U.S. particle physicists recently recommended a list of major research projects that they hope will receive federal funding.
After a multi-year review, the U.S. particle physics
community has announced its vision for research spanning the next five to ten
years. The various projects could, if funded, help researchers develop a much
better understanding of the laws of nature.
The recommendations were released in a report called
“Exploring the Quantum Universe: Pathways to Innovation and Discovery inParticle Physics.” It was written by the Particle Physics Projects
Prioritization Panel (P5), a sub-panel of the High Energy Physics AdvisoryPanel (HEPAP), and will be submitted to funding agencies like the U.S.
Department of Energy Office of Science and the National Science Foundation to
guide their funding decisions over the next decade.
The future of particle physics
Particle physicists study the behavior of matter under the
most extreme conditions ever achieved in the laboratory. They accelerate
subatomic particles like protons and electrons to nearly the speed of light and
crash them together using large and powerful particle accelerators. At the
world’s most powerful accelerator, scientists can achieve temperatures as hot
as an unfathomable 7 trillion degrees Celsius. That’s well over 100,000 times
the temperature within the center of the Sun and nearly 100 times hotter than
the center of a supernova, which is the explosion of a star so bright that it
can be seen across half the Universe. The last time that temperature was common
throughout the Universe was less than a trillionth of a second after the Big
Bang.
The deep connections between the laws that govern the
quantum realm and those that govern the entire Universe have long been known,
and researchers have been studying them for decades. These sorts of experiments
require very large particle accelerators and detectors, involving thousands of
physicists, engineers, computer professionals, technicians, and various support
staff. Such a considerable effort requires careful planning and independent
oversight.
About every five years, the U.S. particle physics community
evaluates the progress made from the previous five years. It uses that
information to determine which efforts are the most likely to provide progress
in the near term. The community must take into account real-world
considerations, like budgets and whether the necessary technology exists or is
in advanced development. They also consider things like scientific impact. Both
P5 and HEPAP are merely advisory and governmental funding agencies that make the
final determination as to which projects should be pursued.
The P5 report recommends projects of a variety of sizes and
impacts. One of the larger projects is a fourth-generation effort to study the
cosmic microwave background of the Universe. These microwaves are the oldest
detectable remnant of the Big Bang and are a direct look at the Universe in its
infancy. Another big project involves
upgrading the Fermilab accelerator complex to improve its already world-class
neutrino research program. Fermilab is America’s flagship particle physics
laboratory, and it is developing an unprecedented effort to study the behavior
of neutrinos, which interact so rarely that they can pass through the entire
Earth with only a very small chance of interacting. Neutrino studies may shed
light on why the Universe seems to consist of only matter when our best
theories suggest that antimatter should be equally present.
The P5 report also recommends the creation of a
third-generation dark matter experiment, which would search for a ghostly form
of matter that is thought to be five times more prevalent than ordinary matter.
If dark matter exists, it should pass through the Earth with little chance of
interacting. Any hope of detecting this theoretical form of matter will require
a focused effort and advanced technology.
Also recommended is American involvement in a future accelerator in either Europe or Asia that would conduct detailed studies of the
Higgs boson, which is the particle discovered in 2012 that gives mass to other
subatomic particles.
One ambitious recommendation is that scientists explore the
creation of a high-energy muon collider. Muons are similar to electrons but
heavier. Another difference is that muons decay in a fraction of a second. To
make a muon collider, researchers will have to create muons, capture them, and
then accelerate them and smash them together in a very short period. It is not
yet clear that such a facility is possible, but it is suggested that the
nation’s accelerator scientist community collaborate to see if such an
accelerator is feasible.
More modestly priced possible future facilities include an
upgrade of the IceCube detector. IceCube uses a cubic kilometer of ice in
Antarctica to study cosmic neutrinos, including some of the most energetic
neutrinos ever made. The study of cosmic neutrinos can give astronomers an
insight into some very violent astronomical phenomena, including supernovae,
colliding neutron stars, and matter accelerated in the vicinity of huge black
holes. A second generation of IceCube could use as much as ten cubic kilometers
of ice to make even more precise measurements.
While the recommendations of the P5 committee are not
binding, they do reflect the judgment of the American particle physics
community. Before the convening of P5, thousands of physicists worked together
in the Snowmass Process. Over several years, researchers came up with their
best ideas and met in large conferences to discuss them. Through discussion,
criticism, and refinement, the proposals from Snowmass represent some of the
most creative ideas for improving our understanding of the laws of nature.
The P5 committee took the Snowmass proposals — refining some
and pruning others — and will pass the remainder along to funding agencies. The
next step in the process will be for agencies like DOE and NSF to consult with
their international counterparts and consider fiscal realities. Over the next
year or so, it will become clear what the future of particle physics research
in America will look like.