Open Questions:
High-energy Physics

[Home] [Up] [Glossary] [Topic Index] [Site Map]

See also: Mathematics and Physics


The standard model

Successes of the standard model

Open questions

The standard model

Higgs physics

Beyond the standard model


Superstring theory


Higher dimensions of spacetime

Neutrino physics

The vacuum

CP symmetry violation

The strong force and QCD

Magnetic monopoles

Quantum theory

Physics of information and computation

Condensed matter physics

Recommended references: Web sites

Recommended references: Magazine/journal articles

Recommended references: Books


People have a tendency to wonder what things are made of -- at least since the Greeks, and undoubtedly much earlier.

Artisotle called Thales of Miletus (ca. 585 BCE) the founder of physical science and wrote that Thales believed everything to be made of water. That wasn't so far-fetched, considering water can be solid, liquid, or gas in everyday experience. Artistotle himself, following his teacher Plato, thought that matter was based on four fundamental substances: earth, air, and fire, as well as water. In fact, Aristotle added a fifth essence, "quintessence", that supposedly resided in the celestial world. (The term has been adopted by some modern physicists and repurposed.)

Other Greek philosophers took a different approach. Instead of trying to identify the essences of matter, they were content to view matter in terms of the ability to divide it into smaller and smaller units until some ultimate indivisible unit they called the "atom" was reached. Leucippus of Miletus (ca. 450 BCE) and (especially) his student Democritus are recognized for promoting this view.

Leucippus and Democritus, of course, weren't that far off, in the view of modern physics. Except that what we now call atoms are known not to be indivisible. In the early 20th century it was recognized that atoms are made of even smaller particles -- electrons, protons, and neutrons. Somewhat later, in the 1960s, it became apparent that protons and neutrons were themselves composed of smaller particles -- quarks -- though electrons remained indivisible.

There are now various good reasons to think that electons and quarks (together with a few other assorted particles) really are fundamental and indivisible. (Even though, a few decades ago, some physicists toyed with "bootstrap" theories in which divisibility really could go on forever, such notions have since been discarded.)

According to the uncertainty principle of quamtum mechanics, the more precisely the position of a particle is determined, the more uncertainty there must be about its velocity, and hence it's kinetic energy. This inverse relationship means that to probe the structure of matter at very small distance scales, it is necessary to work at very large energies. This seeming paradox is the reason that the study of things which are very small is called high-energy physics.

There is now another reason, besides our curiosity as to what "stuff" is actually made of, for a keen interest in the study of the smallest possible units of matter. This is the realization, which gradually dawned on physicists in the last third of the 20th century, that knowing precisely what matter is on the very smallest scales is essential to undertanding it on the very largest scale as well -- which is the subject of cosmology. The connection is that very high energies are involved in both subjects. In cosmology, this means the highest energy we can imagine, namely that of the big bang itself.

One might almost say that the cosmological applications of high-energy physics is what the subject was "really" about from the beginning, though no one was aware of it. Because the fact is that the energy scale at which the "true" nature of matter becomes manifest existed only for the briefest of instants at the time of the big bang.

The bottom line is that it is impossible to separate understanding the origins of the universe from understanding the behavior of the smallest units of matter. The two are fundamentally connected.

The standard model

Around 1970, physical theories of the fundamental nature of matter (and energy) began to converge on what is now called the "standard model" or particle physics. This model consists of several things.

First, there are the "elementary particles" that make up matter. These consist of electrons, neutrinos, and quarks. It is the electrons and quarks which make up ordinary matter. The neutrinos are uncharged, almost massless particles which exist in great abundance but hardly interact with normal matter at all. Electrons and neutrinos, nevertheless, are closely associated, and each has relatives (metaphorically speaking) which differ only in mass (and don't occur normally, becaue they are unstable). Likewise, there are several different types of quarks besides the two that occur in ordinary matter. And for each of these types of particle there is a corresponding anti-particle, which differs in electrical charge (except for neutrinos, which are uncharged).

Along with the "fundamental" particles there are also "fundamental" forces, which are manifested in interactions between the particles. The precise number of these forces is a little slippery, since it turns out they merge into each other at increasingly high energies. The first force is electromagnetism (which itself was considered two forces, electical and magnetic, before James Maxwell unified them). The second force is gravity. The remaining forces are the "weak" force and the "strong" force, which are observable only in interactions between particles and not a part of direct everyday experience.

For each of these force types, there are additional particles which are said to "carry" or "mediate" the force. That is, the force is regarded as resulting from an exchange of these mediating particles, which are of a type referred to as "bosons". These particles are called "photons", "gravitons", "gluons", and W or Z bosons -- corresponding respectively to the electromagnetic, gravitational, strong, and weak forces.

Finally, there is hypothesized to exist one more type of particle, the "Higgs boson", which has not yet been definitievely observed, but is absolutely essential to the theory. The Higgs particle (as it's also called) is the mechanism by which the non-massless particles acquire their mass. It is currently the most sought-after "fugitive" in all of physics. Even though it might seem that the fact it hasn't shown up yet leaves the standard model in a rather precarious position, the success of the model in predicting all observations to date makes physicists very sure that the Higgs particle must exist. There will truly be Hell to pay if it doesn't.

The standard model is described mathematically in terms of what is called a "gauge field theory". This refers to an elegant piece of mathematics which describes all the particle types not as discrete entities, but in terms of "fields" -- mathematical quantities associated with every point of space. Each particle is viewed as the quantum of its own type of field, just as the photon is the quantum of the electromagnetic field. After all, quantum theory says particles can't be localized to a single point in space, so it is natural to deal with them as something that is "smeared out" rather than a discrete object.

The gauge field theory also includes symmetry properties which are essential to the standard model. A symmetry, in the general sense used here, refers to something that apparently different types of particles have in common, some sort of "likeness". Particles that are related by one of these symmetries can "change into" each other by a symmetry operation that is analogous to a rotation in ordinary space. (This is in addition to any changes which occur during interactions between particles, as when a neutron "decays" into a proton and an electron.)

Successes of the standard model

Above all, the standard model brought some order to the explosive discovery of new "fundamental" particles which were found around 1955-65. In spite of the still significant diversity of leptons (electons and neutrinos), quarks, and bosons, especially when counting anti-particles as well, the situation is much more orderly than before.

Symmetry was a big help. The symmetries which exist, for example, between different types of quarks are saying that each is really a manifestation of the "same" thing, rather than representing truly different types of particles. Further, the symmetries are not just an accounting trick. They are represented in the theory as mathematical operators, and as such enter into computations, so that they help determine the calculated results.

And the calculated results are impressive. In every case where a value that can be computed by the theory can be measured experimentally, there is complete agreement. There are no experimental contradictions to the theory.

The theory also accounts for a variety of phenomena involving the various particles and forces, such as:

Open questions

If it is true all experimental measurements are perfectly consistent with the theory of the standard model, why do physicists still feel it is not a satisfactory and complete theory? Simple. There are still many areas where it just doesn't predict or explain physical observations. There are many observed phenomena which aren't predicted by the theory, even though they aren't ruled out either.

Some of these phenomena have to do with particle physics technicalities. In this category we have:

But there are a number of other problems which come from outside of particle physics itself -- from cosmology -- that the standard model provides little help with, even though a complete theory of matter should. These are phenomena for which we have observational evidence, often in great abundance, from a large diversity of astronomical studies. In this category we have:

And then there's the biggest question in all of physics. Right now there are two broad theories which describe physical behavior, usually in very different areas, but sometimes overlapping -- quantum theory and general relativity. Yet they have proven very incompatible. When will we have a consistent quantum theory of gravity, and what will it look like?

Recommended references: Web sites

Site indexes

Open Directory Project: Particle Physics
Categorized and annotated links. A version of this list is at Google, with entries sorted in "page rank" order.
The World-Wide Web Virtual Library: High-Energy Physics
Links to high-energy physics information resources (mostly laboratories and university departments).
Yahoo High Energy and Particle Physics Links
Annotated list of links.
Fermilab: More About Particle Physics: Resources
Good list of links on particle physics, astrophysics, and general physics. There are also links to other high energy physics research laboratories.
Particle Physics Education Sites
An index of overview & tutorial pages, maintained by PDG at the Particle Adventure site.
SLAC Library: Online Particle Physics Information
An index oriented to technical/professional sites.
Galaxy: High-Energy and Particle Physics
Categorized site directory. Entries usually include descriptive annotations.
Physics Internet Resources
Short annotated list, from the American Physical Society.

Sites with general resources

Particle Physics News and Resources
Known more simply as "A communications resource for the world's particle physics laboratories." In addition to recent news, the site contains special sections for cosmic physics and string theory, an image bank, and links to other reources.
Yahoo News Full Coverage: High Energy and Particle Physics
Links to recent news stories from various sources. Also includes links to sites dealing with high energy physics.
Particle Data Group
"The PDG is an international collaboration that reviews Particle Physics and related areas of Astrophysics, and compiles/analyzes data on particle properties." Publishes the main journal in the field, Review of Particle Physics.
Best of Physics Web: Particle and Nuclear Physics
Directory of best feature articles, news stories, and external links on particle and nuclear physics at the Physics Web site.
Level 5: Particle Physics
Relatively short collection of survey articles and external links to some of the best particle physics sites. (From the Level 5 project.)
Bibliography of Particle Physics Educational Materials
Introductory and intermediate resources.
The Large Hadron Collider Home Page
Information, mostly technical, about the Large Hadron Collider under construction at CERN. There are some press releases.
The ATLAS Experiment
Public site with general information on ATLAS ("A Toroidal LHC ApparatuS"). Site has an overview of the relevant physics, a description of the experiment, and information on the LHC.
"CERN is the European Organization for Nuclear Research, the world's largest particle physics centre." This site contains public information about CERN. It's main research tool will be the Large Hadron Collider. Use the site map to find your way around.
CERN Courier
Monthly journal of high energy physics news. Produced by CERN. Although the journal is print-based, the content (including news stories and articles) is archived online.
Stanford Linear Accelerator Center
This is the principal home page of SLAC. For a general overview the Virtual Visitor Center is a good place to start. There are many other subsidiary pages.
SLAC Virtual Visitor Center
Overview of the facilities, experiments, history, and other information about the Stanford Linear Accelerator Center.
BaBar is the name of an experiement using a special particle detector for study of the phsics of B mesons at SLAC. It is the principal high-energy physics program there for the coming decade.
SLAC Library: Databases and Documents
Databases maintained by the SLAC library on vaious high-energy physics topics.
SLAC Beam Line
Quarterly journal of SLAC and particle physics news. Individual issues are in PDF format.
Fermi National Accelerator Laboratory
The laboratory was once a leading center of particle physics research, now a casualty of U. S. failure to support particle physics. Neverthless, the site still has useful features, such as Inquiring Minds and FermiNews. There is a related site for the Fermilab Education Office, which contains educational outreach material and information.
Fermi National Accelerator Laboratory: Inquiring Minds
This is a collection of resources on particle physics at Fermilab. It includes various tutorials, links to external sites, and other useful resources.
Biweekly newsletter dealing with research and community information related to the Fermi National Accelerator Laboratory. Past issues are archived. Has some good permanent features, such as Particle Physics for Regular People-Recommended Readings.
High Energy Physics Information Center (HEPIC)
Operated by Fermilab. General resources for the high energy physics community.
Particle Physics Picture of the Week
People, places, and things associated with particle physics.
Next Linear Collider Home Page
The NLC is a new facility being planned to answer fundamental questions about the nature of matter. A successor to the Stanford Linear Accelerator (SLAC), it will be 20 miles long, 10 times the length of SLAC. This site contains information about the project as well as background information.
European Particle Physics Outreach Group
"A network of people, representing the CERN member states, who are involved with the popularization of Particle Physics." The site consists mainly of external links, in categories such as "learning about particle physics". However, it is a little hard to use since it is organized at the top level by country.
Research Topics in Theoretical Particle Physics
Capsule summaries of research areas pursued by the DESY Theory Group, such as electroweak physics, supersymmetry, applications to cosmology.

Surveys, overviews, tutorials

Particle physics
Article from Wikipedia. See also Elementary particle, Fundamental force.
The Particle Adventure
Maintained by PDG. "Introduces the theory of fundamental particles and forces, called the Standard Model. It explores the experimental evidence and the reasons physicists want to go beyond this theory. In addition, it provides information on particle decay and a brief history section." Features of the site include Particle Physics News and external links to particle physics education sites. An alternative version of this site is maintained at CERN.
Particle Adventure Glossary
From the Particle Adventure site.
ATLAS Glossary of high-energy physics terms
Useful reference.
To the LHC and beyond
September 2004 article from Physics World, by Peter Rodgers. Relates history, status, and future of the CERN laboratory where the Large Hadron Collider is located. The research program for the LHC is briefly described.
Why Do We Need a Linear Collider
Slide presentation given by Martinus Veltman at the 2001: A Spacetime Odyssey conference. Discusses some open questions that could be studied by the proposed "New Linear Collider".
Particle physics: the next generation
December 1999 article from Physics World, by John Ellis. "Although the basic building blocks of matter and their interactions have been placed on a firm theoretical footing, many fundamental questions remain unanswered and await the experiments of the future."
Elementary Particle Physics PHY-653
Material from lectures by Steve Lloyd. Main topics includes hadrons and quarks, electroweak interactions, particle astrophysics, and developments beyond the standard model. Many lectures are available in PDF format. Also contains a section on Feynman diagrams. There are also a few external links of general interest and more specialized.
Introduction to Particle Physics
It's very elementary, but nicely done. From York University (UK).
An Introduction to Particle Physics
Overview, including material on the top quark and connections with the Big Bang theory, by Phil Bradley.
Introduction to Particle Physics
Outline of a college course at Tel Aviv University.
HEPAP'S Subpanel on Vision for the Future of High-Energy Physics
More succinctly known as the Drell Report. A detailed report on the state of high-energy physics in 1994.
Is There a Theory of Everything?
Elementary overview by Michio Kaku.
The ATLAS Experiment Homepage
Simple overview of the basic concepts of high-energy physics: particles, forces, fields. Produced by the team working on an experiment to be performed at the CERN Large Hadron Collider. Much of the same information is at U. S. Atlas Education Pages hosted at CERN.
UCT PHY400/1W Particle Physics
Notes and information related to a course at the University of Cape Town, by D. G. Aschman.
High Energy Physics: A Challenge to Uncover the Secrets of the Creation and Evolution of Our Universe
Good overview produced by the JLC project.
Big Bang Science ...exploring the origins of matter
Overview produced by the UK Particle Physics and Astronomy Research Council.
Particle Physics
Materials, including lecture notes and PDF files from a course, by Niels Walet.
Small Stuff
Brief overview in outline form of some topics in high-energy physics. Part of a physics course by Jess Brewer.
SLAC Virtual Visitor Center: Theory
Hypertext document that gives an overview of high-energy physics.
The Particle Detector BriefBook
Evertything you ever wanted to know about particle detectors.
CERN prepares for the LHC and beyond
May 2000 article from Physics World, by Peter Rodgers about CERN's plans for the next generation of particle accelerators.

Recommended references: Magazine/journal articles

Particle Accelerators Test Cosmological Theory
David N. Schramm; Gary Steigman
Scientific American, June 1988, pp. 66-72
Three distinct "generations" of quarks and leptons are known to exist. Comparisons of the observed abundance of a few light atomic isotopes with calculations of big bang nucleosynthesis indicate that there can be only three generations. Observations of the decay rates of Z0 bosons in accelerator experiments may soon provide a completely independent confirmation of this.

