Mesons

• Mesons are particles with zero or integral spin so they are Bosons.
• The lightest meson is pion or π-meson, with other meson masses ranging beyond proton mass.
• All mesons are unstable and decays in various ways.

π-mesons

• This particle is to transmit nuclear forces, it must interact strongly with nuclei, and therefore it should be scattered and absorbed quickly by matter through which it passes.
• π mesons are thus hypothetical particles responsible for the nuclear forces and had properties pridicted by Yukawa.
• Protons and neurtons can be transferred into one anotherby emitting or absorbing one of these particles.
• There are three kinds of pions π⁺, π^- and π⁰. π^- is the anti particle of π⁺.
• These new particles can be thought of making bonds between (n,n) , (p,p) and (n,p) or (p,n)

K mesons

• These are heavier unstable particles and have a great variety of different decay modes.
• THere are six different ways that K⁺ mesons commonly decay, in each case giving two or three less massive particles

  K⁺→π⁺+π⁰

  K⁺→μ⁺+ν_μ

  K⁺→π⁺+π⁺+π⁰

  K⁺→π⁺+π⁰+π⁰

  K⁺→e⁺+ν+π⁰

• Mass of K⁺ is 966m_e.
• K^- are anti particles of K⁺ mesons and have the same decay modes with appropriate exchange of decay products for their anti particles.
• K⁰ and $\overline{K^0}$are anti particles.

Mesons (B=0) Bosons

Particle	Symbol	Mass MeV/c2	Mean life (s)	spin	S	Y	I	I3
Pion	π+	140	2.6×10-8	0	0	0	1	+1
	π0	135	8.7×10-17					0
	π-	140	2.6×10-8					-1
Kon	K+	494	1.2×10-8	0	+1	+1	1/2	+1/2
	K0	498	9×10-11
	K-	494	5×10-8					-1/2
Eta	η0	549	6×10-19	0	0	0	0	0

Leptons

• particles which are untouched by strong forces and which participates in weak interactions are called leptons.
• All these particles (leptons) also have their corresponding anti particles.
• Unlike baryons and mesons no heavier leptons have been detected.
• Electrons with its anti particle positron along with its associated neutrino and anti nutrino are leptons.
• Muon with their respective neutrino and anti particles of these are also leptons. Another pair of leptons known as tau,τ and its associated neutrino.
• All tau's are charged and decay into electrons, muons or pions along with appropriate neutrino.
• The breakdown of parity conservation in weak interaction has important consequence for leptons.
• The particles are left handed and anti particles are right handed.
• In normal weak interactions the particles (electrons, negative muons, neutrinos) behaves as if they were left handed screws; i.e., observer think they spin clockwise when they are travelling towards him.
• Anti particles are right handed screws; an observer thinks they are spinning counter clockwise as they approaches him.
• Nature of weak interactions, with its voilation of such established symmetries as parity, charge conjunction, isotopic spin and strangeness is physics greatest problems.
• Muons were first discovered in decay of charged pions

  Charged pion decay:

  π⁺→μ⁺+ν_μ

  π^-→μ^-+$\overline{{\nu }_{\mu }}$

  Neutral pion decay:

  π⁰→γ+γ

• Muon decay

  μ⁺→e⁺+ν_e+$\overline{{\nu }_{\mu }}$

  μ^-→e^-+ν_μ+$\overline{{\nu }_e}$

Leptons have (B=0) (fermions) and their properties are

Particle	Symbol	Spin	Le	Lμ	Lτ	Mass (in MeV)	Mean Life
electron	e-	1/2	+1	0	0	0.511	stable
muon	μ-	1/2	0	+1	0	106	2.2×10-6
tau	τ-	1/2	0	0	+1	1784	3.4×10-25
electron neutrino	νe	1/2	+1	0	0	0	stable
μ neutrino	νμ	1/2	0	+1	0	0	stable
τ neutrino	ντ	1/2	0	0	+1	0	stable

Photons

• It only participates in EM interactions.
• Strong and weak interactions are not in photon domain of experience.
• When particle annihilate with anti particles the end product is often protons.
• Photon is its own anti particle.
• Under some circumstances it can disappear and can create particle -anti particle pair.
• Rest mass of photon is zero.
• Photon is a boson with angular momentum equal to 1.
• Spin of photon is 1

elementary particles and fundamental interactions

• Elementary particles are those microscopic elementary constituents out of which all matter in this universe is made of.
• Bound neutron is stable but unbound neutron is unstable and it decays according to equation

  $n\to p+e^{-}+\overline{{\nu }_e}$ (anti-nutrino)

  half life of free neutron is 14 min 49 sec.

Fundamental interactions

There are four fundamental interactions between particles

(1) strong

(2) electromagnetic

(3) week

(4) gravitational

Interaction	Particles affected	range	relative strength	particles exchanged	Role in universe
(1) Strons	Quarks	∼10-15m	1	gluons	Holds quarks togather to form nucleus
	Hadrons			Mesons	Holds nucleons togather to form atomic nuclei
(2) Electromagnetic	charged particles	infinite	∼10-2	photons	determine structure of atoms, molecules , solids etc., Important factor in astronomical universe.
(3) Weak	Quarks and leptons	∼10-17m	∼10-5m	Intermediate bosons	mediates transformations of quarks and leptons; helps determine composition of atomic nuclei
(4) Gravitational	all	infinite	∼10-39m	gravitons	Assemble matter into planet , galaxies and stars

• Elementary particles can be divided into four groups

  (1) photons

  (2) leptons

  (3) mesons

  (4) baryons

Anti particles

• A particle identical with proton except for negative charge, i.e., negative proton or antiproton was created by bombarding protons in a target with 6 GeV protons thereby inducing the reaction

  p+p+energy(6 GeV)→p+p⁺

• Particle and anti particle annihilate each otherto give rise to a form of energy.
• positron is the anti particle of electron.
• There must be an anti oarticle corresponding to each particle.
• From the collection of anti particles a world of anti matter could be created.

Reletionship between particle and anti particles is

property	relationship
(1) mass	same
(2) spin	same
(3) magnetic monemt	of opposite sign but same magnitude
(4) charge	of opposite sign but same magnitude
(5) mean life in free decay	same
(6) annihilation	in pair
(7) creation	in pair
(8) total isotopic spin	same
(9) intrinsic parity	same for bosons but opposite for fermions
(10) strangeness number	of opposite sign but same magnitude