A BRIEF INTRODUCTION TO PARTICLE PHYSICS
A BRIEF INTRODUCTION TO PARTICLE
PHYSICS..................................................... 1
What is Particle Physics?..................................................................................... 2
What about the nature of our
Universe?.................................................... 6
What is Particle Physics?
Protons, electrons, neutrons, neutrinos
and even quarks are often featured in news of scientific discoveries. All of
these, and a whole "zoo" of others, are tiny sub-atomic particles too
small to be seen even in microscopes. While molecules and atoms are the basic
elements of familiar substances that we can see and feel, we have to
"look" within
atoms in order to learn about the "elementary" sub-atomic particles
and to understand the nature of our Universe. The science of this study is
called Particle Physics, Elementary Particle Physics or sometimes High Energy
Physics (HEP).
Atoms were postulated long ago by the
Greek philosopher Democritus, and
until the beginning of the 20th century, atoms were thought to be
the fundamental indivisible building blocks of all forms of matter. Protons,
neutrons and electrons came to be regarded as the
fundamental particles of nature when we learned in the 1900's through the
experiments of Rutherford and others that atoms consist of mostly empty space
with electrons surrounding a dense central nucleus made up of protons and
neutrons.
Inside an Atom: The central nucleus
contains protons and neutrons which in turn contain quarks. Electron clouds surround the nucleus of an atom
The science of particle physics surged
forward with the invention of particle accelerators that could accelerate
protons or electrons to high energies and smash them into nuclei — to the
surprise of scientists, a whole host of new particles were produced in these
collisions.
By the early 1960s, as accelerators reached higher energies, a
hundred or more types of particles were found. Could all of
these then be the
new fundamental particles? Confusion reigned until it became clear late in the
last century, through a long series of experiments and theoretical studies,
that there existed a very simple scheme of two basic sets of particles: the quarks and leptons (among the leptons are electrons and neutrinos), and a set of fundamental forces that allow these to
interact with each other. By the way, these "forces" themselves can
be regarded as being transmitted through the exchange of particles called gauge bosons. An
example of these is the photon, the quantum of light and the transmitter of the
electromagnetic force we experience every day.
Together these fundamental particles form
various combinations that are
observed today as protons, neutrons and the zoo of particles seen in accelerator
experiments. (We should state here that all these sets of particles also include their anti-particles, or in plain language what might roughly
be called their complementary opposites. These make up matter and anti-matter.)
Matter is composed of tiny particles called
quarks. Quarks come in six varieties: up (u), down (d), charm (c), strange (s), top (t), and bottom (b). Quarks also have antimatter counterparts
called antiquarks (designated by a line over the letter symbol). Quarks combine
to form heavier particles called baryons, and quarks and antiquarks combine to form mesons. Protons and neutrons,
particles that form the nuclei of atoms, are examples of baryons. Positive and
negative kaons are examples of mesons.
Today, the Standard Model is the theory that describes the role of these fundamental
particles and interactions between them. And the role of Particle Physics is to
test this model in all conceivable ways, seeking to discover whether something
more lies beyond it. Below we will describe this Standard Model and its salient
features.
What about the nature of
our Universe?
A Hubble
Telescope photograph of galaxies deep in Universe
Here is our present understanding, in a nutshell. We believe that
the Universe started off with a "Big Bang", with enormously high energy and temperature concentrated in an
infinitesimally small volume. The Universe immediately started to expand at a
furious rate and some of the energy was converted into pairs of particles and
antiparticles with mass— remember Einstein's E= mc2 . In the first tiny fraction of a second, only a mix of radiation
(photons of pure energy) and quarks, leptons and gauge bosons existed. During the very dense phase, particles and antiparticles
collided and annihilated each other into photons, leaving just a tiny fraction
of matter to carry on in the Universe. As the
Universe expanded rapidly, in about a hundredth of a second it cooled to a
"temperature" of about 100 billion degrees, and quarks began to clump
together into protons and neutrons which swirled around with electrons, neutrinos
and photons in a grand soup of particles. From this
point on, there were no free quarks to be found. In the next three minutes or so, the Universe
cooled to about a billion degrees, allowing protons and neutrons to clump
together to form the nuclei of light elements such as deuterium,
helium and lithium. After about three hundred thousand years, the Universe
cooled enough (to a few thousand degrees) to allow the free electrons to become
bound to light nuclei and thus formed the first atoms.
