ELEMENTARY PARTICLES

By:Anju sharmaAssociate Professor in PhysicsP.G.G.C.G.Sec- 11 Chandigarh

Physics Paper C BSc. III

DEFINITION

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).

TIMELINE OF DISCOVERY OF FUNDAMENTAL PARTICLES

At the beginning of 19th 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.

Rutherford’s Alpha Scattering apparatus:

qa-source

vacuum

gold foil

zinc sulphide detector

The AtomThe atom consists of two parts:

1. The nucleus which contains:

2. Orbiting electrons.

protonsneutrons

TIMELINE CONTINUED….

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.

SIGNIFICANCE OF POSITRON DISCOVERYThe 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 andform pure energy (EM Radiation).

Carl Andersonaward Nobelprize for thediscovery of thepositron

Carl Anderson1905-1991

THE PICTURE BY ~1932

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

DISCOVERIES IN COSMIC RAYS

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.

DISCOVERY OF THE MUON 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 [ms]

(m e + n + n)

Note that many muons are able to reach the earth from the upper atmosphere because of time dilation ! Because of their large speed, weobserve that their “clocks” run slow they can live longer !!!

DISCOVERY OF THE PION 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 p-meson or pion (p) for short.

p

Pion (p)comes to restbhere, and thendecays: p + + m n n

m

Two neutrinos are also produces but escapeundetected.b

e

Muon (m)comes to resthere, and thendecays: m e + + n n

Two more neutrinos are also produced but also escapeundetected.

Cecil Powell1903-19691950 NobelPrize winner

THE PLETHORA OF PARTICLES

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 newparticles.

Most of these particles areunstable and decay very quickly, and hence had not been seen in cosmic rays.

Notice the discovery of theproton’s antiparticle, theantiproton, in 1955 !

Yes, more antimatter !

TIMELINE CONTINUED…

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.

TIMELINE CONTINUED…

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 Xi particles were discovered.

Gradually the list of elementary particles grew up. Till date , there are around 200 elementary particles discovered.

MATTER & FORCES

MatterMatter

LeptonsLeptons

ChargedCharged NeutrinosNeutrinos

ForcesForces

WeakWeak EMEM

StrongStrongGravityGravity

HadronsHadrons

BaryonsBaryons MesonsMesons

QuarksAnti-Quarks

QuarksAnti-Quarks

Fundamental Forces

THE FOUR FUNDAMENTAL FORCES

1. Gravity

2. Weak Force

3. Electromagnetic force

4. Strong Force

Wea

ker

Stronger

All other forces you know about can be attributed to one of these!

Doesn’t that looklike George W. ?

THE NEW CONCEPT OF FORCE

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.

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 (g)The force carrier of the electromagnetic force is the

photon (g)

Richard Feynman, 1918-19881965 Nobel Prize in Physics

GRAVITATIONAL INTERACTIONSGravitation 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.

ELECTROMAGNETIC INTERACTIONS

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.

WEAK INTERACTIONS

1.Radioactive decays like Beta- Decay is a kind of weak interaction.

2.These are 1013 times weaker than the strong interactions.

3.The exchange particles for these is predicted to be W+ and W – bosons.

STRONG INTERACTION

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.

COMPARISON OF FOUR INTERACTIONS

Interaction Relative Magnitude

Range Carrier Particles

Characteristic Time (sec)

Gravitational Interaction

10-39  ∞ Graviton 10-16

Electromagnetic Interaction

10-3 ∞ Photon 10-20

Strong Interaction

1 10-5 m Pion, Kaon(Gluons)

10-23

Weak Interaction

10-14 10-17 m Intermediate Boson

10-10

CLASSIFICATION OF ELEMENTARY PARTICLES

Photons

Leptons

Mesons

Baryons

Classification Of Elementary

Particles

PHOTONS

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.

LEPTONS

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 .

MESONS

These are the particles which possess rest mass intermediate between 250 me and 1000 me. 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.

BARYONS

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(Λ0), 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.

Summary of Properties of Elementary Particles

Elementary Particles

Bosons

Massless Bosons

Photons

Gravitons

Mesons

0 mesons

-mesons

K-mesons

Fermions

Leptons

e-,e+,-,+,e,e

Baryons

nucleons

Proton, antiprot

on

Neutron,antineutr

on

Hyperons

Lambda,sigma,

Cascade,Omega

PARAMETERS

Rest Mass: Every particle by virtue of its existence exhibits the property of mass.

E=m0c2

This is known as rest mass energy. mo 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

PARAMETERS

(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 = 2Q

(vii) Hypercharge – Y Y is defined as twice the average charge Q of each particle

group. Y = 2QAlso, Y = S + B

CONSERVATION LAWS

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 (Iz) after and before the reaction remains the same.

CONSERVATION LAWS

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.

Quarks

HOW DO WE KNOW ANY OF THIS?

Recall that high energy particlesprovide a way to probe, or“see” matter at the very smallestsizes. (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 detail2) Can produce more massive objects, via E=mc2

FUNDAMENTAL PARTICLES

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] = 1x1012 [eV].

To what wavelength does this correspond? l =hc/E = (6.6x10-34)(3x108) / 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 theyare at least 1000 times smaller than the proton ! Same is truefor electron quarks (and electrons) are considered “fundamental”

Quark masses

up

down

strange

charm

bottom

top

0.001

0.01

0.1

1

10

100

0 1 2 3 4 5 6 7

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/c2])

The heavier quarks are notstable, and decay to lighter quarksquite rapidly

Example: t b (~10-23 [s]) b c (~10-12 [s])

c s (~10-12 [s]) su (~10-7-10-10 [s])

Mas

s [G

eV/c

2 ]

ANTI-PARTICLES TOO !

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:

, ,

, ,

u c t

d s b

Q= +2/3

Q= -1/3Particle

Q= -2/3

Q= +1/3Anti-

Particle

, ,

, ,

u c t

d s b

BACK TO MATTER & QUARKS…

Generations

I II III

Charge =-1/3

d(down)

s(strange)

b (bottom)

Charge =+2/3

u(up)

c (charm)

t(top)

QUARK CONFINEMENT

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…

PROTONS & NEUTRONS

To make a proton:We bind 2 up quarks of Q = +2/3and 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= -1/3with 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!

HADRONS/BARYONS

The forces which hold the protons and neutrons together in thenucleus 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!

MESONS

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?

ud

Q=+1, and it’s called a p+

M~140 [MeV/c2]Lifetime~2.6x10-8 [s]

What’s the charge of this particle?

sd

Q= 0, this strangemeson is called a K0

M~500 [MeV/c2]Lifetime~0.8x10-10 [s]

COMBINATION OF QUARKS