Download From Subatomic Particles to the Cosmos Mark Oreglia

From Subatomic Particles to the Cosmos
(Particle Physics Today)
Mark Oreglia
The Enrico Fermi Institute
The University of Chicago
Outline:
Particle and Forces
Recent Cosmology
Synthesis
The New Tools
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Particle Physicists are concerned
with the very small:
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Atoms can be imaged
• With the best
electron microscopes
we can make very
believable images of
atoms
• For smaller objects
we use similar
instruments … but do
not create such a
graphic image
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STM image of nickel atoms in crystal
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Accelerators: 1890-now!
• Thompson’s electron tube is the basis for the discovery of the
electron, the TV picture tube, … and all current accelerators
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Discovery of The Nucleus
α particle
Rutherford 1909
Led to new model of atom
α particle
size:
Nucleus
Fly
= Cathedral
Atom
nucleus
‘like a fly in a cathedral’
electron
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Scattering = Diffraction
• The pattern of scattered radiation tells you
about the structure it is scattering off
Water wave diffracting as
it passes through a slit
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The Elementary Particles
• So, from 1890 to
1990 we established
a hierarchy of:
– non-composite (i.e.,
elementary) types of
matter
– 4 distinct force
carriers that allow
matter to interact
– Plus the antimatter
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The 4 Known Forces
•The forces are
characterized by their very
different strengths
• STRONG: affects the
quarks; manifested in alpha
radioactivity; holds nucleus
together
• strength 1, but only at
short distances
• EM: affects quarks and
charged leptons; holds atom
together
• strength 1/100
• WEAK: affects quarks and
leptons; evident in beta
radioactivity
• strength 1/10000
• GRAVITY: affects all(?)
• strength 10-38!!!
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Common Matter: quarks + leptons
• QUARKS: combine in pairs or triplets to form
larger objects called hadrons:
– qqq ~ baryons (p = uud; n = udd, etc)
– qq ~ mesons (pion = u + anti-u)
• LEPTONS: the family consisting of:
– the electron and her heavier partners (muon, tau)
• Hundreds of muons are passing through your
body every second … the cosmic rays
– The electrically neutral partners (neutrinos)
• Even more neutrinos are bombarding you
• And their antimatter counterparts
– A basic symmetry of nature!
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How we know
• Years of experiments
with particle
accelerators
– Bring electrons or
protons to high energy
and let them scatter off
the objects we want to
study … or let them
break the target apart
into the ultimate
constituents
– A basic law of nature
tells us that the probe
particle has to have
higher energy to image
smaller targets:
resolution ~ 1/E
De Broglie
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More than mere classification
• By 1970 we had a
really good theory of
the particles and
forces … so good, we
call it the
“Standard Model”
– Specific mathematics
describing the ways
quarks and leptons
interact via strong,
EM and weak forces;
predictive and
testable!
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Einstein’s Dream Partially Realized
• The SM theory succeeded in
unifying 2 of the forces:
EM+Weak into a single theory
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The Problem with Theories
• Predictions are a nuisance – you’ve gotta
verify them!
– The Standard Model unified EM+Weak by
introducing a new particle field nobody has seen
yet … the celebrated Higgs Boson
• This “field” is the entity which could give
mass to all the matter particles
Peter Higgs
(not a boson
(Thanks to Prof. David Miller)
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Higgs Boson, where art thou?
• Searching for the Higgs Boson
has become the #1 priority in
particle physics
• Searches
in the 1990’s hinted
at its existence just outside the
reach of current accelerators
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We expect to see it soon
We expect to see it in 2008 or
2009 when the new “LHC”
accelerator comes online at
the CERN laboratory in
Geneva, Switzerland …
… more on that later
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Theories mean more theories
• Despite the success of the SM, it opened a
Pandora’s box of esoteric problems, such as:
– Where do all the parameters come from?
– How do they get their particular values
– Why don’t we have a theory of gravity!!!
