This image shows the first Omega-minus particle to be
discovered. An incoming K- meson collides with a proton in a
liquid hydrogen bubble chamber producing the Omega-minus, a K+
meson, and a K0 meson. Neutral particles leave no tracks in
the chamber and are denoted by dashed lines. The positions of
the neutral particles are inferred from the visible decay
products and by the use of the laws of conservation of energy
and momentum.
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Discovery of the strange quark
One of the most important observations in particle physics was
the discovery in 1964 of the Omega-minus baryon at Brookhaven
National Laboratory. In the two decades leading up to this
point over 27 supposedly "fundamental" particles had been
found. In 1961 Murray Gell-Mann and Yuvall Ne'eman
independently noticed that these particles could be grouped by
a symmetry called SU(3)—the special unitary group in
three dimensions—into a new "periodic table" called "the
Eightfold Way." In this scheme the particles with the same
quantum numbers of spin and parity are collected into
singlets, octets, and decuplets (groups of 1, 8, and 10).
Within these groups, the particles differed in their mass,
charge, and a funny quantum number called strangeness. In 1964
the only particle missing from the decuplet was the one that
had three units of strangeness. Using the known baryons, this
particle was predicted to have spin 3/2, charge -1, and a mass
of roughly 1.7 GeV/c^2. The subsequent discovery of the
Omega-minus with exactly these properties confirmed that there
was an underlying symmetry that described matter. Further, the
difference in masses of the particles broke the symmetry in a
systematic and predictive manner.
After the discovery of the Omega-minus, Gell-Mann and George
Zweig proposed that this SU(3) symmetry was actually an
artifact of the fact that the baryons were made of three
"quarks"—the up, down, and strange quarks—which
had charges +2/3, -1/3, and -1/3, respectively. Hence, the
Omega-minus is a collection of three strange quarks. The quark
model was born.
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Computer reconstruction of the decay of a Psi-prime meson into
J/Psi, pi+, and pi- mesons. The shape of this event resembles
the Greek letter Psi and was jokingly offered as proof that
the particle knew its name.
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Discovery of the charm quark
Prior to 1970, it was understood that the weak force could
cause a strange quark (charge -1/3) to decay into an up quark
(charge +2/3), a muon (charge -1), and a neutrino (charge 0).
It was a puzzle, however, as to why the strange quark was
never observed to decay into a down quark (charge -1/3) plus
two neutrinos. Along with this puzzle was the observation that
while physicists had discovered four leptons—electron,
muon (a heavy version of the electron), electron neutrino, and
muon neutrino—they had seen only three quarks—up,
down, and strange. In 1970 Sheldon Glashow, John Iliopoulos,
and Luciano Maiani proposed that if there was a fourth quark
with charge +2/3, then a symmetry would forbid these "neutral
current" decays.
In 1974 physicists at both Brookhaven National Laboratory and
the Stanford Linear Accelerator Center discovered a new meson
called the J/Psi. This meson was seen as a peak in the
annihilation of electron/anti-electron pairs at a mass of 3.1
GeV/c^2. The striking feature of this discovery was that the
particle lived over 1,000 times longer than expected. This
meant that it could not be just a composition of the up, down,
and strange quarks but was composed of a charm/anti-charm
quark pair. The discovery of the charm quark left no doubt
that there was a deep symmetry that forced the number of
leptons and quarks to be the same.
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Computer reconstruction of the decays of a B meson and an
anti-B meson from the BABAR experiment at the Stanford Linear
Accelerator Center. One B meson (gold tracks) decays into a
J/Psi (which decays to mu+ and mu-) and K-short (which decays
to pi+ and pi-). The other B meson (red tracks) decays into a
K- and three pions.
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Discovery of the bottom quark
When the tau lepton was discovered in 1975, the number of
known leptons was increased to five (electron, muon, tau,
electron neutrino, and muon neutrino). Immediately a search
began for a third generation of quarks: the bottom (or beauty)
quark and top (or truth) quark. In 1977 physicists discovered
a new meson called the Upsilon at the Fermi National
Accelerator Laboratory. This meson was immediately recognized
as being composed of a bottom/anti-bottom quark pair. The
bottom quark had charge -1/3 and a mass of roughly 5 GeV/c^2.
