A breakthrough particle accelerator could collide electrons with heavy ions or protons at nearly the speed of light to create rapid-fire, high-resolution “snapshots” of the force binding all visible matter.
Deep within the protons and neutrons of an atomic nucleus, powerful and poorly understood gluons flit in and out of existence. These fundamental particles carry the strong nuclear force, which acts as a kind of subatomic “glue,” binding quarks together. And since protons and neutrons make up most of the visible matter in the universe, this glue, in essence, is what holds together everything we see, from stardust to planets to people. But the behavior of this crucial, pervasive binding force is remarkably difficult to draw into focus. A new electron-ion collider could rise to the challenge, bending time and launching light-speed probes to unravel the mysteries of the glue.
Related background: "The Glue that Binds Us All"
Physics can get violent – just imagine head-on collisions at nearly the speed of light. When particle colliders slam high-energy beams together, rare fundamental particles can pop into existence, new matter emerges, and physics gets profoundly strange. Brookhaven’s Relativistic Heavy Ion Collider (RHIC) explores this uncharted territory, and it surprised the world with a breakthrough discovery: colliding beams of gold ions melted the ions' many protons and neutrons into a liquid-like quark-gluon plasma. This friction-free liquid existed only at the birth of the universe, and it holds clues about the dawn of time. But the mechanics underlying the primordial plasma remain elusive.
Colliding protons or heavy ions hints at the interactions that ripple out and guide the formation of visible matter. But the collision itself is a ripple, the product of those pre-smash particles flying at relativistic speeds. By examining accelerated ions directly, scientists might clearly identify physics phenomena that existed just after the Big Bang and later rippled out into the current structure of the universe. In theory, a new kind of collider could gaze far back into the dawn of time, uncovering hints of the initial conditions behind our cosmic origins.
Accelerating particles close to the speed of light appears to slow down time itself, stretching out otherwise undetectable subatomic processes. This is a direct effect of time dilation, part of Einstein’s theory of relativity. At relativistic speeds, the gluons inside protons perform a remarkable bit of quantum trickery, popping in and out of the vacuum with increasing abundance. Eventually, a balance is reached in a steady state of maximal gluon concentration physicists call color glass condensate. High-luminosity electron beams could reveal details of these gluon “walls,” which when collided melt into quark-gluon plasma.
A camera would need femtometer-scale resolution to capture fundamental particles – that’s 10-15 meters, or a millionth of a billionth of a meter. Today's colliders can produce some insights, but only work by destroying the subtle and important quantum activities of gluons. What’s needed, then, is a way to catch the gluons at their light-speed trickiest, to send a subtle spy into that time-stretched landscape.
A proton or heavy ion beam, swollen with gluons, can be probed by another fundamental particle: the electron. In an electron-ion collider, super-bright electron beams accelerated to the same blazing, relativistic speed can bombard protons or heavy ions without destroying the entire delicate system. This proposed collider would probe the heart of matter as we know it, revealing subtle behavior at a range of energy levels. With high luminosity, or high particle interaction rates, the electrons can capture “snapshots” of the fleeting gluons at their most abundant and, in many ways, most interesting.
The atomic world is never still, and even without light-speed collisions protons carry an intrinsic motion called “spin.” Curiously, previous collider experiments showed that the total spin of a proton, a fundamental quantum characteristic, is not comprised solely of the sum of the spins of the quarks inside. Deepening the mystery, gluons appear to contribute only a small fraction of the overall proton spin at the energy levels already explored. But no one has explored the spin contributed by gluons once they reach that extraordinary light-speed saturation point, an ideal task for an electron-ion collider.
Brookhaven Lab offers a history of groundbreaking physics, accelerator expertise, and a fully functional, productive proton/heavy-ion accelerator with unparalleled versatility – the Relativistic Heavy Ion Collider (RHIC). The RHIC community consists of international collaborations of physicists eager to continue their explorations of matter. Adding an electron ring and other components to the existing accelerator complex would be a cost-effective, practical strategy for achieving the scientific goals of an electron-ion collider.