1. By:Anju sharma Associate Professor in Physics P.G.G.C.G. Sec- 11 Chandigarh Physics Paper C BSc. III

2.  Elementary particles or the fundamental particles are those microscopic elementary constituents out of which all matter in the universe is made of.  A particle is elementary if it has no inner structure (i.e not “made” of some even smaller entities).

3.  At the beginning of 19 th century, it was Dalton who put forward his atomic hypothesis to explain the structure of an atom. According to this hypothesis, atom is the ultimate constituent of matter.  In 1897, J.J Thomson found that electron {fundamental particle} is the constituent of matter.  Later on Rutherford discovered that an atom consists of positively charged nucleus and negatively charged electrons revolving around it.  In 1919, proton was discovered.  In 1922, neutral particle named neutron was discovered by Chadwick.

4. Rutherford’s Alpha Scattering apparatus:   -source vacuum gold foil zinc sulphide detector

5. The Atom The atom consists of two parts: 1. The nucleus which contains: 2. Orbiting electrons. protons neutrons

6. Thus the no. of elementary particles became four i.e.  electron  proton  neutron  photon  The photon is the quantum unit of radiation.  In 1930, P.M.Dirac predicted theoretically another particle called positron.

7. The positron discovery was the first evidence for ANTIMATTER. That is, the positron has essentially all the same properties as an electron, except, it’s charge is positive ! If an electron and a positron collide, they ANNIHILATE and form pure energy (EM Radiation). Carl Anderson award Nobel prize for the discovery of the positron Carl Anderson 1905-1991

8.  Electrons were discovered ~1900 by J. J. Thomson.  Protons being confined in a nucleus was put forth ~1905  Neutrons discovered 1932 by James Chadwick.  Quantum theory of radiation had become “widely accepted”, although even Einstein had his doubts

9.  1932 : Discovery of the antiparticle of the electron, the positron. Confirmed the existence and prediction that anti-matter does exist!!!  1937 : Discovery of the muon. It’s very much like a “heavy electron”.  1947 : Discovery of the pion.

10.  The muon was discovered in 1937 by J. C. Street and E. C. Stevenson in a cloud chamber.  Again, the source is cosmic rays produced in the atmosphere.  The muon behaves identally to an electron, except: It is about 200 times as massive  It’s unstable, and decays in about 2x10 -6 [s] = 2 [  s] (   e + + )  Note that many muons are able to reach the earth from the upper atmosphere because of time dilation ! Because of their large speed, we observe that their “clocks” run slow  they can live longer !!!

11.  C Cecil Powell and colleagues at Bristol University used alternate types of detection devices to see charged tracks (called “emulsions”) in the upper atmosphere.  In 1947, they annouced the discovery of a particle called the  -meson or pion (  ) for short.  Pion (  ) comes to rest bhere, and then decays:       Two neutrinos are also produces but escape undetected.b e Muon (  ) comes to rest here, and then decays:   e  Two more neutrinos are also produced but also escape undetected. Cecil Powell 1903-1969 1950 Nobel Prize winner

12. Because one has no control over cosmic rays (energy, types of particles, location, etc), scientists focused their efforts on accelerating particles in the lab and smashing them together. Generically people refer to them as “particle accelerators”. (We’ll come back to the particle accelerators later…) Circa 1950, these particle accelerators began to uncover many new particles. Most of these particles are unstable and decay very quickly, and hence had not been seen in cosmic rays. Notice the discovery of the proton’s antiparticle, the antiproton, in 1955 ! Yes, more antimatter !

13.  In 1935, Yukawa, a Japanese physicist predicted the existence of another fundamental particle called  meson when he explained the nuclear force due to which the nucleons remain inside the nucleas. These particles were later on discovered by Anderson and Nedermeyer in 1938.

14.  In 1950, two new particle was discovered, named as Lambda particle and other was called K-meson or Kaon.  Later on, other elementary particles lik sigma and X i particles were discovered.  Gradually the list of elementary particles grew up. Till date, there are around 200 elementary particles discovered.

15. Matter Leptons Charged Neutrinos Forces Weak EM Strong Gravity Hadrons Baryons Mesons Quarks Anti-Quarks Quarks Anti-Quarks

17. 1.Gravity 2.Weak Force 3.Electromagnetic force 4.Strong Force Weaker Stronger All other forces you know about can be attributed to one of these! Doesn’t that look like George W. ?

