1 The Ultimate Structure of Matter Chapter 13 Presented 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.

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

3 Of What is the Universe Made?

4 The Library Superficial description –Fundamental building blocks Books Rules for organization Basic –Words –grammar Ultimate description –Letters –Spelling

5 Reductionism –Ultimate building blocks How simplicity gives rise to complexity True nature –Thought, experiment, observation

6 The Building Blocks of Matter Fundamental building blocks –Changed over time Atom Nuclei and electrons Elementary particles

7 Discovering Elementary Particles

8 Cosmic Rays –Particles emitted by stars Used to understand nucleus –Found new elementary particles

9 Particle Accelerators: The Essential Tool Particle Accelerator –Artificial cosmic rays Ernest O. Lawrence –cyclotron Synchrotron Linear accelerator

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11 The Elementary Particle Zoo

12 Leptons, Hadrons, and Antimatter

13 Quarks and Leptons Quark –Fundamental building blocks of hadrons –Fractional electrical charge –Only six kinds Leptons –Six kinds –Outside nucleus

14 Quark Confinement Individual quarks –No experimental isolation of a quark Elementary particles –Difficult to isolate

15 Reviewing Subatomics

16 Reviewing Subatomics

17 Reviewing Subatomics

18 Reviewing Subatomics

19 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.

20 Neutrino Factoids The earth receives about 40 billion neutrinos per second per cm 2 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 cm 3 of the universe! By comparison there are about protons per cm 3 in the universe. Our body emits about 340 million neutrinos per day from 40 K. Neutrinos don’t do much when passing through matter. Remember, it is very difficult to observe neutrinos.

21 Neutrino Detection The neutrino is observed by detecting the product of its interaction with matter. e Electron  Muon

22 The Four Fundamental Forces

23 The Four Fundamental Forces

24 Force as an Exchange Forces cause matter to accelerate Gauge particle –Produce fundamental forces

25 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

26 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.

27 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.

28 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 m exp = 0.00…………….…005 theory m th =

29 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

30 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).

31 String Length Could strings be as large as the present-day experimental limit l s = l w ? –Non-gravitational physics is characterized by the weak scale: G F = l w 2 = (10 -3 fm) 2 –Why is l w = fm so much larger than l p = fm? Why is gravity so weak? Why are stars so big? Weak-Scale Strings: can l s = l w ? –G eff is nonetheless small enough! If n = 6 then l s = l w requires a = 10 5 fm, If n = 2 then l s = l w requires a = 0.1 mm!! Arkani-Hamad, Dvali & Dimopoulos

32 String Length and Gravity Could strings be as big as they can be: l s = l w ? –Non-gravitational physics is characterized by the weak scale: G F = l w 2 = (10 -3 fm) 2 –If so, strings may be experimentally discovered ‘tomorrow’! –If so, does this explain why l w is so much larger than l p = fm? ie: Why is gravity so weak? Why are stars so big? Arkani-Hamad, Dvali & Dimopoulos Gravity would be weak because the extra dimensions are large: If n = 2 then l s = l w requires a ~ 0.1 mm!! If n = 6 then l s = l w requires a ~ 10 5 fm,

33 Intermediate-Scale Strings Q: Can l s and a be similar in size? A: Yes. l s / a = 0.01 works if : –I: l s = √( l w l p ) = fm. l w = l s 2 / l p 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 CB, Ibanez & Quevedo

34 String Summary String theory is the only known theory where gravity and quantum mechanics co-exist at high energies. –Major Lesson of the 20 th 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: l s = √( l w l p ) = fm. We may all be Brane bound.