elementary  particles    The fundamental constituents of matter e.g. electron, proton, neutrons etc. Studies with cosmic rays and later  with  accelerators* revealed  that there exist over 200 other short  lived  particles  (some  of  them with very small lifetime 10^-21 s  called  resonances). The elementary particles are classified into four groups   -   the   photon,   leptons, mesons  and  baryons;  the classification  arising  from  the particle spin mass and type of interaction.

The  first group has only one member - the photon which has zero  mass  and spin 1  (in  unit of h ). The leptons (lepton - light)  is  a family of particles with spin 1/2. The members of the  lepton family are, the electron and its associated neutrino, the  muon (m ) and its associated neutrino and recently discovered tau lepton   and   its  associated  neutrino,  along with  their antiparticles.  The  table EI  summarizes  the charge, mass and lifetime  for all the members of this group. The charged leptons participate  in both electromagnetic and weak interactions, while the neutrinos with zero electric charge can only interact by weak force.

The  mesons  (medium  mass)  are a group of eight particles, consisting  of  pions p ^-, p ⁺,p ^o, the kaons K⁺, K^-, K^o, K^o and h ^o. More  mesons  have  been  added  to the list (called the charmed particles  and  the  upsilon)  which  we will discuss later. The baryons (heavy mass) consist of proton, neutron, lambda L ^o , sigmas S ⁺,S ^-,  S ^o, xi's or cascades X ^o, X ^- and their antiparticles. The mesons and the baryons are collectively called hadrons  because they interact through strong force (the force that binds the nucleus). In table EII, the quantum numbers masses, life times for all hadrons are listed.

Table EI

             Table EII. The hadrons

Mesons(spin 0)

Baryons (spin 1/2)

Charmed  particles : In 1974, two groups, one led by Ting and the other  led  by Richter  found a new meson with mass 3.1 GeV.  This  particle  was named J (or y ). Such a particle was suggested  by  Glashow,  from considerations of symmetry. It was the  beginning  of  another  series  of particles both mesons and baryons  to  be  discovered soon afterwards. These particles were given  the name charmed particles, and a new quantum number charm was assigned to them.

The  upsilon  meson  :  In  yet  another  experiment at Fermilab, scientists  have  discovered a new particle of mass 9.5 GeV which is called upsilon, .

Quantum number and conservation laws : The conservation laws of mass, energy, linear momentum, angular momentum, charge also apply to  elementary particle reactions and decays, as any other physical process (see under conservation laws). In addition the conservation  of  baryon  number,  lepton  number, isotopic spin, strangeness and parity are also observed in elementary particle processes. Amongst  these conservation of baryon  number  and  lepton number are strong conservation laws, i.e. they always hold.

Conservation  of baryons : The baryon number B is defined +1 for baryons,  -1 for anti baryons and zero for all other particles. The  total  baryon  number  in any  reaction  or decay is always conserved.

D B = O

example: n ® p + e^- + n _e

B 1 = 1 + 0 + 0

Conservation  of  leptons : Lepton number for a lepton is +1 and for  anti lepton -1.  The  total lepton number of electrons and associated   neutrinos,   muons   and associated  neutrinos  are separately conserved.

example - (a) n ® p + e^- + n _e

L_e 0 = 0 + 1 - 1

(b) m ^- ® e^-+ n _e+ n m

Lm 1 = 0 + 0 + 1

L_e 0 = 1 - 1 + 0

Conservation  of  isotopic  spin:  Mesons  and  baryons occur in multiplets  e.g.  pions (p ⁺, p ^-, p ⁰ ), nucleons (n, p) etc. (see table  EII). These multiplets have approximately equal mass. This led  scientists  to  conclude  that  these  multiplets are different   charged   states   of the  same  particle.  A  quantity isotopic spin  (or isospin) is defined in analogy with angular momentum. For a value of isotopic spin I we have 2I+1 values of  I_z; - I, -I +1, ...........0, .........I-1, I. A group is represented by I. Members of the group are distinguished with I_z. For example for  nucleons (n and p) we have I= 1/2, and I_z  =+1/2  for proton and - 1/2 for neutron. Similarly for a group of  three particles like the pions (p ⁺,p ⁰,p ^-), I = 1 and I_z = +1, 0 and -1.

