Theory and experiment combine to address the fundamental questions of particle physics. <em>Photo: Reidar Hahn, Fermilab</em>
Twenty-first century questions

Theory and experiment combine to address the fundamental questions of particle physics. Photo: Reidar Hahn, Fermilab

What message do neutrinos bring from the beginning of time?

When matter and antimatter meet, they annihilate in a puff of energy. At the birth of the universe, such puffs of energy should have produced particles and antiparticles in equal numbers. Yet somehow we live in a universe made entirely of matter. Could neutrinos, tiny particles permeating the universe, be responsible? Are neutrinos the reason we exist? The race is on to find out, using neutrino beams as well as neutrinos from reactors, the atmosphere and the sun.

Bonnie Fleming, Yale University / Fermilab, USA

What’s the matter with antimatter?

Matter consists of elementary particles such as the electron and proton. Every particle has its antiparticle. Together, the antiparticles compose antimatter. Strikingly, the observable universe is exclusively dominated by matter rather than antimatter, posing one of the greatest puzzles in physics: where’s the antimatter? Experimentalists at CERN, Fermilab, KEK and SLAC are producing antimatter in accelerators and studying its subtle differences from matter to illuminate the startling absence of antimatter in the universe.

Hong-Jian He, Tsinghua University, China

How can we solve the mystery of dark energy?

Observations of light emitted near the horizon of the universe reveal that everything seems to be flying apart with increasing velocity. Big Bang cosmology attributes this to “dark energy” that fills the entire universe— an amazing phenomenon! Is the Big Bang model too simple? Should Einstein’s equations be modified? Is there an unknown fundamental force? As the answers emerge, I expect that in the next decade physicists will solve the mystery of dark energy.

Wilfried Buchmueller, DESY, Germany

Are we on the threshold of a whole new understanding of nature’s particles and forces?

Is there an invisible force that swarms around matter to give it mass? Finding Higgs particles and studying their properties can tell us the answer. The Higgs holds secrets about how the forces of nature are related to each other, how matter became matter, and what is the origin of the atomic scale that makes life possible.

Joe Lykken, Fermilab, USA

What is the trajectory of our universe? How did it evolve?

Einstein’s theory tells us the universe must expand. We believe that everything started with the big bang from a single point, and indeed, we can observe that the distance between the galaxies is increasing. How did energy, matter and forces in the early universe influence how our universe is expanding today? Experiments at the Large Hadron Collider and observations of particles around us will help to solve the mystery of the evolution of the universe.

Mihoko Nojiri, KEK, Japan

Do invisible processes leave their imprint on the world we can observe?

Besides directly seeing new particles, physicists look for traces of exotic phenomena that may lurk unseen in experiments. These rare but crucial processes can leave traces of their existence in seemingly ordinary data and observations. For example, physicists inferred the W boson’s existence from observations of radioactive nuclear decay that heralded the particle’s existence 80 years before experimenters observed it directly. What patterns in today’s experiments will herald the next breakthrough?

Doug Bryman, University of British Columbia/TRIUMF Laboratory, Canada

What is dark matter?

Ordinary matter accounts for only five percent of the total inventory of the universe. From observing the rotational velocities of visible objects in the galaxies, cosmologists have “weighed” about 21 to 23 percent of the total energy of the universe as formed by invisible matter, called dark matter. The most attractive explanation postulates that dark matter is made of yet-undiscovered massive particles interacting very weakly with ordinary matter.

Lucia Votano Laboratory INFN of Gran Sasso, Italy

Are there extra dimensions of space?

We all thought we lived in a universe with three dimensions of space. But to come up with a unified theory of all forces, a feat that even Einstein couldn’t accomplish, some modern-day physicists propose a whopping six more dimensions to the universe, curled up in such small sizes that so far we can’t see them.

Hitoshi Murayama, Institute for the Physics and Mathematics of the Universe, Japan

Is there a simple explanation for it all?

Probably not. “It all” should include the origin of space and time, as well as the basic properties of all known elementary constituents of matter, all emerging out of a more basic conceptual substratum. If there are large extra dimensions accessible to the LHC, today’s experiments could address these questions. Perhaps the ultimate reductionist dream can be realized.

Luis Alvarez-Gaume, CERN, Switzerland