Recommended references: Books

Lisa Randall – Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions
HarperCollins Publishers, 2005
This may be the book to read (as of its publication date) to learn about the whole expanse of modern elementary particle physics. It begins at the beginning, with relativity, quantum mechanics, and the standard model. From there is leaps into the more speculative stuff – supersymmetry, grand unified theories, string theory, and M-theory. A major theme throughout is dimensionality and the way that the most sophisticated theories of modern physics seem to require more than 3 or 4 dimensions.
[Book review]
Lawrence M. Krauss – Hiding in the Mirror: The Mysterious Allure of Extra Dimensions, from Plato to String Theory and Beyond
Viking, 2005
This book, like Lisa Randall's, is themed around the idea of higher dimensions of spacetime. But Krauss writes more briefly, having already covered some of the prerequisite ideas in other books of his. He also adopts a more detached, philosophical, and (somewhat) skeptical attitude towards some of the trendier concepts like superstrings, branes, and "theories of everything".
[Book review]
Martinus Veltman -- Facts and Mysteries in Elementary Particle Physics
World Scientific, 2003
Veltman won a Nobel Prize for his work on the mathematical consistency of the standard model, and this book demonstrates his command of the subject. It is largely a history of the experiments and theories of the middle decades of the 20th century which underlie the standard model. Capsule biographies are provided of many of the important participants in this development. Along the way, the major concepts are explained clearly, but without the more challenging details. The author's predisposition is to stick with physics that has been experimentally confirmed, so more speculative ideas such as supersymmetry and string theory are carefully avoided.
R. Michael Barnett; Henry Mühry; Helen R. Quinn -- The Charm of Strange Quarks: Mysteries and Revolutions of Particle Physics
Springer-Verlag, 2000
This is an introductory book for general readers, but with a good deal of meat in it. It mostly dispenses with historical filler on who did what when, in order to focus on essential concepts: the standard model, quarks, leptons, fundamental forces, and the workings of particle accelerators. There's also a chapter on the relation of particle physics to cosmology. However, a lot of the "good stuff" is ensconced in one 50-page appendix (along with other appendices, such as a table of the Greek alphabet). The bibliography is nice too.
Frank Close -- Lucifer's Legacy: The Meaning of Asymmetry
Oxford University Press, 2000
Yet another introduction to high energy physics for the general audience, with an emphasis on the concept of symmetry. The spontaneous breaking of exact symmetry receives special attention. As the book explains, the breaking of symmetry at low temperatures ultimately results in the fact that particles of matter have mass. The world as we know it follows from that.
Richard Morris -- The Universe, the Eleventh Dimension, and Everything: What We Know and How We Know It
Four Walls Eight Windows, 1999
This is a lightweight, but relatively recent, account of high energy physics and how it relates to the origins of the universe itself. A substantial part of the discussion concerns philosophical questions about physics and science in general. The book is a good choice for a first introduction to these topics.
Gordon Kane -- The Particle Garden
Addison Wesley, 1995
Short, elementary, non-mathematical, and relatively recent overview of particle physics. Good explanations of topics like the meaning of "understanding", Higgs bosons, supersymmetry, unification. Good glossary. Kane is one of the leading theorists of supersymmetry, and his book is one of the best and meatiest available to explain the difficult issues that the standard model cannot handle.
Leon Lederman, Dick Teresi -- The God Particle: If the Universe Is the Answer, What is the Question?
Bantam Doubleday Dell Publishing Group, 1993
Lederman is the former director of Fermilab. The title refers to the long-sought Higgs particle -- and (in spite of the title), this is not one of those books of metaphysical speculation. It's about the business of doing high energy physics -- the history, the accelerators, and the underlying science. Eventually it even discusses the Higgs. And the author has a sense of humor.
David Lindley -- The End of Physics: The Myth of a Unified Theory
Basic Books, 1993
Will a "theory of everything" be nothing more than a new kind of mythology? As the title indicates, this is a somewhat skeptical view that reflects on the philosophical questions involved.
Steven Weinberg -- Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature
Pantheon Books, 1992
General survey of the idea of a "theory of everything". The book is more about the philosophy of such a theory than specific details such as superstrings.
T. D. Lee -- Symmetries, Asymmetries, and the World of Particles
University of Washington Press, 1988
A very short essay on the role of symmetry in particle physics.
Harvey R. Brown, Rom Harré -- Philosophical Foundations of Quantum Field Theory
Oxford University Press, 1988
Essays from a philosophical perspective on topics such as virtual particles, renormalization, and gauge theory.
Michio Kaku, Jennifer Trainer -- Beyond Einstein: The Cosmic Quest for the Theory of the Universe
Bantam Books, 1987
One of the early popular expositions of superstring theory. Considers the problems of "grand unified theories" and develops the notions of symmetry and sypersymmetry.
Robert K. Adair -- The Great Design: Particles, Fields, and Creation
Oxford University Press, 1987
Thorough introduction with some mathematics to fundamental concepts used in particle physics, such as special and general relativity, symmetry and conservation laws, gauge invariance, and unification.
Richard P. Feynman, Steven Weinberg -- Elementary Particles and the Laws of Physics
Cambridge University Press, 1987
Two introductory lectures. Feynman's lecture is on "The reason for antiparticles." Weinberg's is on progress towards a theory of everything.
A. Zee -- Fearful Symmetry: The Search for Beauty in Modern Physics
Macmillan Publishing Company, 1986
Symmetry is a fundamental concept in physics. It is not only about beauty -- in fact it is a basic organizing principle of theory. Symmetry, for instance, is closely associated with conservation laws.
Richard P. Feynman -- QED: The Strange Theory of Light and Matter
Princeton University Press, 1985
Quantum electrodynamics is Feynman's own theory of the electromagnetic force through the exchange of photons -- the theory for which Feynman diagrams were invented.
Heinz Pagels -- The Cosmic Code: Quantum Physics as the Language of Nature
Simon and Schuster, 1982
The exposition covers two related but distinct areas: quantum theory and particle physics. Although the material is somewhat dated, the author's expertise and writing skill provide many lucid insights. The coverage of "quantum reality" is especially good.


Copyright © 2002-04 by Charles Daney, All Rights Reserved