Nuclear and Particle Physics Module 6 By Dr. Manjula Sharma Asstt. Prof. (Physics) Deptt. of Physical Sciences and Languages COBS, CSKHPKV, Palampur, HP-176062 Mail Id: [email protected] Contact no: 7018006656 Muons (μ-) : This is the short name of μ–meson . Mesons were discovered in cosmic radiations. For several years these particles were mistaken for the particle whose exchange generated the nuclear forces. But these particles interacted very weakly with nuclei in contradiction to the expected property of mesons. Muons has mass 206 me and charge 1 unit (i.e. 1.6x10-19C), angular momentum (spin) ħ/2 . The muons are unstable particles with a half life 1.5x10-6s. μ+ is its antiparticle and there is no neutral muons. Muons decay according to : - - μ e + vμ + ve + + μ e + vμ + ve Hadrons. In particle physics,a hadron is a subatomic composite particle made of two or more quarks held together by the strong force in a similar way as molecules are held together by the electromagnetic force . Hadrons are categorized in two families : Mesons and Baryons. (a) Mesons (strong interacting bosons) : In particle physics mesons are hadronic sub-atomic particles composed of one quark and one anti-quark bound together by strong interactions . They have a diameter of roughly one femtometer (10-15 m). The mesons all have zero or integral spin i.e. 0,1,2,etc . They obey Bose-Einstein statistics. They have mass intermediate between leptons and nucleons . They are subjected to all three types of interactions i.e., strong ,weak and electromagnetic. Members of this group are the following : + 0 - + - 0 0 π-mesons (π ,π ,π ) , K mesons (K ,K ,K1 ,K2 ) , and η-mesons(ηo). (i) π-meson(or pions) : The pion is a spinless boson which exists in all the three states viz positive , negative and neutral. Pions can be produced in the laboratory and the simplest possible production equations for the three pions are : p + ϒ π+ + n p + ϒ πo + n d + ϒ π- + n Pions have following properties : Particle Mass Charge Spin Magnetic Parity Mean Moment life o -7 π 264me 0 0 0 odd 7x10 s + -8 π 273me +e 0 0 odd 1.8x10 s - -8 π 273me -e 0 0 odd 1.8x10 s πo is the anti-particle of itself and π- is the anti-particle of π+ . Decay modes : πo ϒ + ϒ (98.8 %) e+ + e- + ϒ (1.2%) π+ μ+ + ν (99.ss9 %) e+ + ν (0.01%) π- μ- + ν (99.9 %) e- + ν (0.01%) (ii) K-mesons(or Kaons): K-mesons are also called heavy mesons because their masses are about 970 times the electron mass. There are three types of kaons which are positive , negative , and neutral. Kaons have the following properties : Particle Mass Charge Spin Mean life + -8 K 976me +e 0 1.2x10 s - -8 K 976me +e 0 1.2x10 s 0 -8 K 975me 0 0 9x10 s K0 is anti-particle of itself and K- is anti-particle of K+ . (iii) η-mesons(ηo) : This meson is a neutral particle having rest mass 1073me and is unstable particle having mean life 7x10-19s and spin 0. it is the antiparticle of itself. Decay modes : ηo 2ϒ (~38%) πo + πo + πo (~30%) π+ + π- + πo (~24%) πo + π- + ϒ (~5%) πo + ϒ + ϒ (~3%) s Baryons(strongly interacting fermions) : These are fermions of half integral spin i.e. ½ . They have masses equal to or more than that of nucleon mass. Particle of this class which are heavier than nucleons are collectively known as hyperons. They are subjected to all three types of interaction i.e., strong , weak and electromagnetic. The members of this group are : 1.Nucleon(proton and neutron) . 2.Omega hyperons . 3.Cascade hyperons . 4.Sigma hyperons . 5.Lambda hyperons. (i) Nucleons : Protons and neutrons are called nucleons . (a) Protons (p): A proton is a stable particle having rest mass 1836 me and charge 1 unit . Its spin is ½ . p is the antiparticle of p. (b) Neutrons (n): A neutron is heavier than proton having rest mass 1839 me and no charge. Its spin is ½ . Anti-neutron(n) is the antiparticle of neutron(n). It is unstable having mean life 8.82x102 sec. (ii) Hyperons: Hyperons consist of following particles : (a) Lambda hyperons(Ʌ0). Lambda hyperons have rest mass 2184me and no charge. Its spin is ½ . Unstable particles having mean life 2.5x10-10 s. Ʌ0 is anti particle of Ʌ0 . Decay mode . Ʌ0 p + π- n + π+ (b) Sigma hyperons (Σ). There are three types of sigma hyperons Σ+ , Σo , Σ- and their antiparticles are Σ+ , Σo , Σ- respectively. The properties of various sigma hyperons are : Particle Mass Charge Spin Mean life + -11 Σ 2328me +e ½ 8.0x10 s + -14 Σ 2334me 0 ½ 10 s + -10 Σ 2342me -e ½ 1.5x10 s Decay modes : Σ+ p + π0 n + π+ Σo Ʌ0 + ϒ Σ- n + π- (c) Cascade hyperons : it is of two types neutral and negatively charged. The neutral hyperons has rest mass 2571me , spin ½ and unstable particle having mean life 3.0x10-10s and is antiparticle of itself . The negatively charged particle has 1 unit negative charge and rest mass 3290me . It has spin 1/2 and is unstable particle having mean life 1.7x10-10 . (d)Omega hyperons(Ω-) : Omega hyperon is a negatively charged particle having charge -e and rest mass 3290me . It has spin 3/2 and is unstable particle having mean life 1.3x1010s Ω+ is antiparticle of Ω- . Gravitons: The gravitational force of attraction between material particles due to their masses is assumed to be due to exchange of a particle called graviton. Till today , graviton is not confirmed experimentally . In string theory , believed to be a consistent theory of quantum gravity , the graviton is a massless state of a fundamental string . The graviton follows Bose – Einstein statistics. It is a spin –2 boson . The mass of a graviton is zero . It has zero electric charge . The term graviton was originally coined in 1934 by the Soviet physicists Dmitri Blokhintsev and F.M. Gal’perin. .