Paul H. Wrote:
-------------------------------------------------------
> The article is in English and I still need a
> translation to understand it.
>
> Direct Photons Point to Positive Gluon
> Polarization
> Brookhaven National Laboratory, June 21, 2023
> [www.interactions.org]

This is how Chat summarised it:

***

The article discusses a new analysis conducted by the PHENIX Collaboration at the Relativistic Heavy Ion Collider (RHIC) that sheds light on the alignment of gluon "spins" with the spin of the proton they are in. Gluons are particles that hold quarks together within protons and neutrons. The study, published in Physical Review Letters, provides evidence that gluon spins align in the same direction as the proton's spin, contributing to its overall spin value.

The research addresses a longstanding question regarding the contribution of gluons to proton spin. Previous experiments revealed that quarks alone cannot account for the total spin value of protons, leading to a "spin crisis" in the field. The hypothesis was that gluons might carry the remaining spin.

The study took place at the RHIC facility at Brookhaven National Laboratory and involved collaborations between various institutions, including Stony Brook University and Japan's RIKEN laboratory. The researchers conducted measurements of "direct photons," which are particles of light emitted during collisions of polarized protons. By comparing the number of direct photons produced when the protons' spins were aligned versus when they were opposite, the researchers could determine the alignment of gluon spins. The results indicated that gluon spins align in the same direction as the proton spin, making a significant contribution to its overall spin value.

The analysis required sophisticated techniques to isolate direct photons from other photon sources. The PHENIX experiment's finely grained electromagnetic calorimeter played a crucial role in differentiating direct photons from those generated through other processes. By eliminating background photons and applying statistical analysis, the researchers obtained a sufficient number of direct photons for their analysis.

While the findings provide insights into gluon spin alignment, they do not fully solve the proton spin mystery. The remaining contribution to proton spin likely arises from the orbital motion of quarks and gluons within protons, which cannot be precisely measured using collisions of longitudinally polarized protons. The upcoming Electron-Ion Collider (EIC), a facility for nuclear physics research, is expected to provide higher precision measurements of gluon contributions to spin by colliding polarized electrons with protons. The EIC experiments will complement the RHIC findings and offer a more comprehensive understanding of the proton spin puzzle.

**

As far as I can tell, the scientists want to determinie the alignment of gluon spins (well, I'm sure we all do), although apparently that's not the whole anwer to the proton spin question.

Hermione
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