Scientists discover that protons partner with neutrons more often than with other protons

Fast-moving protons are much more likely to pair up with fast-moving neutrons than with other protons in the nuclei of atoms, according to a recent experiment performed at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility. The experiment confirms previous theoretical research led by Mark Strikman, a professor of physics at Penn State. Strikman's theory predicts that fast-moving protons have a nearly 100-percent tendency to form pairs with other fast-moving protons or neutrons, and that the majority of these pairings are between protons and neutrons. Strikman also suggested the experimental strategy that the researchers at the Thomas Jefferson National Accelerator Facility used to make their discovery.

Strikman's theory, along with its recent experimental confirmation, may help physicists to understand the structure of nuclear systems, from light nuclei to massive neutron stars. Results of the experiment will be published later this year in the journal Science, which recently posted the paper on its Web site.

Two decades ago, Strikman and his collaborator Leonid Frankfurt of Tel Aviv University in Israel suggested that the most direct way to look for pairings of two high-momentum nucleons--a nucleon is a proton or a neutron in the nucleus of an atom--would be to knock a fast-moving nucleon out of an atom's nucleus and to identify the nucleon that is left behind. This method was used in an experiment conducted by researchers at the Brookhaven National Laboratory, and results were published in 2003.

In 2006, Strikman and his colleagues published a paper in the journal Physical Review Letters in which they described the development and results of a detailed model that analyzed the Brookhaven National Laboratory data. The team's analysis indicated that when a proton is moving faster than one-quarter the speed of light, 90 percent of the time it is paired with a neutron, while 10 percent of the time it is paired, presumably, with another proton. "Although we expected a large asymmetry between the number of proton-neutron pairs and proton-proton pairs, the magnitude of the difference was startling to us because it was a significant departure from what estimates using current models of nucleon-nucleon interactions told us it would be," said Strikman.

Recently, a direct measurement was performed by researchers at the Thomas Jefferson National Accelerator Facility to verify the accuracy of the 90-percent to 10-percent ratio predicted by the model developed in 2006 by Strikman and his colleagues. In their experiment, the scientists sent a beam of energetic electrons into a thin sheet of carbon to knock protons out of the nuclei. Next, they counted the number of times that each proton's remaining partner was a neutron versus another proton. Results of the experiment supported Strikman's and Frankfurt's predictions.

"Although we predicted a very large probability of pairing between fast-moving nucleons, it is amazing to me that 100 percent of fast-moving nucleons are paired," said Strikman. "It also is amazing that so little of this pairing is between two protons."

Strikman added that he is excited to finally see experimental evidence in support of the theory that he has spent much of his career establishing. "Previous methods could not distinguish between proton-proton and proton-neutron correlations," he said. "But this new experiment has achieved something that never has been done before. The discovery is the most clear-cut example that the high-energy processes, which we have been advocating for many years, are truly effective tools and can be used for future discoveries."

Source: Penn State

Related stories:

CERN sticks to opening gala despite atom-smasher troubles
Europe's leading physicists will swap their lab coats for their best suits next month for the official inauguration of a multi-billion dollar machine designed to shed light on the "Big Bang".
BOSS: The Baryon Oscillation Spectroscopic Survey
The Sloan Digital Sky Survey (SDSS) uses a 2.5-meter telescope with a wider field of view than any other large telescope, located on a mountaintop in New Mexico called Apache Point and devoted solely to mapping the universe. We now know that some three-quarters of the universe consists of dark energy, whose very existence was unsuspected when telescope construction began in 1994 and still controversial when the first Sloan survey started in 2000.
Hackers 'find black hole in atom smasher computers'
Hackers claim they have broken into the computer system of the Large Hadron Collider, the mega-machine designed to expose secrets of the cosmos, British newspapers reported on Saturday.
Maxwell's demons may drive some biological systems
(PhysOrg.com) -- According to the second law of thermodynamics, entropy always increases. For example, two bodies of different temperatures, when brought into contact, will eventually mix together to result in a uniform temperature.
Hawking bets CERN mega-machine won't find 'God's Particle' (Update)
Renowned British astrophysicist Stephen Hawking has bet 100 dollars (70 euros) that a mega-experiment this week will not find an elusive particle seen as a holy grail of cosmic science, he said Tuesday.
Countdown starts in quest to pierce secrets of Universe
Particle physicists believe they will throw open a new frontier of knowledge on Wednesday when, 100 metres (325 feet) below ground, they switch on a mega-machine crafted to unveil the deepest mysteries of matter.
Michigan integral to world's largest physics experiment
After 20 years of construction, a machine that could either verify or nullify the prevailing theory of particle physics is about to begin its mission. CERN's epic Large Hadron Collider (LHC) project currently involves 25 University of Michigan physicists and students. More than 100 U-M researchers have been involved in the project over the years. CERN is the European Organization for Nuclear Research, located in Geneva, Switzerland.
Physicists Rule Out the Production of Dangerous Black Holes at the LHC
(PhysOrg.com) -- On August 8, the world's largest particle accelerator, the Large Hadron Collider near Geneva, Switzerland, began the process of slowly throttling to full power. When its proton beams are circling at full speed and collisions begin, scientists from around the world will finally be able to start collecting data.

News discussion:

Physics news