Seeking Answers to the Puzzle of Proton Spin

Each year since 2002, RHIC has taken a short break from its primary mission -- slamming ions together in the quest for understanding the state of matter at the birth of the universe -- to explore this question. This year, however, the entire experimental run will be dedicated to understanding proton spin, as researchers collide beams of polarized protons (protons that are spinning in the same direction) at various collision energies. During the last week of the 20-week run, polarized protons will be accelerated to the highest energies ever recorded -- 250 billion electron volts (GeV), or two-and-a-half times the typical proton collision energies studied at RHIC.

“We are able to achieve these high collision energies because of the improvements we have made to the accelerator complex over the past few years,” said Mei Bai, an accelerator physicist in Brookhaven’s Collider-Accelerator Department. “These improvements make it possible for the proton bunches to retain a high percentage of polarization at high energies, which is required for our physics measurements.”

Scientists know that protons are made of smaller particles called quarks, which are held together by emitting and absorbing particles called gluons. The physicists also know that all of these particles have an intrinsic property known as spin, somewhat analogous to Earth spinning on its axis. Physicists have long thought that the spin of a proton was simply the sum of the spins of its three component quarks. But experiments have shown that the quarks account for only about 20 percent of the proton’s spin. That has left the scientists with a puzzle: What accounts for the missing 80 percent?

Scientists believe that the gluons that hold the quarks together may contribute to the proton's spin, and the orbital motion of the quarks around each other, like the bodies in a solar system, can also add to the spin. RHIC is the world's only machine capable of colliding high-energy beams of polarized protons, and is a unique tool for separating these contributions and answering the question.

Bai explained that keeping the beams polarized as they make their way through the chain of five accelerators that make up the RHIC complex is incredibly difficult. Four of these accelerators are composed of magnets that can interfere with polarization, and there is no way to “re-polarize” the protons once their spin is disrupted. So, the goal is to minimize, as much as possible, polarization loss along the protons’ path.

Specialized sets of magnets called “Siberian snakes” make this possible. The magnets’ corkscrew-like design causes the direction of the magnetic field to spiral along the direction of the beam. As the beam moves through the two snakes in each of RHIC’s 2.4-mile-circumference rings, the magnetic field flips the polarization, or direction of spin, and simultaneously averages out many smaller effects of RHIC’s accelerator magnets, which otherwise would depolarize the proton beams. This allows the scientists to maintain stable polarization of the beam.

This setup has been in place since 2002, but since the snakes were all located at the “end of the line” in RHIC, they did not address polarization loss occurring in the Alternating Gradient Synchrotron (AGS), the accelerator that injects the beams into RHIC’s two rings. So, over the past two years, two additional partial snakes were installed in the AGS to minimize loss in that machine. “These two partial snakes work like a charm, and we’ve seen a big improvement in luminosity” said Bai. “The AGS is now much more stable than it used to be, and we are able to accelerate proton beams of much higher bunch intensity with much higher polarization transmission efficiency.”

In the last two proton runs at RHIC, researchers got their first exciting glimpses of results that depend on the contribution of gluons to the proton's spin. During this run they expect to obtain important data on this contribution and also begin looking at the contribution made by the proton’s orbital motion. The stage will also be set for running protons at much higher energies, so these studies can be carried much further in future runs.

“We have made great progress, but we still have room to improve,” said Bai. “In a perfect world, we would be able to correct for all the potential depolarization mechanisms. I’m very confident that within the next few years, we should be able to achieve our goals.”

Bai will discuss the RHIC complex improvements and the performance of RHIC as a polarized-proton collider at the American Physical Society April meeting in Dallas, Texas.

RHIC is funded primarily by the Office of Nuclear Physics within the U.S. Department of Energy’s Office of Science.

Source: Brookhaven National Laboratory

eRHIC gets to the heart of the matter
At the U.S. Department of Energy's Brookhaven National Laboratory, scientists have proposed a new way of studying the structure of matter down to a level never before observed. Their proposal is the "eRHIC" collider, a planned upgrade to Brookhaven's giant particle accelerator, the Relativistic Heavy Ion Collider (RHIC).
Thomas Roser Wins 2005 Particle Accelerator Science and Technology Award
Thomas Roser, Associate Chair for Accelerators and Accelerator Division Head at the U.S. Department of Energy’s Brookhaven National Laboratory, has been selected as one of two recipients of the 2005 Particle Accelerator Science and Technology Award from the Nuclear and Plasma Science Society of the Institute of Electrical and Electronics Engineers, Inc. The award, which consists of a plaque and $2,000 for each recipient, will be given to Roser and to Ronald Davidson of Princeton University, on May 18, at the biennial particle accelerator conference to be held in Knoxville, Tennessee.
Racing Ahead at the Speed of Light
Imagine trying to catch up to something moving close to the speed of light - the fastest anything can move - and sending ahead information in time to make mid-path flight corrections. Impossible? Not quite. Physicists at the Relativistic Heavy Ion Collider (RHIC), a particle accelerator at the U.S. Department of Energy's Brookhaven National Laboratory, have achieved this tricky task - and the results may save the Lab money and time in their quest to understand the inner workings of the early universe.
New Materials for Making 'Spintronic' Devices
An interdisciplinary group of scientists at the U.S. Department of Energy's Brookhaven National Laboratory has devised methods to make a new class of electronic devices based on a property of electrons known as "spin," rather than merely their electric charge. This approach, dubbed spintronics, could open the way to increasing dramatically the productivity of electronic devices operating at the nanoscale.
Direct Photon Properties Reveal Secrets of Extreme Nuclear States
When atomic nuclei are smashed together at great speed, resulting temperatures exceed one trillion degrees, 200 million times hotter than the surface of the sun. Scientists who study nuclear matter under extreme conditions have a particular interest in the properties of particles of light called photons, which reveal valuable information because they don’t interact strongly with other particles following a nuclear collision.
U.S., China cooperate on high-energy physics experiment
With $5 million in new U.S. funding and a $2 million commitment from the Chinese Academy of Sciences, Rice University physicists are organizing the joint U.S.-Chinese production of an innovative set of detectors for one of the Department of Energy's (DOE) premier particle accelerators, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) on Long Island, New York. The project represents the most significant collaboration to date between the U.S. and China in high-energy particle physics detector research.
Detecting the Traces of Mystery Matter
Using high-speed collisions between gold atoms, scientists think they have re-created one of the most mysterious forms of matter in the universe -- quark-gluon plasma. This form of matter was present during the first microsecond of the Big Bang and may still exist at the cores of dense, distant stars.
Argonne-designed instruments vital in RHIC discovery
Argonne researchers played a significant role in research that led to the surprising finding of a possible ideal liquid instead of the expected quark-gluon plasma at Brookhaven National Laboratory's Relativistic Heavy-Ion Collider (RHIC).
On April 18, each of the four major experiments at RHIC released white papers summarizing the first four years of RHIC operation and their findings from high-energy collisions of gold nuclei. At the incredibly high temperatures and pressures created in the collisions, physicists expected to create "quark-gluon plasma" - a gaseous state of matter thought to have existed in the first few microseconds after the Big Bang. Instead, the matter created inside the detectors behaved like a liquid - a completely unexpected result.

Seeking Answers to the Puzzle of Proton Spin

Related stories:

News discussion: