Physics 7730: Particle Physics

Transcription

1 Physics 7730: Particle Physics! Instructor: Kevin Stenson (particle physics experimentalist)! Office: Duane F317 (Gamow tower)! Phone: ! Web page: Classroom: Duane G2B21 (here)! Text: Quarks & Leptons by Halzen and Martin! Web site: Today is an introduction to particle physics. PHYS Fall

2 Homework and Final! Real understanding of physics comes about from solving problems. Physics is about understanding the universe and applying that understanding to solving problems.! Weekly assignments will be available online, generally by 5pm on Wednesday. You need to print out your own copy. Homework is due the following Tuesday in class at 12:30pm. Solutions will be posted by Wednesday. You are encouraged to work with other students on the homework but you need to write up your own solutions.! There will be a take home final at the end of the semester. PHYS Fall

3 Material to be covered! A basic understanding of the physics behind the Standard Model including the constituents of matter and the forces between them. Topics include:! Quantum Electrodynamics! Quantum Chromodynamics! Weak interactions! Electroweak theory! Higgs mechanism! Using the above knowledge to calculate real quantities like cross sections and decay rates.! Some exploration of physics beyond the Standard Model including supersymmetry! Introduction to particle detector design and other aspects of experimental particle physics. PHYS Fall

4 Course philosophy! This is my first time teaching this course. I would like this class to be a collaborative experience. I don t pretend to have all of the answers but I am happy to work through anything. In this spirit, please don t hesitate to do the following:! Let me know if you have issues with the class (too hard, too easy, too fast, too slow, too boring, too in depth, too superficial, etc.) or other suggestions.! Propose changes to the material to be covered. We can add or remove material as well as dig in more depth on a particular topic.! Interrupt whenever you have questions or something looks wrong.! Come by my office if you have questions I will post office hours later. PHYS Fall

5 Outline for today! Basic constituents of matter! Basic forces! Some implications of the forces and matter particles Please read Chapter 1 of Halzen and Martin by Thursday. I recommend getting the Particle Data Group (PDG) review of particle physics. It is a terrific reference for all things related to particle physics. Probably want to get the booklet and the book. From follow link to Order PDG Products. You can also use the online version. PHYS Fall

6 Natural units! = c = 1! In particle physics, generally use natural units where! If you remember E 2 = (pc) 2 + (mc 2 ) 2 you can figure out that since GeV is a unit of energy, GeV/c is a unit of momentum and GeV/c 2 is a unit of mass. For convenience, drop the c. So a proton has a mass of GeV.! Length (!/mc) and time (!/mc2 ) can be measured in GeV -1.! Useful conversion factor:!c = 197 MeV! fm PHYS Fall

7 Inside a helium atom Atoms are made up of a nucleus surrounded by electrons. Neutral helium has 2 electrons. The nucleus contains protons and neutrons which are in a class of particles called baryons. n p p n The protons and neutrons are composed of up and down quarks. d d u d u u PHYS Fall

8 The Fundamental Particles Although only 2 quarks (up and down) are needed to make up normal matter, there are two other pairs for a total of 6. The name gives the quark flavor. This 3 family structure extends to leptons. 2 particles like the electron plus a set of 3 neutrinos. 2 nd and 3 rd generation have more mass and are unstable so decay (eventually) into 1 st generation particles in 10-6 s for muons and s for top quark. Neutrinos behave differently. +2/3 "1/3 0 "1 Particles are spin! so they obey Fermi-Dirac statistics and are called fermions. There are also antiparticles for each particle (same mass but opposite electric charge). PHYS Fall

9 Fundamental forces Gravity: important for big objects & long distances; negligible in particle physics. Electromagnetism: causes opposite charges to attract (keeps negative electrons from escaping the positive nucleus). Strong force: causes quarks to attract, keeps quarks inside proton and neutron and keeps protons and neutrons inside the nucleus. Weak force: Causes beta decay. PHYS Fall

10 Summary of matter and force particles The force fields are quantized and with the quanta carrying (mediating) the force. The quanta are gauge bosons.! Electromagnetism (QED) carried by massless photons; felt by charged particles! Massive W ± and Z 0 bosons mediate weak force; felt by quarks & leptons! Strong force (QCD) carried by massless gluons; felt by quarks

11 Quantum Chromodynamics (QCD) QCD is the theory of the strong interaction and is similar to QED (quantum electrodynamics) Just like particles which have electric charge feel the electromagnetic force, particles which have color charge feel the strong force. A quark can have one of three colors: red, green, or blue: Antiquarks are opposite: antired, antigreen, or antiblue: R, G, or B R, G, or B Eight types of gluons (the color singlet state cannot be a gluon): RG, RB, GR, GB, BR, BG, ( RR! GG) / 2, ( RR + BB! 2GG) / 6 QED is an Abelian gauge theory (photon is neutral). QCD is a non-abelian gauge theory (gluons have color charge) PHYS Fall

12 Two electrons exchange a photon Some interactions Two quarks exchange a gluon The color flow for the two quarks exchanging a gluon An example of the self-coupling of the gluon. PHYS Fall

13 Hadrons! Hadrons are composite particles made up of quarks. There are two known types:! Baryons are made up of three quarks (antibaryons are made up of three antiquarks). Examples are spin! proton (uud) and neutron (udd), spin 3/2 # ++ (uuu), # + (uud), # 0 (udd), # " (ddd), spin! $ 0 (uds), etc. Baryons have a baryon quantum number of +1 (which is a conserved quantity).! Mesons are made up of a quark and an antiquark. Examples are spin 0 % + ( ud ), % " ( ud), % 0 ( ud + ud ) / 2, K + ( us ). Mesons have baryon number = 0.! Quarks and gluons do not exist outside of hadrons. This principle is called confinement and can also be understood by the statement that only colorless states can exist (technically color singlet states). Can make a color singlet with one of each color or a color+anticolor.! Proton could be (u R u G d B ) or (u R u B d G ) or (u G u B d R )! % + could be ( u R d ) R PHYS Fall

14 One piece of evidence for color! Color explains the existence of # ++ (uuu).! Since quarks are spin! and # ++ has spin 3/2, this means all three quarks have the same flavor (up) and the same spin (+1/2).! Pauli exclusion forbids identical fermions from occupying the same space.! If the three quarks have different colors they are no longer identical. PHYS Fall