We owe our existence to the fact that the universe does not
have the same amount of matter and antimatter. Despite this
observation, it was commonly believed that the laws of physics
were subject to a symmetry known as CP, which states that if
matter is exchanged with antimatter and space is inverted
(think of a reflection in a mirror), then the laws of physics
should not change. In 1964 James Cronin and Val Fitch
discovered that this symmetry is violated by the weak force at
1 part in 1,000 in mesons that had strange quarks. In 1973
Makoto Kobayashi and Toshikide Maskawa showed that if there
were six quarks, then this CP violation could be explained by
mixing between the down, strange, and bottom quarks. The
mixing leads to prediction of a large asymmetry in the decays
of B mesons to J/Psi and K-short mesons. The observation of
this asymmetry provided strong support for the
Kobayashi-Maskawa hypothesis.
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The first image shows a pair of top quarks reconstructed in
the Collider Detector at Fermilab (CDF). Each top quark decays
to a W boson and a b quark. The pink tower in the wide view
identifies a positron (an anti-electron) from one W decay; the
inset shows displaced decays of two b particles (red tracks).
The second image shows a pair of top quarks reconstructed in
the DZero experiment at Fermilab. This end view shows the
final decay products: two muons (turquoise), a neutrino
(pink), and four jets of particles. The height of the boxes
denotes the amount of energy deposited in the detector in each
wedge.
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Discovery of the top quark
After the discovery of the bottom quark in 1977, physicists
expected to quickly find its partner called the top quark. But
it required 18 years of dedicated work to identify the
production of just 20 pairs of top quarks. In 1995 physicists
identified the top quark by both the CDF and DZero experiments
at Fermilab. Many physicists were amazed to find that the mass
of the top quark was 175 GeV/c^2 (similar to a gold atom),
which is about 35 times as massive as the bottom quark and
about 35,000 times as massive as the up and down quarks.
The standard model of particle physics predicts that early in
the history of the universe, before the electroweak force
split into the electromagnetic and weak forces, all
fundamental matter particles (quarks and leptons) were exactly
massless. When the forces split, a symmetry relating left- and
right-handed particles called chirality was also broken,
allowing particles to acquire masses. Remarkably, if we assume
that all unknown numerical coefficients should be
approximately equal to 1, then the top quark mass is predicted
to be 175 GeV/c^2! Unfortunately, this would also predict that
all other matter particles would have masses of around 175
GeV/c^2, which is in rather poor agreement with observation.
One of the most profound mysteries of nature is why most of
matter is so light compared to the top quark.
As the top quark continues to be studied at Fermilab, a unique
property of the decay of the top quark will play a vital role
in understanding what physics could lie beyond the standard
model. It turns out that the top quark decays before it has
time to bind to another quark. This implies that measurements
of the decays probe the actual properties of the quark itself,
and are not washed out by additional interactions. A
significant fraction of theoretical models that go beyond the
standard model predict significant deviations of the measured
rates of production, the angles measured between the decay
products, and even what the decay products will be. The
standard model predicts most of these quantities to high
precision, and any deviation will be a sign of new physics.
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A tau lepton appearing in a tau neutrino interaction as
reconstructed from an emulsion (photographic) measurement. The
tau lepton (red track) appears in the middle of the detector
with two associated particles (most likely pions in gray). The
tau travels a short distance and decays into an electron
(green track) and two invisible neutrinos, which causes a kink
in the track.
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Discovery of the tau neutrino
The tau neutrino was the last matter particle of the standard
model to be observed. After the discovery of the tau lepton in
1975, there was little doubt that the tau neutrino existed.
Later measurements of the invisible decay of the Z boson led
to an indirect measurement that there were exactly three light
neutrinos. However, it was not until 2001 that a tau neutrino
was directly detected for the first time by the DONUT
experiment at Fermilab. The scale of the problem is best
described by realizing that 20 trillion neutrinos pass through
your living room every second, and yet it is likely that none
will ever interact with anything. In order to "see" a tau
neutrino, an experiment has to look for a tau lepton that
appears out of nowhere but points back to the source of tau
neutrinos.
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One method of measuring neutrino oscillation is to compare the
difference in the number of muon neutrinos produced by the KEK
proton synchrotron accelerator in Tsukuba, Japan, and the
number that reach the Super-Kamiokande detector 155 miles
away. In this image a muon neutrino interacts inside the
1,000-ton water tank at KEK and creates a muon. The muon emits
a ring of Cerenkov light as it slows down in the water. The
color coding denotes the time of arrival of the light and is
used to verify that the neutrino came from the direction of
the accelerator.
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Neutrino mass and oscillation
One of the great successes of astrophysics is the solar
standard model. This model predicts with high accuracy how
fusion of elements in the sun ultimately produces the light
that we see. Electron neutrinos are emitted in the process of
fusion. These neutrinos interact so weakly that only a handful
of the billion billion that pass through a detector each day
are seen. However, it was realized as early as 1967 that
physicists were finding only half of the electron neutrinos
predicted to be coming from the sun.
