Transcript gravitational interaction

Introduction to particle physics
Hal
Particle physics want to answer

What are the “elementary” constituents
of Matter?

What are the forces that control their
behaviour at the most basic level?
What is elementary means ?

The word “elementary” is
used in the sense that such
particles have no known
structure, they are
“pointlike. ”

“elementary” depends on
the spatial resolution of the
probe used to investigate
possible structure.
Units in particle physics
History of Constituents of Matter
History of Constituents of Matter
Дми́ трий Ива́нович Менделе́ев
History of Constituents of Matter
Electrons were first discovered
as the constituents of cathode
rays. In 1897 British physicist J.
J. Thompson showed the rays
were composed of a previously
unknown negatively charged
particle, which was named
electron.
History of Constituents of Matter
fluorescent
screen
radioactive
source
aparticles
target
(very thin Gold foil)
Detector
(human eye)
Proton discovery
An atom
consistsatomic
of
For
Thomson’s
model
a positively
the
electriccharged
chargenucleus
“seen” by the
cloudindependent
of electrons
asurrounded
– particleby
is azero,
of impact parameter
 no significant scattering at large angles is expected
a - particle
impact
parameter
b
Atom: spherical distribution
of electric charges
History of Constituents of Matter
James Chadwick's 1932
discovery of the neutron
Neutron
Neutron discovery
Neutron discovery:
observation and measurement of nuclear recoils in an “expansion chamber”
filled with Nitrogen at atmospheric pressure
scattered neutron
(not visible)
incident
neutron
(not visible)
recoil nucleus
(visible by ionization)
Neutron discovery
Incident
neutron
direction
Plate containing
free hydrogen
(paraffin wax)
proton tracks ejected
from paraffin wax
Recoiling Nitrogen nuclei
Assume that incident neutral radiation consists
of particles of mass m moving with velocities v < Vmax
Determine max. velocity of recoil protons (Up) and Nitrogen nuclei (UN)
from max. observed range
Up =
2m
Vmax
m + mp
Up
m + mN
=
UN
m + mp
UN =
2m
Vmax
m + mN
From non-relativistic energy-momentum
conservation
mp: proton mass; mN: Nitrogen nucleus mass
From measured ratio Up / UN and known values of mp, mN
determine neutron mass: m  mn  mp
Present mass values : mp = 938.272 MeV/c2; mn = 939.565 MeV/c2
Antimatter
i
 c  

