SLAC researchers want to answer fundamental questions about neutrinos: What is the mass of the three known types of neutrinos? Is there a fourth type that could be linked to dark matter? Could neutrinos explain why there is more matter than antimatter in the universe? Are neutrinos their own antiparticles?

SLAC physicists are taking on key roles in several accelerator-based neutrino experiments including MicroBooNE, the future ICARUS at Fermilab in Illinois, and are involved in planning the Deep Underground Neutrino Experiment (DUNE), which will send neutrinos 800 miles through the Earth to detectors in South Dakota.

At CERN's Large Hadron Collider (LHC), the world's largest and most powerful particle collider, researchers smash proton beams into one another at record high energies and analyze the debris to reveal some of nature’s best-kept secrets. They already discovered the Higgs boson in 2012, a particle that explains why other fundamental particles have mass. Now they study it in detail to better understand its properties. Particle physicists also search for exotic new particles. Some of these could explain the nature of dark matter, the invisible substance that makes up 85 percent of all matter in the universe.

SLAC builds and operates detector components for ATLAS, one of the two experiments involved in the Higgs discovery. We also develop sophisticated tools to analyze complex proton collisions and host a computing center for ATLAS data.

Of all particles known to scientists, neutrinos are among the most mysterious. They are extremely difficult to study because they can pass through a layer of lead nearly 6 trillion miles thick without leaving a trace. SLAC led the construction and operation of the Enriched Xenon Observatory, located deep in a New Mexico salt deposit. The experiment is searching for a theorized type of particle decay that, if it exists, would happen only once in 100 billion times the age of the universe to any given xenon atom. Seeing this decay would prove that the neutrino is its own antiparticle.

Scientists know from the motions of galaxies that the universe contains about five times more dark matter than visible matter, but don’t know much about the form it takes. Undiscovered dark matter particles could also be attracted, repelled or otherwise affected by unknown dark forces.

SLAC wants to find out more as a key player in the Heavy Photon Search (HPS), an experiment at Jefferson Lab in Virginia. HPS is searching for a dark, heavy version of particles of light known as photons. These dark photons could be potential carriers of dark forces between dark matter particles. SLAC designed and built a crucial part of the HPS detector. SLAC researchers also developed the theories that led to HPS and other heavy photon searches around the world.

The International Linear Collider (ILC) is proposed as the next major particle physics facility, intended to complement the Large Hadron Collider at CERN and shed more light on recent discoveries such as the Higgs boson. The Silicon Detector (SiD) is being designed to exploit the physics discovery potential of the high-energy electron–positron collisions this facility will provide. SiD is a multi-purpose precision detector, optimized for a broad range of physics processes, but designed to be robust and affordable.

SLAC plays a significant role in the design of the Silicon Detector. Detailed simulations of the physics processes and the overall detector performance are carried out by the Linear Collider Detector Physics and Detectors group. Understanding the interplay between the ILC and SiD is the role of the Machine-Detector Interface group. Design of the silicon sensors and readout chips used in both the tracking systems and calorimeters is also underway at SLAC.

Theory is the fundamental tool that explains what scientists observe in experiments and gives them a better idea of where to look for the next big discovery. SLAC theorists explore important topics in particle physics, particle astrophysics and cosmology, including searches for new phenomena, extra dimensions, collider physics, neutrino physics, dark matter and cosmic inflation. These theories advance our understanding of nature, from the properties of tiny particles to the expansion of the entire universe.