LHC – the greatest experiment

1. LHC – the greatest experiment
Prof Nick Evans
& the origin of mass
University of Southampton
on Earth

2. The ring is 27km round and on average 100m
underground
CERN - Geneva Probing the structure of matter
LHC
will
begin
science
in 2008

3. 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!

4. Proton-Proton collisions (hydrogen atom nuclei)
100 billion protons per bunch
20 collisions per crossing
1 crossing every 25ns
600 million collisions per second
14 TeV centre of mass energy
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

5. Detectors at collision
sites:
Atlas
CMS
LHC-b
Alice
Track particles
Measure energy
Measure momentum

6. 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
CERN is 20
European
member
states plus
many
international
contributors
Spin Offs – CERN invented the www
and gave it away…
Amazon revenue 2006 - £7bn

7. The Story So Far
Electrons and their electromagnetic
interactions are responsible for
chemistry and day to day forces

8. 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 = mc2
1
1-v /c
2 2
Rest mass = energy

9. Quantum Theory- Wave-Particle
The photon is the quantum of the electromagnetic field/ light
Duality
The energy in waves comes in lumps or quanta – E = h f
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 > h
t E > h

10. 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 anti-particle
Every particle has a twin of the same mass but precisely
opposite charges – particles and anti-particles annihilate into
photons.

11. 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.

12. Understanding Mass - The Quantum Vacuum
E t > h
The vacuum can borrow energy for short periods
E = mc2
The borrowed energy can be used to create particles
The quantum vacuum is a seething mass of particles appearing and
disappearing constantly….
(You can’t just create an electron because of charge
conservation - but can create electron positron pair)

13. How Can You Tell?
The effective charge
seen in two electron
scattering depends on
the separation of the
electrons.
The “virtual” particle
pairs interfere in
electron scattering
processes.
g-2 is tested to 13 sig
figs!

14. The Strong Nuclear Force
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…
Quarks come in 3 colours!
“asymptotic freedom”
Gross, Politzer, Wilczek

15. 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 anti-
quark pairs!

16. The vacuum is really full of quark anti-quark pairs with a density
like that of an atomic nucleus (10 grams/cm ) !!
15
The Proton Mass
The quark pairs are responsible for the proton’s mass
Interaction
energy provides
proton mass
3

17. 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
BUT relativistic strings like to
live in 10 dimensions!
String theory contains
quantum gravity
A string is a one dimensional
object with tension

18. Gauge Gravity Duality
Maldacena
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…
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
If the space-time stretches it
the quark becomes massive
Classical General Relativity
computations solve strongly
coupled quantum problems!
Is real gravity a hologram??

19. 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

20. 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!

21. What else have we found?
Why do otherwise
identical particles have
different masses?
Massive gauge
bosons for the
weak nuclear
force!

22. The Origin of Mass
The strong nuclear force cannot explain the mass of the electron
though…
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?
Or very heavy quarks top mass = 175 proton mass
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

23. 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…

24. 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…

25. Summary
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
Coming soon!
As reviewed in the Independent & Times
Higher Education Supplement!
A FREE Popular science novel
www.hep.phys.soton.ac.uk/~evans/NL