HCP: Particle Physics Module, Lecture 2 Plan for today: 1. Brief review of last time hierarchy of matter recap of particle physics history 2. How are particle physics experiments done? 3. Compare "classical" theory of forces to "modern" theory of forces. 4. Electromagnetic force 5. Why are such big accelerators needed for these studies? 6. Quark model

Review of the Hierarchy of Matter Molecule: size ~ 10-9 m Collection of atoms bound together by electrical forces Atom: size ~ 10-10 m Collecton of electrons orbiting a small massive atomic nucleus. The electromagnetic force binds the negatively charged electrons to the positively charged nucleus. Nucleus: size ~ 10-14 m = 10 F Collection of protons (positively charged) and neutrons (zero charge) bound together by the strong force Proton (p), neutron (n), +, -, K+, K-, o  some of the ~ 260 Hadrons (size ~ 1 F) : Baryons + Mesons Quarks: "pointlike" as best we can tell currently  bind together through the strong force to make baryons: q q q mesons: q q

Brief Recap of the History of Particle Physics 1. Thomson (1897): Discovery of the electron (e-) 2. Rutherford (1911): Discovery of the atomic nucleus 3. Rutherford (1919): Discovery of the proton (p) 4. Chadwick (1932): Discovery of the neutron (n) 5. Anderson (1933): Discovery of the positron = antielectron (e+)  By 1933, the constituents of everyday, stable matter (e , p, n) were known. Why did people continue particle physics studies after that? 6. 1920's  1950's: Used cosmic rays ("Nature's accelerator") and particle accelerators (manmade) to make lots (~ 260) unstable, short-lived (10-6  10-22 seconds), more massive particles 7. Quark model (1964): model that brought organization to the "particle zoo" and deeper understanding of the strong force

How are particle physics experiments done? Components: 1. Accelerator: To accelerate particles (like protons or electrons) so that they have a lot of kinetic energy (the energy of motion). 2. Target: Collection of nuclei at rest (in liquid, solid, or gas form) to be bombarded and smashed apart by the accelerated particles. 3. Detector: Large device that allows one to observe the reaction products of the above collisions. Fixed target experiment (target at rest): Target High energy particle beam Accelerator Detector Collider: Two counter-rotating beams: p + e- e+ + e- p + p Reaction products Collision Detector (almost completely surrounds collision point)

Demonstration: How Thomson Discovered the Electron By looking at the radius of curvature of the electron orbit in the magnetic field, Thomson was able to determine its charge-to-mass (e/m) ratio. Electrons get bent in magnetic field and detected (“detector”). Electrons get accelerated here by a voltage source (“accelerator”) Electrons get generated here by a heater (thermionic emission)

Fermilab – near Chicago

Stanford Linear Accelerator Center

CERN – near Geneva, Switzerland

DESY, in Hamburg, Germany

HERA tunnel

H1 collision detector at HERA

H1 Event

HERMES fixed target detector

How do these fundamental particles interact with each other? Current picture of fundamental “matter constituents”: 3 families of leptons + 3 families of quarks How do these fundamental particles interact with each other?  Through the four fundamental forces

Classical Description of Forces Particles feel a force “field” emanating from other particles Examples: M1 M2 R Q1 Q2 R

Modern Description of Forces “Quantum Field Theory”  Forces between particles are caused by the exchange of discrete particles (called exchange particles or exchange bosons) The type of exchange particle depends on the force examples: electromagnetic force: photon () m = 0 weak force: Z boson: mZ = 90 GeV/c2 BUT doesn’t this violate energy conservation?

“Virtual” Exchange Particles The exchange particles (“force carriers”) are referred to as virtual exchange particles because if they were actually detected, they would violate energy conservation. We can only observe them indirectly.  The Heisenberg uncertainty principle makes this possible: If the existence time of the virtual particle is small enough, then we can’t measure the energy accurately enough to know if we violated energy conservation!

