A joint Fermilab/SLAC publication

LHC basics: What we can learn from lead-ion collisions

A lead-ion collision recorded by the ALICE experiment. This was a high-multiplicity collision that produced thousands of particles, including muons recorded in the forward muon spectrometer on the left.

A lead-ion collision recorded by the ALICE experiment. This was a high-multiplicity collision that produced thousands of particles, including muons recorded in the forward muon spectrometer (left). © CERN for the benefit of the ALICE collaboration.

The LHC has been smashing lead ions since Sunday, Nov. 7, 2010, and physicists from the ALICE, ATLAS and CMS experiments are working around the clock to analyze the aftermath of these heavy-ion collisions at record energies and temperatures.* Last week we walked you through the process of creating, accelerating and colliding lead ions. Now we’ll talk about the big question: Why spend one month each year colliding heavy ions in the LHC?

The building blocks and mini 'big bangs'

To understand why physicists around the world study heavy-ion collisions, we need to review some basic particle physics and discuss what happens when two nuclei collide at ultra-high energies.

First, the basics. Everything you see around you is composed of atoms, which are themselves composed of protons and neutrons bound together in an atomic nucleus, surrounded by a cloud of electrons. Electrons are one of the basic building blocks of matter, but protons and neutrons are not—they are in turn composed of elementary particles called quarks.

The strong force binds quarks inside composite particles, such as protons, via other elementary particles called gluons. Thus lead nuclei, which are made up of protons and neutrons, are really composed of many quarks and gluons.

When two lead nuclei slam into each other at high enough energies, they form a fireball of hot, dense matter. The temperatures created in the fireball are so great that they ‘melt’ the protons and neutrons. The result is a state of matter called the quark gluon plasma, in which quarks and gluons roam freely. The QGP exists for only an instant before the fireball expands and cools to the point where quarks and gluons once again form composite particles.

The LHC is not the first accelerator to create QGP. CERN announced indirect evidence for the "new state of matter" in 2000, a result of experiments in the 1980s and 1990s at the Super Proton Synchrotron (now used as the injector for the LHC). Next up was Brookhaven National Laboratory’s Relativistic Heavy Ion Collider. In 2005 scientists from the RHIC experiments tentatively claimed the creation of a QGP in collisions of gold nuclei. Physicists at the LHC are now studying collisions of lead nuclei—and the properties of the QGP that should be produced in those collisions—at energies more than 13 times higher than at RHIC.

From bang to being—why we care

Snapshot of two lead nuclei just after impact. Click on the image to view animations of lead-ion collisions at the LHC's design energy (twice the current LHC collision energy).

Snapshot of two lead nuclei just after impact. Click on the image to view animations of lead-ion collisions at the LHC's design energy (twice the current LHC collision energy). © CERN/Henning Weber.

Physicists believe that the universe was filled with QGP millionths of a second after the big bang, until it expanded and cooled enough for the very first composite particles to form. This process is mirrored at very small scales in heavy-ion collisions, allowing scientists to study in the laboratory one of the early stages of the universe’s evolution. By creating millions of QGPs over the next month, scientists will learn more about how the basic building blocks of matter—quarks and gluons—come together to form particles, which in turn form all the matter in the universe.

One specific area of interest for LHC physicists is confinement. It appears to be impossible to observe quarks and gluons in isolation. Outside the quark gluon plasma, they are always confined within composite particles. The exact mechanism that causes this confinement is unknown. Without confinement, none of the composite particles that make up the world around us exist. So pinning down aspects of this mechanism is critical to understanding how matter evolved into the form it has today.

QGP will also be used to study the mystery of mass. Physicists are eagerly hunting for the Higgs boson in proton collision data, but finding the Higgs would only explain how the elementary particles such as quarks get their masses.  Another unsolved puzzle is where most of the mass of a proton or neutron comes from. Protons and neutrons are made up of three quarks each, but adding up the masses of the three quarks only accounts for about 1 percent of the proton or neutron mass. Physicists hope to find clues to the missing 99 percent in the behavior of the QGP.

One step at a time

A lead-ion collision as recorded by the CMS detector at the LHC. <i>Image copyright CERN for the benefit of the CMS collaboration.</i>

A lead-ion collision as recorded by the CMS detector at the LHC. © CERN for the benefit of the CMS collaboration.

Even with higher-energy collisions and more sophisticated detectors than ever before, LHC scientists won’t solve all the mysteries of quark confinement or the proton’s missing mass. But with only five days’ worth of data collected, physicists in the ALICE, ATLAS and CMS experiments are already working feverishly to produce the first new QGP measurements that will improve our understanding of the strong force.

With the very first data, physicists will study the heavy-ion collision environment at the highest energies ever produced in the laboratory. Some of the very first measurements to be published from the ALICE, ATLAS and CMS experiments with lead-ion collision data will describe the number of particles produced in such collisions, which will confirm or rule out various theoretical predictions.

With a bit more data, scientists will measure the “flow” of the QGP. In 2005 the physics community was surprised by RHIC physicists' announcement that the QGP created in gold-ion collisions behaved like a liquid rather than a gas. Scientists don’t yet know—but will very soon—whether the QGP created in these higher-energy lead-ion collisions will flow like a liquid or will act more like a gas.

With even more data, physicists will measure the behavior of certain types of particles as they travel through the QGP. Physicists at RHIC have shown that certain particles are suppressed as they move through the QGP, which shows that the plasma interacts very strongly with these particles. The higher energies at the LHC and the more sensitive particle detectors will allow a greater number and variety of particles to be measured.

The ALICE experiment published its first paper using proton collision data mere days after the first collisions in November 2009, so the first papers with measurements from the LHC's lead-ion collisions should be popping up in physics journals very soon.

* A note about temperatures
If you’ve been reading the news reports on the first lead-ion collisions at CERN, you will have seen scientists and journalists claiming that LHC collisions will create temperatures anywhere from 100,000 to 1 million times hotter than in the center of the sun. Here’s an attempt to set the record straight.

  • Based on measurements made at previous accelerators, the temperature at which a quark gluon plasma is created is about 100,000 times hotter than the sun’s core temperature. Above this temperature quarks and gluons are no longer confined inside other particles. (The sun’s core temperature is about 20 million Kelvin, equal to about 36 million degrees Fahrenheit.)
  • At RHIC, physicists discovered that the temperature generated in the very first instant after two gold ions collide was up to 2.5 times higher than that necessary to create the quark-gluon plasma – about 250,000 times higher than the sun’s core temperature.
  • This month's lead-ion collisions at the LHC will take place at energies more than 13 times higher than at RHIC. Physicists estimate that these collisions could create temperatures up to twice as high as at RHIC, thus about 500,000 times the sun’s core temperature. When the LHC eventually runs at design energy, the temperatures could rise even higher.
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