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The Ultimate Structure of Matter

Chapter 13Presented 15 October 2015 Prof. Geller

Great Idea:All matter is made of quarks and leptons, which are the most fundamental building blocks of the

universe that we know.

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Chapter Outline

• Of What is the Universe Made?• Discovering Elementary Particles• The Elementary Particle Zoo• The Four Fundamental Forces

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Of What is the Universe Made?

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The Library

• Superficial description– Fundamental building blocks

• Books• Rules for organization

• Basic– Words– grammar

• Ultimate description– Letters– Spelling

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Reductionism

• Reductionism– Ultimate building blocks

• How simplicity gives rise to complexity

• True nature– Thought, experiment, observation

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The Building Blocks of Matter

• Fundamental building blocks– Changed over time

• Atom• Nuclei and electrons• Elementary particles

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Discovering Elementary Particles

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Cosmic Rays

• Cosmic Rays– Particles emitted by stars

• Used to understand nucleus– Found new elementary particles

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Particle Accelerators:The Essential Tool

• Particle Accelerator– Artificial cosmic rays

• Ernest O. Lawrence– cyclotron

• Synchrotron• Linear accelerator

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The Elementary Particle Zoo

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Leptons, Hadrons, andAntimatter

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Quarks and Leptons

• Quark– Fundamental building blocks of hadrons– Fractional electrical charge– Only six kinds

• Leptons– Six kinds– Outside nucleus

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Quark Confinement

• Individual quarks– No experimental isolation of a quark

• Elementary particles– Difficult to isolate

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Reviewing Subatomics

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Reviewing Subatomics

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Reviewing Subatomics

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Reviewing Subatomics

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Reviewing Subatomics• Neutrinos are produced

in the “Weak Interaction”, for example– Neutrinos from the earth

• natural radioactivity

– “Man-made” neutrinos• accelerators, nuclear

power plants.

– Astrophysical neutrinos• Solar neutrinos• Atmospheric neutrinos• Relic neutrinos

– left over from the big bang.

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Neutrino Factoids• The earth receives about 40 billion

neutrinos per second per cm2 from the sun.– About 100 times that amount are passing

through us from the big bang.• This works out to about 330 neutrinos in every cm3 of

the universe!• By comparison there are about 0.0000005 protons per

cm3 in the universe.

• Our body emits about 340 million neutrinos per day from 40K.

• Neutrinos don’t do much when passing through matter.

• Remember, it is very difficult to observe neutrinos.

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Neutrino Detection

• The neutrino is observed by detecting the product of its interaction with matter.

eElectron

Muon

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The Four Fundamental Forces

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The Four Fundamental Forces

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Force as an Exchange

• Forces cause matter to accelerate• Gauge particle

– Produce fundamental forces

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Unified Field Theories

• Unified Field Theories– Fundamental forces seen as different

aspects of one force– Four fundamental forces may be

different aspects of a single force

• Electroweak force• Standard model• Theories of Everything

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String Theory: the idea

• All matter consists of small one-dimensional objects (strings).– Strings look like

particles when not resolved closely enough

– All particle types are different normal modes of the string.

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String Theory: the interactions

• All interactions consist of the splitting and joining of these elementary strings.– This is currently the best

description of the scattering of gravitational waves at very high energies!

– Looks like General Relativity plus other interactions at low energies.

– No parameters: string length sets units.

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Possible Downsides

• At first sight there appeared to be a number of different kinds of string theories.– Open, closed, heterotic, Type I, Type IIA,...

• Predicts we live in 10 spacetime dimensions!– Experimental update: number of (large) dimensions = 4

• Very difficult to experimentally test so far.– Strings are so short that once the symmetries and

spectrum are gotten right, most of the details are usually also right.

– Calculation gets known masses right, but…..• experiment mexp = 0.00…………….…005

• theory mth = 0.00000000

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D-Branes

• String theory is bigger than previously thought. – Normally, open strings satisfy

Neumann boundary conditions, • string ends move at light

speed.

– Dirichlet boundary conditions also make sense

• string ends live on a surface.

– This surface is interpreted as a large massive object, a D-brane, in spacetime, much like a monopole.

Polchinski

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Why Do This?

• Good Things Happen if the theory has both strings and D-branes:– Previously-hidden duality symmetries emerge, with all

known string theories dual to one another under these symmetries!

– Some weakly-interacting string theories are the duals of the strong-coupling limit of others!

– Led to discoveries of similar symmetries amongst ordinary particle theories.

• Conjecture: all known string theories are different solutions to a more fundamental (11-dimensional) theory (M Theory).

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String Length and Gravity

• Could strings be as big as they can be: ls = lw? – Non-gravitational physics is characterized by the weak scale:

GF = lw2 = (10-3 fm)2

– If so, strings may be experimentally discovered ‘tomorrow’! – If so, does this explain why lw is so much larger than lp = 10-19 fm?

• ie: Why is gravity so weak? Why are stars so big?

a

effspl

2 / n

s

Arkani-Hamad, Dvali & Dimopoulos

• Gravity would be weak because the extra dimensions are large:

If n = 2 then ls = lw requires a ~ 0.1 mm!! If n = 6 then ls = lw requires a ~ 105 fm,

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Intermediate-Scale Strings

• Q: Can ls and a be similar in size?

• A: Yes. ls / a = 0.01 works if:– I: ls = √(lw lp) =10-11 fm.

• lw = ls2 / lp naturally arises if supersymmetry breaks on another brane, and is transmitted to our brane by gravity.

– II: n = 6 extra dimensions.• No heirarchy need be dialed in.• Other nice things also happen if so:

String axions can solve strong CP problem, neutrino masses similar to experiments, etc

plw

pl

2

s2 eff

2 sa

n

eff

sa

0.01

CB, Ibanez & Quevedo

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String Summary

• String theory is the only known theory where gravity and quantum mechanics co-exist at high energies.– Major Lesson of the 20th Century: Relativity and

Quantum mechanics are almost inconsistent, and so together impose extremely strong self-consistency conditions.

• The string length is likely much longer than the Planck length.– Size Matters: much better prospects for comparison with

experiments.– The intermediate scale is well motivated on particle-

physics grounds: ls = √(lw lp) =10-11 fm. • We may all be Brane bound.