Download Quarks, Leptons, Bosons the LHC and All That

Quarks, Leptons, Bosons, the
LHC and all that.
Tony Liss
OLLI Lecture
September 23, 2008
Some HE Physicist Principles
• We are reductionists (and proud of it!)
– Our worldview is that there are a small
number of fundamental constituents,
interacting via a small number of forces, that
make up the Universe as we know it.
– This picture has worked extremely well for
about 2000 years.
– The modern version has been untangled
using particle beams of ever increasing
energy.
The Standard Model
The matter
around us is made
up of “quarks”
and “leptons”
A proton is made of
uud
Add an electron to
make a hydrogen atom
Electromagnetic
Strong
Weak
And held together
by four forces, each
with a force carrier:
????
Gravity
The Standard Model
The matter
around us is made
up of “quarks”
and “leptons”
Helium Atom
particleadventure.org
The marriage of quantum
mechanics and special
relativity required that
antiparticles exist.
Why High Energy?
• From quantum theory we know
l ~ 1/p Wavelength is inversely
proportional to momentum
If you want to see small things you need short wavelengths (that’s why
electron microscopes were invented) and short wavelengths means high
momentum (and energy).
• From relativity we know
E=Mc2
If you want to create a heavy particle (large M) you need a lot of energy.
Unification of the Forces
Electric
Magnetic
Weak
Strong
“Low
Energy”
Higgs Bosons
born here?
Electromagnetic
Electroweak
“High
Energy”
Theory works up
to ~here
That’s the region
we want to probe
with the LHC.
“Very
(very)High
Energy”
Fermilab
Protons & antiprotons collide at
2 TeV (2 x 1012
electron volts)
The worlds
highest energy
particle
accelerator!!
Fermilab Makes Top Quarks
The heaviest known
elementary particle.
Discovered in 1994!
Why is it so
heavy??
We don’t know
pp  tt
W b  e  eb



W b  udb
The Large Hadron Collider
The world’s largest, highest energy, accelerator 300 feet
underground outside of Geneva, Switzerland. The LHC collides
intense beams of protons 40 million times per second at “14 TeV”
France
Grapes
Switzerland
Cows
Inside the Tunnel
ATLAS Detector at CERN
ATLAS is VERY BIG
ATLAS
Who Is ATLAS?
• ATLAS is one of four large experiments at
LHC
– The ATLAS collaboration consists of
•
•
•
•
~2500 physicists including
~700 graduate students from
169 different institutions in
37 different countries
ATLAS is a United Nations of particle physics.
ATLAS, The Movie
http://atlas.ch
Some of What LHC Can Study
• Higgs Boson
– Understanding M
• Supersymmetry
– Dark Matter?
• Extra Dimensions
– Quantum Gravity/String Theory
• Dark Energy
– We don’t even know how to look for this
• Heavy gauge bosons
– New forces?
• Precision top quark studies
– New physics?
• Diboson production
– From the Higgs?
• Quark and lepton substructure
– Are fundamental particles fundamental?
• etc. etc.
Let’s Pick Two
• Higgs Boson
• Supersymmetry
What is “The Higgs”?
• Named after Peter Higgs
• It “gives mass” to the fundamental
particles (if, in fact, it exists)
Without the Higgs (or something) the theory
requires that all these fundamental particles have
M=0. But we know that’s not the case.
F=Ma
The idea is that the Higgs field exists throughout all
space. As particles try to move through this field they
interact with it and are “slowed down”. Heavier
particles are those that interact more strongly
M=F/a
In quantum mechanics there is a particle associated with a
field (quantum of the field). The photon is the quantum of the
electromagnetic field. The Higgs boson is the quantum of the
Higgs Field.
Finding the Higgs
• The Higgs “couples to mass”
– It decays to the heaviest particles available
Hard, but copious
Easy, but rare
This is a simulation of the
production and decay of a Higgs
to two Z bosons.
The Z bosons themselves decay,
one to a pair of electrons and the
other to a pair of muons.
m+m-
pp  H 0  Z 0 Z 0
e+e-
Supersymmetry (SUSY)
• Every quark, lepton and force
carrier has a SUSY partner
(sparticles).
– Sparticles would be made
copiously in the early (HOT)
universe.
– They all decay away quickly,
except for the lightest one
(neutralino), which has nowhere
to go.
Make SUSY
particles at an
accelerator:
pp  
www.science.doe.gov/hep/EME2004/03-what-is.html
Why is this an attractive idea?
SUSY & Unified Forces
• SUSY helps with unifying the forces.
Einstein’s
dream of
a “Unified
Field
• •SUSY
is a necessary
ingredient
of quantum
gravity
theories.
Theory”,
now needs SUSY:
• We know that the universe is filled with dark
matter. No SUSY
SUSY
EM matter is not made of quarks and leptons – the
– Dark
Standard Model has no dark matter candidates.
– Dark matter interacts very weakly with normal matter
weak
(or else
we would have found it already).
strong
– The lightest SUSY particle is a perfect candidate.
Energy
Energy
Dark Matter
Dark Matter’s Everywhere
In Galaxies
And
clusters of galaxies
Speed of stuff
Motion
out here
of a galaxy
out here
Doesn’t match luminous
matter in here!
Doesn’t agree with
luminous matter in here
The “Hydra” Galactic Cluster
Physics 211
• Momentum is “conserved”
– Before the protons collide they have equal and
opposite momentum: The total momentum is zero.
– Therefore: The total momentum of all the stuff created
in the collision must also be zero.
A Simulated SUSY Event
Missing momentum carried away by
invisible particle
About Those Black Holes…
• Creating microscopic black holes at the
LHC would be
– A MAJOR BREAKTHROUGH IN SCIENCE!
– INCREDIBLY EXCITING
– NOBEL PRIZE STUFF
– NOT AT ALL DANGEROUS
– REALLY
About Black Holes
• The microscopic black holes that might be
created at LHC are so small they
evaporate instantly according to Steven
Hawking.
– But what if Hawking’s wrong?
• Cosmic rays reach much higher energies than the
LHC and have been having collisions for billions of
years – any black holes created have not done
much damage.
About Black Holes
• Black holes don’t suck everything in.
– Only stuff inside the Schwarzschild radius.
– If the sun suddenly became a black hole, the
Earth’s orbit would not change (but we’d get
very cold). The Schwarzschild radius of the
sun is 3 kilometers.
– The Schwarzschild radius of a 1 TeV black
hole is about 10-18 m – that’s about 1/1000th of
the size of a proton.
• There’s nothing around such a black hole for it to
suck in!
Final Words
• After 25 years of planning and 15 years of
design and construction, the LHC is finally
about to turn on.
• This is the chance of a lifetime.
• Our understanding of the way the
Universe works is about to be
revolutionized.
– We just don’t know exactly how…