 
  a1 1  a 2 2  a 3 3   a 4 mc2
t
i  x
x
x 
E   m 2c 4  p 2c 2
Dirac equation
P.A.M. Dirac
two solutions
1928
For each solution of Dirac’s equation with electron energy E > 0
there is another solution with E < 0
What is the physical meaning of these “negative energy” solutions ?
Motion of an electron in an electromagnetic field: presence of a term
describing (for slow electrons) the potential energy of a magnetic dipole
moment in a magnetic field
 existence of an intrinsic electron magnetic dipole moment opposite to
spin
electron spin
electron
magnetic dipole
moment me
e
me 
 5.79  10 -5 [eV/T]
2me
Antimatter discovery
Cloud chamber photograph by
C.D. Anderson of the first positron
ever identified. The positron must
have come from below since the
upper track is bent more strongly
in the magnetic field indicating a
lower energy
Carl D. Anderson
1932
23 MeV positron
6 mm thick Pb plate
63 MeV positron
Cosmic-ray “shower”
containing several e+ e– pairs
Neutrinos
A puzzle in b – decay: the continuous electron energy spectrum
n  p  e - (Two body decay) electron total energy E = [M(A, Z) – M(A, Z+1)]c2
First measurement by Chadwick (1914)
Radium E: 210Bi83
(a radioactive isotope produced
in the decay chain of 238U)
b- decay: n  p + e- + n
(E. Fermi, 1932-33)
First neutrino detection
(Reines, Cowan 1953)
n + p  e+ + n
detect 0.5 MeV g-rays from e+e–  g g (t = 0)
Eg = 0.5 MeV
neutron “thermalization” followed by capture
in Cd nuclei  emission of delayed g-rays
(average delay ~30 ms)
2m
H2O +
CdCl2
I, II, III:
Liquid scintillator
Event rate at the Savannah River
nuclear power plant:
3.0  0.2 events / hour
(after subracting event rate measured
with reactor OFF )
in agreement with expectations
History of Constituents of Matter
Late 1950’s – early 1960’s:
discovery of many particles
at the high energy proton
accelerators (Berkeley Bevatron,
BNL AGS, CERN PS),
all with very short mean life times
(10–20 – 10–23 s,collectively named
“hadrons”)
ARE HADRONS ELEMENTARY PARTICLES?
History of Constituents of Matter
1964 (Gell-Mann, Zweig): Hadron
classification into “families”; observation
that all hadrons could be built from three
spin ½ “building blocks.” (named “quarks”
by Gell-Mann) The three quarks are
u(+2/3),d(-1/3),s(-1/3). And three
antiquarks ( u , d , s ) with opposite
electric charge.
Mesons: quark – antiquark pairs
Examples of non-strange mesons:
  ud ; -  u d ; 0  (dd - uu ) / 2
Examples of strange mesons:
K -  su ; K 0  sd ; K   s u ; K 0  s d
Baryons: three quarks bound together
Antibaryons: three antiquarks bound together
Examples of non-strange baryons:
proton  uud ; neutron  udd
Examples of strangeness –1 baryons:
  suu ; 0  sud ; -  sdd
Examples of strangeness –2 baryons:
0  ssu ; -  ssd
What are the “elementary” constituents
of Matter?
3 x 6 = 18 quarks
+ 6 leptons
= 24 fermions (constituents of matter)
+ 24 antiparticles
48 elementary particles
consistent with point-like dimensions
within the resolving power of present
instrumentation ( ~ 10-16 cm)
What are the forces that control their
behaviour at the most basic level?
Electromagnetic
interaction (all
charged particles)
Infinite
interaction
radius
Gravitational
interaction (all
particles)
Totally
negligible in
particle
physics
No static fields of forces
In Relativistic Quantum Mechanics static fields of forces DO NOT EXIST ;
the interaction between two particles is “transmitted” by intermediate particles
acting as “interaction carriers”
Example: electron – proton scattering (an effect of the electromagnetic interaction)
is described as a two-step process : 1. incident electron  scattered electron + photon
2. photon + incident proton  scattered proton
The photon ( g ) is the carrier of the electromagnetic interaction
Energy – momentum conservation:
Eg = 0
pg = p – p ’
scattered electron
( Ee , p’ )
incident electron
( Ee , p )
q
g
( | p | = | p ’| )
incident proton
( Ep , – p )
scattered proton
( Ep , – p’ )
“Mass” of the intermediate photon: Q2  Eg2 – pg2 c2 = – 2 p2 c2 ( 1 – cos q )
The photon is in a VIRTUAL state because for real photons Eg2 – pg2 c2 = 0
(the mass of real photons is ZERO ) – virtual photons can only travel over
very short distances thanks to the “Uncertainty Principle”
Weak interaction
The weak interaction is the only force affecting neutrinos
(except for gravitation). Its most familiar effect is beta
decay.
The weak interaction is unique in a number of respects:
1.It is the only interaction capable of changing flavour.
2.It is the only interaction which violates parity symmetry
P (because it almost exclusively acts on left-handed
particles).

3.It is mediated by massive gauge bosons.
W
W

W
-
W
-
Strong interaction
In particle physics, the strong interaction, or strong force, or
color force, holds quarks and gluons together to form
protons, neutrons, baryons and mesons. The interaction
radius  10 –13 cm.The theory about strong force is
quantum chromodynamics (QCD).
Each quark exists in three states of a new quantum number named “colour”
Particles with colour interact strongly through the exchange of spin 1
particles named “gluons”, in analogy with electrically charged particles
interacting electromagnetically through the exchange of spin 1 photons.
Strong interaction
Free quarks, gluons have never been observed experimentally;
only indirect evidence from the study of hadrons – WHY?
CONFINEMENT: coloured particles are confined within
colourless hadrons because of the behaviour of the colour forces
at large distances
The attractive force between coloured particles increases with
distance  increase of potential energy  production of
quark – antiquark pairs which neutralize colour  formation
of colourless hadrons (hadronization)
END
Conclusion
1937: Theory of nuclear forces (H. Yukawa)
Existence of a new light particle (“meson”)
as the carrier of nuclear forces
Relation between interaction radius and meson mass m:
Rint


