What IS Matter ?

1. Hadrons Baryons Mesons Quarks Anti-Quarks What IS Matter ? • Matter is all the “stuff” around you! • Here’s the picture we’re going to uncover(not all today though) Matter Leptons Forces Charged Neutrinos Gravity Strong Weak EM

2. u d u Proton The Quarks – a Recap • Quarks can have 3 color values: red, green & blue • Quarks have total spin S = ½ (SZ = -½ or +½) • Anti-quarks have the same mass as their quark does. • Hadrons = Baryons + Mesons • Baryons (antibaryons) contain 3 quarks (3 antiquarks) • Mesons contain a quark and an antiquark

3. Why quarks? Murray Gell-Mann 1969 Nobel Prizein Physics Why should nature be this complicated? To simplify the picture, and still account for this plethora of particles which were observed, Murray Gell-Mann proposed all these particles were composed of just 3 smaller constituents, called quarks.

4. But even Gell-Mann doubted that they were real… An excerpt from Gell-Mann’s 1964 paper: “A search for stable quarks of charge –1/3 or +2/3 and/or stable di-quarks of charge –2/3 or +1/3 or +4/3 at the highest energy accelerators would help to reassure us of the non-existence of real quarks”. In 1969, an experimentat SLAC uncovered thefirst evidence thatprotons in fact hadsubstructure

5. If neutrons & protons are not fundamental, what about electrons?Are they made up of smaller constituents also ? As far as we can tell, electrons appear to be indivisible.

6. Leptons • Electrons belong to a general class of particles, called “Leptons” • As far as we can tell, the leptons are “fundamental”. • Each charged lepton has an uncharged partner called the “neutrino” • The leptons behave quite differently than the quarks - They don’t form hadrons (no binding between leptons)

7. Are there other types of charged leptons (like the electron) ? • 1932: Discovery of the positron,the “anti-particle” of the electron. Anti-particles really exist !!!!! • 1937 – Muons (m+and m-) discovered in cosmic rays. • M(m) ~ 200*M(e) • The muon behaves very similarly to the electron (i.e., it’sa lepton).

8. p n e Neutrinos 1934: To account for the “unseen” momentum in the reaction (decay): n  p + e-+ X X Nobel Laureate: Enrico Fermi Fermi proposed that the unseen momentum (X) was carried off by a particle dubbed the neutrino (n ). (means “little neutral one”)

9. Discovery of the neutrino • 1956: Existence of the neutrino confirmed at a nuclear reactor. (Nobel Prize) Photon detectors Fred Reines and Clyde Cowan, 1956 Detector: H2O w/Cadmium Chloride

10. p+ m+ n Collide these neutrinos into protons n n p Never anelectron! m+ Neutrinos which were produced in associationwith a m only produce muons, never electrons ! There are at least 2 kinds of neutrinos nm m ne e How many n types are there ? 1962: An experiment at Brookhaven National Lab showed thatthere were in fact at least 2 types of neutrinos.

11. Lepton Picture up to now

12. Three happy families… • In 1975, researchers at the Stanford Linear Accelerator discovereda third charged lepton, with a mass about 3500 times that of theelectron. It was named the t-lepton. • In 2000, first evidence of the t’s partner, the tau-neutrino (nt)was announced at Fermi National Accelerator Lab. 3 families, just like the quarks… interesting !!!

13. Anti-lepton (anti-particle) Lepton (particle) positron muon-plus tau-plus electron anti-neutrinomuon anti-neutrinotau anti-neutrino electron muon-minus tau-minus electron neutrinomuon neutrinotau neutrino This all looks Greek to me ?

14. So here’s the big picture • Quarks and leptons are the most fundamental particles of nature that we know about. • Up & down quarks and electronsare the constituents of ordinary matter. • The other quarks and leptons can be produced in cosmic ray showers or in high energy particle accelerators. • Each particle has a correspondingantiparticle.

15. Summarization of Matter Video Clip (~5 min)

16. Introduction to Forces

17. Weaker Stronger The Four Fundamental Forces • Gravity • Weak Force • Electromagnetic force • Strong Force Doesn’t that looklike George W. ? All other forces you know about can be attributed to one of these!

18. Gravity Gravity is the weakest of the 4 forces. The gravitational force between two objects of masses m1 and m2, separated by a distanced is: F = Gm1m2/d2 G = gravitational constant = 6.7x10-11[N*m2/kg2] d = distance from center-to-center The units of each are: [Force] = [Newton] = [N] [mass] = [kg][distance] = [meters] Gravity is only an attractive force

19. + + + Electric Force – The Classical Picture- In classical physics, one charge “exerts” a force on anotherby establishing a field at the location of the other charge The “electric field”of the charge on theright exerts a force onthe one to the left. +

20. The Electric Charge and Force The form of the electric force law between two charges q1 and q2separated by a separation d is given by: F = kq1q2 / d2 Like the gravitational force, F a (1/d2) k = electric constant = 9x109[N*m2/C2] d = distance from center-to-center Units: [Force] = [Newton] = [N]; [charge] = [Coulombs] = [C][distance] = [meters] The electric force can be attractive or repulsive !

21. Direction of Electric Forces Opposites charges attract + - Like charges repel + + - -

22. Strong Force • The strong force is the strongest of the known forces. • It is responsible for the binding of quarks intobaryons and mesons. Its residual effects also account for the binding of protons & neutrons in the nucleus. • This force behaves more like a “spring”. That is, the the force actually gets stronger as quarks move apart! • This in striking contrast to the EM & Grav. Force. Their forces decreases with separation (recall F a 1/d2)

23. Weak Force • The weak force is the weakest of the known forces. • It is responsible for neutron decay, and decays of heavyquarks to the lighter quarks (we’ll see more of this later) • It’s interaction is very short range (as opposed to thelong range interactions of the EM and gravitational force)(we’ll see why this is so later, when we talk about the weak force)

24. Summary EM STRONG WEAK