The Weak Force ? EM STRONG WEAK The Force Carriers Like the Electromagnetic & Strong forces, the Weak force is also mediated by “force carriers”. For the weak force, there are actually 3 force carriers: W+ W- Z0 These “weak force” carriers This “weak force” carrier carry electric charge also ! is electrically neutral The “charge” of the weak interaction is called “weak charge” 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: (a) Carry weak charge (b) The sum of their charges equals the charge of the W I will mainly talk about the W in the context of decays… Comparison of the Force Carriers Property EM Strong Weak Force Photon Gluon Carrier W+, W- Z0 (g) (g) Charge of Electrical & None Color Weak force carrier Weak Quarks & All quarks All quarks Couples to: Charged Quarks only & & leptons all leptons all leptons Infinite <10-14 [m] Range < 2x10-18 [m] < 2x10-18 [m] (1/d2) (inside hadrons) Notice that the weak force only operates at distances ~ < 10-18 [m] ! Particles & Forces Charged Neutral quarks leptons leptons (e,m,t) (n) Strong Y N N Electro- Magnetic Y Y N Weak Y Y Y Quarks carry strong, weak & EM charge !!!!! 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 it wasn’t there! Neutron decay This is how neutron decay really proceeds through the Weak Interaction e- u u u W- d d u d u d ne Neutron Proton Proton u u d d u d + e- + ne Neutron Proton Neutron Decay (cont) u u d d u d + W- Neutron Proton e- + ne n p + e- + ne But in fact, what’s really going on is this: d u + e- + ne Feynman diagram for weak decay du+e -+n e ne e- W- d u d d “Spectator u u quarks” Spectator quark(s): Those quarks which do not directly participate in the interaction or decay. Feynman diagram for weak decay (continued) Since the spectator quarks do not directly participate in the decay, we can just omit them… This yields the “quark-level” Feynman diagram! Is charge conserved ? ne 0 e - -1 W- d u -1/3 +2/3 Is Le conserved ? Decays of “heavy” quarks The heavy quarks decay to the lighter ones by “cascading down” t Q=+2/3 b Q=-1/3 c Q=+2/3 s Q=-1/3 u Q=+2/3 d Q=-1/3 What about the decay of a b-quark? bc+m -+n m Is charge conserved ? nm 0 m- -1 W- b c -1/3 +2/3 Notice: Here, the W- decays to a m- and nm Is Lm conserved ? What about the decay of a c-quark? cs+m ++n m Is charge conserved ? nm 0 m+ +1 W+ c s 2/3 -1/3 Notice: Here, I have the W+ decaying to a m+ and nm (could have been an e+ and ne as well). Is Lm conserved ? What about the decay of a b-quark? su+e -+n e Is charge conserved ? ne 0 e - -1 W- s u -1/3 +2/3 Is Le conserved ? 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. Quark Charge Mass [GeV/c2] top +2/3 ~175 tb W+(100%) bottom -1/3 ~4.5 b c W- (~90%) charm +2/3 ~1.5 b u W- (~10%) strange -1/3 ~0.2 up +2/3 ~0.005 c s W+ (~95%) c d W+ (~5%) down -1/3 ~0.010 s u W- (~100%) d u W+ (~100%) “Leptonic” Decay of W Once the W is produced, it must decay W- e- ne W+ e + n e W- m- nm W+ m + n m W- t - nt W+ t + n t 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 … “Hadronic” Decay of W Since quarks also carry weak charge, we can also get: - + W ud W ud It’s call “hadronic decay” because the W is decaying to quarks, which will form hadrons! u Check charge: W- (-2/3 + -1/3 = -1) d b c 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 ? One possibility… u Can, in fact, W- d form a p- b c B- u D0 u B- D0 Meson Meson == p d - u B- D0 p- “Hadronization” The process by which quarks “dress themselves” into hadrons u u u p0 W- W- u u u d d p- d d 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) ! Hadronization (cont) In fact, this process whereby quark-antiquark pairs are created can 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 with the strong force “snaps”, it always produces quarks and antiquarks of the same type. They are usually the lighter quarks, since they have lower mass, and thus are created more easily: uu , dd , ss Again Energy being transformed into mass ! Feynman Diagrams involving W force carriers Decay of a B- Meson Could end up as: u B- D0 p- W- B- D0 p-p0 d b c B- D0 p- p+ p- B- u D0 u etc Additional particles are created when the strong force produces more quark-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! W Decays Leptonic Hadronic W- e- ne W- hadrons W- m- nm Can be 1 or more W- t - nt hadrons produced W+ follows in an analogous way… see previous slides 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 + e n e +n e + - e- ne W- e+ ne time Check lepton number, charge conservation… The main points 1. Neutrons decay to protons through the weak interaction 2. The electron and neutrino in fact come from the Wene decay. 3. The W can decay into either lepton pairs or a quark-antiquark pair (ud). In the latter case, the quarks undergo hadronization into hadrons 4. Heavy quarks decay to lighter quarks via emission of a W particle. Since the W has charge, we get bcW- for example, but NOT bdW- !