LHC

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LHC the greatest experiment
on Earth
the origin of mass
Prof Nick Evans
University of Southampton
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CERN - Geneva
Probing the structure of matter
LHC will begin science in 2008
The ring is 27km round and on average 100m
underground
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The Large Hadron Collider will collide the nuclei
of atoms with 10 times higher energy than has
previously been achieved (14 TeV)
1232, 35 ton, superconducting dipole magnets
accelerate ions and focus them into bunches for
collision
36,000 tons of coolant below 2K!
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Proton-Proton collisions (hydrogen atom nuclei)
14 TeV centre of mass energy
100 billion protons per bunch 20 collisions per
crossing 1 crossing every 25ns 600 million
collisions per second
To store all collision data would involve storing
10 Petabytes of data a year ie a 20km high stack
of CDs more than can be made
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Detectors at collision sites Atlas
CMS LHC-b Alice
Track particles Measure energy Measure momentum
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LHC Budget - 2.6 billion over 10 years
CERN is 20 European member states plus many
international contributors
Football on Sky for 3 years - 1bn Film Titanic
has grossed - 1bn Tesco 2006 revenue
- 19bn UK NHS yearly expenditure -
20bn UK Army yearly budget - 35bn
Spin Offs CERN invented the www
and gave it away
Amazon revenue 2006 - 7bn
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The Story So Far
Electrons and their electromagnetic interactions
are responsible for chemistry and day to day
forces
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Special Relativity
The speed of light is the same for any observer
This means nothing can travel with light
nothing can reach vc!
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2
E mc
2
2
1-v /c
Rest mass energy
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Quantum Theory- Wave-Particle
Duality
The energy in waves comes in lumps or quanta E
h f
The photon is the quantum of the electromagnetic
field/ light
Sine wave has definite momentum but indefinite
position
A definite position state is a Fourier sum of all
momentum sine waves
A compromise wave packet satisfies
x p gt h
t E gt h
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Diracs Legacy
Electrons can absorb photons
But in Relativity, rotating this in space-time
gives
The electron travelling back in time is a hole or
anti-particle
Every particle has a twin of the same mass but
precisely opposite charges particles and
anti-particles annihilate into photons.
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Gauge Theory - QED
How do you know which to call particle and which
anti-particle?
Nature has the same problem it may make a
different choice in causally disconnected bits of
space
Nature has invented an interaction so that two
charged particles can probe the choice each other
made that force is electro-magnetism.
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Understanding Mass - The Quantum Vacuum
E t gt h
The vacuum can borrow energy for short periods
E mc
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The borrowed energy can be used to create
particles
(You cant just create an electron because of
charge conservation - but can create electron
positron pair)
The quantum vacuum is a seething mass of
particles appearing and disappearing constantly.
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How Can You Tell?
The virtual particle pairs interfere in
electron scattering processes.
g-2 is tested to 13 sig figs!
The effective charge seen in two electron
scattering depends on the separation of the
electrons.
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The Strong Nuclear Force
Quarks come in 3 colours!
The strong nuclear force is described by a gauge
theory except that the 8 gauge fields, gluons,
carry colour charge..
This difference changes the way in which the
vacuum is polarized so that
asymptotic freedom
Gross, Politzer, Wilczek
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Confinement
You can never pull hard enough to liberate a
quark from a proton
The Quantum Vacuum
Every so often quantum effects create a quark
anti-quark pair. The attractive force is so
strong that
binding energy gtgt mass energy
The vacuum has lower energy if it fills itself
with quark anti-quark pairs!
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The vacuum is really full of quark anti-quark
pairs with a density like that of an atomic
nucleus (10 grams/cm ) !!
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The Proton Mass
The quark pairs are responsible for the protons
mass
Interaction energy provides proton mass
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Strongly coupled QCD is a tough maths problem
how do we compute beyond perturbation theory?
QCD Strings
String theory gets meson properties right because
a q anti-q pair look like a string
A string is a one dimensional object with tension
BUT relativistic strings like to live in 10
dimensions!
String theory contains quantum gravity
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Gauge Gravity Duality
In recent years we have realized that strings in
10d are in fact the QCD string a weird and
wonderful alternative description of quarks and
glue
Maldacena
The extra dimensions are holographic creations.
EG a quark is a string with an up label on one
end and a colour label on the other
Classical General Relativity computations solve
strongly coupled quantum problems!
If the space-time stretches it the quark becomes
massive
Is real gravity a hologram??
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Quarks in a Dense QCD Plasma
Computations of gravity wave propagation tell us
about transport properties of a quark gluon plasma
Larry Yaffes calculations of the shock wave
produced by a moving quark
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The ALICE Connection
A lead-lead collider at LHC
In heavy ion collisions we squeeze quarks
together testing asymptotic freedom. At LHC
energies the quark gluon plasma is a strongly
coupled liquid
Gauge gravity duality is currently our best tool
to describe this mayhem!
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What else have we found?
Why do otherwise identical particles have
different masses?
Massive gauge bosons for the weak nuclear force!
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The Origin of Mass
The strong nuclear force cannot explain the mass
of the electron though
Or very heavy quarks top mass 175
proton mass
The Higgs Boson
We suspect the vacuum is full of another sort of
matter that is responsible the higgs. a new
sort of matter a scalar?
To explain the W mass the higgs vacuum must be
100 times denser than nuclear matter!!
It must be weak charged but not electrically
charged
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The Search for the Higgs
EG look for Higgs decay to two photons
There are variants.
Is the Higgs some new quark anti-quark pair bound
by a new ultra strong force?
Should we embrace a new symmetry that requires a
scalar for every fermion Supersymmetry
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No Loose
What if our theories are wrong and there is no
higgs?
Without the higgs our theory of WW interactions
predicts scattering cross sections greater than
one there must be something there
What could it be? extra space-time dimensions
- a bigger gauge
symmetry SU(2)xSU(2)x
- something new
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Summary
Coming soon!
The LHC is an amazing technological project That
will explore the frontier of our understanding of
the building blocks of nature It will test our
understanding of QCD as the generator of mass And
search for the Higgs boson the missing link of
the Standard Model Mostly we hope for the
unexpected though we want new insights into
scientific law and natural philosophy
As reviewed in the Independent Times Higher
Education Supplement!
A FREE Popular science novel
www.hep.phys.soton.ac.uk/evans/NL