FROM THE VERY SMALL TO THE VERY LARGE

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FROM THE VERY SMALL TO THE VERY LARGE

• Theoretical High Energy Physics in the 21st
  Century

Michael Dine June, 2007
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New York Times

• Reports a debate among cosmologists about the Big
  Bang.
• lll1.html

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Dr. Tyson, who introduced himself as the
Frederick P. Rose director of the Hayden
Planetarium, had invited five "distinguished"
cosmologists into his lair for a
roasting disguised as a debate about the Big
Bang. It was part of series in honor of the late
and prolific author Isaac Asimov (540 books
written or edited). What turned out to be at
issue was less the Big Bang than
cosmologists' pretensions that they now know
something about the universe, a subject about
which "the public feels some sense of ownership,"
Dr. Tyson said. "Imagine you're in a living
room," he told the audience. "You're
eavesdropping on scientists as they argue
about things for which there is very little data."
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Dr. James Peebles, recently retired from
Princeton, whom he called "the godfather" Dr.
Alan Guth from the Massachusetts Institute of
Technology, author of the leading theory of the
Big Bang, known as inflation, which posits a
spurt of a kind of anti-gravity at the beginning
of time and Dr. Paul Steinhardt, also of
Princeton, who has recently been pushing an
alternative genesis involving colliding universes.
Rounding out the field were Dr. Lee Smolin, a
gravitational theorist at the Perimeter Institute
for Theoretical Physics in Waterloo, Ontario,
whom Dr. Tyson described as "always good for an
idea completely out of left field - he's here to
stir the pot" and Dr. David Spergel, a
Princeton astrophysicist.
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But Dr. Smolin said the 20th-century revolution
was not complete. His work involves trying to
reconcile Einstein's general relativity, which
explains gravity as the "curvature" of
space-time, with quantum mechanics, the strange
laws that describe the behavior of atoms.
"Quantum mechanics and gravity don't talk to
each other," he said, and until they do in a
theory of so-called quantum gravity, science
lacks a fundamental theory of the world. The
modern analog of Newton's Principia, which
codified the previous view of physics in 1687,
"is still ahead of us, not behind us," he said.
Although he is not a cosmologist, it was fitting
for him to be there, he said, because "all the
problems those guys don't solve wind up with us."
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Today, you are listening to someone seemingly
more out in left field -- a particle
physicist. Particle physics seeks to determine
the laws of nature at a microscopic really
submicroscopic, level. What does this have to
do with the Big Bang?
EVERYTHING!
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• With due respect to the New York Times, articles
  like
• this give a very misleading impression.
• We know
• There was a Big Bang
• This even occurred about 13 Billion Years Ago
• We can describe the history of the universe,
• starting at t3minutes
• There is now a huge amount of data and a picture
• with great detail.

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• There are lots of things we dont know. With due
  respect to Lee Smolin, the correct address for
  these questions is Particle Physics.
• What is the dark matter?
• Why does the universe contain matter at all?
• What is the dark energy?
• What is responsible for inflation?
• What happened at t0?
• We cant answer any of these questions without
  resolving mysteries of particle physics. We need
  to know that laws of nature which operate at the
  smallest distances we can presently imagine.

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Physical Law What are we after?

• Newton Fma FG M1M1/R2
• Probably the most famous physical laws.
  UNIVERSAL

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Newton could use his laws to explain the motion
of the planets, the moon. Haley comets.
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Electricity and MagnetismMaxwell
Wrote down the laws of electricity and
magnetism Maxwells equations. Light, radio
waves (Maxwell predicted), and other radiation
all part of the same set of phenomena.
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So two sets of laws. These describe most of the
phenomena of our day to day experience gravity,
light, electricity, magnetism With these,
scientists of the late 19th century understood
the motion of the planets in great detail, and
made great technical progress. They started, as
well (somewhat inadvertently) to explore the
world of atoms. The end of the 19th century saw
the discovery of the first elementary particle,
by Thompson the electron.
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EINSTEIN
Excited by Maxwells equations and also puzzled.
There seemed to be a maximal speed at which light
could travel. Puzzled, also by the problem of
the photoelectric effect the emission of
electrons by light. Also wondered about the
existence of atoms. Were they real, or just a
trick to understand the periodic table?

