Getting to the Heart of Matter:
New Ways to Understand the Mass We See
and Don't See in the World Around Us
Extraordinary progress has been made over the past decade in understanding
masses of "elementary" particles, the building blocks that make up all matter we
know about in the universe. That progress was accompanied by great surprises:
supposedly mass-less neutrinos have been shown to possess mass. Most of the
mass in our galaxy and elsewhere in the universe appears to be in the form of "dark
matter", particles not yet detected in the laboratory. In the "Standard Model" of
particle physics, mass derives from interactions of quarks and leptons -- the
constituents of all known matter -- with a new kind of particle or field, called the
"Higgs particle". The Higgs particle also remains elusive.
New scientific tools -- including the Large Hadron Collider which will be coming
online next year -- may be able to shed light on the nature of dark matter and the
Higgs particle. If history is any guide, the new tools will also create their own
surprises.
The talk will cover some of the recent highlights in the historic quest to understand
the basic forces and building blocks of all matter and energy and it will preview the
new tools and how they plan to carry on this quest.
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Einstein (1905, ff.)
E
m= 2
c
E=
hc
λ
m=m
and, the World is made of atoms!
Last year, we celebrated the centenary of Einstein’s annus mirabilis: 5 remarkable
papers that changed physics. We will use 3 equations of Einstein in our
examination of mass In the universe.
The first, and to most physicists, the most radical of that year, proposed that light—
radiation—was not continuous, but consisted of particles we now call photons which
carry a definite quantum of energy depending on their wavelength. This is the work
for which he received the Nobel Prize. His formula for representing this idea is the
centerpiece of this slide.
The formula on the left came from his final paper of 1905 in an addendum to his
“relativity” paper; in slightly different form, it is the most famous formula in science.
In the present form, however, it achieves its “Einsteinian” significance: “The mass of
a body is a measure of its energy content;” He went on to say: “If theory agrees
with the facts, then radiation carries inertia between emitting and absorbing bodies.”
(Comment: energy from the stars; accelerators.)
The third formula is Einstein’s somewhat later postulate, the equivalence principle,
stating that the two Newtonian concepts of mass, inertial and gravitational, are
equivalent.
Taken as icons, these three formulas represent modern physics, the joining of
relativity and quantum theory to describe our world of atoms and nuclei in all their
diversity and Einstein’s theory of gravity, “General Relativity.”
His other two papers that year essentially closed any remaining debate on the
atomic picture of the structure of matter.
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Goals of particle physics to find out:
What is the world made of?
How does it work?
Answer:
it depends on where you look.
Physics recognizes 4 basic forces in nature: gravity, electromagnetism, a strong
force responsible for binding nuclei of atoms and a weak force involved in certain
kinds of radioactive decay. At the largest scales we know about, gravity is the
dominant force, shaping the universe around us. At our scale, gravity is important,
but we begin to sense the importance of the other forces and by the time we
examine physics at the scale of atoms and smaller, gravity is completely negligible
and the other forces dominate.
20th century physics revealed the nature of these forces at the atomic scale and
smaller, the particles that make up all the matter around us, and the theoretical tools
needed to understand their interactions down to distances ~1/1000 the size of
atomic nuclei, our present horizon of understand of the very small. At much smaller
distances—the “Planck scale”—it is expected that gravity will return to be the
dominant force.
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m=
hc
E
E
=
λ
c2
Progress in understanding the structure of atoms and their nuclei during the 2nd half
of the 20th century was achieved by remarkable advances in particle accelerators
and detectors. The photo above shows Fermilab, outside Chicago, which has
operated the world’s highest energy accelerator—the Tevatron—over the past 20
years (along with other, lower-energy machines).
In the Tevatron, counter-rotating beams of protons and their antiparticle,
antiprotons, collide in the center of large detectors, such as “CDF” shown, which
analyze particles produced in the collisions. Physicists use detectors to “see” trails
of ionization left by charged particles produced in the collisions as they pass through
matter.