• So we expand on the theory
– Invent new particles (eg, supersymmetry) purely on
the basis of mathematical aesthetics
– Invent new theories (strings) purely on the basis of
mathematical aesthetics
• Job of accelerators: see if new particles exist
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Meanwhile, in a Gaxaxy Far Away
• Enter a noteworthy astronomer
from Marshfield: Edwin Hubble
Riess, Press, Kirshner 1996
Slope is equal to 1/t, where t is the
amount of time the expansion has
been occurring
Thus, t = 1.5 billion years
← 1 Mpsc = roughly earth to α-Centuri
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Hubble Expansion implies Big Bang
• All objects in the cosmos are moving away
from each other
– Not exactly like an explosion…if you go to any
reference point, the rest of the universe appears
to be receding from you in the same way we see it
– Visualize galaxies as proteins in a lump of bread
dough as it rises, expanding in all directions
Slice from Sloan Digital Sky Survey
• The universe appears the
same in all directions and in
all regions … “homogeneous
and isotropic” … this is the
clincher: space itself is
stretching … consequence of
how gravity distorts space
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How We Know
• It is only fairly recently that we can make
reliable surveys of the space around us
• Digital processing and automation of
telescopes permits us to make billions of
measurements
• The speed of distance objects is measured
using the Doppler Shift of the spectrum
Here is what a spectrum looks
like from sources moving away or
towards you
v = c x (wavelength shift) / (wavelength)
Hubble: if you know v, you know distance
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Global Cooling
• If space is stretching, then it used to be
smaller
– That means things were closer together (denser)
at earlier times, likewise the energy density … and
therefore the temperature increases as we go
back in time
– Here is where we can start interpreting the
picture in terms of chemistry and particles
• First an ultra hot universe so energetic that
only the most elementary particles existed
• As it expands, it cools; particles “condense” and
form atoms, molecules, suns, planets, galaxies
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Cosmological Fine Print
• The Big Bang did not occur at a single point in space as an
"explosion." It is better thought of as the simultaneous
appearance of space everywhere in the universe. That region of
space that is within our present horizon was indeed no bigger
than a point in the past. Nevertheless, if all of space both inside
and outside our horizon is infinite now, it was born infinite. If it
is closed and finite, then it was born with zero volume and grew
from that. In neither case is there a "center of expansion" - a
point from which the universe is expanding away from.
• By definition, the universe encompasses all of space and time as
we know it, so it is beyond the realm of the Big Bang model to
postulate what the universe is expanding into. In either the open
or closed universe, the only "edge" to space-time occurs at the
Big Bang, so it is not logically necessary to consider this
question.
• It is beyond the realm of the Big Bang Model to say what gave
rise to the Big Bang… where the stuff came from, etc
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The evolving universe
•
10 -43 sec, size = 10 -30 today, temp = 10 32 Kelvin
•
10 -35 sec, size = 10 -26 today, temp = 10 28 Kelvin
•
10 -12 sec, size = 10 -15 today, temp = 10 15 Kelvin
•
10 -6 sec, size = 10 -12 today, temp = 10 12 Kelvin
•
10 sec, size = 10 -9 today, temp = 10 9 Kelvin
•
3.7×10 5 years, size = 10 -3 today, temp = 3×10 3 Kelvin
•
10 8 years, size = 10 -1 today, temp = 30 Kelvin
•
9×10 9 years, size = 5×10 -1 today, temp = 6 Kelvin
•
13.7×10 9 years, size = 10 0 today, temp = 2.74 Kelvin
– The Planck era. Quantum gravity is important; current theories are inadequate. We
can't get any closer to the Big Bang at earlier times
– Inflation. A temporary period of domination by a form of dark energy at an ultrahigh energy scale. A speculative theory.
– Electroweak phase transition. At high temperatures, electromagnetism is unified
with the weak interactions; now they become distinct.
– Quark-gluon phase transition. Quarks and gluons become bound into the protons
and neutrons we see today.