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Einsteins Extraordinary year 1905

• Photoelectric effect the idea that lights come
  in packets of energy the beginnings of the
  photon concept
• Explanation of the Brownian motion basic to
  physics, chemistry, biology clinched the idea
  that atoms were real.
• Special relativity time and space are relative
  concepts depend on the observer. But the speed
  of light is absolute all observers agree about
  it.

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General Relativity
Now, a deeper understanding of the laws of
electricity and magnetism. But Einstein didnt
know how to reconcile Newtons laws with the
rules of relativity. E.g. in Newtons laws,
action at a distance. Didnt make sense
electricity and magnetism dont work this
way. Einsteins clue the equality of
gravitational and inertial mass. Inertia
something to do with space and time. So gravity?
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Fma FG mM/R2
Gravitation
Inertia
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The equality of gravitational and inertial mass
was first tested in experiments by the Hungarian
scientist Eotvos in the late 1800s
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Einstein and the General Theory of Relativity

• After almost eleven years of struggle, Einstein
  announced his general theory of relativity in
  1916. A theory in which gravity arises as the
  distortion of space and time by energy.
  Proposed three experimental tests
• Bending of light by the sun
• Perihelion of Mercury
• Red Shift

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General Relativity and the Universe
Gravity is unique among the forces in that it is
always attractive. So it acts on things at the
surface of the earth, on the planets, on stars,
and on the universe as a whole. So Einstein and
others tried to apply his theory to the
universe. But the universe is complicated,
varied. How to proceed?
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Einstein Copernicus
Assume the universe is homogeneous and isotropic
no special place or direction.
Einsteins equations have no Static
solutions. The universe expands!
Einstein was very troubled remember that at
that time (c. 1920) Astronomers didnt know about
galaxies!
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HUBBLE (1921)
Galaxies move away from us at a speed
proportional to their distance
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The Cosmic Microwave Background
In the past, the universe must have been much
hotter Big Bang. Gamow, Peebles if true,
there should be a glow left over from this
huge explosion (but of microwave radiation, not
light). Objects give off a characteristic
spectrum of electromagnetic radiation depending
on their temperature blackbody. The
temperature then was 10,000 degrees today it
would be about 3 degrees Discovered by Penzias
and Wilson (1969). Today thanks to COBE
satellite, the best measured black body spectrum
in nature.
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Artists Rendering of COBE
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COBE measured the temperature of the universe
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More detailed study of the CMBR

• From satellites and earth based (balloon)
  experiments. Most recently the WMAP satellite.

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Detailed information about the universe
Aside we understand this as a quantum
phenomenon the measurement was done a long
time ago.
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COMPOSITION OF THE UNIVERSE

• From studies of CMBR, of distant Supernova
  explosions, and from Hubble and Ground-Based
  observations we know
• 5 Baryons (protons, neutrons)
• 35 Dark Matter ??? (zero pressure)
• 65 Dark Energy ???? (negative pressure)

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A Confusing Picture Where Do We Stand?

• We have a good understanding of the history of
  the universe, both from observations and well
  understood physical theory, from t180 seconds.
• BUT
• We dont know why there are baryons at all!
• We dont know what constitutes 95 of the energy
  of the universe.
• We know that the universe underwent a period of
  violent expansion (inflation) at about 10-30
  seconds after the big bang. What caused this?

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But weve gotten ahead of our story.

• We started out talking about laws of nature. We
  had Newton, and with him an understanding of the
  planets then Maxwell, and an understanding of
  the electromagnetic spectrum, and now Einstein,
  and we have started to think about the universe
  as a whole. But a lot happened between 1905 and
  these discoveries.

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New particles, new laws

• 1895 discovery of the electron
• 1911 - discovery of the atomic nucleus
• 1920s quantum mechanics
• 1930s the neutron, and understanding of the
  atomic nucleus.
• 1930s discovery of antimatter.

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LOOKING STILL DEEPER

• By the 1940s, much progress, but much not well
  understood
• Photons -- the quantum mechanics of
  electrodynamics (QED)
• The precise laws underlying the nuclear forces
• To go further theoretical developments
• Experiments probing distances smaller
• than the size of nuclei

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Quantum Electrodynamics

• Feynman, Schwinger, Tomanaga detailed
  understanding of how quantum mechanics and
  electricity and magnetism work together.
  Predictions with awesome precision. E.g. the
  magnetism of the electron explained in terms of
  the electrons charge and mass to one part in
  1012.

