Particles and Strings

Nick Evans

“The search for a fundamental theory of the building blocks of nature and their interactions”

University of Southampton

Reality Check!

Particles & Strings

Hard Science

Mathematically well defined

Experimentally verified

New experiments - LHC

Search for a missing piece – the higgs particle

Speculative

A theory of quantum gravity

Maths rules still up for grabs

No experimental verification

No clear experiment to do!

Throws up fascinating possibilities!

The Particle World

Forces• Electromagnetism

• Strong nuclear

• Weak nuclear

• Gravity

ParticlesThree families (copies) of

2 quarks 1 electron 1 neutrinos

plus their antiparticles

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

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

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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….

Quantum GravityIf the vacuum is full of all this stuff shouldn’t we be pulled gravitationally by it?

Since it is uniformily distributed there is no net pull (equal space to all sides)

But General Relativity says the energy should uniformily curve space-time… the Universe should be the size of a grapefruit!!

There’s something big we don’t understand about quantum gravity

Gravity is so weak that we’ve never measured it’s effects between individual particles or on distance scales smaller than 1mm!

Gravity is different to the other forces – it’s only attractive…

In General Relativity this shows up in that gravitational waves have different polarizations to electromagnetic waves

What fundamental theory can encompass both types of fields?

String Theory

String theory is A quantum theory of gravity.. Is it THE quantum theory of gravity? - entering realm of speculation!

String theory unifies gravity and other forces

What are these strings?We describe them only in terms of a fundamental tension – as for a rubber band

How big are they?A particle of energy E has a wavelength

E = h c /

So can probe down to scales of order …. So far we’re down to 1/1000th size of atomic nucleus… strings could be 10 times smaller!

T

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Extra DimensionsSurprisingly the mathematics of string theory only makes sense in 9 spatial dimensions and 1 time dimension!

A prediction…. But wrong!!

CompactificationWe can imagine a space where directions are curled up

We study A string in this space not ALL the strings that make the space…. Why are 6 dimensions compact… by what mechanism? UNKNOWN!

The radius must be at least 1000 times smaller than an atomic nucleus!

Membranes too…Recently we have discovered that open strings may be restricted to a sub-space or membrane

Electromagnetic particles live on a sub-space or “brane”

Gravitons live in a higher dimension “bulk” “Existence proof” for

such a world

Could Our Universe Be A Brane?

2 R

US

The strength of gravity is determined by the number of spatial dimensions (gravitons spread out around mass)

F = G M m

r 2 D=3+1

But…. we don’t know anything about gravity on length scales below 1mm… R could = 1mm… and we wouldn’t know it!

If so we’ve miscalculated the strength of gravity (G) – it could become strong in our particle accelerators at any new energy!!

Matter Loss To The BulkOur Universe will slowly lose matter into the bulk…

In particle physics gravity is too weak to ever see these events

In fact the energy loss from the centre of stars is too low to see too!

Other BranesWhy should there be only one brane in the higher dimension spacetime?

The matter on the other brane will only interact with our world gravitationally – it’s dark matter….

Until we can produce high energy gravitons that are strongly interacting there’s little way to directly probe this idea though

US

NEW

There could be entire Universes only mm away!

Brane CollisionsThere’s no reason branes should be static in the extra dimensions…. So they could collide!

This would be catastrophic!

Huge amounts of energy would be dumped into our Universe….

Could that have triggered the currently observed expansion of the Universe??!!

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

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

• Our theories of particles don’t fit with theories of gravity

• String Theory is an attempt to construct a sensible theory of both

• We are realizing it predicts wild possibilities

• Extra dimensions and other Universes

• Many exciting possibilities but very hard to experimentally test because gravity is very very weak….