– Primordial nucleosynthesis. Universe cools to a point where protons and neutrons
combine to form light atomic nuclei, primarily Helium, Deuterium, and Lithium.
– Recombination. Universe cools to a point where electrons can combine with nuclei to
form atoms; becomes transparent. CMB is a snapshot of this era.
– The dark ages. Ripples in the density gradually assemble into stars and galaxies.
– Sun and Earth form. From the existence of heavy elements in the Solar System, we
know that the Sun is a second-generation star, formed about five billion yrs ago.
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Nucleosynthesis in the early Universe
• The predicted abundance of
deuterium, helium and lithium
depends on the density of ordinary
matter in the early universe
• About 24% of the ordinary matter
in the universe is helium produced
in the Big Bang
– in very good agreement with
observations and is another
major triumph for the Big Bang
theory
• In order for the predicted yields
of the other light elements to come
out in agreement with observations,
the overall density of the ordinary
matter must be roughly 4% of the
critical density (… next slide)
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Amount of Matter affects Outcome
• The stretching of
the universe is a
balance of
outward forces
and gravitational
attraction
• Thus, the total
amount of
gravitating stuff
determines the
kind of evolution
• Critical density is
a unique point
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Open, Flat, or Closed?
• The amount of matter, and the fractions of
different types of matter, determine
whether the universe expands forever or
eventually collapses
– Radiation: composed of massless or nearly
massless particles that move at the speed of light.
Known examples include photons (light) and
neutrinos. This form of matter is characterized by
having a large positive pressure.
– Baryonic matter: this is "ordinary matter"
composed primarily of protons, neutrons and
electrons. This form of matter has essentially no
pressure of cosmological importance.
– Pure Energy … like Einstein’s cosmological constant
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Einstein’s “big mistake”
• The expanding nature of the Big Bang theory
derives from Einstein’s general theory of
relativity
• Einstein originally wrote an equation for how
space must expand under relativity:
acceleration = - const/6R2 + 1/3 R Λ
The “cosmological” constant was a term Einstein
inserted on aesthetic grounds – to stop the
expansion !!! He later called it his greatest mistake.
(we now think it does indeed exist … an energy sea)
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Luminous Matter
• Most of the larger collections of matter in
the universe emit detectable radiation, from
x-rays to radio frequencies
• But most of the photons in the universe are actually
not emitted by stars, gas, or any other object in the
contemporary universe. Rather, they are the lowenergy photons left over from the Big Bang. This relic
radiation is primarily in the microwave region of the
spectrum, and is known as the Cosmic Microwave
Background (CMB). Photons from the CMB pervade
space, providing a background buzz that serves as a
constant reminder of the hot, dense state in which
our universe began
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The Cosmic Microwave Background
• At startling discovery in 1965: we are bathed
in a sea of photons equivalent to those emitted
by an object at temp=2.7 Kelvin
•Blackbody radiation:
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Why Isotropic Photons?
• The Cosmic Microwave Background (CMB)
strongly supports the Big Bang model
– These would be the photons created from particle
collisions after the density of matter became so
dilute the photons were not being absorbed any
longer
– It therefore makes sense they should be coming
uniformly from all directions (more or less)
– They were created about 370,000 yr after the Big
Bang, when the universe had a temperature of
3,700 K (we see the photons redshifted to 2.7 K)
– These photons came from the “surface of last
scattering” – the edge of the visible universe
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In pictures:
• So, we see a
consistent
model where
as the
universe
expands the
fractions of
different
types of
matter
evolve
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The Latest Measurements
• In recent years we have been able to perform
very precise studies of the CMB
– Space probes and Antarctic telescopes
– Can see small variations in the CMB (10-5 level)
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The CMB Fluctuations
• Here’s a plot of the amount
of graininess at different
distance scales
• The first peak results from
the interplay between
gravity and acoustic shock
• As the universe cooled, matter
collected preferentially in certain
areas … like acoustic nodes
• the scale plot fits beautifully to a
theory … which gives the parameters
•… and a problem
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Meanwhile, stargazers had a problem
Not Newton
Newton
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• 1990s: Vera Rubin and
others measured the
rotation speed of visible
objects in galaxies and
clusters
• The results were not
consistent with what we’d
expect for a mass
concentration in the
center
• There is a LOT of “dark”
matter everywhere
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Another Indication of DM
• This is supported by
images made using x-rays
• X-rays are coming from
reqions of space where
we did not expect there
to be much matter
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Gravitational Lensing
• Further evidence of
some forms of Dark
Matter are evident
through telescopes
• Light from some
visible objects is
distorted by the
gravitational field of
very heavy compact
objects which are
invisible (emit no
visible light)
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Finally, Hubble redux:
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The expansion is accelerating!
