quarks, leptons and force intro
Post on 22-May-2017
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What IS Matter ?• Matter is all the “stuff” around you!
• Here’s the picture we’re going to uncover(not all today though)
Matter
Leptons
Charged Neutrinos
Forces
Weak EM
StrongGravity
Hadrons
Baryons Mesons
QuarksAnti-Quarks
The Quarks – a Recap
Proton
uu
d
Quarks Antiquarks
Q = +2/3 Q = -1/3 Q = -2/3 Q = +1/3
u d
c s
t b
u
c
t
d
s
b
Quarks can have 3 color values: red, green & blue Quarks have total spin S = ½ (SZ = -½ or +½) Anti-quarks have the same mass as their quark does.
Hadrons = Baryons + Mesons Baryons (antibaryons) contain 3 quarks (3 antiquarks) Mesons contain a quark and an antiquark
Why quarks?
Murray Gell-Mann 1969 Nobel Prize
in Physics
Why should nature be this complicated? To simplify the picture, and still account for this plethora of particles which were observed, Murray Gell-Mann proposed all these particles were composed of just 3 smaller constituents, called quarks.
But even Gell-Mann doubted that they were real…
An excerpt from Gell-Mann’s 1964 paper:“A search for stable quarks of charge –1/3 or +2/3 and/or stable di-quarks of charge –2/3 or +1/3 or +4/3 at the highest energy accelerators would help to reassure us of the non-existence of real quarks”.
In 1969, an experimentat SLAC uncovered thefirst evidence thatprotons in fact hadsubstructure
If neutrons & protons are not fundamental, what about
electrons?
Are they made up of smaller constituents also ?
As far as we can tell, electrons appear to be indivisible.
Leptons
Electrons belong to a general class of particles, called “Leptons”
As far as we can tell, the leptons are “fundamental”.
Each charged lepton has an uncharged partner called the “neutrino”
The leptons behave quite differently than the quarks
- They don’t form hadrons (no binding between leptons)
Are there other types of charged leptons (like the electron) ?
1932: Discovery of the positron,the “anti-particle” of the electron.
Anti-particles really exist !!!!!
1937 – Muons (and ) discovered in cosmic rays.
M() ~ 200*M(e)
The muon behaves very similarly to the electron (i.e., it’sa lepton).
Neutrinos
Fermi proposed that the unseen momentum (X) was carried off by a particle dubbed the neutrino ( ).
1934: To account for the “unseen” momentum in the reaction (decay):
Nobel Laureate: Enrico Fermi
np
e X
n p + e- + X
(means “little neutral one”)
Discovery of the neutrino
Detector: H2O w/Cadmium Chloride
Fred Reines and Clyde Cowan, 1956
1956: Existence of the neutrino confirmed at a nuclear reactor. (Nobel Prize)
Photon detectors
How many types are there ?1962: An experiment at Brookhaven National Lab showed thatthere were in fact at least 2 types of neutrinos.
p
n
Collide these neutrinos into protons
e e
Neutrinos which were produced in associationwith a only produce muons, never electrons ! There are at least 2 kinds of neutrinos
Never anelectron!
Lepton Picture up to now
Family Leptons AntileptonsQ = -1 Q = 0 Q = +1 Q = 0
1 e- e e+ e
2
Three happy families… In 1975, researchers at the Stanford Linear Accelerator discovered
a third charged lepton, with a mass about 3500 times that of theelectron. It was named the -lepton.
In 2000, first evidence of the ’s partner, the tau-neutrino () was announced at Fermi National Accelerator Lab.
Family Leptons AntileptonsQ = -1 Q = 0 Q = +1 Q = 0
1 e- e e+ e
3 families, just like the quarks… interesting !!!
This all looks Greek to me ?
e
e
electron
muon-minus
tau-minus
electron neutrino
muon neutrino
tau neutrino
Lepton (particle)
e
e
positron
muon-plus
tau-plus
electron anti-neutrino
muon anti-neutrino
tau anti-neutrino
Anti-lepton (anti-particle)
So here’s the big picture Quarks and leptons are the most fundamental particles of nature that we know about.
Up & down quarks and electrons are the constituents of ordinary matter.
The other quarks and leptons can be produced in cosmic ray showers or in high energy particle accelerators.
Each particle has a correspondingantiparticle.
Introduction to 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. ?
GravityGravity is the weakest of the 4 forces. The gravitational force between two objects of masses m1 and m2, separated by a distance d is:
F = Gm1m2/d2
G = gravitational constant = 6.7x10-11[N*m2/kg2]d = distance from center-to-center
The units of each are:[Force] = [Newton] = [N] [mass] = [kg] [distance] = [meters]
Gravity is only an attractive force
Electric Force – The Classical Picture-
+
In classical physics, one charge “exerts” a force on anotherby establishing a field at the location of the other charge
+
The “electric field”of the charge on theright exerts a force onthe one to the left.
++
The Electric Charge and ForceThe form of the electric force law between two charges q1 and q2 separated by a separation d is given by:
F = kq1q2 / d2
Like the gravitational force, F (1/d2)
k = electric constant = 9x109[N*m2/C2]d = distance from center-to-center
Units: [Force] = [Newton] = [N]; [charge] = [Coulombs] = [C] [distance] = [meters]
The electric force can be attractive or repulsive !
Direction of Electric Forces
Opposites charges attract
+ -
+ +
Like charges repel
- -
Strong Force
The strong force is the strongest of the known forces.
It is responsible for the binding of quarks intobaryons and mesons. Its residual effects also account for the binding of protons & neutrons in the nucleus.
This force behaves more like a “spring”. That is, the the force actually gets stronger as quarks move apart!
This in striking contrast to the EM & Grav. Force. Their forces decreases with separation (recall F 1/d2)
Weak Force
The weak force is the weakest of the known forces.
It is responsible for neutron decay, and decays of heavyquarks to the lighter quarks (we’ll see more of this later)
It’s interaction is very short range (as opposed to thelong range interactions of the EM and gravitational force)(we’ll see why this is so later, when we talk about the weak force)
Summary
EM
STRONG
WEAK
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