LHC – the greatest experiment

Prof Nick Evans

& the origin of mass

University of Southampton

on Earth

The ring is 27km round and on average 100m underground

CERN - Geneva Probing the structure of matter

LHC will begin science in 2008

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)

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

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

CERN is 20 European member states plus many international contributorsSpin Offs – CERN invented the www

and gave it away…

Amazon revenue 2006 - £7bn

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 = mc211-v /c2 2

Rest mass = energy

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 > ht E > h

Dirac’s LegacyElectrons 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.

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 = 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)

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!

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

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!

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

QCD & StringsStrongly 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

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??

Quarks in a Dense QCD PlasmaComputations 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 ConnectionA 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?

Why do otherwise identical particles have different masses?

Massive gauge bosons for the weak nuclear force!

The Origin of MassThe strong nuclear force cannot explain the mass of the electron though…

The Higgs BosonWe 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

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

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