18. In the 1960’s, a new theory of interactions was developed. At the heart of it is the concept that: Forces are the result of the exchange of “force carriers” between the two particles involved in the interaction. The force carrier of the electromagnetic force is the photon (  ) Richard Feynman, 1918-1988 1965 Nobel Prize in Physics

19. Gravitation was the first interaction to be described mathematically.. 1.It is the weakest of all the forces and attractive in nature. 2.It depends only on inertia or mass of the objects. 3.The exchange particles for these interactions is predicted to be graviton with spin (-2) though it is not discovered experimentally.

20. 1.They are charge dependent,attractive as well repulsive in nature. 2.The quantum of electromagnetic radiation is photon. 3.They are 100 times weaker than strong interaction.

21. 1.Radioactive decays like Beta- Decay is a kind of weak interaction. 2.These are 10 13 times weaker than the strong interactions. 3.The exchange particles for these is predicted to be W + and W – bosons.

22. 1. It explains the stability of nucleus. 2. Their exchange particle is pion. 3. These interactions are charge independent and spin dependent. 4. Their strength is maximum.

23. InteractionRelative Magnitude RangeCarrier Particles Characteristi c Time (sec) Gravitational Interaction 10 -39 ∞ Graviton10 -16 Electromagnetic Interaction 10 -3 ∞ Photon10 -20 Strong Interaction 110 -5 mPion, Kaon(Gluons) 10 -23 Weak Interaction 10 -14 10 -17 mIntermediate Boson 10 -10

24. PhotonsLeptons MesonsBaryons Classification Of Elementary Particles

25. A photon is a quantum unit of radiation emitted or absorbed by electrons in the outer structure of electrons or by the nucleus within the nucleus. Each quantum or photon has an energy h, where h is Planck’s constant and is its frequency.

26. The particles whose masses are smaller than the masses of pions and possess angular momentum of ½(h) are called lepton (light particles). Electrons, protons, neutrinos and muons are the members of this group.

27. These are the particles which possess rest mass intermediate between 250 m e and 1000 m e. The angular momentum of meson is zero. Most of them owe their existence to cosmic rays. Π- mesons or pions, k-mesons or kaons are the members of this group.

28. The term baryon applies to all elementary particles having a mass equal to or greater than a nucleon. The angular momentum of these particles is ½(h). Baryons have been grouped into classes known as 1. Nucleons : protons. Anti protons, neutrons and anti neutrons are the members of this class. 2. Hyperons: the term hyperons refer to particle whose masses are greater than those of nucleons. The known particles belonging to this class are Lambda particle(Λ 0 ), Sigma particle(Σ), Xi- particle(Ξ) and omega particle(Ώ). The various hyperons may decay in a variety of ways but the end product is always a proton or neutron.

29. Summary of Properties of Elementary Particles

30. Elementary ParticlesBosonsMassless BosonsPhotonsGravitonsMesons  0 mesons  -mesons K-mesonsFermionsLeptons e-,e+,  -,  +, e, e BaryonsnucleonsProton, antiproton Neutron, antineutron Hyperons Lambda, sigma, Cascade, Omega

31.  Rest Mass: Every particle by virtue of its existence exhibits the property of mass. E=m 0 c 2 This is known as rest mass energy. m o is known as rest mass. Quantum numbers: (i) The nucleon number: N= number of nucleon (ii) The lepton number: L = +1 for leptons L =-1 for antileptons L= 0 for other particles. (iii) The baryon number: B = +1 for nucleons and hyperons B= -1 for anti nucleons and anti hyperons B= 0 for all other particles

32. (iv) Spin quantum number: The intrinsic angular momentum of a particle is called spin and is represented by a quantum number s. (v) Isospin: An isotopic spin can be defined just like the intrinsic spin in which strong interactions will be invariant under rotation. (vi) Strangeness – S S is zero for particles which are not strange and have non-zero value for strange particles. S = 2  Q (vii) Hypercharge – Y Y is defined as twice the average charge Q of each particle group. Y = 2Q Also, Y = S + B

33. In addition to the conservations of mass, energy, charge, linear momentum and total angular momentum other conservation laws which are obeyed by elementary particles are: 1. Law of conservation of Leptons : In any reaction, the total number of leptons is conserved. Every lepton particle is counted as +1 lepton whereas each antiparticle(lepton) is counted as -1 lepton. 2. Law of conservation of Baryon : In every reaction, the total number of baryons is conserved. Proton and neutron are counted as +1 baryon whereas anti-proton and anti-neutron are counted as -1 baryon. 3. The isospin conservation law : For any strong interaction, the toatl iso-spin (I) magnitude after and before the reaction remains the same. For any strong or electromagnetic interaction, the z-component of iso-spin (I z ) after and before the reaction remains the same.

