Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Jefferson Lab experiments shed light on proton spin mystery

04.07.2002


It’s a conundrum that’s confounded the curious for several decades. In the past, some called it a crisis. More recently, it’s come to be known as a puzzle: a mystery that has occupied the minds of thousands of researchers worldwide.



Call it the Case of the Missing Spin. A mathematical property of all subatomic particles, including quarks, spin is roughly equivalent to the physical rotation of an object in the macroscopic world.

Physicists have long wondered how the properties (including the spin) of the protons and neutrons inside an atomic nucleus can be explained in terms of quarks, their most elementary building blocks.


Although seemingly limited to the realm of the infinitesimal, those properties eventually affect all things of "normal" size, since ordinary matter is comprised of these smallest building blocks. Now, a series of experiments recently concluded in Hall B may be leading researchers toward a better understanding of the spin of protons and neutrons.

"The effort in the past has always been to see how much of the proton spin was coming from the quarks," says Volker Burkert, Hall B senior scientist and an EG1 spokesperson. "The goal was and remains learning about the internal structure of protons and neutrons. The extremely surprising thing we found out is that the spin of the quarks is not contributing much to the proton or neutron spin: maybe 25 percent, or even less. That is contrary to the expectation of most scientists working in the field, and is unexplained in simple models."

In a landmark program called "EG1" (comprised of several related experiments) scientists used the sophisticated particle detector at the heart of Jefferson Lab’s Hall B to acquire a much more detailed picture of the proton’s internal structure. Called CLAS, for CEBAF Large Acceptance Spectrometer, the detector’s components include time-of-flight counters, energy-measuring calorimeters and particle-tracking drift chambers. CLAS records, on average, 2,200 particle interactions per second on 40,000 data channels. And during EG1, there were times the detector actually recorded up to 4,000 interactions per second.

To derive their EG1 data, researchers collected 26 billion "events," or interactions, over the total running time of 10 months. The first series of experiments, EG1a, took three months and concluded in December 1998, with 3 billion events on tape. The second run, EG1b, took seven months to complete, ending this past April 20 and recording 23 billion events.

"By the time we analyze the data we just took, we should end up making a major contribution to the understanding of proton structure," says Sebastian Kuhn, the EG1 experimental coordinator and associate professor of physics at Old Dominion University. "What we’re looking at is the transition between the microscopic and the submicroscopic."

The medium view

Seen individually, at high resolution and at small distances, quarks are point-like, and appear independent from each other. But subatomic particles (like protons) are essentially "built" at medium distances; that is, the interactions of one or more quark varieties at intermediate ranges determine the structures of protons and neutrons. It is at these distances that the quarks are coupled to one another, like springs in a mattress that move together in response to weight.

Scientists think that the "missing" spin could in fact be hidden in plain view, a component of the complex, extended structures that include quarks and the quark-binding particles known as gluons. As quarks move around, they may exchange gluons at medium distances. By understanding such interactions, and how they determine and affect spin, scientists will have a far clearer knowledge of how protons and neutrons hold together inside a nucleus, enabling atoms, and eventually molecules, to form and endure.

"If you think of a motor or a watch, you can take it apart and see all the pieces," Kuhn says. "But that doesn’t tell you how the watch works. You need to know how all the parts work together. That’s the point of the EG1 experiments, which probe medium distances so we see what’s happening in the transition region, where quarks are no longer free and individual, but conspire to form protons and their resonant excited states."

A technical challenge and accomplishment of the EG1 experiments was the construction of a polarized target with which photons generated during the experiment could interact, producing far more useful data than was the case in past attempts. Polarization refers to the alignment of spin, insuring that particles within an atomic nucleus spin in the same direction. An international collaboration of researchers from JLab, and universities in Virginia and Italy used ammonia that, when place in a strong magnetic field, created the desired polarization.

"This experiment was a massive effort, involving more than 100 collaborators preparing the apparatus, planning each step of the experiment, and taking data, around the clock, for 7 months," Kuhn comments. This work will be matched by an equally large effort to analyze the data stored on computer tapes and to extract the vast amount of information they contain.

"This work will go on for several years (at least) and should enable us to take the next big step in our understanding of proton and neutron structure. Additional experiments are ongoing or planned in all three halls at Jefferson Lab," Volker Burkert notes. "Together with the results from many other JLab experiments, and utilizing new developments in theory, we hope to make a major contribution toward a fundamental understanding of the nucleon structure from the smallest to the largest distance scale. This is exactly the kind of science Jefferson Lab was built to do."

Linda Ware | EurekAlert!

More articles from Physics and Astronomy:

nachricht Neutron star merger directly observed for the first time
17.10.2017 | University of Maryland

nachricht Breaking: the first light from two neutron stars merging
17.10.2017 | American Association for the Advancement of Science

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Breaking: the first light from two neutron stars merging

Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.

Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....

Im Focus: Smart sensors for efficient processes

Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).

When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...

Im Focus: Cold molecules on collision course

Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.

How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...

Im Focus: Shrinking the proton again!

Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.

It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...

Im Focus: New nanomaterial can extract hydrogen fuel from seawater

Hybrid material converts more sunlight and can weather seawater's harsh conditions

It's possible to produce hydrogen to power fuel cells by extracting the gas from seawater, but the electricity required to do it makes the process costly. UCF...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ASEAN Member States discuss the future role of renewable energy

17.10.2017 | Event News

World Health Summit 2017: International experts set the course for the future of Global Health

10.10.2017 | Event News

Climate Engineering Conference 2017 Opens in Berlin

10.10.2017 | Event News

 
Latest News

Plant escape from waterlogging

17.10.2017 | Life Sciences

Study suggests oysters offer hot spot for reducing nutrient pollution

17.10.2017 | Life Sciences

Breaking: the first light from two neutron stars merging

17.10.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>