New tool for proton spin

May 06, 2011
Figure 1: When two beams of protons collide (center), the electrons (e-) produced by the W- boson-mediated interactions provide information about the different ‘flavor’ quarks of a proton, such as the anti-up (ū) quark and the down (d) quark. Credit: Brookhaven National Laboratory

How the particles that constitute a proton give rise to is to its rotation, or ‘spin’, is an intriguing open question of contemporary particle physics. A technique that could provide some answers has been developed using the world’s only polarized proton–proton collider. The work was published by the PHENIX Collaboration, which includes researchers from the RIKEN Brookhaven National Laboratory (BNL) Research Center in Upton, USA.

Nowadays, the most popular theory for subatomic particles is the Standard Model: a menagerie of fundamental particles including quarks, which come in six different types or flavors, and four fundamental forces. These forces include the ‘weak’ force that is mediated by called W bosons, which are created, albeit only briefly, when protons collide. The researchers discovered that these W bosons are a sensitive probe of the quarks that make up a

To investigate proton spin, the PHENIX team fired two beams of high-energy protons at one another using the Relativistic Heavy Ion Collider at BNL. “Most of the interactions that take place when the protons collide are ‘strong’ interactions,” explains Okada. “But our experiment was sensitive enough to detect ‘weak’ interactions too.” The researchers identified two such weak reactions: detection of an electron indicated the decay of a negatively charged W boson (Fig. 1); and detection of a positron—a positively charged electron—indicated the decay of a positively charged W boson. By counting the number of resulting electrons and positrons, the researchers could calculate the probability of each type of interaction.

The PHENIX team then performed two experiments simultaneously. In one, they made protons spin parallel to the axis of the beam; and in the other, they made them spin in the opposite direction. The difference in the rate of weak interactions in each experiment provided information about the spin direction of the quarks in the proton. “The asymmetry of the production rates is connected to the probability that the spin of a particular flavor of quark is aligned to the proton spin direction,” says Okada. This approach could soon be extended to identify the spin contribution of all the proton’s quarks.

Next the team hopes to improve the sensitivity of the experiment. “This time, we only caught electrons and positrons that emerged at 90 degrees to the beam axis,” explains Okada. “We are preparing new detectors to extend this detection region for a more complete analysis.”

Explore further: New insights found in black hole collisions

More information: Adare, A., et al. Cross section and parity-violating spin asymmetries of W± boson production in polarized p + p collisions at √s = 500 GeV. Physical Review Letters 106, 062001 (2011).

Related Stories

How does the proton get its spin?

Feb 17, 2010

( -- At a meeting this week of the American Physical Society in Washington, MIT Associate Professor of Physics Bernd Surrow reported on new results from the STAR experiment at the Relativistic ...

Protons - Everything Revolves Around Spin

Dec 17, 2007

Current understanding of the spin structure of protons has been summarised in a single book for the first time. The book examines attempts to solve one of the greatest puzzles of physics. Models and experiments ...

Seeking Answers to the Puzzle of Proton Spin

Apr 24, 2006

Thanks to a series of machine upgrades, researchers at the Relativistic Heavy Ion Collider (RHIC), the newest and largest particle accelerator at the U.S. Department of Energy's Brookhaven National Laboratory, ...

Moving Quarks Help Solve Proton Spin Puzzle

Sep 11, 2008

( -- New theory work at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility has shown that more than half of the spin of the proton is the result of the movement of its building blocks: ...

Quarks take wrong turns

Apr 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, ...

Recommended for you

New idea for Dyson sphere proposed

39 minutes ago

(—A pair of Turkish space scientists with Bogazici University has proposed that researchers looking for the existence of Dyson spheres might be looking at the wrong objects. İbrahim Semiz and ...

Turning back time by controlling magnetic interactions

1 hour ago

In many materials, macroscopic magnetic properties emerge when microscopically small magnets align in a fixed pattern throughout the whole solid. In a publication in Nature Communications, Johan Mentink, Karste ...

New insights found in black hole collisions

Mar 27, 2015

New research provides revelations about the most energetic event in the universe—the merging of two spinning, orbiting black holes into a much larger black hole.

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

1 / 5 (1) May 06, 2011
An alternative and simpler model of the proton even predicts its 1/2-spin:
1 / 5 (1) May 07, 2011

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.