Forces & Particles (PHYSICS)

7/30/2019 Forces & Particles (PHYSICS)

1/24

Fu n dam en t a l For ces

&

Elem en t ar y Par t i cles


2/24

For ces o f N at u r e

Four forces responsible for all phenomena Gr av i ta t i ona l f o r ce ( 10 - 4 5)

interaction between masses (all particles)

most familiar to us

W eak f o r ce ( 1 0 - 8)

responsible for some nuclear decays and reactionsin stellar interiors

Elect rom agne t i c f o rce ( 10 - 2)

restricted to electrically charged particles holds atoms/molecules together

St r o n g ( n u cl ear ) f o r ce ( 1 )

holds nuclei together


3/24

How d oes a f or ce w or k ?

I ssu e : How is a force transmitted between particles not in directphysical contact with each other?

I n classi ca l phy si cs use concep t o f f i e ld

( r esu l t i ng i n act i on at a d ist ance ) :

A particle, by virtue of its presence somewhere, modifiesthe space around it, i.e. it creates a field

A second particle, a distance r away, is embedded in thisfield

The field acts on this second particle

Result: The second particle experiences the force acted

on it by the first particle

I n 2 0 t h cen t u r y p h y si cs ( q u an t u m m ech a n ics)use concep t o f exchange fo r ce :

Two particles interact with each other by exchanging a(virtual) particle between them


4/24

Vi r t u a l Ex ch an g e Par t i cle

An exchange particle (field quantum) is:

created (and emitted) by one of the interacting particles, isabsorbed by the other. This process produces the interaction

specific to an interaction (different for different interactions)

How can energy be conserved during this creation? QM: Energy measured in t is uncertain by E

Heisenber g Uncer t a in t y Pr inc ip le E t h / 2 No extra energy needed to create it! May exist for

short enough t between creation and absorption for

its energy E to obey HUP and thus not violate

energy conservation

is called a v i r t u a l particle (we never see it)

This exchange leads to a change in the momentum

and energy of the interacting particles (force)


5/24

Ex am p le: Yak aw a ( st r o n g ) Fo r ce

Prediction of exchange particle for nuclear (strong) force

Use range of nuclear force: 1.5 fm = 1.5 x 10-15 m

The longest time t a particle could exist, if moving withspeed of light c, and the corresponding E, using theH.U. P., would be:

Predicted (1935) new particle of 131 MeV rest energy

Discovered pion (1947) and measured rest energyEo = 140 MeV! (Rest mass mo: 140 MeV/c

2)

MeV

eVJxsx

sJx

t

E

sxx

x

c

xt

131

/106.1

1

105

1005.1

105103

105.1

1324

34

24

8

15

=

==

===

h


6/24

Ex ch an g e Par t icle Mass v s

Ran ge o f I n t er act i on Field quantum may have zero or non zero mass

The greater the mass the more energy needed for its

creation the shorter time it can exist (to not violateE-conservation, and be within HUP limits)

the shorter the range of the corresponding force

For zero mass, the range is infinite

CONCLUSI ON: The range of the force associated withthe exchange of an virtual particle is inverselyproportional to the mass of this particle

p

pn

n

+

e -e -

e -e -

Feynman Diagrams


7/24

Th e Fie ld Qu an t a

FORCE STRENGTH QUANTUM MASS(GeV/c 2)

RANGE(m)

Grav i ta t i ona l 10-45 Graviton? Zero Infinite 1/r2

Weak 10-8

W

, Z0

80, 91


8/24

St r u ct u r e o f Mat t er

( Up t o t he la t e 60 s)

Atoms consist of nuclei surrounded by

electrons bound to the nucleus through theelectromagnetic force

Nuclei consist of protons and neutrons bound

together by the nuclear force

The nuclear force is understood in terms of an

exchange of mesons Basis of successful models of nuclear structure

p

p

n

n

+


9/24

Cu r r en t Un derst an d ing

o f St r u ct u r e o f Mat t er

Protons, neutrons and mesons are not elementary particles They are composites of quarks

The most fundamental constituents of matter are quarks, leptons

Quarks interact through the exchange of gluons Individual quarks do not exist in isolation

Always bound together to form nucleons and mesons

Theory for nuclear force: Quantum Chromodynamics (QCD)


10/24

Par t i cle Sp in

Each nuclear particle has a property called spin Intrinsic angular momentum (rotation about their own axis)

One specific, fixed (not arbitrary) value for each particle

Comes in units of = h/2 (h = 6.626x10-34

J.s) It can only be either an integer or half-integer multiple of

h-bar (0, 1, 2 or 1/2, 3/2, 5/2)

Spin may serve as a criterion for classifying

particles Different statistics for each type of spin value

Half-integer spin particles are called Fermions

Obey Fermi-Dirac Statistics No two-particles in exactly the same state (Pauli Exclusion Principle)

Examples: e, p, n, quarks

Integer spin particles are called Bosons

Obey Bose-Einstein Statistics Examples: photon, gluons

h


11/24

Par t i cle Classi f i ca t i on - Par t i cle Zoo

Many particles are known (100s) - most are not elementary

Detecting patterns in data very useful - remember periodictable?

May classify nuclear particles by t he ir i n t e ract i on :

( 1 ) Had r o n s: They may experience a l l f ou r forces.Are NOT e lem ent a ry particles, have structure and size. Two

categories: Baryons - heavy particles (p, n, , , , antiparticles)

All have half-integer spin (fermions)

Some are stable (do not decay)

Mesons - less heavy (, , , K, antiparticles)

All have integer spin (bosons)

All are unstable


12/24


( 2 ) Lep t o n s: Do NOT experience the s t r o n gforce but experience all other three forces

Are elem en t a ry particles, no internal structure, zerosize (


13/24


( 3 ) Qu ar k s: Experience al l four forces Are e lem en ta ry pa r t i cl es, no internal structure, zero size

Are the const i t uen t s of h ad rons ( ba ry ons and m esons)

Come in 6 types (f l a v o rs): u (up), d (down), s (strange), c(charmed), t (top), b (bottom), plus a set of antiquarks

Have f rac t i ona l electric charge (+ 2/3 e, -1/3 e)

Have color charge (red, blue, green) Needed to satisfy Pauli Exclusion Principle ( -, sss, 3/2 )

Same colors repel, opposites (color-anticolor) attract

Different colors attract (less so)

All have spin 1/2 (fermions) Are no t f ound i so l at ed in the laboratory

Strong force increases with distance between quarks

Baryons are made of 3 quarks, mesons of 2 (qq-bar pair)

3 colors make up white = colorless

h


14/24


( 4 ) Fiel d Quan t a ( o r Gauge Bosons) :

Elect r om agne t i c i n t e ract ion

W + , W - , Zo W eak I n t er a ct i o n

Carry weak charge

8 g lu on s St r on g ( co lo r ) I n t er act ion

6 carry color 2 colorless

g r av i t on ? Gr av i t at ion al I n t er act ion

Not observed yet They are the force carriers

All have spin 1 (graviton 2) - (bosons)

All are elementary, no internal structure, no size


15/24

Som e Par t i cle H ist o r y

The plethora of hadrons led to the search for a more fundamentalset of particles out of which baryons and mesons would be built.

1963 - Gell-Mann and Zweig proposed such a model, where

baryons and mesons are composites of elementary constituents,labeled quarks. Baryons: 3 quarks. Mesons: one quark, one anti-quark.

For each quark there is a corresponding antiparticle, all

properties the same except for opposite electric charge. 1963 quarks proposed : u p , d o w n , s t range . Discovered early 70s

1967 c ha rmed quark proposed - discovered in 1974

cc-bar in J/psi SLAC/BNL).

1975 - Tau lep t on (SLAC) discovered

Led to proposal of 2 more quarks top, bottom.

1977 - B o t t o m quark discovered (bb-bar in Y-, Fermi Lab)

1995 - Top quark discovered (Fermi Lab)


16/24

Som e Par t i cle Pr op er t ies

CATEGORY PARTICLE MASS SPIN LIFETIME (s)

Hadrons Proton (p) 938.3 Stable

Neutron(n) 939.6 889

Omega (-) 2285 3/2 0.82x10-10

Pion (+,-) 139.6 0 2.6x10-8

Kaon (K+,K-) 494 0 1.2x10-8

Lep tons Electron (e-,e+) 0.511 Stable

Muon (-,+) 105.7 2.2x10-6

Tau (-,+) 1784 3.0x10-13

Neutrino () small StableFie ld Quant a Photon () 0 1 Stable

Z 91117 1 ~10-25

QUARK CHARGE SPI N MASS2


17/24

( e) (h/2) (MeV/c2)

