December 9, 2011
Last week the Office of High Energy Physics sponsored a workshop titled "Fundamental Physics at the Intensity Frontier." More than 500 scientists from the U.S. and international communities gathered for three days in Rockville, MD, to take stock of the exciting science opportunities presented by a broad array of precision particle physics experiments.
The Standard Model of particle physics rests on an interwoven fabric of precision experiments, whose consistency underpins our understanding of the forces and building blocks of nature. The future evolution of this precision approach, and its capability for probing indirectly the frontiers of discovery, remains an exciting opportunity as amply demonstrated by the workshop.
The U.S. High Energy Physics community has subdivided the fields of particle physics, particle astrophysics and cosmology into three broad, overlapping areas of research: the Energy, Cosmic and Intensity Frontiers. The scientific goals and anticipated impacts of the Energy and Cosmic Frontiers are readily explained:
- Projects at the Energy Frontier, such as the Large Hadron Collider (LHC), allow experimenters to probe for fundamental particles of ever-higher mass and approach the threshold for producing new physics last seen in the early stages of the Big Bang.
- Cosmic Frontier experiments, which study the universe at cosmological scales, have revealed compelling evidence that it is composed largely of dark matter and dark energy, and not the familiar ordinary matter around us.
- The Intensity Frontier involves many diverse lower-energy precision experiments; and its discovery potential, being less direct, is therefore harder to understand.
So what is the Intensity Frontier, and why should we place as much importance on these types of experiments as on the other Frontiers?
Many processes in particle physics involve quantum loops: fluctuations in the vacuum, consistent with the laws of quantum mechanics, where virtual Standard Model force carriers or known particles fleetingly appear and contribute to the predicted rate for the process. Other processes are very rare, or even forbidden, within the Standard Model. Physics beyond the Standard Model can introduce new particles into quantum loops, thereby changing predicted rates or properties for processes.
Likewise, new physics can substantially raise the rate for very rare or forbidden processes to observable levels. Thus, Intensity Frontier experiments, by looking for measurable deviations from Standard Model predictions, provide an indirect but essential probe for new physics at mass scales far beyond those directly accessible in Energy Frontier machines.
Particle physics has a long history of interplay between these kinds of indirect probes, which provide early evidence for new physics, and later direct discoveries at the Energy Frontier experiments of the day, and we fully expect this to continue. If the LHC discovers new particles, precision measurements will be critical to understanding the underlying physics. If it does not, precision measurements will still allow us to indirectly probe mass scales a thousand times heavier than the LHC can reach.
The SLAC high energy physics program reflects this interplay. We are partners on the Energy Frontier with the ATLAS experiment at the LHC. Our Cosmic Frontier program includes the Fermi Gamma-ray Space Telescope, participation in relic dark matter searches with the Super Cryogenic Dark Matter Search experiment, and the future Large Synoptic Survey Telescope as a means of elucidating the nature of dark energy.
We also have a distinguished history of contributions to the Intensity Frontier with the physics of the bottom and charm quarks and the tau lepton, with BaBar and the PEP-II B Factory, and more recently with exploring the nature of the neutrino through precision searches for rare neutrino-less double-beta decays with the Enriched Xenon Observatory (EXO).
In preparation for the Intensity Frontier Workshop, PPA has been mapping out a future for our Intensity Frontier program as part of the national high energy physics strategy. Our internal examination of future opportunities has led us to continue a small effort towards quark flavor physics measurements at next-generation flavor factories, a commitment to developing a much larger, tonne-scale version of EXO and pursuing future heavy photon search experiments.
In addition, as a national laboratory we have begun exploring possible participation in the Long-Baseline Neutrino Experiment as the centerpiece of the national program. As a result, SLAC physicists were not only active participants in many aspects of organizing the workshop, but also contributed to working groups on neutrinos, heavy quarks, charged leptons and heavy photons.
Indeed, SLAC’s JoAnne Hewett was co-convener of the overall workshop with Harry Weerts of Argonne National Laboratory, and several working group conveners were also from SLAC.
The Intensity Frontier workshop was an invigorating reminder of the central role precision experiments have played, and will continue to play, as the foundation of particle physics and of our understanding of forces and fundamental particles. The Intensity Frontier provides a rich tapestry of interconnected, redundant measurements whose consistency is the underpinning for our confidence in the Standard Model and, in the future, for new physics beyond.