F.A.Q's: Particle Physics



  1. Why are the laws of physics not symmetrical between left and right, future and past, and between matter and antimatter?  I.e., what is the mechanism of CP violation, and what is the origin of parity violation in Weak interactions?  Are there right-handed Weak currents too weak to have been detected so far?  If so, what broke the symmetry?  Is CP violation explicable entirely within the Standard Model, or is some new force or mechanism required?
  2. Why is there more matter than antimatter, at least around here?  Is there really more matter than antimatter throughout the universe?  This seems related to the previous question, since most attempts at explaining the prevalence of matter over antimatter make use of CP violation.
  3. Are there really just three generations of leptons and quarks?  If so, why?  For example, the muon is a particle almost exactly like the electron except much heavier, and the tau particle is also almost the same, but heavier still.  Why do these three exist and no more?  Or, are these unanswerable questions?
    Besides the particles that carry forces (the photon, W and Z boson, and gluons), all elementary particles we have seen so far fit neatly into three "generations" of particles called leptons and quarks.  The first generation consists of:
    • the electron
    • the electron neutrino
    • the up quark
    • the down quark

    The second consists of:

    • the muon
    • the muon neutrino
    • the charmed quark
    • the strange quark

    and the third consists of:

    • the tau
    • the tau neutrino
    • the top quark
    • the bottom quark

    How do we know there aren't more?
    Ever since particle accelerators achieved the ability to create Z bosons in 1983, our best estimates on the number of generations have come from measuring the rate at which Z bosons decay into completely invisible stuff.  The underlying assumption is that when this happens, the Z boson is decaying into a neutrino-antineutrino pair as predicted by the Standard Model.  Each of the three known generations contains a neutrino which is very light.  If this pattern holds up, the total rate of "decay into invisible stuff" should be proportional to the number of generations!
    Experiments like this keep indicating there are three generations of this sort.  So, most physicists feel sure there are exactly three generations of quarks and leptons.  The question then becomes "why?"—and so far we haven't a clue!
    For details see:
    Honesty compels us to point out a slight amount of wiggle room in the remarks above.  Conservation of energy prevents the Z from decaying into a neutrino-antineutrino pair if the neutrino in question is of a sort that has more than half the mass of Z.  So, if there were a fourth generation with a very heavy neutrino, we couldn't detect it by studying the decay of Z bosons.  However, all three known neutrinos have a mass less than 1/3000 times the Z mass, so a fourth neutrino would have to be much heavier than the rest to escape detection this way.
    Another bit of wiggle room lurks in the phrase "decaying into a neutrino-antineutrino pair in the manner predicted by the Standard Model".  If there were a fourth generation with a neutrino that didn't act like the other three, or no neutrino at all, we might not see it.  However, in this case it would be stretching language a bit to speak of a "fourth generation", since the marvelous thing about the three known generations is how they're completely identical except for the values of certain constants like masses.
  4. Why does each generation of particles have precisely this structure: two leptons and two quarks?
    If you're familiar with particle physics, you'll know it goes much deeper than this: the Standard Model says every generation of particles has precisely the same mathematical structure except for some numbers that describe Higgs couplings.  We don't know any reason for this structure, although the requirement of "anomaly cancellation" puts some limits on what it can be. If you're not an expert on particle physics, perhaps these introductions to the Standard Model will help explain things:
    The second is much more detailed and technical than the first.
  5. Do the quarks or leptons have any substructure, or are they truly elementary particles?
  6. Is there really a Higgs boson, as predicted by the Standard Model of particle physics?  If so, what is its mass?  If not, what breaks the symmetry between the electromagnetic and weak forces, and gives all the elementary particles their masses?
    The Standard Model predicts the existence of a spin-0 particle called the Higgs boson, which comes in two isospin states, one with charge +1 and one neutral.  (It also predicts that this particle has an antiparticle.)  According to the Standard Model, the interaction of the Higgs boson with the electroweak force is responsible for a "spontaneous symmetry breaking" process that makes this force act like two very different forces: the electromagnetic force and the weak force.  Moreover, it is primarily the interaction of the Higgs boson with the other particles in the Standard Model that endows them with their masses!  The Higgs boson is very mysterious, because in addition to doing all these important things, it stands alone, very different from all the other particles.  For example, it is the only spin-0 particle in the Standard Model.  To add to the mystery, it is the only particle in the Standard Model that has not yet been directly detected!On the 4th of July, 2012, two experimental teams looking for the Higgs boson at the Large Hadron Collider (LHC) announced the discovery of a previously unknown boson with mass of roughly 125-126 GeV/c2. Using the combined analysis of two interaction types, these experiments reached a statistical significance of 5 sigma, meaning that if no such boson existed, the chance of seeing what they was less than 1 in a million.
    However, it has not yet been confirmed that this boson behaves as the Standard Model predicts of the Higgs. Some particle physicists hope that the Higgs boson, when seen, will work a bit differently than the Standard Model predicts.  For example, some variants of the Standard Model predict more than one type of Higgs boson.  LHC may also discover other new phenomena when it starts colliding particles at energies higher than ever before explored.  For example, it could find evidence for supersymmetry, providing indirect support for superstring theory.
    So, stay tuned.  But meanwhile, try these:
  7. What is the correct theory of neutrinos?  Why are they almost but not quite massless?  Do all three known neutrinos—electron, muon, and tau—all have a mass?  Could any neutrinos be Majorana spinors?  Is there a fourth kind of neutrino, such as a "sterile" neutrino?
    Starting in the 1990s, our understanding of neutrinos has dramatically improved, and the puzzle of why we see about 1/3 as many electron neutrinos coming from the sun as naively expected has pretty much been answered: the different neutrinos can turn into each other via a process called "oscillation".  But, there are still lots of loose ends.  For details, try:
    The first of these has lots of links to the web pages of research groups doing experiments on neutrinos.  It's indeed a big industry!
  8. Is quantum chromodynamics (QCD) a precise description of the behavior of quarks and gluons?  Can we prove using QCD that quarks and gluons are confined at low temperatures?  Is it possible to calculate masses of hadrons (such as the proton, neutron, pion, etc.) correctly from the Standard Model, with the help of QCD?  Does QCD predict that quarks and gluons become deconfined and form plasma at high temperature?  If so, what is the nature of the deconfinement phase transition?  Does this really happen in Nature?
    Most physicists believe the answers to all these questions are "yes".  There are currently a number of experiments going on to produce and detect a quark-gluon plasma.  It's believed that producing such a plasma at low pressures requires a temperature of 2 million million kelvins.  Since this is 10,000 times hotter than the sun, and such extreme temperatures were last prevalent in our Universe only 1 microsecond after the Big Bang, these experiments are lots of fun.  The largest, the Relativistic Heavy Ion Collider on Long Island, New York, began operation in 2000.  It works by slamming gold nuclei together at outrageous speeds.  For details, see: But, in addition to such experimental work, a lot of high-powered theoretical work is needed to understand just what QCD predicts, both in extreme situations like these, and for ordinary matter.  In fact, it's a great challenge to use QCD to predict the masses of protons, neutrons, pions and the like to an accuracy greater than about 10%.  Doing so makes heavy use of supercomputers, but there are also fundamental obstacles to good numerical computations, like the "fermion doubling problem", where bright new ideas are needed.  See for example:
  9. Is there a mathematically rigorous formulation of a relativistic quantum field theory describing interacting (not free) fields in four spacetime dimensions?  For example, is the Standard Model mathematically consistent?  How about Quantum Electrodynamics?  Even the classical electrodynamics of point particles does not yet have a satisfactory mathematically rigorous formulation.  Does one exist or is this theory inconsistent?
    These are questions of mathematical physics rather than physics per se, but they are important.  At the turn of the millennium, the Clay Mathematics Institute offered a $1,000,000 prize for providing a mathematically rigorous foundation for the quantum version of SU(2) Yang-Mills theory in four spacetime dimensions, and proving that there's a "mass gap"—meaning that the lightest particle in this theory has nonzero mass.  For details see:
  10. Is the proton really stable, or does it eventually decay?
    Most "grand unified theories" (GUTs) predict that the proton decays, but so far experiments have (for the most part) only put lower limits on the proton lifetime.  As of 2002, the lower limit on the mean life of the proton was somewhere between 1031 and 1033 years, depending on the presumed mode of decay, or 1.6 x 1025 years regardless of the mode of decay. Proton decay experiments are heroic undertakings, involving some truly huge apparatus.  Right now the biggest one is "Super-Kamiokande".  This was built in 1995, a kilometer underground in the Mozumi mine in Japan.  This experiment is mainly designed to study neutrinos, but it doubles as a proton decay detector.  It consists of a tank holding 50,000 tons of pure water, lined with 11,200 photomultiplier tubes which can detect very small flashes of light.  Usually these flashes are produced by neutrinos and various less interesting things (the tank is deep underground to minimize the effect of cosmic rays).  But, flashes of light would also be produced by certain modes of proton decay, if this ever happens.
    Super-Kamiokande was beginning to give much improved lower bounds on the proton lifetime, and excellent information on neutrino oscillations, when a strange disaster happened on November 12, 2001.  The tank was being refilled with water after some burnt-out photomultiplier tubes had been replaced.  Workmen standing on styrofoam pads on top of some of the bottom tubes made small cracks in the neck of one of the tubes, causing that tube to implode.  The resulting shock wave started a chain reaction in which about 7,000 of the photomultiplier tubes were destroyed!  Luckily, after lots of hard work the experiment was rebuilt by December 2002.
    In 2000, after about 20 years of operation, the Kolar Mine proton decay experiment claimed to have found proton decay, and their team of physicists gave an estimate of 1031 years for the proton lifetime.  Other teams are skeptical.
    For more details, try these:
  11. Why do the particles have the precise masses they do?  Or is this an unanswerable question?
    Of course their mass in kilograms depends on an arbitrary human choice of units, but their mass ratios are fundamental constants of nature.  For example, the muon is about 206.76828 times as heavy as the electron.  We have no explanation of this sort of number!  We attribute the masses of the elementary particles to the strength of their interaction with the Higgs boson (see above), but we have no understanding of why these interactions are as strong as they are.
  12. Why are the strengths of the fundamental forces (electromagnetism, weak and strong forces, and gravity) what they are?  For example, why is the fine structure constant, that measures the strength of electromagnetism, about 1/137.036?  Where do such dimensionless constants come from?  Or is this an unanswerable question?
    Particle masses and strengths of the fundamental forces constitute most of the 26 fundamental dimensionless constants of nature.  Another one is the cosmological constant—assuming it's constant.  Others govern the oscillation of neutrinos (see below).  So, we can wrap a bunch of open questions into a bundle by asking: Why do these 26 dimensionless constants have the values they do?Perhaps the answer involves the Anthropic Principle, but perhaps not.  Right now, we have no way of knowing that this question has any answer at all!
    For a list of these 26 dimensionless constants, try:
  13. What is the explanation of the Pioneer anomaly?
    The Pioneer 10 and Pioneer 11 spacecraft are leaving the the Solar System.  Pioneer 10 sent back radio information about its location until January 2003, when it was about 80 times farther from the Sun than the Earth is.  Pioneer 11 sent back signals until September 1995, when its distance from the Sun was about 45 times the Earth's.The Pioneer missions have yielded the most precise information we have about navigation in deep space.  However, analysis of their radio tracking data indicates a small unexplained acceleration towards the Sun!  The magnitude of this acceleration is roughly 10−9 meters per second per second.  This is known as the "Pioneer anomaly".
    This anomaly has also been seen in the Ulysses spacecraft, and possibly also in the Galileo spacecraft, though the data is much more noisy, since these were Jupiter probes, hence much closer to the Sun, where there is a lot more pressure from solar radiation.  The Viking mission to Mars did not detect the Pioneer anomaly — and it would have, had an acceleration of this magnitude been present, because its radio tracking was accurate to about 12 meters.
    Many physicists and astronomers have tried to explain the Pioneer anomaly using conventional physics, but so far nobody seems to have succeeded.  There are many proposals that try to explain the anomaly using new physics — in particular, modified theories of gravity.  But there is no consensus that any of these explanations are right, either.  For example, explaining the Pioneer anomaly using dark matter would require more than 0.0003 solar masses of dark matter within 50 astronomical units of the Sun (an astronomical unit is the distance between Sun and Earth).  But, this is in conflict with our calculations of planetary orbits.
    For more information, see:
  14. Are there important aspects of the Universe that can only be understood using the Anthropic Principle?  Or is this principle unnecessary, or perhaps inherently unscientific?
    Very roughly speaking, the Anthropic Principle says that our universe must be approximately the way it is for intelligent life to exist, so that the mere fact we are asking certain questions constrains their answers.  This might "explain" the values of fundamental constants of nature, and perhaps other aspects of the laws of physics as well.  Or, it might not.Different ways of making the Anthropic Principle precise, and a great deal of evidence concerning it, can be found in a book by Barrow and Tipler:
    • John D. Barrow and Frank J. Tipler, The Cosmological Anthropic Principle, Oxford U. Press, Oxford, 1988.
    This book started a heated debate on the merits of the Anthropic Principle, which continues to this day.  Some people have argued the principle is vacuous.  Others have argued that it distracts us from finding better explanations of the facts of nature, and is thus inherently unscientific.  For one interesting view, see:
    In 1994 Lee Smolin advocated an alternative but equally mind-boggling idea, namely that the parameters of the Universe are tuned, not to permit intelligent life, but to maximize black hole production!  The mechanism he proposes for this is a kind of cosmic Darwinian evolution, based on the (unproven) theory that universes beget new baby universes via black holes.  For details, see:
    More recently, the string theorist Leonard Susskind has argued that the "string theory vacuum" which describes the laws of physics we see must be chosen using the Anthropic Principle:
  15. Do the forces really become unified at sufficiently high energy?
  16. Does some version of string theory or M-theory give specific predictions about the behavior of elementary particles?  If so, what are these predictions?  Can we test these predictions in the near future?  And: are they correct?
    Despite a huge amount of work on string theory over the last decades, it still has made no predictions that we can check with our particle accelerators, whose failure would falsify the theory.  The closest it comes so far is by predicting the existence of a "superpartner" for each of the observed types of particle.  None of these superpartners have ever been seen.  It is possible that the Large Hadron Collider will detect signs of the lightest superpartner.  It's also possible that dark matter is due to a superpartner!  But, these remain open questions.It's also interesting to see what string theorists regard as the biggest open questions in physics.  At the turn of the millennium, the participants of the conference Strings 2000 voted on the ten most important physics problems.  Here they are:
    1. Are all the (measurable) dimensionless parameters that characterize the physical universe calculable in principle or are some merely determined by historical or quantum mechanical accident and uncalculable?
    2. How can quantum gravity help explain the origin of the universe?
    3. What is the lifetime of the proton and how do we understand it?
    4. Is Nature supersymmetric, and if so, how is supersymmetry broken?
    5. Why does the universe appear to have one time and three space dimensions?
    6. Why does the cosmological constant have the value that it has, is it zero and is it really constant?
    7. What are the fundamental degrees of freedom of M-theory (the theory whose low-energy limit is eleven-dimensional supergravity and which subsumes the five consistent superstring theories) and does the theory describe Nature?
    8. What is the resolution of the black hole information paradox?
    9. What physics explains the enormous disparity between the gravitational scale and the typical mass scale of the elementary particles?
    10. Can we quantitatively understand quark and gluon confinement in Quantum Chromodynamics and the existence of a mass gap?
    For details see:

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