Understanding Antimatter: Properties, Creation, and Annihilation in Particle Physics

1. PH 103 Dr. Cecilia Vogel Lecture 24

2. Review • Nuclei • decay rate and number of nuclei • Matter Particles • leptons & quarks Outline • More Particles • antimatter • creation and annihilation • baryons & mesons

3. Antimatter Properties • For every matter particle there is an antimatter particle with • the same mass • always positive mass!!! • the same spin • opposite charge

4. Antimatter Matters • Antimatter is just accounting: • Lepton # = The number of leptons minus the number of antileptons is constant. • two electrons: lepton # is 2 • an electron and a positron: lepton # is zero • two positrons: lepton # is -2 • ALSO • Baryon # = (The number of quarks minus the number of antiquarks)/3 is constant.

5. Antimatter Matters • Imagine that Augustana currently has 50 more female students than male • & they decide to keep it that way forever • Then, if Augustana wants to admit a female, • would also have to admit a male • If a female student drops out or graduates • would also have to lose a male student • That’s how matter and antimatter work

6. Antimatter Matters • Antimatter is just accounting: • You can’t create a lepton • without creating an anti-lepton. • You can’t create a quark • without creating an anti-quark. • Note: you need to have enough energy to create the mass of both particle & antiparticle • Example: beta-plus decay • You can’t create a positron alone, • because, if you create anti-lepton (positron), you have to create a lepton, too. • Thus a positron is created, and so is a neutrino

7. Antimatter Matters • Antimatter is just accounting: • You can’t lose a lepton without losing an anti-lepton. • You can’t lose a quark without losing an anti-quark. • Example annihilation • If a particle meets its antiparticle, • the two can both disappear. • mass is lost! • The mass energy turns into other forms of energy • such as photons

8. PET • PET = Positron Emission Tomography • Beta-plus emitter emits a positron • which immediately annihilates with an electron • (electrons are everywhere) • mass of electron and positron disappears • two photons are produced • energy of photons =hf1+hf2 • Comes from the energy of the positron and electron = KE + 2mec2

9. It came from here PET • The two photons are detected. • Like with binocular vision, • if you have two rays, you can tell direction and distance! • 3-D image observed observed

10. Antimatter • Symbols for antimatter • For charged leptons, just show the opposite charge in superscript. • example e+, t+. • For all others, put a bar over the symbol • example ne, u

11. Antimatter e+ (positron) m+ t +

12. Quark Content • The charge of a particle made of quarks • is the sum of the charges of the quarks • The baryon # of a particle • also comes from adding the baryon #’s of the quarks • Quark content of a few particles can be found in text • otherwise figure it out from quark properties adding up

13. Baryons • Particles made up of three quarks • are called baryons • ex: proton is a baryon • made up of quarks uud • charge 2/3 + 2/3 - 1/3 = 3/3 = +1 • baryon # =1/3 + 1/3 +1/3 = 1 • ex: neutron is a baryon • made up of quarks udd • charge 2/3 - 1/3 - 1/3 = 0/3 = 0 • baryon # =1/3 + 1/3 +1/3 = 1

14. Mesons • Particles made up of a quark and an antiquark • are called mesons • ex: p+ is a meson • made up of ud • charge 2/3 + 1/3 = 3/3 = +1 • baryon # = 1/3 -1/3 =0

15. Integer Charge • Baryons all have integer charge • Any combination of 3 quarks will make -1, 0, +1, or +2 • -1/3 - 1/3 - 1/3 = -1 • -1/3 - 1/3 + 2/3 = 0 • -1/3 + 2/3 + 2/3 = +1 • +2/3 + 2/3 + 2/3 = +2

16. Integer Charge • Mesons all have integer charge • Any combination of quarks and anti-quark will make -1, 0 , or +1 • -1/3 - 2/3 = -1 • -1/3 + 1/3 = 0 • +2/3 - 2/3 = 0 • +2/3 + 1/3 = +1