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Looking Deeper

• The late 1940s launched the era of large
  particle accelerators. Over the next 50 years,
  numerous elementary particles, understanding of
  the basic constituents of matter and the forces
  between them the Standard Model. Critical
  interplay between theory and experiment.

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Success of the Standard Model

• Standard Model is extremely successful
• Experimental discovery of all of its matter
  constituents and force carriers
• Simple common approach to describe all
  (relevant) forces gauge principle
• Self-consistent at the
• level of quantum
• corrections

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Detailed Comparison with Experiment
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One missing piece the Higgs particle

• In SM, responsible for the masses of the quarks,
  leptons, W, Z0
• Mass not predicted by the model, but know
  something from experiment.
• Should be discovered at the LHC.
• Much work on this by H. Haber

Peter Higgs
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time year

• Last missing particle in SM
• (EW symmetry breaking mass)
• Light SM Higgs preferred

MH 126 73 -48 GeV lt 280 GeV
(95 CL)
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Fundamental open questions
Is There a Higgs particle? The hierarchy
problem why is Higgs mass so small?
(dimensional analysis mH 1019 mp )
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One possible new phenomenonSupersymmetry

• A new symmetry among the elementary particles.
  Fermions ! bosons bosons ! fermions.
• Not only a new symmetry of nature, but if this
  idea is right, then it explains what the dark
  matter is! Banks, Dine, Haber

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... doubled particle spectrum ... ?
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Consequences of Supersymmetry

• Automatically provides an explanation of the dark
  matter
• Predicts one of the fundamental constants of
  nature.
• Provides an understanding of why there is more
  matter than antimatter in the universe
• Predicts lots of phenomena observable at the LHC.

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without SUSY

• ... BUT some Standard Model
• Problems solved ...
• ... extension in string theory is
• candidate for Grand Unified Theory ...
• ... lightest SUSY particle stable
• ?candidate for dark matter ...
• ... unification of forces ...

with SUSY
Interaction energy in GeV
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l
c
q
q
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c
g
q
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Production and decay of superparticles at the
LHC. Here, jets, Leptons, missing energy.
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In a broad class of supersymmetric models, the
lightest new particle is stable (R-parity)
typically the partner of the Higgs or Z boson or
photon. Produced in early universe.
N
W
N
W-
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Range of supersymmetry parameters consistent with
dark matter density here partner of photon is
essentially the dark matter.
Range of supersymmetry parameters consistent with
dark matter constraints
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• I am a fan of the supersymmetry hypothesis I'm
  not alone. About 12,000 papers in the SPIRES
  data base. If true, quite exciting a new
  symmetry of physics, closely tied to the very
  nature of space and time. Dramatic experimental
  signatures. A whole new phenomenology, new
  questions. But neither the limited evidence nor
  these sorts of arguments make it true there is
  good experimental as well as theoretical reason
  for skepticism.
• This is not the only explanation offered for the
  hierarchy, and all predict dramatic phenomena in
  this energy range.
• Large extra dimensions
• Warped extra dimensions
• Technicolor
• Its just that way (anthropic?)

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Hypothetical answers to a set of fundamental
questions

• Too many parameters
• Charge quantization
• Quantum general relativity
• Dark Matter
• Dark energy
• Baryogenesis

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STRING THEORY

• String theory has pretensions to attack the
  remaining problems on this list
• A consistent theory of quantum gravity
• Incorporates gauge interactions, quarks and
  leptons, and other features of the Standard
  Model.
• Parameters of the model can be calculated, in
  principle.
• Low energy supersymmetry emerges naturally all
  of this proliferation, which seemed artificial,
  almost automatic.

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Has string theory delivered?

• String theory is hard. We dont have a
  well-understood set of principles. Some
  problems of quantum gravity are resolved, but
  many of the challenges remain.
• String theory seems able to describe a vast
  number of possible universes, only a small
  fraction of which are like ours.
• Until recently, no progress on one of the most
  difficult challenges to particle physics the
  dark energy, but this has changed.

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We are at the dawn of a very exciting era. We
may resolve some of our fundamental questions.