In these experiments, direct use is made of Einstein’s relationship between mass
and energy. New massive particles are created from the energy achieved by the
accelerator. The study of the new particles and their interactions reveals details of
the underlying physics down to scales of distance related to energy by Einstein’s
“photon” relationship.
The 50 “golden-years” of accelerator-driven particle physics can be broken into two
major periods: 1947-1974 experimental and theoretical discoveries leading to the
“Standard Model”, post-1974 testing the SM and searching for physics “beyond the
Standard Model”
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Standard Model
ca. Nov. 1974 — today
ne
e
nm
m
nt
t
Leptons
+ forces unified through hidden
symmetries, all described by
relativistic quantum field theory
u
d
c
s
t
b
Quarks
Mass of quarks and charged leptons
acquired through interactions with a
new field filling all space: The Higgs
The Standard Model represents the culmination of the 20th Century quest to
understand the basic building blocks of all matter and the forces that operate
between building blocks. It posits matter as being made from two classes of
objects: quarks and leptons. Quarks participate in the strong nuclear force and are
the distinguishing constituents of the nuclei of atoms. Leptons do not respond to
the strong force, but interact with the other known forces, electromagnetism, weak
and gravity, in the same way as do quarks. The Standard Model is based on
symmetry principles requiring the quarks and leptons to have exactly zero mass. It
accommodates the evident fact that particles carry mass by introducing a new field
which fills all space, the “Higgs field”. Quarks and the charged leptons acquire their
masses by they way they “attach” to the Higgs field.
The precise date I’ve given to the acceptance of the Standard Model corresponds to
the start of a revolutionary set of experiments which began on this day in 1974 at
the Stanford Linear Accelerator Center. That was a Saturday; by the following
Monday the world of particle physics had entered its “November Revolution” which,
over the next few weeks and months, joined in a remarkable consensus on the
validity of the Standard Model. To date, there are no experimental conflicts with
predictions of the SM, but as we shall see, there certainly are important questions
not understood within the Standard Model.
Review what was so special about the Nov. revolution and the next generation of
discoveries to where we stand today.
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Weighing Nothing
m=
E
c2
E=
hc
λ
How mass is described in the Standard Model:
The SM uses elegant symmetry arguments to describe and unify the strong,
electromagnetic and weak forces. The symmetries invoked rely on having all
particles—quarks and leptons—exactly massless. The symmetry is broken in our
world by the introduction of a new field filling all space, the Higgs field. Interactions
between the quarks and charged leptons and the Higgs field given them their
apparent mass. Nuclei of atoms gain additional mass from the energy content
associated with binding quarks in nuclei. The diversity of masses between, say, the
electron and the top quark is large, about 1 : 1/3 million
The three neutrinos sit outside this picture. They don’t stick to the Higgs field in the
way quarks and charged leptons do. They don’t bind together to acquire internal
energy and, hence, mass. They were expected to retain their masslessness in the
real world.
It was known since the early days of nuclear physics that neutrinos must have tiny
masses at most, much less than the electron, for example.
Starting nearly 40 years ago, there started to appear data indicating something was
wrong with neutrinos that travel long distances, first from the sun, then in the upper
reaches of the earth’s atmosphere and now in accelerator experiments. 2002 NP,
SNO & MINOS @ UT
Unavoidable conclusion: neutrinos have definite mass, small but not zero.
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Weighing Nearly Everything
m=m
For decades, astronomers have sought to measure the total mass “out there” in our
universe. In both Newton’s theory of gravity and Einstein’s, an object in orbit
around other things must move with a speed that depends on the total mass inside
its orbit. Too slow and it falls in; too fast and it sails off. So, by measuring the
speed of stars orbiting on the fringes of galaxies, one can determine the total mass
of all the stuff in the galaxy!
(Naturally, some simple equations from Einstein tell how to relate shifts in the
apparent color of stars to their speed around their galaxy—just like Doppler radar!)
Historically big putdown: what we actually see in galaxies—all the stars and other
exotic things—represents only a few percent of the mass of the galaxy, itself!