Free photons and neutrinos continue to stream throughout the Universe, meeting
and interacting occasionally with the atoms in galaxies, stars and in us!
We see now that to understand how the Universe evolved we really
need to understand the behavior of the elementary particles: the quarks,
leptons and gauge bosons. These make up all the known recognizable matter in our Universe.
Beyond that, the Universe holds at least two dark secrets: Dark Matter and Dark Energy! The total amount of luminous matter (e.g., stars, etc.) is not
enough to explain the total observed gravitational behavior of galaxies and
clusters of galaxies. Some form of mysterious Dark Matter has to be found. Below we will see how
new kinds of particles may be discovered that fit the description. Recent
evidence showing that the expansion of the Universe may be accelerating instead
of slowing down leads to the conclusion that a mysterious Dark Energy may be the culprit. Perhaps some new
form of interaction may be responsible for that.
So how do we get to study quarks and such, if they don't exist freely now?
Just as in the Big Bang, if we can manage to make high enough
temperatures, we can create some pairs of quarks & anti-quarks, by the conversion of
energy into matter. (Particles & anti-particles have to be created in pairs
to balance charge, etc.)
When particles of matter and antimatter collide they
annihilate each other, creating conditions like those that might have existed
in the first fractions of a second after the big bang.
This is where high energy accelerators
come in. In head-on collisions between high-energy particles and their
antiparticles, pure energy is created in "little bangs" when the
particles and their antiparticles annihilate each other and disappear. This energy is
then free to reappear as pairs of fundamental particles, e.g., a
quark-antiquark pair, or an electron-positron pair, etc. Now electrons and
their positron antiparticles can be observed as two distinct particles. But
quarks and antiquarks behave somewhat like two ends of a string — you can
cut the string and have two separate strings but you can never separate a string into
two distinct "ends". Free quarks cannot be observed!
So when a quark-antiquark pair is
produced in a head-on collision with excess energy (i.e., E > 2mq c2 ) the quark and antiquark fly off in opposite
directions until "the string breaks into two" and each of the pair
finds itself bound with another quark. What we actually observe is a pair of mesons being produced, each meson consisting of
a quark and an antiquark bound together. With enough excess energy, larger
clumps of quarks and antiquarks can be produced: protons, neutrons and heavier
particles classed as baryons. These mesons and baryons make up the
zoo of particles discovered earlier.
What we have thus found is that to study
quarks, one has to create them in high energy collisions, but they can only be
observed clumped into mesons and baryons. We have to infer the properties of individual quarks
through the study of the decay and interactions of these mesons and baryons.
Baryons and
Mesons contain combinations of quarks and anti-quarks.
The Standard Model
Particle physicists now believe they can describe the behavior of
all known subatomic particles within a single theoretical framework called the
Standard Model, incorporating quarks and leptons and their interactions through
the strong, weak and electromagnetic forces. Gravity is the one force not
described by the Standard Model.
The Standard Model is the fruit of many years of international
effort through experiments, theoretical ideas and discussions. We can summarize
it this way:
All of the known matter in the Universe today is made up
of quarks and leptons, held together by fundamental forces which are
represented by the exchange of particles known as gauge bosons.
One guiding principle that led to current
ideas about the nature of elementary particles was the concept of Symmetry. Nature points the way to many of its
underlying principles through the existence of various symmetries.
Quarks
The quark scheme was suggested by the symmetries in the way the
many mesons and baryons seemed to be arranged in families.
Theorists Gell-Mann and Zweig independently proposed in 1964 that just three
fundamental "constituents" (and their anti-particles) combined in
different ways according to the rules of mathematical symmetries could explain
the whole zoo. Gell-Mann called these constituents quarks, and the three types were named up, down and strange quarks. Evidence for quark-like constituents of protons and
neutrons became clear in the late 1960s and 1970s. In 1974, a new particle was
unexpectedly discovered at SLAC (Stanford Linear Accelerator Center). It was given the unwieldy
dual name J/Psi, because of its simultaneous discovery
by two groups of experimenters! The J/Psi was later shown to be a bound state of a completely new
quark-antiquark pair, which nevertheless had been predicted on the basis of a
subtle phenomenon. The new fourth quark was named charm. (We do not wish to comment here on the choice of names!)
The four-quark scheme was extended to its
present state of six
quarks by the addition
of a new pair, in a prediction by theorists Cabbibo and independently,
Kobayashi and Maskawa (collectively known as CKM). So now we have the six
quarks: up, down, strange, charm, bottom and top quarks and they each have their partner anti-quarks. The
quarks are usually labeled by their first letters: u, d, s, c, b and
t. In various combinations they make up all
the mesons and baryons that have been seen. The six-quark prediction was
fulfilled when in 1977 a new heavy meson called the Upsilon was discovered at Fermilab and later
shown to be the bound state of the bottom and anti-bottom quark pair. The B meson, containing an anti-b quark and a u or d
quark was discovered by the CLEO experiment at Cornell in 1983. Finally, in
1998, conclusive evidence of the existence of the super heavy top quark was obtained at Fermilab.
Leptons
What about leptons? Only the electron, muon and neutrino were known before the 1960s. These behave differently from the
mesons and baryons. First, they are much less massive. The mass of the electron
is almost 2,000 times smaller than the mass of the proton, and the muon appears
to be just a heavier version of the electron, its mass being nine times smaller
than that of the proton. The neutrino has almost no mass at all, and up until
recently, its mass was thought to be truly zero. Hence the name "leptons" or light particles. Second, the
electron and muon interact with matter mainly through their electric charges;
the neutrino being neutral, hardly at all. They all have a weak interaction with the matter in nuclei and, in high energy
collisions, they do not produce the profusion of new mesons and baryons that
protons and neutrons do when colliding with nuclei.
In 1962, the first experiment using a
high-energy neutrino beam (the PhD thesis of this author) showed that the
electron has its own electron-neutrino, and the muon its own distinct muon-neutrino. This was the very first evidence that
there could be families or generations of pairs of fundamental particles. This
notion was dramatically extended in 1974, when shortly after the discovery of
the J/Psi, a new heavy lepton was discovered, called the tau,
almost twice as
massive as the proton, but behaving like the other leptons, sharing the weak
interaction property!
This was the first evidence that three pairs or families of leptons existed: the electron and electron-neutrino, the muon and muon-neutrino and the tau and tau-neutrino.
A Note on Masses
& Energies: We give all masses in terms of the proton mass. Since energy is
related to mass by E= mc2 the proton mass is
given in energy units as 938 MeV (Million electron Volts), the energy required to create a
proton, or approximately 1GeV (Giga electron volt), which will henceforth serve as the unit of
energy too.
Quarks and leptons have an intrinsic
angular momentum called spin, equal to a half-integer (1/2) of the
basic unit and are labeled as fermions. Particles that have zero or integer
spin are called bosons.
|
QUARKS |
(u) up-quark mass = 0.005 (d) down-quark mass = 0.009 |
(c) charm-quark mass = 1.5 (s) strange-quark mass = 0.16 |
(t) top-quark mass = 186 (b) bottom-quark mass = 5.2 |
Charge = +2/3 Charge = -1/3 |
|
LEPTONS |
elec-neutrino mass ~ 0 (e) electron mass = 0.00054 |
muon-neutrino mass ~ 0 ( mass = 0.11 |
tau-neutrino mass ~ 0 ( mass = 1.9 |
Charge = 0 Charge = -1 |
Table 1: The Quark and Lepton families. All masses are given relative to the proton mass, which is 938 MeV. All of the above have a spin (angular momentum) of 1/2 unit.
However, the fundamental questions still
remain: why are there
quarks and leptons, with different charges and interaction characteristics? Why
are there three generations, and so many different masses?
Forces and Interactions
Now we must tackle the fundamental
forces or interactions among the quarks and leptons: Gravity, the Weak Force, Electromagnetism, and the Strong Force. Of these, our everyday world is controlled by gravity and
electromagnetism. The strong force binds quarks together and holds nucleons
(protons & neutrons) in nuclei. The weak force is responsible for the
radioactive decay of unstable nuclei and for interactions of neutrinos and
other leptons with matter.
The intrinsic strengths of the forces can
be compared relative to the strong force, here considered
to have unit strength (i.e., =1.) In these terms, the electromagnetic
force has an intrinsic strength of (1/137). The weak force is a billion times weaker than the strong
force. The weakest of them all is
the gravitational force. This may seem strange, since it is strong enough to
hold the massive Earth & planets in orbit around the Sun! But we know that that the gravitational
force between two bodies a distance r
apart is proportional
to the product of the two masses (M & m) and inversely proportional
to the distance r squared:
We see now what is meant by intrinsic strength. It is given by the magnitude
of the universal force constant, in this case, G, independent of the masses or distances involved. In similar
terms, the electromagnetic force between two particles is proportional to the
product of the two charges (Q & q)
and inversely to the distance r squared:
Here the universal constant alpha, 
, gives the
intrinsic strength.
We can compare the relative strengths of
the electromagnetic repulsion and the gravitational attraction between two protons of unit charge using
the above equations. Independent of the distance, the ratio turns out to be 1036 ! Thus the two
protons will repel each other and fly apart, easily overcoming the puny gravitational attraction.
As we noted before, forces can be
represented in the theory as arising from the exchange of specific particles
called gauge bosons, the quanta of the "force field". Just as photons are real (i.e., quanta of light!) and can be
radiated (shaken off) when charged particles are accelerated or decelerated,
the other gauge bosons (see below) can also be created and observed as real
particles. All the bosons have 0 or integer spins.
|
FORCE |
Relative Strength |
Gauge Boson |
Mass (rel. to proton) |
Charge |
Spin |
|
Strong |
1 |
Gluon (g) |
0 |
0 |
1 |
|
Electromagnetic |
1/137 |
Photon ( |
0 |
0 |
1 |
|
Weak |
10 -9 |
W ± , Z |
86, 97 |
± 1, 0 |
1 |
|
Gravity |
10 -38 |
Graviton (G) |
0 |
0 |
2 |
Table 2:
Forces and their quanta, the gauge bosons. Charge is in units of electron charge.
The carriers of the strong force are called gluons, the glue
that holds quarks together in protons and neutrons and also helps form nuclei.
The carriers of the weak force come in three forms, and are called weak bosons: the W± and the Z0 . The carriers of
the gravitational field are called gravitons
and are unique in having a spin of 2.
Unification!
For a universal theory, four forces are too many. Why is there not
just one universal
force? For decades physicists have been striving for the unification of the four forces into one universal
force that existed at least in the primordial stage of the Universe. In such a
picture, the four forces we observe today are just manifestations of the
original single force. However, we must understand that our existence depends
on having these different forces now. If gravity were not so weak, there might
only have been one massive black hole instead of galaxies, stars and planets.
If electromagnetic forces were not in delicate balance with the strong force,
nuclei would disintegrate — no atoms or molecules, chemistry or biology!
The weak force allows more subtle phenomena —the slow burning of stars
like our Sun may not be possible without the weak interaction; supernova
explosions which create all elements heavier than iron also depend on just the
right strength of neutrino interactions; and radioactivity in its bowels allows
the Earth to remain a warm hospitable body.
It is not quite satisfactory to have four different theories to
account for these four forces. The electromagnetic interaction of particles is
explained by a well established modern theory of Quantum Electrodynamics (QED). The weak interaction had its own theory but these two
have now been combined as the
Electroweak Theory in
the Standard Model. The strong interaction between quarks and gluons has
another theory called Quantum Chromodynamics (QCD), where the equivalent of electric charge is named
"color". And Einstein's General
Theory of Relativity explains how the
gravity we know is a manifestation of the basic geometry of space-time.
Just as Maxwell showed that electricity
and magnetism were manifestations of the same basic phenomenon of
electromagnetism, the Electroweak theory, which in 1979 won the Nobel
Prize for Glashow, Salam and Weinberg,
succeeds in unifying the Weak and Electromagnetic interactions into what
is called the Electroweak force. When we noted the intrinsic strengths of the four different
interactions in Table 2, we omitted to say that these strengths could depend on
the "temperature" or energy level of the interaction. Although these
strengths are quite different at present temperatures (e.g., at 300K or equivalent energy of about 1/40 eV), the weak interaction depends strongly on
the energy, and in collisions at near 1000 GeV, it gets just as strong as the
electromagnetic interaction! The Electroweak theory of the Standard Model
explains all this. The basic equations are symmetric in the way the two interactions
occur and in fact the masses of all the quanta are zero. However, as the
temperatures drops, the symmetry is broken and the quanta split up into four
different gauge bosons of different masses: the W+ and W– ( both 80 GeV), the Z0 (91 GeV) and the photon 
with zero mass.
At "room temperature" , the massive W and Z do not play an important
part. But at very high energies of 300 GeV or more, the difference between the
zero mass photon and the heavier W and Z bosons is erased,
and they all act equally strongly. In 1983 the W boson and in 1984 the Z boson were observed
at the CERN laboratory in Geneva, in high energy collisions of protons with
antiprotons. They had the predicted masses. The Standard Model was on its way!
There is however one piece of evidence
yet to be found. We mentioned above that the basic symmetry of the electroweak theory is broken as
the temperature drops and the forces separate in strength as the bosons gain mass.
The culprit that causes this is actually a new field called the Higgs field. It is possible to visualize how this
works. Recall that mass is a manifestation of inertia or resistance to acceleration. If a
Higgs field suddenly permeates all of space as the Universe cools, it can act
as a drag on every particle moving in space, the
drag depending on how well each interacts with the Higgs field. This drag shows
up as inertia and thus a measurable mass of the particles that were originally massless. But now we
have to look for the boson that carries this field — the Higgs boson.
This is now the one feature of the Standard Model still needed to clinch
the picture. It is expected to have a mass of about 100 GeV, within the reach
of the largest accelerators planned for the immediate future.
Beyond the Standard Model
Theories, called "Grand Unification
Theories" or GUTs, have been proposed to unify the electroweak force with the strong force. But so far no concrete evidence
has been found for them. Beyond that,
the holy grail of unification has long been the
unification of gravity with all the
other forces. Einstein himself labored in vain to fit gravity into a scheme
where it could be compatible with quantum theory.
The theory of Supersymmetry requires a whole new set of particles
beyond the Standard Model complement: a heavy partner for each quark, lepton
and gauge boson of the old set, together all of them making up one great
super-family of particles. The three forces strong, electromagnetic and weak
all have exactly
equal strengths in this theory at a very high energy. And of course, it gives
experimentalists a whole new game of looking for new particles. It is just
possible that one of these new super particles is a primordial relic of the Big
Bang and makes up the Dark
Matter in the Universe,
a further incentive to discover these super-partners.
Meanwhile theoretical studies range far
and wide in a search for the
Theory Of Everything (TOE). Most familiar is String Theory, which pictures particles as infinitesimal little vibrating loops
of strings in 10 dimensions. Further refinements lead to Membrane Theory, with the entire Universe regarded as
existing on multidimensional sheets or membranes, with particles as loops
anchored on "our" sheet
and gravitons ranging into the continuum between
sheets. We await predictions that can be tested.
Particle Physics Experiments
Throughout the history of Physics, experimental discoveries and
theoretical ideas and explanations have moved forward together, sometimes
playing leap-frog, but always drawing inspiration one from the other. Modern
versions of Rutherford's table-top experiment on the scattering of alpha
particles occupy many square kilometers of land, with massive and costly
apparatus in underground tunnels tens of kilometers long. These are the particle accelerators that speed protons, antiprotons,
electrons, or positrons to near the speed of light and then make them collide
head-on with each other or with stationary targets.
In an accelerator, focusing
magnets and bending magnets guide the beam of particles around a ring. (Only a
few of the bending magnets are shown here). High frequency microwave (RF) cavities accelerate the beams as they
pass through.
The quest has mostly been for higher and
higher collision energies. To make a pair of massive new particles and observe
them flying apart, one has to generate excess energy over and above the
equivalent of the mass (2mX) of the pair :
Ecollision >
2mX
c2 . High energy is also needed to probe
deeper and deeper to smaller length scales in studying the unknown — this
is the equivalent of using X rays of shorter wave-lengths to probe smaller crystal
structures. On the other hand, to
look for rare phenomena, it is
necessary to increase the intensity of particle beams and the collision rates. So accelerators have proceeded along
parallel paths of ever higher energies and ever higher intensities.
To observe and interpret the results of collisions, particle detectors have to be developed that can track and
analyze the particles that fly apart and disappear in nanoseconds. The detector
consists of many different types of complex apparatus and electronics,
requiring a cadre of experts in every conceivable technology. Collider
experiments use large detectors completely surrounding the "interaction
point" where high energy
particles and antiparticles collide head-on.
Typical are electron-positron colliders, proton-antiproton colliders and massive detectors at the
interaction points.
Other experiments study the collisions of
intense beams with fixed (stationary) solid targets. Typical are several
experiments with intense high energy neutrino beams and massive detectors in which neutrinos
can interact. Many are studying the conversion
of one type of neutrino (the muon-neutrino) into another (e.g., the tau-neutrino). Evidence for this is now pretty
definite after decades of research, and precise measurements may pin down the
non-zero mass of each neutrino. Relic neutrinos from the Big Bang populate the
Universe, and even a tiny mass can explain some of the Dark
Matter.
The art and science of particle accelerators and detectors has
depended heavily on technology. The technology of solid state devices,
superconducting magnets, electronics, computers and exotic materials, all have
played leap frog with developments
in experimental particle physics, sometimes driving and sometimes being
driven by the inventions of particle physicists.
All these very complex detectors are
built and operated by large numbers of physicists, in collaborations ranging
from 100 to almost 1000 personnel. The collaborations extend across boundaries
of countries and continents, in a typical illustration of science extending the
hand of cooperation and friendship across national and political barriers.
Looking to the Future
One of the primary goals for the new and
upgraded facilities in Fermilab near Chicago (the Tevatron) and CERN in Geneva Switzerland (the
Large Hadron Collider or LHC) is to find the Higgs boson, the one missing element of the Standard
Model.
Evidence for supersymmetric partners of the known particles is a goal in all
experiments, as part of the search for the true particle theory beyond the
Standard Model. Beyond that is the need to find anything that can point to a
real Grand Unification with the gravitational force.
A different kind of e+e- collider is being planned
internationally — the International Linear Collider or ILC, a very high energy linear collider,
with two opposing linear accelerators tens of kilometers long. The technical
challenges are many and this is likely to be the first truly world-wide
accelerator collaboration.
Where to Get More Information
BOOKS:
1.
The
Particle Odyssey: A Journey to the Heart of the Matter
by Michael Marten, Christine Sutton,
Frank Close. Oxford Press (2002)
2.
The
Charm of Strange Quarks : Mysteries and Revolutions of Particle Physics
by R. Michael Barnett, Henry Muehry,
Helen R. Quinn. American Institute of Physics, (2000)
WEBSITES:
1.
The
Particle Adventure (Lawrence Berkeley Lab) http://www.particleadventure.org/particleadventure/
2.
Inquiring
Minds (Fermi National Lab.) http://www.fnal.gov/pub/inquiring/index.html
3.
The
World of Beams (Center for Beam Physics, Lawrence Berkeley Lab), http://cbp-1.lbl.gov/
4.
Big
Bang Science, (Particle physics & Astronomy Research Council, UK) http://hepwww.rl.ac.uk/pub/bigbang/part1.html
FOR A GLOSSARY OF TERMS, see: http://www.particleadventure.org/particleadventure/frameless/glossary.html#top