From the Director of PPA: Snowmass and Future Directions for Particle Physics


August 16, 2013

Over the last 10 months, the Division of Particles and Fields of the American Physical Society sponsored an extensive community-led planning exercise for U.S. high energy physics. Organized around the energy, intensity and cosmic frontier themes for particle physics research and supported by facility, instrumentation, and computing frontier working groups, the work culminated recently in a nine-day intensive 2013 Community Summer Study in Minneapolis. An education and public outreach group was also prominently engaged. The workshop was dubbed “Snowmass on the Mississippi” in reference to earlier community studies held in Snowmass, Colo., where many previous community summer meetings of this kind have been conducted over the years. This time Snowmass was preceded by 57 specialized meetings and workshops over spring and summer 2013 that identified, commissioned, organized and performed extensive supporting studies and documentation in advance of the Minneapolis meeting itself.

Attended by some 700 physicists from the U.S. and around the world, including some 45 scientific staff from SLAC, Snowmass was an opportunity to frame the direction of the field over the next 20 years around major science questions: What is dark matter and how can we detect and study it? How can we solve the mystery of dark energy? What are the neutrinos telling us? What happened to all the antimatter in the universe? What is the Higgs boson telling us about the origins of mass and the nature of the vacuum? How can we glimpse the physics of inflation from the earliest stages of the universe?

These questions can be addressed by experiments at existing and new accelerator facilities, deep underground laboratories, observatories at the South Pole and even satellites in space. The Large Hadron Collider (LHC) will continue to explore the properties of the Higgs boson and search for new particles at the highest energies. The International Linear Collider would provide complementary high-precision measurements of the Higgs boson, and thereby fundamental insight into the physics mechanism behind mass. The Long-Baseline Neutrino Experiment offers the potential for discovering CP violation in neutrinos and a glimpse at physics well beyond energies directly accessible with accelerators. Discovery of dark matter would benefit from complementary approaches through underground detectors, searches for dark matter annihilation with present and future gamma-ray observatories and ongoing searches at the LHC for new particles that may be dark matter candidates.

My assessment is that Snowmass has been very successful in laying the foundation for the next phase of budget-constrained planning for the U.S. high energy physics program. It is anticipated that a subpanel of the High Energy Physics Advisory Panel (HEPAP) ­– the Particle Physics Project Prioritization Panel, or P5 – will be launched in September by the Department of Energy and the National Science Foundation, with a charge to provide a long-range strategy for high energy physics under various budget scenarios. Steve Ritz from UC-Santa Cruz has already been designated chair of the subpanel and the full membership of P5 will be finalized over the coming weeks.

The main output from Snowmass is a comprehensive documentation of existing and future experiments and their physics potential. Many new studies and specifically commissioned work provide an outstanding foundation for evaluation of priorities. In particular, fresh new ideas and strategies for addressing some of the fundamental questions have emerged through the Snowmass process, which are the beginnings of several exciting new experimental programs.

Snowmass also brought together the whole field in a way that will bring broader appreciation of the full spectrum of approaches to the fundamental questions. While it is sometimes convenient to classify our research along the three axes of the energy, intensity and cosmic frontiers, these are really just different and often complementary approaches to the same underlying physics questions. Emphasizing the science and educating the community broadly about these opportunities lays a foundation of support and appreciation for the priorities that will be identified and articulated by P5. Comparable planning efforts in Europe and Japan have been completed, so an integrated world view of the future of the field will emerge from these processes.

SLAC physicists figured prominently at all levels in many of these studies and outcomes. Michael Peskin and JoAnne Hewett had major leadership roles as co-conveners of the energy and intensity frontier working groups, respectively. Our accelerator physics community provided a wealth of expertise to evaluating future accelerator options. Kavli Institute for Particle Astrophysics and Cosmology scientists, working with our colleagues at other labs and universities, defined an exciting array of new dark matter, dark energy and cosmic microwave background experiments to address the big questions. The success of Snowmass in reaffirming the excitement of the many opportunities for understanding the forces and particles governing the universe around us was palpable and infectious.