The solution to the mystery of the disappearing neutrinos
begins with the 1998 observation by the Super-Kamiokande
collaboration in Japan that a large fraction of the muon
neutrinos produced in the atmosphere were oscillating into tau
neutrinos before they reached the Earth. In 2002 the Sudbury
Neutrino Observatory in Ontario, Canada, confirmed that the
total number of neutrinos coming from the sun is exactly
consistent with the prediction of the solar standard model,
but that only half of these are electron neutrinos.
The combination of these observations leads to the conclusion
that the rest of the electron neutrinos coming from the sun
oscillate into either muon neutrinos or tau neutrinos. While
neither of these experiments measure the actual neutrino
masses, the fact that neutrinos mix overturns a 50-year-old
assumption that the masses were exactly zero. Several ongoing
experiments are attempting to understand the exact nature of
neutrino mixing and to determine the neutrino masses.
The discovery that neutrinos of different generations can turn
into each other violates a symmetry known as lepton family
number conservation. In all known particle physics experiments
performed so far, the total number of leptons of a given type
is always the same. Therefore, if a muon (muon-L = 1) decays
into an electron (electron-L= 1), then there is always a muon
neutrino (muon-L = 1), and an electron anti-neutrino
(electron-L = -1) as well. If a muon neutrino can become an
electron anti-neutrino, then the decay of a muon to an
electron plus a photon should be possible. This has never been
seen, however, because the probability of such a decay
occurring is less than one part in a million trillion trillion
trillion trillion. It is unlikely that this has ever occurred
in our galaxy.
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The first image is a reconstruction of a W boson produced in a
proton/anti-proton collision. The W boson quickly decays into
a tau lepton and tau anti-neutrino. The pencil-like jet in the
center is the tau lepton, the neutrino is inferred from energy
conservation, and the curved tracks are the remnants of the
proton and anti-proton. The second image is a reconstruction
of a proton/anti-proton collision that produced a Z boson. The
Z boson decayed to an electron and a positron (blue dotted
tracks). The other tracks are the remnants of the proton and
anti-proton.
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Electroweak symmetry
The electromagnetic force between charges (responsible for
both chemical binding and the emission of light) is carried by
a massless photon. The weak force is responsible for both the
fusion of hydrogen in the sun into deuterium and the decay of
a neutron into a proton, electron, and anti-neutrino. In the
1960s Sheldon Glashow, Abdus Salam, and Steven Weinberg
realized that a single theory of electroweak interactions
could describe these two forces. This theory showed that the
weak force was weak because it occurred by the exchange of
very massive W and Z bosons, which have a very short range.
The W boson changes one particle into another with a different
charge—for example, an electron (charge -1) into an
electron neutrino (charge 0), or a strange quark (charge -1/3)
into an up quark (charge +2/3). The Z boson is similar to the
photon in that it does not change particles into each other.
However, the Z boson can interact with neutrinos, whereas the
massless photon only interacts with charged particles.
In 1967 Steven Weinberg showed that there was a fixed
relationship between masses of the W and Z bosons. In 1983 the
W and Z bosons were found by the UA1 collaboration at the
Super Proton Synchrotron at CERN with exactly the masses they
were predicted to have. Today many of the predictions of the
electroweak theory have been tested to high precision.
However, there is one additional neutral particle called the
Higgs boson that is predicted by the theory, but has not been
found. There are no exact predictions of the mass of the Higgs
boson, but indirect measurements suggest that this particle
should be seen in the near future. A tantalizing prospect is
that the same indirect measurements suggest that the theory
may not be complete. Hence, the search for the Higgs boson may
soon completely change our understanding of the forces of
nature.
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In the first image, the production of three jets (or isolated
groupings) of particles indicates the discovery of the gluon.
Two jets are the remnants of quarks, and one is from a gluon.
As the second image indicates, modern detectors can see gluons
by the production of three jets. The height of the boxes
indicates the amount of energy deposited in the detector, and
the colors indicate how far into the detector the energy was
deposited.
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Production of the gluon
The quark model explained why mesons are made of
quark/anti-quark pairs, and baryons are made of three quarks
or three anti-quarks. This appears to violate the Pauli
exclusion principle, which forbids putting even two identical
quarks in the same state. And yet the Omega-minus baryon
(composed of three strange quarks) was discovered in 1964. The
solution to this puzzle, proposed by Oscar Greenberg in 1964,
is that the quarks each carry a "color" charge (red, blue, or
green). Hence, baryons are composed of one red quark, one blue
quark, and one green quark, and mesons are composed of one red
quark and one anti-red anti-quark (or green/anti-green, or
blue/anti-blue). All observable particles must have no net
color (red + green + blue = 0).
