conservation law. For every conservation law, there must exist a continuous symmetry. • Invariance under • Time translation – Energy • Space Translation – Momentum • Rotation – Angular momentum • These quantities are obeyed in any system – on any level • Easiest assumption is that they are obeyed locally!

10. Waves and Particles • Electromagnetic forces are propagated by fields between charges • Classically characterized by waves that carry energy & momentum & spin • Quantum mechanics describes particles as a wave packet. • The wave packet carries energy, momentum, and spin • The quantum theory of fields (Quantum Field Theory) describes the fields which couple to particles  as particles!

11. Fundamental Matter Particles LEPTONS QUARKS

12. What is a Force? • Every law of physics you have learned boils down to involving two classes of phenomena: • Conserved quantities: • Mechanical • Energy, momentum, angular momentum • Related to time, translation, and rotation invariance • Number • Charge conservation, law mass action in chemistry

13. Forces of Nature • Now we know what there “is” • How do they talk to each other? We have managed to find four forces:

15. How did we get here? • This picture of the world didn’t just emerge naturally • It is the synthesis of a wide variety of experimental data • It is worthwhile to consider how certain things were discovered

16. Radioactivity • End of the 19th century • Discovery of three “particles” emitted by nuclei • Alpha  Turned out to be 4He • Beta  Turned out to be an electron • Gamma  Turned out to be a photon • Amazing – already the strong, weak, and electromagnetic interactions were visible • But they were not distinguishable at this point

17. Proton & Neutron • Rutherford identified the proton as the nucleus of the hydrogen atom • Neutron was discovered by James Chadwick by bombarding beryllium with alpha particles

18. Nucleus • Before Rutherford, people thought the atom was a diffuse cloud of protons and neutrons • Rutherford found that there was scattering off of a point source in the atom • Short distances allowed large momentum transfers – even back-scattering • Like firing a cannonball at tissue paper, and having it bounce back!

19. The Electron • Thomson identified the cathode rays as a new type of matter • Same charge as a proton • Much lighter!

23. Mesons & The Strong Force • But what held the nucleus together • Coulomb forces should repel the protons • Something stronger must be present • Yukawa postulated a force similar to the photon, but massive • Strong, but limited in range • Nuclear size suggested

24. Particles from the Sky! • Up in the mountains of Europe, scientists detected high-energy particles in emulsion and cloud chambers • Discovered new particles which were lighter than nucleons but much heavier than electrons • New particles • Pion • Muon • Similar in mass, but interacted very differently

25. The Muon • Did not suffer nuclear interactions • Rather, was quite penetrating • Like an electron, but slower (more massive) at the same momentum Ionization energy lossof charged particles

26. The Pion • Other meson events appeared to show a negative particle which stopped in the emulsion, was absorbed by a nucleus, and then “exploded” into “stars” (D.H. Perkins was one who observed these!) • The positive particles seemed to stop and then decay into the previously-seen muons • These had a similar mass to the mesons, but clearly had different interactions • Recognized as strongly-interacting particles, more like Yukawa’s predictions!

27. Antimatter • As soon as Dirac combined • Special Relativity • Quantum Mechanics in a way that was symmetric in space & time, he found that his equation described spin-1/2 particles • It also predicted negative energy solutions for fermions • Predicted “anti-particles” in nature, with opposite charge but same mass • Anti-electron  positron was discovered in cosmic rays • Anderson’s cloud chamber • Curvature gives momentum • Length gives rate of energy loss Only consistent withlight positive particle

28. Accelerators and Detectors • In order to probe down to smaller distances, you need large energies • Development of accelerator technology was rapid in the first half of 20th century • Three major types • Linear accelerators • Cyclotrons • Synchrotrons • With increasing energy,require increasingsophistication of tools usedto detect particles • Detector technology

29. Accelerators Cyclotron Linear Accelerator Synchrotron

30. Detectors • Making subatomic particles visible to human senses • Most commonly-used principles • Scintillation – charged particle produces light • Ionization – charged particle produces charged ions • Magnetic spectrometers – tracking a particle through a magnetic field: p (MeV) = .3 qB(kG)R(cm)

31. Bubble Chamber The bubble chamber was the most instructive detector of the early years Liquid kept under overpressure, but below the boiling point When particles passed through, stopper pulled out, reducing boiling point and bubbles formed around tracks Photograph of tank created a full image of the event However, slow and difficult to extract only the events you wanted (e.g. for rare particles) These days, the granularity and complexity of the collisions have made the bubble chamber obsolete But excellent for pedagogy!

