2.4 Particle interactions

Transcription

1 2.4 Particle interactions Particles interact through the four fundamental forces the electrostatic force, the strong nuclear force, the weak nuclear force and gravitational force. All articles with charge feel the electromagnetic force. All articles with mass feel the gravitational force. Only articles containing quarks exerience the strong nuclear force. Both hadrons and letons feel the weak nuclear force. A famous American hysicist, Richard Feynman suggested that the interaction is achieved by the articles exchanging virtual articles. In the case of charged articles (e.g. roton-roton), the electrostatic force exerienced by the articles comes from an interaction achieved by the exchange of virtual hotons. Feynman came u with a visual way of describing these interactions. They are called Feynman diagrams. Below is a Feynman diagram showing the electrostatic interaction between two rotons. time γ sace These diagrams should be read from bottom to to. This shows the events as time rogresses. We can see from the examle above that two rotons move towards each other, exchange a virtual gamma (γ) hoton and then move aart. (they reel each other). The wavy line reresents the exchange of a virtual gamma hoton. A model for exchange Below is a model to icture how exchange of virtual articles results in a force on two interacting articles. A 2016 fliedaroundhysics.com B Skater A throws a heavy ball to skater B. Skater A exeriences a force to the left. On catching, skater B exeriences a force to the right.

2 (1)! Using Newton s third law, exlain why A moves to the left on throwing the ball. (2)! Using Newton s third law, exlain why B moves to the right on catching the ball. (3)! Exlain how the momentum of the system (containing two skaters) is conserved. The second diagram models the effect of attraction between articles (e.g. electrostatic interaction between the electron and roton). A B Person A throws a boomerang to the left, which ushes them to the right. Person B catches the boomerang, which moves them to the left. The overall effect is attractive. The strong nuclear force The strong nuclear force acts between all articles containing quarks, which are constituents of articles (neutrons and rotons) in the nucleus. It acts over a very short range. The exchange article acting between nucleons is the virtual ion. (4)! Over what range is the strong nuclear force attractive? Gravitational force The exchange article for the gravitational force is the graviton. These haven t been detected yet. The weak nuclear force The weak nuclear force is resonsible for interactions which result in changes to the articles involved. W bosons are the exchange articles for the weak nuclear force fliedaroundhysics.com

3 Consider an interaction between an electron neutrino (υ " ) and a neutron (n). (Such interactions are extremely rare.) n + ν " + β ( We can see that the electron neutrino interacts with the neutron to roduce a roton () and a beta minus article (β ( ). The Feynman diagram for this interaction is shown below: n W ( υ " β ( Note: It is a good idea to draw an arrow to show the direction the exchange article moves. In this case, we could substitute a W * boson moving in the oosite direction. We can see the W minus exchange article (W ( ) is involved in changing a neutron into a roton. Note that the charge is conserved; the overall charge is zero before and after the interaction. (5)! An anti-electron neutrino (υ " ) can also (rarely) interact with a roton, roducing a neutron and a beta lus (β * )article. A W * boson is the exchange article. Draw a Feynman diagram for this interaction. The weak nuclear force is resonsible for changes which occur to nucleons during beta decay. For examle, in beta minus decay, a neutron decays to a roton and emits a beta minus article and an anti-electron neutrino: 2016 fliedaroundhysics.com n + β ( + υ "

4 We can show this weak interaction using a Feynman diagram: n W ( β ( υ " In this case the exchange article is a W ( boson. You can see that charge is conserved as the W ( boson carries a negative charge away from the neutron to the β ( (an electron). In beta lus decay, a roton changes to a neutron and emits a beta lus article and an electron neutrino. (6)! Write a symbol equation for beta lus decay. (7)! Draw a Feynman diagram for beta lus decay. Electron cature In some cases, where a nucleus is roton rich, a roton will cature an inner shell electron and transform to a neutron. (8)! What other article is roduced by electron cature? (9)! Draw a Feynman diagram for electron cature fliedaroundhysics.com

5 Electron-roton collision In a collision between an electron and a roton, a neutron can be roduced and another article. (10)! What other article is roduced by electron-roton collision? (11)! Draw a Feynman diagram for electron-roton collision fliedaroundhysics.com