KAHIBARO
Table of Contents
Purpose and Basic Idea
Heavy ion colliders are particle accelerators designed to collide the nuclei of heavy atoms, such as lead or gold, at extremely high energies. Unlike proton colliders, which usually study simpler particle collisions, heavy ion colliders are used mainly to investigate how nuclear matter behaves under very extreme temperature and density conditions.
When two heavy nuclei collide at high speed, they can create a tiny region of matter so hot and dense that protons and neutrons no longer remain intact. Their constituent quarks and gluons can move more freely for a very short time. This state is called the quark gluon plasma. The details of quarks, gluons, and the strong interaction belong to other chapters, so here the focus is on why heavy ion colliders are special as accelerator facilities.
Why Heavy Ions Are Different
A heavy ion is an atom that has been stripped of some or all of its electrons, leaving a positively charged nucleus. In a collider, these nuclei are accelerated and brought into head on collision. Because each nucleus contains many protons and neutrons, a single collision is much more complex than a proton proton collision.
The collision outcome depends strongly on how directly the nuclei hit each other. A nearly central collision, where the nuclei overlap strongly, usually produces many more particles than a grazing collision.
In heavy ion colliders, the goal is often not just to produce a few new particles, but to create a large volume of extremely hot, dense nuclear matter for a very short time.
Collision Geometry
A useful idea in heavy ion physics is the impact parameter, usually denoted by $b$. It is the transverse distance between the centers of the two nuclei if they were to continue on straight paths without interacting.
If $b \approx 0$, the collision is central. If $b$ is large, the collision is peripheral. Central collisions typically involve more participating nucleons and produce more secondary particles.
What Is Accelerated
Heavy ion colliders do not accelerate neutral atoms. They accelerate ions, because electric fields act on charged particles. A heavy atom must first be ionized, often completely stripped of electrons, before acceleration.
The accelerated beam is characterized by the nuclear charge $Z$ and mass number $A$. For example, lead has $Z = 82$ and a commonly used isotope has $A = 208$. Since the beam particle is a nucleus and not a single proton, its motion in electromagnetic fields depends on both its charge and its mass.
A useful quantity is the energy per nucleon, because each nucleus contains many nucleons. If a nucleus has total energy $E$, then the energy per nucleon is approximately
$$
\frac{E}{A}
$$
This helps compare heavy ion collisions with collisions involving lighter nuclei.
Beam Preparation and Acceleration
Heavy ion beams are more difficult to prepare than proton beams. The ions must be produced, stripped of electrons, pre accelerated, and then passed through a sequence of larger accelerators before injection into the collider ring.
Because heavy ions have large mass and multiple charge, accelerator operation becomes more demanding. Beam losses can be more serious, and electromagnetic interactions between the beams and the machine can be stronger. Also, heavy ions can fragment if they interact with residual gas or machine components.
Luminosity in Heavy Ion Colliders
As with other colliders, an important quantity is luminosity, because it determines how often collisions occur. The event rate is
$$
R = L \sigma
$$
where $L$ is the luminosity and $\sigma$ is the cross section for the process of interest.
Heavy ion colliders generally have lower luminosity than proton colliders. One reason is that it is harder to store intense, tightly focused beams of heavy nuclei. However, each collision can produce a very large number of particles, so even lower collision rates can still provide rich physics data.
The event rate in any collider is given by
$$
R = L \sigma
$$
A lower luminosity can still be useful in heavy ion physics because each collision is highly complex and information rich.
Important Facilities
Several major accelerator laboratories have operated heavy ion collider programs. Two especially important examples are the Relativistic Heavy Ion Collider, RHIC, at Brookhaven National Laboratory in the United States, and the Large Hadron Collider, LHC, at CERN in Europe, which also runs heavy ion collisions in addition to proton collisions.
RHIC
RHIC was built specifically to collide heavy ions at relativistic energies. It has collided gold nuclei most famously, though it has also operated with other species. RHIC played a central role in establishing strong evidence for the creation of quark gluon plasma.
LHC Heavy Ion Program
The LHC is best known as a proton collider, but it also runs heavy ion collisions, especially lead lead collisions. Because the LHC reaches higher beam energies than RHIC, it can produce even hotter and denser collision systems. This allows physicists to study how the produced matter behaves over a wider energy range.
What Physicists Measure
Heavy ion collider experiments do not usually focus on a single outgoing particle. Instead, they often measure global properties of the event, such as how many particles are produced, how energy is distributed, and whether the outgoing particles show collective patterns.
Common observables include multiplicity, transverse energy, flow patterns, and the suppression or modification of certain particle signals as they pass through the dense medium created in the collision. These signatures help reveal the properties of the matter formed in the collision zone.
Centrality
A practical concept in heavy ion experiments is centrality. Centrality is an experimental way of classifying how head on a collision was. It is not measured directly from $b$, but inferred from observables like particle multiplicity or deposited energy.
A central collision usually has high multiplicity and corresponds to a small impact parameter. A peripheral collision usually has lower multiplicity and corresponds to a larger impact parameter.
| Collision class | Impact parameter | Overlap of nuclei | Typical particle production |
|---|---|---|---|
| Central | Small | Large | High |
| Mid central | Intermediate | Moderate | Moderate |
| Peripheral | Large | Small | Lower |
Experimental Challenges
Heavy ion colliders create events with enormous numbers of produced particles. Detectors must handle this high track density without losing too much information. Data analysis is also more difficult, because the background is much larger than in simpler collisions.
Another challenge is separating effects caused by the hot medium from effects that already exist because nuclei contain many nucleons. For this reason, experiments often compare heavy ion collisions with proton proton and proton nucleus collisions.
Energy Scales and Comparison
Heavy ion collisions are often described by the center of mass energy per nucleon pair, written as $\sqrt{s_{NN}}$. This is more useful than just quoting the total beam energy, because it allows comparison between different nuclear species.
For heavy ion collisions, the standard quoted energy is usually the center of mass energy per nucleon pair,
$$
\sqrt{s_{NN}}
$$
not simply the total energy of the whole nucleus.
Scientific Importance
Heavy ion colliders allow scientists to study nuclear matter under conditions similar to those that existed very shortly after the early universe began expanding. They also test our understanding of the strong interaction in many body systems.
Their importance is therefore twofold. They are accelerator machines of great technical sophistication, and they are unique laboratories for studying matter at extreme temperature and density. In this sense, heavy ion colliders connect accelerator physics, nuclear physics, and particle physics in a very direct way.