introductory elemetary particle[1] group h

8/6/2019 Introductory Elemetary Particle[1] Group H

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GROUP H.



GROUP HGROUP H MUAZAM SHAH B. HASSAN

MUHAMMAD FARHAN B. ISHAK

MUHD.NOR AKBARVILDAN B.KAMARUDIN

NADIYAH BT. MD. YUSOFF



1. What makes a particle elementary ?

� A particle is elementary if it has no inner structure (i.e. not ³made´ of some even

smaller entities).



2. Which particles were consideredelementary throughout History?

Antiquity : Four elements. Unsuccessful attempt at an atomistictheory during the 5th century BC (Democritus).

18th century : Lavoisier and Dalton verify experimentally the validity

of the atomic structure. 1868 : Mendeleev proposes his chart of elements, containing the 63

atoms known at the time. The empty cases he left were soonfiled. By 1896, 77 atoms have been discovered, and are consideredelementary.

1897 : Discovery of the first subatomic particle by J.J Thompson :the electron. The search for its positive counterpart begins, until

1911 : Rutherford discovers the nucleus. Transmutation reactionsshowed that the hydrogen nucleus played a specific role (4

2He +14

7N --> 189F --> 17

8O + 11p) . Rutherford named it proton (protos =

first)



1932 : Chadwick discovers the neutron, which is not stable when isolated, and decays as follows : n p + e-

(+ ¯e) . The proton, electron and neutron account for

all the atoms of all the elements in the Universe.

This was the simplest elementary particle set everdescribed. A small number of particles, a small numberof interactions.

LEPTON (leptos = light) : e-

BARYONS (baryos = heavy) : p , n



However, some problems were already present.

1. The photon : Photoelectric effect ; Compton

scattering.2. Antiparticles : Discovery of the positron by Anderson(1932), studying cosmic rays. Many more particles wouldbe discovered in cosmic rays

3. Mesons : These particles were first postulated by Yukawa

(1935) to explain the force that binds the nucleustogether. Being of intermediate masses, they were calledmesons (mesos = middle).

4. Neutrinos : Necessary to preserve E conservation in

decay



From the particle garden to the jungle :In 1937, Anderson discovered the muon . The proved to be some sort of

heavier electron (lepton).

I.I Rabi, Nobel 1944

Who ordered THAT ?The muon decays into through decay:

+ e- +¯e

In 1947, pions (mesons) were detected in cosmic rays. They were thought of as Yukawas mediator particle for the strong interaction. The Universe was inorder again, except for the muon, which played no visible role.

In December 1947, new mesons were found : the kaons. The place got crowded again

With the use of particle accelerators in the 50s, many new particles werediscovered. Some of them were « strange » because they were produced bythe strong force but decayed through the weak force.



Moreover, some rules seemed to be missing to predict if a decay could occur or not :

Why is - + p+ K + + - possible , When - + p+ K 0 + n is impossible ?

In 1953, Gell-Mann and Nishijima came with a simple

and elegant idea. Each particle was to be assigned a«strangeness », and the overall strangeness had to beconserved during a collision (not through decay).

There were then THREE laws of conservations for

reactions : Charge

Baryonic number (proton like particles)

Strangeness



Still, there were dozens of elementaryparticles by 1960, either pion like(mesons) or proton like (baryons).

Mesons do not feel the strong interaction,whereas baryons do. Either type can bestrange or non-strange.In 1955, Willis Lamb started his NobelPrize acceptance speech by saying that

maybe physicists discovering a newparticle ought to be fined 10 000$

There was a strong need for

simplification, which Gell Mann providedin 1961. He acted like Mendeleev haddone a century before for chemistry. HisPeriodic Table was known as...

Fine them !



The Quark Model (1964)

ud

¯u ¯d

s

¯s

S=0

S=-1

S=1

Q=2/3Q=-1 /3

Q=-2/3 Q=-1 /3



Quark Hypothesis

Mesons are bound states of quark-antiquark : + is u ¯d. Baryons are bound states of three quarks : p is

uud.

The quarks as a model were confirmed by thediscovery of the - sss baryon of strangeness -3in 1964.

The existence of the quarks as particles wasconfirmed experimentally by Rutherford-like

experiments at SLAC in 1968 (Friedman,Kendall, Taylor). They are todays «elementary»particles, with the leptons and the mediatorparticles.



3. New particles again, but some symmetryand order gained...

Quark dynamics was understood later, andbrought 8 photon like mediator particles :gluons.

After a few years of quiet, the NovemberRevolution (1974) brought a new quark

(charm quark) through the discovery of theJ / meson (c ¯c). In 1975, the lepton was discovered. In 1977, the meson (b ¯b) was

discovered, introducing the bottom quark.

In 1983, the mediators for the weakinteraction were discovered at CERN : W+-

and Z0

The symmetry of six quarks and six leptonswas completed with the top quark in 1995.



Top quark discovery (Fermilab 1995)



Leptons The electron, the muon and their neutrinos, like the quarks,

appear to be fundamental. That is, so far, we are unable to find

that they are made up of anything smaller.

However, they behave very differently than the quarks. They have integral charge (0 or ±1).

They do not ³bind´ to form hadrons.

They do not participate in the strong interaction.

The electron, muon and neutrino belong to a general classof particles called LEPTONS.



Lepton have half ±integral spin. It can carry either one unit

electric charge or neutral.

The charges are electron, muons and taus.

Each of these types has a negative charge and a distinct mass.

Electrons is the lightest and has a mass only 0.0005.

Muons are heavier and have 200 times mass from electrons.

Taus, in turn has approximately 3700 times more massive than

electron.



Elementary particles today



Orders of magnitude for distances



4. Tomorrow

�There are no theoreticalreasons for the quarks to be the

final elementary particles.

� The electron is still being

probed, in search of an internal

structure.

� New accelerators (LHC) will

provide higher energies to

explore yet uncharted territories

(³small Big Bangs´) and maybe

discover new particles (Higgs

Boson). The Higgs is predictedby the Standard Model.



Conclusion

It is impossible to do justice to the subtlety and

richness of this field in a brief survey, or to give their

due to the thousands of researchers who have

painstakingly uncovered the beautiful structure that is

emerging. The interplay of imaginative

experimentation and daring conjectures has been a

source of wonder to all who have witnessed the

growth and maturing of elementary-particle physics.

introductory elemetary particle[1] group h

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