Physics Issues Working Groups
P1: Working Group on Electroweak Symmetry Breaking

Working Group Convenors: Marcela Carena (FNAL), David Gerdes (Michigan), André Turcot (Brookhaven), Peter Zerwas (DESY)

This group has within its purview three fundamental issues to be tackled by high-energy physics during this decade and the next:

  1. What is the mechanism of electroweak symmetry breaking?

  2. What is the relation between electroweak symmetry breaking and the origin of quark and lepton masses? How many problems of mass are there?

  3. What scale of new physics is associated with electroweak symmetry breaking?

Physics scenarios that address these questions can be roughly divided as:

  1. standard-model Higgs mechanism

  2. composite Higgs boson from new strong dynamics

  3. the "no Higgs" scenario of strong WW scattering

  4. MSSM Higgs mechanism in various representative regions of the MSSM parameter space

  5. extended Higgs sectors with extra pseudoscalars, singlets, radions (or other bulk scalars), Kaluza-Klein modes, etc.

  6. other supersymmetric models with extended Higgs sectors


  1. Tree of questions. Produce a tree of more narrowly posed questions, each of which can be decided by a well-defined experimental measurement or series of measurements, and which, taken as a whole, will serve to answer (A) -- (C) above with a high degree of confidence and insight. The number of branchings of this tree will obviously be limited by practical considerations, but at the first level of fine-graining it should include the ability to discriminate among the physics scenarios outlined above.

  2. Experiments to illuminate the questions. Identify, for each question in this tree, what kinds of experiments at what kinds of machines could plausibly answer them, and with what accuracy and confidence level. Critical machine parameters or detector capabilities should be identified where appropriate.

  3. Comparison and plan. Integrate the information laid out for the first two tasks into a coherent plan. Where multiple experimental strategies at different machines address the same questions, compare and contrast them. Address the challenges of integrating information from different sorts of experiments.

A major task in responding to these charges will involve the broad topic of precision measurements. The group should clearly delineate the constraints on models of electroweak symmetry breaking coming from current data, particularly electroweak precision data. The group should examine how future precision measurements, not just, e.g., of the Higgs sector, but also, e.g., of the masses and couplings in the gauge-boson, top-quark, and SUSY partner sectors, will specifically address the question-tree developed for the first charge. Does the top quark, because of its great mass, provide a special window on electroweak symmetry breaking?

For all the topics within the province of the group, describe what calculations will be needed and propose a plan for assuring that they are done.

The final product of this group will be a comprehensive, coordinated, and aggressive plan for discovery and understanding of the physics related to electroweak symmetry breaking and the generation of fermion masses, based upon our best current knowledge.

The Tree of Questions should be developed, to the extent possible, before the beginning of Snowmass 2001. This activity should be coordinated with the convenors of the instrument-oriented physics groups. During Snowmass, many of the specific experimental questions can be addressed in the instrument-oriented physics sessions, reserving the EWSB sessions for other issues, comparisons, synthesis, and discussion. Coordination with the other physics working groups, in particular Flavor Physics and Scales beyond 1 TeV, will be important.

Organizing Committee Contacts: Joe Lykken, Sally Dawson

P2: Working Group on Flavor Physics

Working Group Convenors: Belén Gavela (Madrid), Boris Kayser (NSF), Clark McGrew (Stony Brook), Patricia Rankin (Colorado)

The Flavor Physics Working Group encompasses both quark and lepton flavor physics. At an operational level, the two major areas of interest are quark flavor physics and the CKM matrix, and neutrino flavor physics and the analogous MNS matrix for neutrino mixing. The CKM (quark-mixing) matrix has been studied for several decades and is entering an era of increasingly precise measurements in which the CP violating phase will be determined and unitarity can be tested. Experimental evidence for the presence of non-diagonal elements in the MNS (neutrino-mixing) matrix, on the other hand, is rather recent and we cannot even be certain about which neutrinos mix or whether mixing is limited to the three known neutrino species. We need to learn whether there are sterile neutrinos. It is also an open question whether CP is violated in the neutrino sector and whether it may be experimentally observable.

