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 - £5 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

Where we are now….

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

Space - Time

A flash of light causes a spherical wave front even if you move relative to source

*

This only makes sense if space and time mix!

t ‘ = (1 – v /c ) ( t – v x / c )2 22

x ‘ = (1 – v /c ) ( x – v t)2 2

Quantum Rules of Motion

Energy comes in lumps

E = h f

Fields can look like particles

The photon is the quantum of the electromagnetic field/ light

Quantum Dynamics

The quantum in some sense travels by both paths….

There is an uncertainty in the position and momentum of the quantum

Heisenberg’s Uncertainty Principle x p > ht E > hOr equally

Dirac’s LegacyElectrons can absorb photons

But in relativity observers do not agree on time ordering of events… so can we have

What does it mean for an electron to travel backwards in time? We only measure charge…

It looks like a +ve charge electron moving forward in time

We have discovered anti-particles!

Accelerator Physics

Electron positron annihilation to a photon allows us to convert their energy to look for all the particles that make up nature

What have we found?

Why do otherwise identical particles have different masses?

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.

The Strong Nuclear Force

The strong nuclear force is described by a theory that is similar to electromagnetism… except that the fields carry (colour) charge…..

This difference changes the way in which the vacuum is polarized so that…

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

The Origin of MassThe 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….

Or very heavy quarks such as the top quark

top mass = 175 proton mass

To explain the top mass the higgs vacuum must be 100 times denser than nuclear matter!!

The Search for the HiggsTo find the higgs we must “excite” the vacuum – produce a higgs particle… we collide electrons, protons etc so there is 100 times nuclear energy density in some region….

The Large Hadron Collider in Switzerland will switch on in 2008…

We haven’t found it so far but…

There are many versions of the “higgs theory” – when we find it we can study its properties in detail….

No Lose

What if our theories are wrong and there is no higgs?

Without the higgs our theory of WW interactions predicts scattering probabilities greater than one… there must be something there…

What could it be? – extra space-time dimensions

- new forces…

- something entirely new…

Overview• Particle physics has a concise description of matter and forces

• The particles obey very strange laws at high energy and small scales

• The proton mass is a result of the vacuum being full of quarks

• The missing element is the higgs that generates other masses (plus explanation of why the building blocks are what they are)

• Still much to do - our theories of particles don’t fit with theories of gravity

• The LHC will switch on soon and begin to provide answers…