Introduction to Particle Physics Particle Physics This is

Introduction to Particle Physics

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 appreciate the history and new results in the field

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

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

What is a particle? • Not an easy question! • • • Is Is Is a speck of dust a particle? an atom a particle? a nucleus a particle? a proton a particle? 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?

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?

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 e. V (Hydrogen) • Nuclear physics – 10’s of Me. V (Binding energy) • Particle physics – 100’s of Me. V (exciting proton structure) 100’s of Ge. V (Electroweak unification) • At the lower scales, particles are really particles since you do not perceive their substructure or excited states

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!

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!

Fundamental Matter Particles LEPTONS QUARKS

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

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

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

Radioactivity • End of the 19 th century • Discovery of three “particles” emitted by nuclei • Alpha Turned out to be 4 He • 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

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

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!

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

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

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

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

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!

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 with light positive particle

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 20 th century • Three major types • Linear accelerators • Cyclotrons • Synchrotrons • With increasing energy, require increasing sophistication of tools used to detect particles • Detector technology

Accelerators Cyclotron Linear Accelerator Synchrotron

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 (Me. V) =. 3 q. B(k. G)R(cm)

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!

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”

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

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

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 S DS- • 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 Do D+ So q q q S+ + - D++ I 3

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

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

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

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

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 Ge. V • The massless photon is induced by the presence of a condensate of “Higgs” bosons, that spontaneously breaks the symmetry of the interaction

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 Ge. V!) 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…) mp p y m+

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 Ge. V. • Suggestive of yet another quark m~5 Ge. V • A new family was found • Required another neutrino and another quark • Took around 20 years to find both!

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

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

Colliders in Use HERA e+p 30+900 Ge. V LEP, e+e- 91 -209 Ge. V Tevatron, p+p 2 Te. V RHIC, Au+Au 200 Ge. V/N

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 Ge. V) was a problem for many years • Only in 1995 was the top quark identified in p+p collisions at Fermilab • Mass of 170 Ge. V – 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…

Neutrino Oscillations • Super-Kamiokande is originally designed to search for proton decay • 50 k tonnes of water • 11 k phototubes to detect light • ’ 98 Detected a significant deficit of muon neutrinos, especially when coming through the earth • Fit hypothesis of neutrinos oscillating – changing flavor • Not part of the standard model – yet!

The Higgs • The Higgs particle, couples to all massive particles (quarks and leptons) Higgs Condensate M=0 M=m • However, direct searches for the Higgs have been without success • The data may suggest MH~114 Ge. V… • The LHC is the ultimate hope for understanding the origin of mass

The Future? ? • As we push towards a deeper understanding of nature, our laboratories are seeming less and less sufficient • Much recent progress in particle physics comes from the side of cosmology • Kind of ironic • Many subatomic particles seemed to come from space (pion, muon, etc) • We learned all about the world at hand through the patterns these particles made • Now we are heading back to space, to see what more we can figure out!

What is left (i. e. What I may not cover!) • Heavy Ion Physics • Search for quark-gluon matter • Supersymmetry • Symmetry between Bosons & Fermions • Dark Matter / Dark Energy • Seems to require new particles, which are clearly all around us! • Superstrings / Extra Dimensions • Physics of the 21 st century that appeared miraculously in the 1980’s • Particles are vibrating strings, embedded in a many-dimensional space where only 4 are allowed to be macroscopic!