Transcript Physics 106P: Lecture 1 Notes

Atom Smashers
Particle Accelerators
and Detectors
Mats Selen, UIUC
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What’s the motivation ?
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How to build a particle accelerator.
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How to build a particle detector.
A great web site with lots of links is:
http://pdg.lbl.gov/particleadventure/english/index.html
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What’s the Motivation ?
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Try to make sense of the way the Universe works
Look for simple explanations to complicated looking
observations !
Complication
Simplification
There is lots of different “stuff” on earth
There are many different kinds of atoms
There are lots of elements
We can make “other” stuff (not made
of protons neutrons and electrons)
We don’t really understand this at a
fundamental level (Standard Model has
too many free parameters)
Small indivisible “Atoms”
Elements (Periodic Table)
Nucleus (protons,
neutrons) and electrons
Everything made
out of quarks & leptons
Smash More Atoms !
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How to build a Particle Accelerator
First get some particles:
ee e
eeee ee e e e
e eee
Electrons are easy: Just heat up a filament
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ENERGIZER
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Protons are also quite easy: Ionize hydrogen
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ENERGIZER
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Now Accelerate Them
We’d like to give them lots of kinetic energy (lets say 10 GeV)
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VanDeGraaff Accelerator
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10,000,000,000 volt
battery
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Do VdG & bubble demo
This can’t provide enough energy
(only a few million volts) !
Need bigger voltage (and a vacuum system) !!
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Linear Accelerator
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Linear Accelerator
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Linear Accelerator
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Linear Accelerator
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Linear Accelerator
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Linear Accelerator
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Linear Accelerator
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Some Examples
LINAC at Bates Lab (MIT)
(0.5 GeV)
LINAC at Fermilab
(0.4 GeV)
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A Bigger Example
SLAC (Stanford)
(50 GeV)
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But some people like to recycle...
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Have the particles go through
the same accelerator cavity
many times
Circular “track”
(high-vacuum beam-pipe)
This is called a
Synchrotron
Small linear
accelerator
Key ingredients:
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Need to make particles go in a circle
Need to keep everything synchronized
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Forces on Charged Particles
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Both electric and magnetic fields influence the motion of charges.
Magnetic field “north” points into screen
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E
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q
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F
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F = qE
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qX
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F = qv x B
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E/m demo
TV demo
Geiger demo
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Synchrotrons
bending magnet
Accelerating
section
Focussing
magnets
Vacuum tube
(beam pipe)
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Fermilab
Booster (8 GeV)
Main Ring (150 GeV)
Tevatron (1000 GeV)
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A slick trick
Two for the price of one
2000 GeV
1000 GeV
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1000 GeV
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CERN
NOW
LEP 200 GeV e+ e-
In 5-6 years
LHC 14000 GeV p p
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LEP Tunnel
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How to build a Particle Detector
Ideally, we want to measure everything (E, Px, Py, Px) about every particle
produced in the “collision”.
Charged Particles
Neutral Particles
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Most Common Approach
(for colliding beam experiments)
Detector “package” looks like a beer can
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The Parts
Vertex Detector to measure the
position of charged particles close to
their point of creation.
Solenoid (Magnet)
(Makes charged particles curve,
which tells us their momentum)
Drift Chamber to measure the
path of charged charged
particles.
Low density…mostly gas…so
particles don’t notice it much.
Calorimeter to measure the
energy of neutral particles.
High density…particles often
stop here.
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How it works
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All of the parts work more or less the same way:
Charged particles ionize the material they pass through,
and we can detect signs of this ionization.
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e-
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edetect electron
Geiger counter
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How it works
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2)
All of the parts work more or less the same way:
Charged particles ionize the material they pass through,
and we can detect signs of this ionization.
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e-
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e-
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scintillation
e-
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detect photon
Scintillator counter
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Lets Try It
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We don’t have an accelerator handy so lets look at cosmic
rays:
p
m
Cloud chamber
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Scintillation Detector
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Photons are produced in the plastic by scintillating
molecules when a cosmic ray muon whizzes through it.
These photons are detected by a photo-multiplier tube,
and result in pulses on the oscilloscope.
Wrapped scintillator
oscilloscope
Photo-multiplier
Singles
coincidence
beta
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Concluding Remarks
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The pursuit of High Energy Physics is motivated by a quest
for fundamental understanding.
Current accelerator and detector technology is very cool.
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The Periodic Table
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The Periodic Table Explained ?
Look carefully
proton
neutron
electron
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The nucleus is much smaller than the atom
This is about 500 times too big
Even so, the nucleus contains > 99.9 %
of the mass of the atom !
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Energy vs Mass
He (m=4.0026 u)
O (M=15.9995 u)
4 x He = 16.01 u
Mass difference = 0.01 u
= binding energy
So energy is the same as mass somehow ??
sure
E = mc2
Units
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Mass from Energy
KE1
KE2
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Mass from Energy
Ebefore = Eafter
Mostly KE
New Mass + KE
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Units
E = mc2
E=m
Physicists are sloppy
1eV = kinetic energy of an electron accelerated
through a 1 volt potential difference
This is a small amount of energy
(more appropriate for chemistry)
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Preferred unit of Energy/Mass = GeV
1 volt
battery
1GeV = 109 eV (a billion eV)
(about the mass of a proton…appropriate scale for us)
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It gets more complicated again...
p2
K* p
a1
K1 a
2
h
f4
a4
D*
W
K
Lc
D*2
f3
B
S
r
D2
D1
w
w
K*3
K*4
f’2
f0
w3
L
K*2
B*
f2
Sc
K3
h1
a0
Bc
K2
fj
D
h0
Ds
f1
Bs
f
h’
r3
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quarks
+ 2/3
- 1/3
up
down
quark charge
charm
strange
&
top
bottom
leptons
-1
0
e
m
t
n
nm
nt
e
lepton charge
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quarks & anti-quarks
+ 2/3
- 1/3
u
d
c
s
t
b
Now we can easily build any of
of the particles we have discovered
(and predict some we haven't)
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u
c
t
d
s
b
+ 1/3
(baryons)
proton
neutron anti-proton
(mesons)
p+
D0
Bs
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Example: CLEO
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Example: L3
=2X
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Vertex Detector (CLEO)
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Drift Chamber
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Calorimeter
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Solenoid
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