Up ( u ) +2/3 2-8

Do w n ( d ) -1/3 5-15

St r a n g e ( s ) -1/3 100-300Ch a r m e d ( c) +2/3 1000-1600

To p ( t ) +2/3 1.8x105

Bo t t o m ( b ) -1/3 4100-4500

LEPTONS CHARGE( e)

SPI N(h/2)

MASS(MeV/c2)

Elect r on ( e -) -1 0.511

Mu o n ( -) -1 106

Tau (

-

) -1 1784Ele ct r o n Ne u t r i n o ( e) 0


18/24

Mor e Had r o n Pr o p er t ies

For Baryon properties:

C:\Documents and Settings\Dimitri\Desktop\baryon.html

For Meson properties:

C:\Documents and Settings\Dimitri\Desktop\meson.html
http://c/Documents%20and%20Settings/Dimitri/Desktop/baryon.htmlhttp://c/Documents%20and%20Settings/Dimitri/Desktop/meson.htmlhttp://c/Documents%20and%20Settings/Dimitri/Desktop/meson.htmlhttp://c/Documents%20and%20Settings/Dimitri/Desktop/baryon.html


19/24

Elem en t a r y Par t i cle Gen er a t ion s

1 2 Elem ent a ry Par t i cl esPlus 4 field quanta

Plus antiparticles3 Genera t i onsMasses of (II) > (I)

Masses of (III) > (II)(I) is for ordinary matterQ: Only three generations?

Only three observed

(1991, CERN)Therefore expect only three

generations


20/24

Un der st an d in g Elem en t ar y

Par t i cles an d t he i r I n t er act i on s ( 1 ) The St anda rd Model - I t in clu des:

Theory of the Elect row eak I n t eract i on

combines Electromagnetic and Weak Interactions

two aspects of a single unified electroweak interaction

same strength at very high energies (10-10 s after Big Bang)

symmetry breaking at low energies (mW,Z 0, m = 0) Spectacular successes (e.g. discovery of W, Z)

Predicts the Higgs boson (undetected at present)

Quantum Electrodynamics (QED)

Theory ofSt r o n g ( co lo r ) I n t er a ct i o n Force between quarks and gluons

Nuclear force is remnant of this force

Quantum Chromodynamics (QCD) - very complicated math


21/24

Un der st an d in g Elem en t ar y

Par t i cles an d t he i r I n t er act i on s ( 2 ) Einst e in s Th eo r y o f

Gener a l Re lat i v i t y Theory ofGrav i t a t i ona l I n t eract i on

Not a quantum theory, expected to fail at smalldistances


22/24

St and ar d Mode l

Man y Rem ain in g Qu est ion s... Are the current elementary particles really elementary?

Why do quarks and leptons have the mass they do?

Why are there only 3 generations of elementary particles?

Why does the electron and the proton have exactly the samecharge? They are different in almost every other way.

Why is the neutron heavier than the proton? The oppositewould be easier to understand - proton has electric charge

Why does the photon have zero mass but W,Z have mass?They mediate one single force (electroweak force)

Why does the W and Z have the mass they have?

Does the Higgs boson exist? It would explain these massesand symmetry breaking. Has not been seen yet (need TeV)


23/24

Fu r t h er Un i f i ca t ion o f For ces?

Elect r ow eak un i f i ca t i on - First successful step

Gr an d Un i f i ca t ion Th eor ies ( GUTs) - Next step

Would merge the Electroweak and Color Force Current Predictions:

Proton Decay (1031 years) - Not seen yet

Neutrinos have mass - Observed 1998

Hopeful signs for ultimate success

Ul t im a te goal : Include Gravity in Unification

Superstring Theory (theory of everything)

Particles: string-like structures; ~10-35 m

Needs 10-dimensional space-time

Extremely complicated math

The jury is still out...


24/24

Ev o lu t ion o f For ces in Na t u r e

Fr om t he Big Bang t o t he Pr esen t

1032

1027

1013

1019

1010

103

3

LHC RHIC

Temperature (K)