July 20, 2015

Physicists have added another type of subatomic particle to the so-called “particle zoo” of quantum mechanics: the pentaquark. This rare, fleeting particle is—as its name implies—made of five smaller particles, called quarks. Physicists at the Large Hadron Collider announced the discovery on July 14, though it was based on detailed measurements taken years earlier during the collider’s atom-smashing operations.

Huge and complex particle accelerators have greatly expanded our understanding of the science of physics. This photograph shows a particle detector at an accelerator called the Large Hadron Collider (LHC). Physicists at the LHC recently announced the discovery of the pentaquark. (© Maximilien Brice, CERN)

Quarks are elementary particles—that is, pieces of matter that do not seem to consist of anything smaller. Instead, quarks form building blocks for larger particles, called hadrons. Protons and neutrons are two types of hadrons, each containing three quarks. Protons and neutrons, in turn, form the nuclei (cores) of atoms. To understand how small quarks are, consider that atoms are already far too small to see with the naked eye. Then consider that the inside of an atom is almost entirely empty space. If a hydrogen atom were 4 miles (6 kilometers) wide, the single proton inside its nucleus would be the size of a tennis ball. Such is the scale of hadrons and the quarks they contain.

Three-quark hadrons—like protons and neutrons—are called baryons. Mesons are another type of hadron, consisting of just two quarks. Scientists have observed almost three hundred distinct types of hadrons, but until recently, all of these hadrons seem to have contained just two or three quarks. Scientists had predicted that five-quark hadrons—pentaquarks—could exist, theoretically. Now they have experimental proof.

Protons and neutrons are the only stable types of hadrons. All other known types of hadrons, once created, tend to disintegrate within a few hundred-millionths of a second. This difficulty, along with the fact that hadrons are so vanishingly small, makes measuring their presence difficult. But the Large Hadron Collider, based at the CERN organization in Switzerland, was specially built for this task. It accelerates hadrons around an underground ring 17 miles (27 miles) in circumference, causing them to travel at nearly the speed of light. At such speeds, collisions between even the tiniest objects are inevitable over time. New types of hadrons, like pentaquarks and other unstable particles, are created in the debris of such collisions. By carefully and accurately measuring signals from the collisions, and observing the patterns over time, physicists have been able to detect the presence of many new types of particles.

In 2012, physicists at the Large Hadron Collider announced they had found evidence for the Higgs boson, a particle believed to give other particles their mass. The discovery of pentaquarks, while not as earth-shattering as that of the Higgs boson, helps flesh out our understanding of how matter behaves at the smallest scales.

Other World Book articles: 

• Physics (2008-a Back in time article)
• Physics (2012-a Back in time article)
• Physics (2014-a Back in time article)

July 5, 2012

Scientists at CERN, Europe’s leading physics laboratory, have announced significant proof of a subatomic particle that may be the long-sought Higgs boson. A boson is any member of a certain class of atomic and subatomic particles. For decades, scientists have searched for the elusive particle, which is believed to give matter its mass. The particle is named for the British physicist Peter Ware Higgs, who posited its existence in 1964.

Mass is an extremely important characteristic of matter. Light is made up of particles called photons, which have no mass. The lack of mass enables photons to move at the fastest possible speed–the speed of light. If the particles that make up matter had no mass, they would also zip across the universe at extremely high speeds. But something provides a drag that prevents matter from doing this, an effect we call mass. Scientists believe that the drag is provided by interactions with Higgs bosons.

The Large Hadron Collider (LHC) smashes together circulating beams of high-energy particles. Scientists use the LHC to improve their understanding of the behavior of subatomic particles (particles smaller than atoms). (© CERN)

Evidence for the newly discovered particle came from two separate experiments being conducted using the Large Hadron Collider (LHC), outside Geneva, Switzerland. The powerful particle accelerator smashes bits of matter together at nearly the speed of light. The LHC scientists will continue working to confirm the results. But the discovery, announced on July 4, has sent ripples of excitement through the scientific community. If the new particle is in fact the Higgs boson, studying it may lead to new answers–and new questions–for our understanding of the nature of matter.

Additional World Book articles:

• Hadron
• Standard Model
• The Dark Side of the Universe (a special report)
• Found—The Top Quark (a special report)
• What Is the Fundamental Nature of Space? (a special report)

Nov. 29, 2011

Incredibly tiny subatomic particles called neutrinos may travel faster than light, according to a series of experiments conducted in Europe. The results were first announced in September and repeated in a refined experiment announced in November. The particles were produced at the CERN laboratory on the German-Swiss border. They then traveled about 450 miles (730 kilometers) through the ground to a giant detector in Italy. The neutrinos reportedly arrived a 0.000000058 second faster than a light beam would have.

In the early 1900′s, the physicist Albert Einstein concluded that nothing could travel faster than the speed of light–about 186,282 miles (299,792 kilometers) per second in empty space. The idea was part of a larger concept called the theory of relativity. The theory has been a cornerstone of modern physics ever since. Many of the ideas in the theory have been proven through different experiments over the years.

The Sudbury Neutrino Observatory consists of a large spherical water tank surrounded by sensors. The sensors detect flashes of light that occur when neutrinos interact with the water. Sudbury Neutrino Observatory.

If it is true that the speed of light is not a dead end, the most widely accepted theories of modern physics may need to be revised. However, neutrinos are rather unusual in nature. They probably have some mass. But this mass is so small that scientists have not yet been able to measure it directly. Neutrinos do not interact with ordinary matter easily. Many millions produced by the sun pass through our body everyday. The ghostly nature of neutrinos or a simple error in the experiment may yet undo these controversial results.

Additional World Book articles:

• Astronomy (Neutrino astronomy)
• Observatory (Neutrino observatories)
• On the Trail of the Elusive Neutrino (a Special Report)