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All the pieces fit!
• When you assemble all
the experimental
evidence, the picture
is consistent:
– Flat universe
– Accelerated expansion
• Implies backgrnd E
• What causes it?
– Consistent with
inflation
• But a surprising
measurement of the
matter …
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The Embarassing Bottom Line
1. So 5/6ths of the
matter is in a
form unknown to
us
2. Even worse: 74%
of the energy in
the universe is
not in any of our
theories!!!
• We might
already have a
solution for #1
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Particle Physics Might Have a Solution
• For a long time, particle physicists have felt
that there should be partners of the
currently known particles
– This family of supersymmetric partners would
provide a simple solution to some problems in the
Standard Model theory
– If they exist, they could have just the right
qualities to be a candidate for the Dark Matter!
– To solve the theory problems, some of them would
have to have masses which the next generation of
accelerators could see
– Another theory posits Extra Dimensions
• Suddenly particle physicists are popular again!
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SUSY
• SuperSymmetry is an idea that has been
around for decades
• To solve some mathematical glitches in the
Standard Model, propose the existence of a
mirror family of partners to the current ones
– The sparticles have oppposite properties of the
particles, causing mathematical cancellations in the
theory
– The sparticles would have larger masses
• And the sparticles could have the right
properties to be the Dark Matter!!!
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2007: the Large Hadron Collider
• To reach the energies needed to create
SUSY particles, we need an accelerator
capable of reaching the 1,000,000,000,000
volt range … the “TeV” range, or Terascale
– We tried to build the SSC in Texas, but it became
too expensive, so we are constructing a cheaper
version in Geneva, Switzerland … the LHC
– LHC will collide protons head-on, permitting them
to sometimes annihilate into any particles …
providing the products have masses satisfying mc2
is less than the collision energy
• Online by end of 2007
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At 7.5xEFermilab, LHC is big!
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LHC Parameters
• The protons are going so fast, it is difficult
to build magnets that can bend them sharply
– hence the large radius (27 km circumference)
– Need 1232 magnets, each 14 m long ($300k ea)
• Because the collision products have high
energies, large detectors are required
– Need lots of material to absorb energetic particles
– Millions of electronic channels to measure the
collisions
– HUGE data rate – using GRID computing network
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The ATLAS Detector
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It will be in place next year
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Expect to see the new particles!
• The energy and
rate of LHC
protons should
produce the
particles we are
seeing hints of
in earlier
experiments and
cosmology
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And After LHC?
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The ILC !!!
• The International Linear Collider
– Being planned now for construction next decade
– A truly international project
– By colliding electrons instead of protons, it will be
much better than LHC for pinning down the
properties of the new particles … and better at
discovering some types of new particles
Its been in the press recently and
getting a lot of attention in
Washington
Also: realization that the US has
fallen behind in scientific
funding
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Epilogue
• I hope I have convinced you:
– We are in a new scientific revolution
– Major discoveries will thrill us in the next few years
• While the results are fascinating in and of
themselves, we all benefit from this research
in many ways
– Education and cutting-edge technical training
– Development of new industrial processes
– … what the future brings…who knows!
• Thank you, and the Linda Hall Library, for this
opportunity!
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