In an important sense, all the fermion masses and mixing angles--today's primary concerns of flavor physics--have their origin in physics beyond the standard model. Accordingly, behind the description of the properties of and behavior of fundamental fermions lie important questions of principle, including

  • The riddle of identity: what makes an electron an electron, and a top quark a top quark?

  • The flavor scale(s): at what energy scales are the properties of the fundamental fermions determined? (Are they the same for neutrinos as for quarks and charged leptons?)

  • The origin of CP violation: How does CP violation arise? What is it telling us?

  • The nature of neutrinos: Is a neutrino its own antiparticle?


  1. Current knowledge. Adopt a convention to describe each matrix in terms of a convenient set of parameters, and summarize our present knowledge. What are the dominant sources of error--theoretical or experimental, statistical or systematic? In the case of neutrinos, comment also on how additional sterile neutrinos enter and what limits are available.

  2. The decade ahead. Extrapolate the expected improvement in our knowledge of the flavor parameters over the next 10 years. Catalogue the existing and projected experiments that will study the issues of quark (prominently strange and bottom, but also charm and top) charged-lepton (largely muon and tau), and neutrino flavor. Are any important opportunities being missed?

  3. Toward a coherent picture.

    1. What are the opportunities and needs for improving our knowledge of the flavor sector for all of the quarks (up, down, charm, strange, top, bottom) and charged leptons (e, m, t)?

    2. What theoretical developments are required to make sense of forthcoming measurements? What are the strengths and limitations of our current tools, including heavy-quark theory, chiral perturbation theory, and lattice gauge theory? What kind and level of theoretical effort is demanded by current and planned experiments? What measurements are needed to test and inform calculations of hadronic matrix elements? This group should coordinate with the working group on QCD and Strong Interactions.

    3. Give examples of how multiple measurements of the CKM parameters over-constrain the standard model, and how they can, in the presence of new physics, lead to conflicting results or conflict with unitarity. Examine how new physics would show itself in the flavor sector.

    4. What future experiments or facilities would be required to establish and further explore new physics that shows up in the quark flavor sector?

    5. For neutrino mixing, what additional information would be provided by future experiments with conventional neutrino beams, a muon storage ring as an intense source of electron and muon neutrinos, reactor experiments, and solar or atmospheric neutrinos? In particular,

      1. Compare the capabilities of a neutrino factory based on a muon storage ring with those of a neutrino beam generated by an intense pion "super beam."

      2. What additional information should be provided by nonaccelerator experiments in a new deep underground facility, e.g., new large-scale double-beta decay or solar neutrino / supernova experiments?

      3. How will searches for lepton flavor violation, precision g-2 measurements, and searches for permanent electric dipole moments add to our knowledge?

  4. The origin of flavor. What do theories of flavor suggest as crucial questions for experiment? How can the accumulating knowledge of the flavor sector--for the quarks, charged leptons, and neutrinos--guide the development of a theory of flavor, and the identification of one or more flavor scales? What significant clues are provided by the structure of the fermion mixing matrices?

  5. Why the Universe is made of matter. Confront the measurements of CP violation with the level of CP violation required to explain the observed baryon asymmetry of the Universe. What further program of measurements (including cosmological measurements) or theoretical developments will be most useful in completing our understanding of baryogenesis? What are the likely cosmological consequences of CP violation in the neutrino sector? These discussions should be coordinated with the Astroparticle and Scales beyond 1 TeV working groups.

The background information for Points 1 and 2 should be developed, to the extent possible, before the beginning of Snowmass 2001. This activity should be coordinated with the convenors of the instrument-oriented physics groups. During Snowmass, many of the specific experimental questions can be addressed in the instrument-oriented physics sessions, reserving the Flavor sessions for other issues, comparisons, synthesis, and discussion. Coordination with the other physics working groups, in particular Electroweak Symmetry Breaking, Scales beyond 1 TeV, and QCD and Strong Interactions, will be important.

Organizing Committee Contacts: Natalie Roe, Hitoshi Murayama

P3: Working Group on Scales beyond 1 TeV

Working Group Convenors: Michael Dine (UCSC), JoAnne Hewett (SLAC), Greg Landsberg (Brown), David Miller (UC London)

In the past decade, we have established the standard model gauge interactions by precision measurements. We are now entering a new decade with a strong emphasis on the physics of electroweak symmetry breaking and the origin of fermion masses and mixings. At the same time, experimentation in this decade could well bring new information beyond the physics of electroweak symmetry breaking and genuine surprises.

There are at least two big reasons why the standard model is incomplete: (i) The hierarchy problem, or why the electroweak scale is so much smaller than the Planck scale. (ii) Gravity is absent from the standard model. Therefore at least two approaches may be fruitful. In the bottom-up approach, we study possible solutions to the hierarchy problem and work out their observable consequences. In the top-down approach, we begin with certain theories of quantum gravity (e.g., string theory) and work out their consequences for low-energy experiments. The experiments may include rare decay studies, b, c, or t factories, electroweak precision measurements, experiments at the energy frontier, searches for proton decay and for dark matter, gravitational-wave detectors, high-energy astrophysics experiments, and experiments yet unknown. The implications of new physics at scales beyond 1 TeV touch all the other physics working groups, so we encourage joint sessions to explore areas of common interest.

Among the usual candidates for new physics beyond the standard model are the collider signatures of supersymmetry, new strong interactions, or extra dimensions, but this group should also consider new frontiers, such as gravity measurements below 0.1 mm. Brainstorming sessions might be useful.


  1. Survey theoretical scenarios that stabilize the electroweak scale far below the Planck scale, including supersymmetry, new strong interactions, large extra dimensions, small extra dimensions, and their combinations. Also examine scenarios motivated by reasons other than the hierarchy problem (e.g., axions, new gauge interactions, etc.), and consider where we might look for surprises. Review the current experimental situation and consider prospects for future improvements.

  2. Study how TeV-scale measurements could give trustworthy information on much higher energy scales, and evaluate what set of measurements (of what quality) would be needed to draw definite conclusions. Example: How could knowledge of the superparticle spectrum discriminate among different mechanisms of supersymmetry breaking or different unification schemes?
    Within the framework of supersymmetry,

    1. What does SUSY tell us about the mechanism of electroweak symmetry breaking?

    2. How do we determine the mechanism of SUSY breaking, the messenger mechanism, and the scales associated with this new physics?

    3. How do we use measurements of SUSY at the TeV scale as a window on the physics of strings, extra dimensions, and unification?

    4. How do we decipher the role of SUSY in flavor physics and in CP violation?

    5. When do you give up SUSY?

  3. Evaluate the current status of unified theories of the strong, weak, and electromagnetic interactions, and survey the important targets for experiment, including proton decay, neutron-antineutron conversion, neutrino properties, and lepton flavor violation. Consider the role of various sorts of precision measurements in testing models of unification.

  4. For a representative set of scenarios:

    1. Work out experimental signatures and study how they might best be observed. To cite a few examples: (i) How could we establish new strong interactions at hadron or a lepton colliders? (ii) How would we observe quantum decoherence due to Planckian physics in the neutral kaon system? (iii) How could we be detect mini-black hole formation at TeV-scale colliders? (iv) What are the prospects for micron-scale gravity measurements?

    2. Study how models of fermion masses might have consequences for rare processes. To cite a few examples: (i) What do fermion-mass models based on "fat" branes imply for rare decays? (ii) What are the properties of leptoquarks that arise in fermionic string constructions? (iii) How would supersymmetry manifest itself in muon-electron conversion? (iv) What is the connection between lepton flavor violation and neutrino oscillations?

    3. Study the implications of new physics beyond the 1-TeV scale for astrophysics and cosmology. Examples: (i) How would violations of Lorentz invariance influence ultrahigh-energy cosmic rays? (ii) What are the cosmological consequences of modifying the gravitational force law? (iii) Catalogue the plausible dark-matter candidates. How would different kinds of dark matter show themselves, and what are their implications for structure formation?