Four Fundamental Forces of Nature

Electromagnetic Force Only acts between electrically charged particles In modern language the force is described by Quantum Electrodynamics (QED) The force is mediated by the exchange of massless photons Example: Two protons interact by exchanging a photon, as shown in this “Feynman diagram” Consider a single electron: In QED, the “vacuum” around the electron is very complicated:

Calculation of Electron’s Magnetic Moment To properly calculate the “magnetic moment” of the electron, one needs to take into account all of the “virtual” particles that exist around it. Magnetic moment of electron: Prediction with no virtual particles:  = 2 (from Dirac) experiment = 2.0115965218 QED theory = 2.0115965214 Experiment and theory agree to 10 digits! Some of the hundreds of “Feynman” diagrams that need to be computed to get this good agreement 

Why do particle accelerators need to have high energies? To create new, more massive particles (example: The top quark (mass = 175 GeV/c2) discovery in 1996 didn’t happen until they built an accelerator with enough kinetic energy in the bombarding particles to create this massive particle) To probe particles at smaller length scales to look for substructure (example: discovery that protons are made up of quarks)  Why are high energies needed to study small length scales?

Why are high energies needed to probe small length scales? Consider an ordinary microscope: We use visible light waves ( ~ 10-6 m) to look at the structure of single cell objects (length ~ 10-5 m)  To look at much smaller length scales we need an “effective microscope” with a much smaller wavelength. So we use matter waves (recall wave-particle duality) de-Broglie relation:  = h / p h = Planck’s constant p = momentum So p = h /  to probe short distances requires small  and therefore large momentum p (and energy)

Quark Model (1964) Around this time (1964), certain particle reactions were not observed (even though there was no conservation law that prohibited them). To explain this, a new quantum number called strangeness (S) was introduced. Example: - + p  K- + p not observed electric charge Q: -1 +1 = -1 +1  conserved strangeness S: 0 0 # -1 0  NOT conserved - + p  K0 + 0 observed electric charge Q: -1 +1 = 0 0  conserved strangeness S: 0 0 = +1 -1  conserved So they introduced strangeness and assigned a strangeness quantum number to each particle so that reactions occurred only if the strangeness quantum numbers add up.

Quark Model, continued In 1964, Murray Gell-Mann plotted all known hadrons on charts according to their electric charge (Q) and strangeness quantum number (S). He concluded that he could explain all of them with just three quarks: up (u) Q = +2/3 e S = 0 down (d) Q = -1/3 e S = 0 strange (s) Q = -1/3 e S = -1 Examples: proton: p = u u d Q: +1 = +2/3 +2/3 -1/3 S: 0 = 0 0 0 neutron: n = d d u Q: 0 = -1/3 -1/3 +2/3 kaon: K- = s u Q: -1 = -1/3 -2/3 S: -1 = -1 0 omega: - = s s s Q: -1 = -1/3 -1/3 -1/3 S: -3 = -1 -1 -1

Baryons: 3 quark objects Baryon decuplet  - predicted by Gell-Mann Baryon octet

Discovery of the - (1964) Based on his quark model, Gell-Mann predicted the existence of a new Particle, the - (sss), which was observed shortly thereafter.

Mesons: 2 quark objects (quark and anti-quark)

Observations of a Pentaquark Object in 2002 - 2004 + = five bound quarks (u u d d s) BUT more recent, “higher statistics” observations in 2005 fail to confirm this

What evidence do we have that quarks actually exist? Recall that Rutherford showed that the atom had a small particle at its center by scattering charged particles off it:  + Au   + Au To look for evidence of quarks, people did an analogous experiment in the late 1960’s e- + p  e- + “stuff” They observed many more electrons scattered at large angles than expected. This was interpreted as scattering of the electrons off small, pointlike objects inside the proton  QUARKS!

Analogy Between Rutherford Experiment and Electron Scattering Experiment that Discovered Quarks Electron-proton collisions at SLAC: analogous to Rutherford experiment; provided direct evidence for quarks

Evidence for Quarks from 1968 Experiment at Stanford 2-mile Long Linear Accelerator (SLAC) detector e- p e- “stuff” Many more events seen at large angle than expected: Indirect evidence for the existence of quarks ~ scattering angle

Bombarding Energy Needed to Probe Inside the Proton How high of an electron bombarding energy do we need to probe a length scale (say ~ 0.025 F) small compared to the size of the proton (~ 1 F)?  We need an electron “matter wave” with a wavelength ~ 0.025 F: