Quantum Physics

30.10 Strange Particles and Strangeness 991Quick Quiz 30.2Which of the following reactions cannot occur?(a) p p : p p p (b) n : p e e(c) : e (d) e : Quick Quiz 30.3Which of the following reactions cannot occur?(a) p p : 2 (b)(c) (d) p : K p : n 0 0 n : K Quick Quiz 30.4Suppose a claim is made that the decay of a neutron is given by n : p e .Which of the following conservation laws are necessarily violated by this proposeddecay scheme? (a) energy (b) linear momentum (c) electric charge (d) leptonnumber (e) baryon number30.10 STRANGE PARTICLES AND STRANGENESSMany particles discovered in the 1950s were produced by the nuclear interactionof pions with protons and neutrons in the atmosphere. A group of these particles,namely the K, , and particles, was found to exhibit unusual properties in theirproduction and decay and hence were called strange particles.One unusual property of strange particles is that they are always producedin pairs. For example, when a pion collides with a proton, two neutral strange particlesare produced with high probability (Fig. 30.8) following the reaction p : K 0 0 p : K 0 nOn the other hand, the reactionhas never occurred, eventhough it violates no known conservation laws and the energy of the pion is sufficientto initiate the reaction.The second peculiar feature of strange particles is that although they are producedby the strong interaction at a high rate, they do not decay into particles thatinteract via the strong force at a very high rate. Instead, they decay very slowly,which is characteristic of the weak interaction. Their half-lives are in the rangefrom 10 10 s to 10 8 s; most other particles that interact via the strong force havelifetimes on the order of 10 23 s.To explain these unusual properties of strange particles, a law called conservationof strangeness was introduced, together with a new quantum number S calledstrangeness. The strangeness numbers for some particles are given in Table 30.2.The production of strange particles in pairs is explained by assigning S 1to one of the particles and S 1 to the other. All nonstrange particles areassigned strangeness S 0. The law of conservation of strangeness states thatwhenever a nuclear reaction or decay occurs, the sum of the strangeness numbersbefore the process must equal the sum of the strangeness numbers after theprocess.The slow decay of strange particles can be explained by assuming that thestrong and electromagnetic interactions obey the law of conservation of strangeness,whereas the weak interaction does not. Because the decay reaction involvesthe loss of one strange particle, it violates strangeness conservation and hence proceedsslowly via the weak interaction.Courtesy Lawrence Berkeley Laboratory, University of CaliforniaFigure 30.8 This drawing representstracks of many events obtainedby analyzing a bubble-chamber photograph.The strange particles 0and K 0 are formed (at the bottom)as the interacts with a protonaccording to the interaction. (Note thatthe neutral particles leave no tracks,as is indicated by the dashed lines.)The 0 and K 0 then decay accordingto the interactions 0 : p andK 0 : . p : 0 K 0 Conservation of strangenessnumber