    4. Consider a representative sample of new phenomena, such as new neutral weak bosons, signals for quark and lepton compositeness, magnetic monopoles, fractionally charged particles, etc. Review thoroughly the current limits and the assumptions that underlie them, and discuss the discovery limits that might be reached in the future.

  5. The first decisive evidence for new phenomena may admit competing interpretations. Explore several scenarios in which the first collider signatures might fit more than one picture (e.g., technicolor and supersymmetry), and devise strategies to unambiguously determine the nature of the new physics.

  6. For the exotic signatures considered, summarize the control over standard-model processes that must be achieved in order to establish and study the "new physics." Identify areas in which major progress is required to make new-physics searches effective and reliable.

  7. For the universe of models investigated, consider how speculations about new physics beyond 1 TeV should inform the discussion of future accelerators and other experimental initiatives.

  8. Starting from theories of quantum gravity, develop scenarios for low-energy experimental consequences, including proton decay, stochastic gravitational waves, violations of Lorentz invariance or CPT symmetry, and black hole physics.

Background information should be developed, to the extent possible, before the beginning of Snowmass 2001. This activity should be coordinated with the convenors of the instrument-oriented physics groups. During Snowmass, many of the specific experimental questions can be addressed in the instrument-oriented physics sessions, reserving the Scales beyond 1 TeV sessions for other issues, comparisons, synthesis, and discussion. Coordination with the other physics working groups, in particular Electroweak Symmetry Breaking and Flavor, will be important.

Organizing Committee Contacts: Hitoshi Murayama, Paul Grannis

P4: Working Group on Astro/Cosmo/Particle Physics

Working Group Convenors: Dan Akerib (Case-Western Reserve), Sean Carroll (Chicago), Marc Kamionkowski (Caltech), Steve Ritz (Goddard)

The Astro/Cosmo/Particle Physics Working Group encompasses a broad range of scientific topics that border on particle physics, cosmology, and astronomy. This area of research has been delineated more by historical accident than by calculated design. One of the goals of this group will be to explore what constitutes astro/cosmo/particle physics. For the purposes of this working group, we will consider research done in the following areas as at least being pertinent:

  • Cosmology and the early Universe

  • Dark matter and dark energy

  • High-energy particle astronomy (using gamma-rays, cosmic rays, and neutrinos)

  • Gravitational waves

  • The search for nucleon instability and the problem of why the Universe is made of matter

Regardless of definitions, during the last ten to fifteen years astro/cosmo/particle physics has enjoyed an explosion of exciting results, along with greatly increased interest. These discoveries have answered some questions, but a number of exciting questions remain, including:

  1. The detection of fluctuations in the microwave background revealed the seeds of structure formation in the early Universe. What information will the next generation of precision cosmology measurements provide, and what self-consistency checks among different measurements will be possible with this new body of data?

  2. From the study of type I supernovae, we have evidence of a new dark energy that acts as a negative pressure to accelerate the expansion of the universe. What is the nature of this dark energy and how does it relate to particle physics?

  3. Individual cosmic ray particles have been detected with energies exceeding 100 EeV. What are these particles and how are they produced?

  4. We have detected high-energy (MeV-TeV) gamma rays from a variety of powerful astrophysical objects, including gamma-ray bursts and active galactic nuclei. How are these objects powered, how do they channel such a large fraction of their power into gamma rays, and do they play a significant role in the origin of the cosmic rays?

  5. New experiments to detect dark matter, high-energy astrophysical neutrinos, and gravitational waves are being comissioned or considered. What are the prospects for this new generation of experiments, that have substantial increases in sensitivity over the previous generation?

One of the most important aspects of this field is its increased connection to particle physics. Historically, astro/cosmo/particle physics has derived both scientific impetus and experimental methodology from high-energy physics, but more recently, it has become clear that astrophysical research will very likely have a profound impact on particle physics. It is largely in this context why it is so essential to have a vigorous working group in astro/cosmo/particle physics at Snowmass.


The basic charge for this working group is to broadly define and review astro/cosmo/particle physics, to examine its connections to, and ramifications for, particle physics, and to consider a vision for future research in the field.

In somewhat more detail, it will be essential to:

  1. Review the field of astro/cosmo/particle physics and summarize the current status of research in the field (theory, phenomenology, and experiments--both operational and under construction).

    1. Try to come up with the defining elements of astro/cosmo/particle physics and how the field relates to particle physics and astronomy. How many people are working in astro/cosmo/particle physics?

    2. Delineate the major sub-areas of the field (e.g. UHECRs, gamma-rays, neutrinos, gravitational waves, dark matter, cosmology, etc.)

    This educational exercise will be important for providing a baseline and common language, as well as for improving understanding of the field in the larger communities of particle physics and astronomy.

  2. Outline a vision for astro/cosmo/particle physics for the next decade and beyond. Among other things, consider:

    1. What are the broad scientific goals, and what are the key measurements to be made? What new experiments are required? What advances in technology are required?

    2. Where is there important overlap (in theory, experimental techniques, instrumentation) between astro/cosmo/particle physics and accelerator based particle physics? What can astro/cosmo/particle physics learn from high energy physics, and vice-versa?

    3. How can ideas and measurements in astro/cosmo/particle physics (e.g., precision cosmology) help identify new energy scales of interest to particle physics?

    4. What are the prospects for astrophysical techniques to detect new fundamental particles or to provide evidence for new interactions?

    5. What information from particle physics is needed to interpret the results of astro/cosmo/particle experiments?

  3. Seek community and agency input on the current mechanisms for project funding and review, and examine the funding matrix for astro/cosmo/particle physics. Which mechanisms are working and which are not? What steps can be taken to improve the situation?

Background information should be developed, to the extent possible, before the beginning of Snowmass 2001. This activity should be coordinated with the convenors of the instrument-oriented physics groups. During Snowmass, many of the specific experimental questions can be addressed in the instrument-oriented physics sessions, reserving the Astro/cosmo/particle sessions for other issues, comparisons, synthesis, and discussion. Coordination with the other physics working groups, in particular Scales beyond 1 TeV, and P2: Flavor, will be important.

Organizing Committee Contacts: René Ong, Maria Spiropulu

P5: Working Group on QCD and Strong Interactions

Working Group Convenors: Brenna Flaugher (Fermilab), Ed Kinney (Colorado), Paul Mackenzie (Fermilab), George Sterman (Stony Brook)

This group should consider the full range of topics associated with the strong interactions, including critical tests of Quantum Chromodynamics, the developing area of hadronic physics including our understanding of hadron (particularly nucleon) structure, the fundamental parameters of QCD including the strong coupling constant and the quark masses, the ramifications of the richness of QCD under unusual conditions, and QCD as a tool for calculations and measurements of cross sections and decay rates.

An important responsibility of this group is to interact with the other working groups on common problems. QCD has a strong influence on almost all measurements in particle physics, via the scattering cross sections and backgrounds at hadron colliders, fragmentation in electron-positron colliders and weak or strong matrix elements in hadron decays. A solid understanding of the QCD issues underlies many measurements (and discoveries) in HEP.


  1. Status and prospects. Provide a compact summary of the current status of QCD, catalogue the new information on QCD that may become available in the next decade, either through experimental measurement or improved theoretical techniques, and--after consultation with the other physics working groups--report on the interrelation of QCD with other topics in particle physics. Assess the current state of our knowledge of the quark masses and the strong coupling constant. What issues surround the precise definition and meaning of quark masses? What are the limitations to current knowledge, and how might they be overcome? How do uncertainties in the QCD parameters propagate into predictions for observables? Survey the range of experimental studies of QCD and ask which important experiments are not yet being undertaken, and what kinds of instruments will be needed to make them happen.