34. 4. Conservation Hypercharge: For any Strong and electromagnetic interactions the total hyper charge after and before the reaction remains the same. 5. Law of conservation of Strangeness: For strong or electromagnetic interactions, the total strangeness is conserved.

36.  Recall that high energy particles provide a way to probe, or “see” matter at the very smallest sizes. (Recall Electron microscope example).  Today, high energy accelerators produce energetic beams which allow us to probe matter at its most fundamental level.  As we go to higher energy particle collisions: 1) Wavelength probe is smaller  see finer detail 2) Can produce more massive objects, via E=mc 2

37.  We consider quarks to be fundamental, because so far we have been unable to “break them apart”.  As we increase the momentum of particles in our accelerators, we are able to resolve, or see, deeper into matter.  We are currently able to accelerate particles to energies of ~1 [TeV] = 1x10 12 [eV].  To what wavelength does this correspond?  =hc/E = (6.6x10 -34 )(3x10 8 ) / 1.6x10 -7 = 1.2x10 -18 [m] So, if quarks were bigger than this, we would be able to discern their substructure. So far, they look to be smaller than this ! That is they are at least 1000 times smaller than the proton ! Same is true for electron  quarks (and electrons) are considered “fundamental ”

38. Quark masses  6 different kinds of quarks.  Matter is composed mainly of up quarks and down quarks bound in the nuclei of atoms.  The masses vary dramatically (from ~0.005 to 175 [GeV/c 2 ])  The heavier quarks are not stable, and decay to lighter quarks quite rapidly Example: t  b (~10 -23 [s]) b  c (~10 -12 [s]) c  s (~10 -12 [s]) s  u (~10 -7- 10 -10 [s]) Mass [GeV/c 2 ]

39. We also know that every particle has a corresponding antiparticle! That is, there are also 6 anti-quarks, they have opposite charge to the quarks. So, the full slate of quarks are: Q= +2/3 Q= -1/3 Particle Q= -2/3 Q= +1/3 Anti- Particle

41. Generations IIIIII Charge = -1/3 d (down) s (strange) b (bottom) Charge = +2/3 u (up) c (charm) t (top)

42. q Hadron Jail Proton  Quarks are “confined” inside objects known as “hadrons”. We’ll learn more about hadrons in a bit…  This is a result of the “strong force” which we will discuss later…  Quarks are “confined” inside objects known as “hadrons”. We’ll learn more about hadrons in a bit…  This is a result of the “strong force” which we will discuss later…

43. To make a proton: We bind 2 up quarks of Q = +2/3 and 1 down quark of Q = -1/3. The total charge is 2/3 + 2/3 + (-1/3) = +1 ! To make a neutron: We bind 2 down quarks of Q= -1/3 with 1 up quark of Q = +2/3 to get: (-1/3) + (-1/3) + (2/3) = 0 ! So, it all works out ! But, yes, we have FRACTIONALLY CHARGED PARTICLES!

44.  The forces which hold the protons and neutrons together in the nucleus are VERY strong. They interact via the STRONG FORCE.  Protons and neutrons are among a class of particles called “hadrons” (Greek for strong). Hadrons interact very strongly with other hadrons!  Baryons are hadrons which contain 3 quarks (no anti-quarks). Anti-baryons are hadrons which contain 3 anti-quarks (no quarks). Me too, me too… Wow, I’m somebody… I’m a Baryon!

45.  Mesons are also in the hadron family.  They are formed when a quark and an anti-quark “bind” together. (We’ll talk more later about what we mean by “bind”). What’s the charge of this particle? u d Q=+1, and it’s called a  + M~140 [MeV/c 2 ] Lifetime~2.6x10 -8 [s] What’s the charge of this particle? s d Q= 0, this strange meson is called a K 0 M~500 [MeV/c 2 ] Lifetime~0.8x10 -10 [s]