32. Strange Particles • In cloud chamber, bubble chamber and emulsion experiments new particles were being discovered at a fast rate in the 40’s and 50’s • Some particles appeared to be • Produced immediately (strong interactions) • Decaying only after a considerable time (weak interaction) • Produced in pairs – looks like a quantum number • Given name “strangeness”

33. Conserved quantities • Without detailed understanding of the interactions, particles were classified by their quantum numbers, in the hope that some scheme would emerge • Multiplicative • Parity – behavior of wave function under spatial inversion • Charge conjugation – symmetry if charges were flipped • Additive • Isospin – used to group particles into doublets and triplets, like an internal spin • Strangeness – characteristic of long lived particles

35. The Particle Zoo • Pre-standard model particle physics was characterized by an increasing particle zoo

36. Quark Model • Gell-Mann and Ne’eman explained the spectrum of hadronic states with similar quantum number by means of “quarks” • Baryons (p, n, L) have 3 quarks • Mesons have one quark, and one anti-quark • Transform states into each other using “rotations” • UpDown • DownStrange • StrangeUp • Particles with similar spin and parity fell into multiplets • SU(3) symmetry increasingly broken with increasing strangeness • Predicted unobserved states, like W S D- Do D+ D++ S- So S+ I3 - + - q q q

40. Neutrinos • Neutrino proposed by Pauli to account for energy released in b-decay • Reines and Cowan showed that neutrinos were actual particles • Steinberger, Schwartz and Lederman showed that muons had their own neutrino New law of nature: Lepton number is conserved separately

41. The Later Years • After the quark model, the zoo reduced to six microbes. Then it became chase after heavier and heavier particles nt

42. Weak and Strong Interactions • While weak and strong interactions were now extensively studied, and theoretical concepts existed for their deeper structure, experiments were still limited in energy • Thus, difficult to probe • Force carriers of weak interactions • Substructure of hadrons

43. Partons • For a long time, quarks were seen as simply a convenient mathematical tool to account for quantum numbers • No evidence for free quarks in nature • Scattering experiments at SLAC did the same thing as Rutherford • Found that large momentum transfers were possible – as if the proton has pointlike consituents • Measured “structure functions” that characterize the momentum distributions of the “pieces” of the proton

44. Electroweak Unification • Many features of the weak interactions • Long lifetimes • Parity violation • Isotropic decays • Explained by • Heavy intermediate bosons (like the Yukawa force, but much shorter range) • Coupled to left-handed fermions • The features were then unified with the electromagnetic force by Glashow, Salam and Weinberg – who received the Nobel in 1979 • The weak force is carried by W and Z bosons of M~90 GeV • The massless photon is induced by the presence of a condensate of “Higgs” bosons, that spontaneously breaks the symmetry of the interaction

45. Charmed Particles • A case where theory led experiment • Weak interactions seemed to require a change of strangeness • “Neutral currents” not seen in decays of kaons to pions  Always a change in charge • This was explained naturally by the existence of a fourth quark • The J/Y particle (M~3.1 GeV!) was found near-simultaneously at BNL and SLAC in 1974! • Not just a new quark: • Completed the second family of quarks and leptons • Nobel prize awarded in 1976 (just two years later…) m- p p y m+

46. Tau & Bottom • As energies increased in both e+e- colliders and fixed target proton beams, new particles started appearing in the mid-70’s • Mark II observed strange events with one electron and one muon • Suggested new lepton that decayed into e or m • Leon Lederman et al observed new peaks around 10 GeV. • Suggestive of yet another quark m~5 GeV • A new family was found • Required another neutrino and another quark • Took around 20 years to find both!

47. Gluons • Still, there were some mysteries • It seemed as if the quarks only carried ½ the momentum of a proton • Moreover, it was clear that quarks could not be the whole story • No way for a particle to be in the uuu state unless each u quark carried a distinct quantum number! • This led to the “colour hypothesis” of Nambu, which evolved into Quantum Chromodynamics in the early 1970’s • Quarks came in 3 colors – so each u quark was a different particle • Another gauge symmetry  “long range” force to maintain it • QCD predicted that gluons could be radiated from quarks (and gluons) just like photons from electrons

48. W&Z • Electroweak unification required W and Z • Found by Carlo Rubbia and collaborators at the CERN SppS exactly where expected! • MW ~ 80 GeV • MZ ~ 90 GeV • Another case of theory leading experiment. • But experimentalists got the Nobel in 1984 (3 years later!) • The collider era had really begun!

49. Colliders in Use HERA e+p 30+900 GeV LEP, e+e- 91-209 GeV Tevatron, p+p 2 TeV RHIC, Au+Au 200 GeV/N

50. The Top Quark • The discovery of the charm quark led us to believe that all quarks come in doublets. • Thus, the lonely bottom quark (5 GeV) was a problem for many years • Only in 1995 was the top quark identified in p+p collisions at Fermilab • Mass of 170 GeV – Almost like a gold nucleus! • Required deep understanding of almost everything before it • Single lepton production • Jet production from W’s • QCD backgrounds (soft & hard) • Essentially completed the standard model • OK, the tau neutrino was only established in 2000…