The Weak Force

1. The Weak Force ? EM STRONG WEAK

2. The Force Carriers • Like the Electromagnetic & Strong forces, the Weak force is alsomediated by “force carriers”. • For the weak force, there are actually 3 force carriers: W+ W- Z0 These “weak force” carrierscarry electric charge also ! This “weak force” carrieris electrically neutral The “charge” of the weak interaction is called “weak charge”

3. Weak Charge of Quarks & Leptons Both quarks & leptons carry weak charge • Both quarks & leptons “couple to” the W and Z force carriers • Since the W’s have a charge of +1 and –1 they cause a “charge-changing” interaction. That is when they are emitted or absorbed, to conserve charge, the “emitting” or “absorbing” particle changes charge by +1 or –1 unit. • The emitting or absorbing particle changes into a different particle. Alternately, when the W decays, it decays into 2 particles which: • Carry weak charge • The sum of their charges equals the charge of the W  I will mainly talk about the W in the context of decays…

4. Comparison of the Force Carriers Notice that the weak force only operates at distances ~ < 10-18 [m] !

5. quarks Charged leptons(e,m,t) Neutral leptons(n) Y Strong N N Electro-Magnetic N Y Y Y Y Y Weak Particles & Forces Quarks carry strong, weak & EM charge !!!!!

6. Neutrinos • From the previous table, you saw that neutrinos only interact via the weak force. • Also, the weak force only “kicks in” for d <10-18 [m]. Recall that the nucleus’ size is about 10-15 [m], so this is 1000 times smaller than the size of the nucleus. • As a result, neutrinos can pass through a lot of matter, and do absolutely nothing!!!! • After all, matter is mostly empty space, right ! • Neutrinos can easily pass right through the earth, almost as if itwasn’t there!

7. W- u u u d d d d u u Proton Proton Neutron Proton Neutron e- u u + + d e- d u d Neutron decay This is how neutron decay really proceeds through the Weak Interaction

8. + W- W- + e- + + n p e - Proton Neutron But in fact, what’s really going on is this: u u + + d u e - d d u d Neutron Decay (cont)

9. Feynman diagram for weak decayd  u + e - + ne e- W- dd u ud u “Spectator quarks” Spectator quark(s): Those quarks which do not directly participate in the interaction or decay.

10. 0 e- -1 W- d u -1/3 +2/3 Feynman diagram for weak decay(continued) • Since the spectator quarks do not directly participate in thedecay, we can just omit them… • This yields the “quark-level” Feynman diagram! Is charge conserved ? Is Le conserved ?

11. Decays of “heavy” quarks The heavy quarks decay to the lighter onesby “cascading down” Q=+2/3 t Q=-1/3 b Q=+2/3 c Q=-1/3 s Q=+2/3 u d Q=-1/3

12. 0 m- -1 W- b c -1/3 +2/3 Notice: Here, the W- decays to a m- and nm What about the decay of a b-quark?b  c + m- + nm Is charge conserved ? Is Lm conserved ?

13. nm 0 m+ +1 W+ c s 2/3 -1/3 What about the decay of a c-quark?c  s + m+ + nm Is charge conserved ? Notice: Here, I have the W+ decaying to a m+ and nm (could havebeen an e+ and ne as well). Is Lm conserved ?

14. 0 e - -1 W- s u -1/3 +2/3 What about the decay of a b-quark?s  u + e- + ne Is charge conserved ? Is Le conserved ?

15. Decays of heavy quarks to u & d • A quark can only decay to a lighter quark. • The W charge has the same sign as the parent quark. t  b W+(100%) b  c W- (~90%) b  u W- (~10%) c  s W+ (~95%) c  d W+ (~5%) s  u W- (~100%) d  u W+ (~100%)

16. W- e-ne W+ e+ne W- m-nm W+ m+nm W- t-nt W+ t+nt “Leptonic” Decay of W Once the W is produced, it must decay It’s call “leptonic decay” because the W is decaying to leptons! The W can decay to leptons because leptons carry weak charge But so do quarks …

17. W- d b c “Hadronic” Decay of W Since quarks also carry weak charge, we can also get: It’s call “hadronic decay” because the W is decaying to quarks, which will form hadrons! Check charge: (-2/3 + -1/3 = -1) But quarks are bound to one another by the strong force, and are not observed as “free” particle. That is, they are bound up inside hadrons… What happens next ?

18. Can, in fact,form a p- B- D0 W- d == p- b c d u One possibility… D0 Meson B-Meson B- D0p-

19. p0 W- W- u d p- d d d “Hadronization” The process by which quarks “dress themselves” into hadrons As the quarks separate, the “potential energy” stored in this “spring-like” force increases. Eventually, the potential energy gets large enough, and nature relives itself by converting this potential energy into mass energy. That is, quark-antiquark pairs are created ! The quarks then pair off and form hadrons (which we can see) !

20. Hadronization (cont) • In fact, this process whereby quark-antiquark pairs are createdcan happen more than once ! • One might therefore get 2, 3, or more hadrons from a hadronic W decay! • It’s important to note that when the “spring” associated withthe strong force “snaps”, it always produces quarks and antiquarks of the same type. They are usually the lighter quarks, since theyhave lower mass, and thus are created more easily: Again Energy being transformed into mass !

21. W- d b c Feynman Diagrams involvingW force carriers Decay of a B- Meson Could end up as:B- D0p-B- D0p-p0B- D0p- p+ p-etc to hadrons B- D0 • Additional particles are created when the strong force produces morequark-antiquark pairs. They then combine to form hadrons! • Notice that the charge of the particles other than the D0 add up to the charge of the W- (Q = -1), as they must!

22. Leptonic Hadronic W- e-ne W- hadrons W- m-nm Can be 1 or morehadrons produced W- t-nt W Decays W+ follows in an analogous way… see previous slides

23. Position time Interactions involving W’s Here is one… Don’t worry about these types of interactions… I want to emphasize the role of W’s in decays of quarks e- W- e+ Check lepton number, charge conservation…

24. The main points • Neutrons decay to protons through the weakinteraction • The electron and neutrino in fact come fromthe Wene decay. • The W can decay into either lepton pairs or a quark-antiquark pair (ud). In the latter case, the quarks undergo hadronization into hadrons • Heavy quarks decay to lighter quarks viaemission of a W particle. Since the W has charge,we get bcW- for example, but NOT bdW- !