mc
mc2  200 MeV
for Rint  10 -13 cm
Hideki Yukawa
Yukawa’s meson initially identified with the muon – in this case m– stopping
in matter should be immediately absorbed by nuclei  nuclear breakup
(not true for stopping m+ because of Coulomb repulsion - m+ never come close enough
to nuclei, while m– form “muonic” atoms)
Experiment of Conversi, Pancini, Piccioni (Rome, 1945):
study of m– stopping in matter using m– magnetic selection in the cosmic rays
In light material (Z  10) the m– decays mainly to electron (just as m+)
In heavier material, the m– disappears partly by decaying to electron,
and partly by nuclear capture (process later understood as m– + p  n + n).
However, the rate of nuclear captures is consistent with the weak interaction.
the muon is not Yukawa’s meson
1947: Discovery of the  - meson (the “real” Yukawa particle)
Observation of the +  m+  e+ decay chain in nuclear emulsion
exposed to cosmic rays at high altitudes
Nuclear emulsion: a detector sensitive to
ionization with ~1 mm space resolution
(AgBr microcrystals suspended in gelatin)
In all events the muon has a fixed kinetic energy
(4.1 MeV, corresponding to a range of ~ 600 mm in
nuclear emulsion)  two-body decay
m = 139.57 MeV/c2 ; spin = 0
Dominant decay mode: +  m+ + n
(and  –  m– + n )
Mean life at rest: t = 2.6 x 10-8 s = 26 ns
 – at rest undergoes nuclear capture,
as expected for the Yukawa particle
A neutral  – meson (°) also exists:
m (°) = 134. 98 MeV /c2
Decay: °  g + g , mean life = 8.4 x 10-17 s
 – mesons are the most copiously produced
particles in proton – proton and proton – nucleus
collisions at high energies
Four events showing the decay of a +
coming to rest in nuclear emulsion
CONCLUSIONS
The elementary particles today:
3 x 6 = 18 quarks
+ 6 leptons
= 24 fermions (constituents of matter)
+ 24 antiparticles
48 elementary particles
consistent with point-like dimensions within the
resolving power of present instrumentation
( ~ 10-16 cm)
12 force carriers (g, W±, Z, 8 gluons)
+ the Higgs spin 0 particle (NOT YET DISCOVERED)
responsible for generating the masses of all particles
Cosmic rays underground





At the surface, muons contribute more than a half to the total
cosmic ray flux.
The energy spectrum of muons: dN / dE E -3.7 (at E>1 TeV).
Only muons and neutrinos can penetrate to large depths
underground.
The background from cosmic-ray muons for underground
experiments will be considered later in the course.
Atmospheric neutrinos are hard to detect due to small
interaction cross-section. Nevertheless their flux has been
measured and the deficit of muon neutrinos has been
observed pointing to the neutrino oscillations - this will be
considered in detail later.
Lectures 1-2, slide 35
Experimental Astroparticle Physics
Dr.Vitaly Kudryavtsev
Cosmic rays underground



Lectures 1-2, slide 36
Review of Particle Physics:
pdg.lbl.gov (Cosmic Rays).
Muon flux as a function of
depth underground:
x (m w. e.) = depth (m)  
(g/cm3)
Neutrino-induced muons
dominate at x > 15 km w. e.
Experimental Astroparticle Physics
Dr.Vitaly Kudryavtsev
Energy loss of muons
dE
 a  bE ; E  E 0 if x  0
dx
where a  3 MeV/(g/cm 2 ) is the energy loss due to ionisation,
the sum of the fractional energy losses due to bremsstrahlung,
-
inelastic scattering;
(Z = 11, A = 22);
dE
 -dx ;
a + bE
b  4  10 -6 (g/cm 2 ) -1 is
pair production and
both numbers are valid for E > 1 TeV and standard rock
E

E0
x
dE
 -  dx ;
a + bE
0
1 a  bE
ln
 -x ;
b a  bE 0

a  bE
a  -bx a
-bx
-bx
 e ; a  bE  (a  bE 0 )e ; E   E 0  e - ;

a  bE 0
b
b
E=
a -bx
e - 1) E 0 e -bx
(
b
Lectures 1-2, slide 37
Experimental Astroparticle Physics
Dr.Vitaly Kudryavtsev
11
Feel weak force
“predicted”  later discovered
Neutrinos
100,000,000,000,000 per second pass
through each person from the Sun
Antiparticles
Equal and opposite properties
“predicted”  later discovered
Annihilate with normal particles
Now used in PET scans
1950s, 1960s
Many new particles created
in high energy collisions
Convert energy to mass. Einstein E = mc2
> 200 new “elementary” (?) particles
Institute of Physics
Peter Kalmus
Particles and the Universe
Thomson (1897): Discovers electron
1x10 -10 m
1x10 -15 m
0.7 x10 -15 m
 0.7 x10 -18 m
Neutron discovery
Neutron discovery:
observation and measurement of nuclear recoils in an “expansion chamber”
filled with Nitrogen at atmospheric pressure
scattered neutron
(not visible)
incident
neutron
(not visible)
recoil nucleus
(visible by ionization)
An old gaseous detector based
on an expanding vapour;
ionization acts as seed for the
formation of liquid drops.
Tracks can be photographed
as strings of droplets
Example: Rutherford’s scattering
fluorescent
screen
radioactive
source
a - particles
target
(very thin Gold foil)
Detector
(human eye)
h
6.626  10 -34 J s
-15
-13



6
.
7

10
m

6
.
7

10
cm
- 27
7
-1
ma v (6.6  10 kg )  (1.5  10 m s )
a-particle
mass
0.05 c
~ resolving power
of Rutherford’s
experiment