The  total  value  of I_z is conserved in elementary particle processes involving strong and electromagnetic interactions

example p ⁰ + p ® p ⁺ + n

I_z 0 +(1/2)= 1 -(1/2)

Conservation  of  strangeness  :  The  kaons and hyperons (heavy baryons  - lambda,  sigma, and cascade particles) are always produced   in pairs  in  strong interactions.  Moreover lifetime of these particles are much greater than 10^-3s, showing they  do not  decay  by  strong  interactions. To explain this phenomenon a new quantum number `strangeness' was introduced and particles with S ¹ 0 are called strange particles (see Table EII).

The total strangeness is conserved in strong and electromagnetic interactions. For processes involving weak interactions, D S is either 0 or 1 (selection rule)

example (a) S ⁰ ® L ⁰ + g (strong)

S - 1 = - 1 + 0

(b) L ⁰ ® p ^- + p

S - 1 ¹ 0 + 0, D S = - 1 (weak)

Instead of strangeness, one may use hypercharge, Y defined as

Y = S + B

Since D B  = 0 physical significance of D S = 0 and D Y = 0 is identical.

Conservation of parity: The quantum mechanical wave function* of  a  particle or a system of particles can either be symmetric (even  parity)  or  anti symmetric  (odd  parity). It was believed  that  parity would be always conserved. It was suggested theoretically by Lee and Yang that parity is not conserved in weak interactions. This was later verified experimentally By Wu et al.

Parity  is    conserved  in strong and electromagnetic interactions but not in weak interactions.

The  quark model : The quark hypothesis was introduced in 1963 by Gell  Mann and  Zweig  in  an  attempt to account for surprising regularities  in  hadron states. When  particles are arranged according to  the  values  of  quantum  numbers they are seen to  fall  into  regular  geometrical  patterns  (see figs. e5, e6 and e7). The two physicists postulated that all hadrons are made of  still smaller constituents called quarks and anti quarks. The quarks were named arbitrarily as up (u), down (d),and strange (s), with  fractional  charge (2/3)e for u and -(1/3)e for d and s. In Table  EIII,  all  the quantum numbers associated with the quarks are  listed. The spin  of a quark is 1/2 which can be oriented parallel   or antiparallel direction. The  three  quarks (u,d,s)  and  their  anti quarks  in various combinations form baryons and mesons whose quantum numbers are obtained by simply adding  those of the constituent quarks. Baryons have three quarks, anti baryons  consist three corresponding anti quarks. The mesons are formed by a quark and an anti quark. The c and b quarks have been  added  in order  to accommodate newly discovered charmed particles  (charm  quark) and upsilon (bottom or beauty quark). There  is expectation that a sixth quark (named top or truth, t)  also  exists, which will keep the symmetry with the number of leptons.

Another  important property of quark states called `color' has  emerged  from the study of hadron spectroscopy. A baryon known  as D <sup>++</sup> (mass 1238 MeV and spin 3/2) contains three identical quarks with parallel spin in a state which is symmetrical  to the exchange of a pair of quarks. This violates Pauli's   exclusion principle.  This  difficulty  has  been resolved  by  introducing  another  degree of freedom, a means to distinguish  otherwise  identical  quarks.  The  quarks are labeled  as  red, blue and green. In this scheme a hadron is a   `white' object. Therefore,  for  example D ⁺⁺ state  is (U_RU_BU_G)  in  which  three primary colors are mixed. In mesons a color  is  mixed with its anti color to give white. Therefore finally  we have 18 quarks, 18 anti quarks and 12 leptons which are basic building blocks of matter.

Even   though   free   quarks   have   never  been  seen, circumstantial evidence in its favor is too strong.

               Table EIII. Properties of quarks