Transcript 20071008133014301

LHC – the greatest experiment
on Earth
& the origin of mass
Prof Nick Evans
University of Southampton
CERN - Geneva
Probing the structure of matter
LHC
will
begin
science
in 2008
The ring is 27km round and on average 100m
underground
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!
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
Detectors at collision
sites:
Atlas
CMS
LHC-b
Alice
Track particles
Measure energy
Measure momentum
LHC Budget - £2.6 billion over 10 years
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
CERN is 20
European
member
states plus
many
international
contributors
The Story So Far
Electrons and their electromagnetic
interactions are responsible for
chemistry and day to day forces
Special Relativity
The speed of light is the
same for any observer
This means nothing can travel with light –
nothing can reach v=c!
E=
1
mc 2
1-v2/c2
Rest mass = energy
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
t
p >h
E >h
Dirac’s Legacy
Electrons can absorb photons
But in Relativity, rotating
this in space-time gives…
The electron travelling back in time is a hole or antiparticle
Every particle has a twin of the same mass but precisely
opposite charges – particles and anti-particles annihilate
into photons.
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.
Understanding Mass - The Quantum Vacuum
E
t >h
The vacuum can borrow energy for short periods
E = mc 2
The borrowed energy can be used to create particles
(You can’t 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….
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.
The Strong Nuclear Force
Quarks come in 3 colours!
This difference changes the
way in which the vacuum is
polarized so that…
“asymptotic freedom”
Gross, Politzer, Wilczek
The strong nuclear force is
described by a gauge theory
… except that the 8 gauge
fields, gluons, carry colour
charge…..
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 >> mass energy
The vacuum has lower energy if it fills itself with quark antiquark pairs!
The vacuum is really full of quark anti-quark pairs with a density
15
3
like that of an atomic nucleus (10 grams/cm ) !!
The Proton Mass
The quark pairs are responsible for the proton’s mass
Interaction
energy provides
proton mass
QCD & Strings
Strongly coupled QCD is a
tough maths problem – how
do we compute beyond
perturbation theory?
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
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!
Is real gravity a hologram??
If the space-time stretches it
the quark becomes massive
Quarks in a Dense QCD Plasma
Computations of gravity wave propagation tell
us about transport properties of a quark gluon
plasma
Larry Yaffe’s calculations of the shock wave produced by a moving quark
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!
What else have we found?
Massive gauge
bosons for the
weak nuclear
force!
Why do otherwise
identical particles have
different masses?
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
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…
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…
Summary
Coming soon!
The LHC is an amazing technological
project
As reviewed in the Independent & Times
Higher Education Supplement!
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
A FREE Popular science novel
www.hep.phys.soton.ac.uk/~evans/NL