  2. The technology of perturbative QCD. Survey the current state of the art in making reliable perturbative calculations in QCD--not just at very high energies, but in all the domains in which QCD is applied. What are the points at which current methods encounter unresolved issues? What are the prospects for major advances over the coming decade? What calculations will be required by the coming generation of experiments? How can we ensure that the needed theoretical work is done?

  3. Nonperturbative methods. Survey the current state of the art in making reliable nonperturbative calculations in QCD--by lattice gauge theory, sum rules, and other approaches. What are the points at which current methods encounter unresolved issues? What are the prospects for major advances over the coming decade? What calculations will be required by the coming generation of experiments? How can we ensure that the needed theoretical work is done?

  4. Confinement and the hadron spectrum. How close have we come to a quantitative understanding of the hadron spectrum through lattice QCD? What are the prospects for a complete solution (including dynamical fermions) over the next decade, and what developments are required to make that happen? What insights into the mechanism of confinement come from developments in string theory and supersymmetric gauge theories, and what do they suggest for investigations (on the lattice, or by other methods) of theories other than four-dimensional QCD that might yield important lessons?

  5. Hadron structure.

    1. Static properties. Briefly summarize what is known about the static properties of the nucleon and other hadrons, and discuss the areas in which improvements are needed. In consultation with the experimental working groups, consider the kinds of measurements (by improvements in traditional methods, using intense neutrino beams, etc.) that could yield the desired information.

    2. Parton distribution functions. Make a critical assessment of the current crop of parton distribution functions, with attention to how well they reproduce the data from which they are extracted, how precisely they respect important theoretical constraints, and how well they serve the needs of their users. Evaluate the newly available parton distributions with uncertainties, and characterize what would be an ideal set of parton distribution functions. What are the current theoretical and experimental limitations on the reliability of parton distribution functions?
      On a related topic, consider what is currently known about fragmentation functions, and what needs to be known for applications that will be important over the coming decade.

    3. Partons and the structure of hadrons. What progress can we expect in relating the parton degrees of freedom in the infinite momentum frame to the structure of hadrons in the rest frame? What are the prospects for developing quantitative tools and physical pictures to make this link?

  6. Hadronic physics. The study of strongly interacting matter is an area of fruitful interaction between nuclear and particle physics, and many important questions involve experimental results and theoretical tools from both disciplines. The QCD working group should report on the state of hadronic physics, considering a few key issues (such as chiral symmetry breaking and the development of sound models and approximations, particularly those based on effective field theory, to QCD) to give form to the discussion. What role can high-energy experiments play in advancing our understanding of hadronic physics? The group should coordinate with the working group on Flavor Physics.

  7. Spin. What is the value of spin observables, and of polarized beams and targets, in probing the implications of QCD and in looking for new phenomena?

  8. Diffraction. What are the important issues in diffractive physics that must be addressed by theory and experiment? Are there special situations in which diffractive phenomena can be an effective tool in the search for new physics?

  9. Compositeness. The idealization that quarks and leptons are elementary is one of the foundations of the standard model. What are the prospects for finding, or setting limits on, a compositeness scale over the next decade and beyond, in all the instruments we might contemplate? Examine theoretical scenarios for composite quarks and leptons (in consultation with the Scales beyond 1 TeV working group). What special considerations might present themselves for the top quark, or for the third generation?

  10. The richness of QCD. Explore the novel phase structure of QCD under unusual conditions, including the prospects for observing and understanding the quark-gluon plasma and the consequences of phenomena such as color superconductivity. What are the implications of heavy-ion experiments for our understanding of QCD? What other experimental approaches might yield similar, or complementary, information? What lessons can we expect for the quark-hadron phase transition and other phenomena in the early universe?

Background information should be developed, to the extent possible, before the beginning of Snowmass 2001. This activity should be coordinated with the convenors of the instrument-oriented physics groups. During Snowmass, many of the specific experimental questions can be addressed in the instrument-oriented physics sessions, reserving the QCD and Strong Interactions sessions for other issues, comparisons, synthesis, and discussion.

Organizing Committee Contacts: Heidi Schellman