Quarks take wrong turns

April 13, 2004

Physicists peering inside the neutron are seeing glimmers of what appears to be an impossible situation. The vexing findings pertain to quarks, which are the main components of neutrons and protons. The quarks, in essence, spin like tops, as do the neutrons and protons themselves.
Now, experimenters at the Thomas Jefferson National Accelerator Facility in Newport News, Va., have found hints that a single quark can briefly hog most of the energy residing in a neutron, yet spin in the direction opposite to that of the neutron itself, says Science News

"That's very disturbing," comments theoretical physicist Xiangdong Ji of the University of Maryland at College Park.

The finding suggests that scientists may have erred in calculations using fundamental theory to predict quark behavior within neutrons, he says. It might also indicate that orbital motions of particles within neutrons, in addition to those particles' spins, are more important than previously recognized. Those motions might be akin to the moon's rotation around Earth as the satellite also spins about its own axis.

Given that neutrons and protons are sister particles, called nucleons, the new findings apply to both, says Xiaochao Zheng, a member of the experimental team who's now at Argonne (Ill.) National Laboratory.

Nucleons are the building blocks of atomic nuclei. A typical nucleon includes three quarks: two down quarks and one up quark for a neutron; two up quarks and one down quark for a proton. In addition to those so-called valence quarks, each nucleon contains multitudes of gluons—particles that bind quarks—and of short-lived quark-antiquark pairs, known collectively as the quark sea.

Each of these constituents of a nucleon carries some share of the nucleon's energy, although the distribution of that energy among the constituents constantly shifts, Ji explains.

From previous experiments, scientists knew that only valence quarks can grab major portions of a nucleon's total energy content, says Jian-Ping Chen of the Jefferson lab, a coleader of the experiment. The new spin-detecting experiment is the first to measure the state of the neutron when most of its energy momentarily resides in a single quark.

Calculations based on the prevailing theory of quark behavior predict that any quark holding more than about half the energy of a nucleon should spin in the same direction as the nucleon. However, when the new experiment probed valence quarks temporarily laden with up to 60 percent of a neutron's energy, it revealed that only the up quarks behaved as expected. The down quarks somehow carried most of the energy yet rotated in a direction opposite to that of the neutron as a whole.

Electrons and entire atoms also have spins. To arrive at the new findings, the experimenters made a target of helium gas in which nearly all atoms were forced to spin in the same direction and bombarded it with a beam of high-energy electrons, whose spins were also forced to have uniform orientations.

The researchers determined the spin orientations of the quarks in the helium atoms by placing detectors in specific positions where they are more likely to make detections when the orientations of the electron's spin and the quark's spin are opposite, Zheng says. She and her colleagues present their results in the Jan. 9 Physical Review Letters.

Explore further: Exploring the Higgs boson's dark side

Related Stories

Exploring the Higgs boson's dark side

July 21, 2015

In 2012 CERN's Large Hadron Collider (LHC) discovered the Higgs boson, the 'missing piece' in the jigsaw of particles predicted by the Standard Model.

The ins and outs of quantum chromodynamics

July 7, 2015

Quarks and antiquarks are the teeny, tiny building blocks with which all matter is built, binding together to form protons and neutrons in a process explained by quantum chromodynamics (QCD).

Recommended for you

The sound of music, according to physicists

July 30, 2015

Joshua Bodon is sick of hearing "Somewhere Over the Rainbow." More specifically, he's sick of hearing one 25-second clip of the song repeated more than 550 times.

Unusual red arcs spotted on icy Saturn moon Tethys

July 30, 2015

Like graffiti sprayed by an unknown artist, unexplained arc-shaped, reddish streaks are visible on the surface of Saturn's icy moon Tethys in new, enhanced-color images from NASA's Cassini spacecraft.

Power grid forecasting tool reduces costly errors

July 30, 2015

Accurately forecasting future electricity needs is tricky, with sudden weather changes and other variables impacting projections minute by minute. Errors can have grave repercussions, from blackouts to high market costs. ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.