Another several percent is probably in non-luminous ordinary matter, like planets
and dust clouds and the like. The rest appears to be distributed in more or less a
large ball of dark matter, coexisting with the beautiful luminous disks we associate
with galaxies. Presumably, the particles of dark matter are passing through us and
everything we see in our own Milky Way galaxy all the time with no hint of their
passage, properties or identities.
This is known as the dark-matter “problem.”
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Weighing Everything
Around the time the Standard Model came together, an intrepid band of particle
physicists, astronomers and cosmologists invented a new sub-field of science which
is now called particle astrophysics. The basic idea was that information being
gained about the basic building blocks of matter and basic forces from particle
accelerators and detectors could be combined with knowledge emerging on the
evolution of the universe—following Einstein’s General Relativity theory—from the
Big Bang to the present day to learn more about both the basic laws of physics and
why our universe, in the large, is the way it is. Particle astrophysics is one of the
most exciting areas in science today; it was recognized earlier this Fall by the
awarding of the Nobel Prize to George Smoot and John Mather for their work in
acquiring the image above of the “early universe” with the COBE satellite.
(Side-light: GS’s role in the November Revolution. His proposal presented at SLAC
on 11/11/1974 was turned down, so he went on to other pursuits!)
Describe what the image tells us.
Since COBE, more superb measurements: WMAP, supernova measurements, …
What has been learned: Universe is “flat”, ordinary matter makes up about 4% of
the total mass/energy of the universe; dark matter about 20-25% and all the rest
appears to be in the form of “dark energy.”
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Everything?
All Matter ?
Standard Model
ne
e
nm
m
nt
t
u
d
c
s
Light and Matter
t
b
Dark Matter
Dark Energy or Einstein’s “greatest mistake”
Representation of the current state of knowledge of the relative abundance of
matter and energy in the universe.
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Coming Soon:
CERN Large Hadron Collider
For the past decade, a new particle accelerator under construction: the Large
Hadron Collider at CERN in Geneva, Switzerland.
World-wide involvement; major participation by US physicists.
Initial operations should begin in about one year.
7X the energy of the Fermilab Tevatron, permitting particles of higher mass to be
produced and discovered! Some of the main scientific goals:
Study, in detail, the Higgs mechanism for mass generation
Discover (?) the Higgs particle
Test the validity of the Standard Model at higher energies/smaller distances.
Look for new physics “beyond the Standard Model”
Dark Matter candidates?
Complete surprises
Stay tuned!
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Some elements of the LHC:
Inside the underground accelerator tunnel showing a section of the
superconducting magnets that contain the counter-rotating beams of protons
which collide to form the new states of matter to be studied.
Cut-away drawing of one of the large particle detectors. There are two major
detectors, Atlas and CMS, being constructed by international teams of ~2000
physicists each!
Simulated Higgs event.
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Where We stand
Matter
• Neutrinos have mass but
shouldn’t
• Only about 4% of the stuff in
the Universe is made out of
“ordinary” matter
• The LHC should tell us about
the Higgs field and its role in
the mass of ordinary matter
• The LHC may reveal the
nature of dark matter
• How do we ever get our
hands on Dark Energy?
This is great news!
An interesting and amusing genre on the periphery of scientific literature comprises
the proclamations of the end of science by some of its greatest heroes, just as they
are about to retire from the stage. An example:
“Physical science is thus approaching the stage when it will be complete, and
therefore, uninteresting. Given the laws governing the motions of electrons
and protons, the rest is merely geography—a collection of particular facts
filling their distribution throughout the portion of the world’s history. The total
number of facts of geography required to determine the world’s history is
probably finite; theoretically, they all could be written down in a log book to
be kept at Somerset House with a calculating machine attached which, by
turning a handle, could enable the inquirer to find out the facts at other times
than those recorded. It is difficult to imagine anything less interesting or
more different from the passionate delight of incomplete discovery. It is like
climbing a high mountain and finding nothing at the top except a restaurant
where they sell ginger beer, surrounded by fog but equipped with a wireless.
Perhaps in the times of Ahmes the multiplication table was exciting.”
-- Bertrand Russell
Fortunately, there is much we don’t yet know!
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