Quarks and Strings

Nick Evans

“On a remarkable connection between quark physics & string theory…”

Theory Group

The Rule of Three

• Electrons

• Protons

• Neutrons

Previously

You have heard about electrons changing orbits & emitting or absorbing energy

Everything is made of just three things!

Quarks!

The strong force binds the quarks together & then the protons together..

Remarkably it gets stronger with quark separation

Protons and neutrons are made from up and down quarks

The Strong Force

Confinement

You can never pull hard enough to liberate a quark from a proton…

The VacuumThe attractive force between quarks is so strong that

binding energy >> mass energy

The vacuum has lower energy if it fills itself with quark anti-quark pairs!

Interaction energy provides proton mass

Quantum Chromodynamics - QCD

There are three identical colours of each quark… and “glue” fields that allow them to probe each other’s colour

The glue fields have properties of two quarks…

Computers needed!

Gravitational & Electrical Orbits are well behaved

QCD orbits are much harder to control

Chris & Jonathan

String Theory

A quark pair can look like a string

BUT

String theory lives in 9 spatial dimensions

Short strings have the charge of two quarks so are glue fields

Closed strings describe gravity fields

This is not QCD (surely?) but an interesting possible theory of everything….

The Cat is Out!

Membranes too… and 3-branes… and…

In string theory branes emerge as areas of space where strings have their ends tied

String Dual of QCD

The open strings tied to a 3-brane make up a theory of QCD glue fields in 3 spatial dimensions

Maldacena conjectured that the remaining strings in 9 dimensions provide an alternative description of QCD

Quarks TooTo have quarks as well we need to anchor one end of the strings somewhere else eg on a 7-brane

The mass of the quark is given by the minimum length of the string

energy = length x tension

To get massless quarks the 7-brane & 3-brane should intersect

Babington, Erdmenger, NE, Erlich, Zuralnik, 2003

A simple (one page) calculation shows that the branes repel – they prefer to generate a dynamical quark mass

Health WarningHealth Warning

We’re still describing the basic features We’re still describing the basic features of QCD… we’re using maths tricks of QCD… we’re using maths tricks which mean we are not yet which mean we are not yet calculating real QCD quantities…. calculating real QCD quantities….

The new framework & understandings The new framework & understandings are exciting though!are exciting though!