The mathematics that describes how the colors can combine is
the same SU(3)—the special unitary group in three
dimensions—that led to the discovery of quarks. Unlike
the approximate symmetry between the quarks, however, this
symmetry is exact. This theory predicted that the quarks are
bound together by a strong force that is carried by a new
massless particle called the gluon. The gluon connects one
color to another in eight possible ways, but since it carries
color charge itself, it is never seen in isolation. In 1979
scientists at the PETRA storage ring at the DESY laboratory in
Hamburg, Germany observed three jets of particles for the
first time. Two of the jets were due to the hadronization (the
process of becoming color neutral) of quarks, and one was from
a gluon.
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This image shows a candidate for the associated production of
a Higgs boson and Z boson. The candidate Higgs decays to a
bottom quark and anti-bottom quark, which in turn decay to the
jets denoted by the green and yellow tracks. The Z boson also
decays to two jets denoted by the red and blue tracks.
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Search for the Higgs boson
The electroweak symmetry that describes both the
electromagnetic and weak forces is not exact at low energy.
This symmetry is broken by the Higgs mechanism, which predicts
the existence of a new particle with no spin called the Higgs
boson. The interaction between Higgs bosons and quarks or
leptons provides the mass of these fundamental matter
particles. Despite 30 years of searching, the Higgs boson has
remained elusive. Indirect evidence suggests that discovery of
the Higgs boson should be within reach of experiments that
will occur over the next decade at Fermilab and the CERN Large
Hadron Collider.
The difficulty in finding the Higgs boson has created a great
deal of excitement in particle physics. Whether or not the
Higgs exists, the standard model is now known to be
incomplete. If the Higgs is found, then a calculation of the
Higgs mass implies that the standard model breaks down at
energies just higher than the mass of the Higgs itself. By
studying the properties of the Higgs boson it is hoped that
clues can be found to what lies beyond. An even more exciting
possibility is that the particle does not exist. In this case
there would be completely new types of matter that have never
been seen before. The structure of space and time itself may
even be different than expected.
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This is a simulation of the production and decay of
supersymmetric particles in a proposed linear collider
detector. The straight line is a lepton, the tracks are two
overlapping jets of particles, and several invisible particles
are inferred by conservation of energy and momentum.
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Search for supersymmetry
One model that has gained a lot of attention as a replacement
for the standard model is the theory of supersymmetry. This
theory states that for every particle that has been found
there are mirror particles that are identical in all respects
except for their spin. Bosons of spin 1—the photon, W,
Z, and gluon—have spin 1/2 partners called the
charginos, neutralinos, and gluino. Fermions of spin
1/2—leptons and quarks—have spin 0 partners called
the sleptons and squarks. There are predicted to be five Higgs
bosons of spin 0, each of which has a partner called a
higgsino of spin 1/2. Since none of these supersymmetric
particles have been found, the symmetry must be broken.
One prediction of this theory is that at least one of the
Higgs bosons must have a mass less than 150 GeV/c^2—well
within the reach of experiments that will occur within the
next decade. If this theory is correct, then several of the
superparticles should also be visible either in current
experiments at Fermilab or upcoming experiments at the CERN
Large Hadron Collider. Supersymmetry is a key prediction of
string theories. The discovery of superparticles would provide
significant insight into how string theories are manifest in
our low-energy world.
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This image is a simulation of the production and decay of a
black hole in a proposed linear collider detector. The black
hole quickly evaporates into every type of matter particle.
The "democratic" selection of decay products is a distinct
signature of black hole decay.
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Search for large extra dimensions
The universe appears to be made of three dimensions of space
and one of time. Objects go up and down, right and left,
forward and backward, and move forward through time. This
appearance, however, has never been tested at distances much
less than the width of a hair, or at energies much above the
mass of the top quark. What if there were additional
directions in space in which particles could move if they just
had enough energy? Einstein showed that the force of gravity
was equivalent to the geometry of space and time. If there are
more than three spacial dimensions, then the force of gravity
is very different than normally experienced.
The gravitational force between two particles is so weak
compared to the other forces that it is generally ignored by
particle physicists. In a world with additional dimensions,
however, this gravity may be very strongly coupled to matter
at high energies. This leads to the intriguing possibility of
producing miniature black holes at collider experiments. These
black holes would quickly decay by Hawking radiation into
several particle/anti-particle pairs. Because gravitation
couples to energy density, there is little preference for the
type of particles that would appear in the decays. Hence, the
signature of black hole production would be the appearance of
nearly every type of particle coming from a point.
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