Along with the neutron, the proton is the basic building block of the atomic nucleus. But, the source of the proton’s spin—a fundamental property, along with charge—has been a long standing puzzle to scientists.
By analyzing data from the BNL Relativistic Heavy Ion Collider (RHIC), the PHENIX collaboration at Brookhaven National Laboratory in Upton, USA, including scientists from the RIKEN BNL Research Center and the RIKEN Nishina Center for Accelerator-Based Science, has now ruled out gluons—the particles that exchange the so-called strong force between the quarks that make up the proton—as the dominant contributor to proton spin.
There are three quarks—two ‘up’ quarks and one ‘down’ quark— in the proton. Each quark has a spin of ½ and they configure with respect to each other—two up and one down—so the sum of their spins is ½. For this reason, scientists thought the spin of the proton, which is also ½, came entirely from quarks.
Experiments in the 1980s, however, showed that quarks only contribute about 25% of the total spin of the proton. “This result was so surprising that it was called the ‘spin-crisis’,” says Yasuyuki Akiba, a member of the PHENIX team. Particle physicists were therefore confronted with a fundamental question: What else contributes to the spin of the proton?
Some models predict that the missing spin comes mainly from gluons, while others suggest that the contribution from the orbital angular momentum of quarks within the proton may also be significant. To determine the contribution from gluons, the PHENIX team analyzed data from a year-long experiment performed at RHIC in 2006.
In this experiment, protons were collided at very high energies approximating 200 GeV, where 1 GeV is equal to one billion electron volts, to produce particles called pions. The researchers derived the gluon contribution to the proton spin from the difference between the number of pions produced by colliding protons with their spins lined up in the same direction to one another versus those lined up in opposite directions.
The important result from the PHENIX team’s work is that the gluon contribution to the proton spin is actually small. “The best estimate is that it is about 40%, but the data don’t rule out that it is zero,” explains Akiba. “Although there is still a significant uncertainty in this result, our data show that models predicting large gluon spin can now be firmly excluded.”
The corresponding author for this highlight is based at the Experimental Group, RIKEN BNL Research Center
Adare, A., Afanasiev, S., Aidala, C., Ajitanand, N.N., Akiba, Y., Al-Bataineh, H., Alexander, J., Aoki, K., Aphecetche, L., Asai, J. et al. (PHENIX Collaboration) Gluon-spin contribution to the proton spin from the double-helicity asymmetry in inclusive π0 production in polarized p + p collisions at √ｓ = 200 GeV. Physical Review Letters 103, 012003 (2009)
Saeko Okada | Research asia research news
Astronomers release most complete ultraviolet-light survey of nearby galaxies
18.05.2018 | NASA/Goddard Space Flight Center
A quantum entanglement between two physically separated ultra-cold atomic clouds
17.05.2018 | University of the Basque Country
So-called quantum many-body scars allow quantum systems to stay out of equilibrium much longer, explaining experiment | Study published in Nature Physics
Recently, researchers from Harvard and MIT succeeded in trapping a record 53 atoms and individually controlling their quantum state, realizing what is called a...
The historic first detection of gravitational waves from colliding black holes far outside our galaxy opened a new window to understanding the universe. A...
A team led by Austrian experimental physicist Rainer Blatt has succeeded in characterizing the quantum entanglement of two spatially separated atoms by observing their light emission. This fundamental demonstration could lead to the development of highly sensitive optical gradiometers for the precise measurement of the gravitational field or the earth's magnetic field.
The age of quantum technology has long been heralded. Decades of research into the quantum world have led to the development of methods that make it possible...
Cardiovascular tissue engineering aims to treat heart disease with prostheses that grow and regenerate. Now, researchers from the University of Zurich, the Technical University Eindhoven and the Charité Berlin have successfully implanted regenerative heart valves, designed with the aid of computer simulations, into sheep for the first time.
Producing living tissue or organs based on human cells is one of the main research fields in regenerative medicine. Tissue engineering, which involves growing...
A team of scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg investigated optically-induced superconductivity in the alkali-doped fulleride K3C60under high external pressures. This study allowed, on one hand, to uniquely assess the nature of the transient state as a superconducting phase. In addition, it unveiled the possibility to induce superconductivity in K3C60 at temperatures far above the -170 degrees Celsius hypothesized previously, and rather all the way to room temperature. The paper by Cantaluppi et al has been published in Nature Physics.
Unlike ordinary metals, superconductors have the unique capability of transporting electrical currents without any loss. Nowadays, their technological...
02.05.2018 | Event News
13.04.2018 | Event News
12.04.2018 | Event News
18.05.2018 | Power and Electrical Engineering
18.05.2018 | Information Technology
18.05.2018 | Information Technology