PWGC Preview Presentation -- 10.20.17 -- https://drupal.star.bnl.gov/STAR/system/files/102017pwgcProposal.pdf

Analysis Homepage: http://www.star.bnl.gov/protected/spin/dilks/paper_FMS_aLL_510GeV/index.html

Current Draft: http://www.star.bnl.gov/protected/spin/trent/aps.pdf

Analysis Note: https://drupal.star.bnl.gov/STAR/blog/dilks/all-analysis-note-0

Analysis Code: https://drupal.star.bnl.gov/STAR/blog/dilks/all-analysis-code

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Title: Longitudinal Double-Spin Asymmetries for $\pi^{0}$s in the Forward Direction for 510 GeV Polarized pp Collisions

PWG: Spin / Cold QCD

PAs: Chris Dilks, Steve Heppelmann, Stephen Trentalange

Target Journal: Physical Review D

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Abstract:

The STAR Collaboration reports measurements of the longitudinal double-spin
asymmetry, $A_{LL}$, for neutral pions produced at forward directions in polarized
proton-proton collisions, at a center-of-mass energy of 510~GeV.  Results cover the
transverse momentum range 2.0$< p_{T} <$~10~GeV/c and two regions in the
pseudorapidity range 2.6$< \eta <$4.0.  The observed asymmetries are generally less
than $\pm$5$\cdot$10$^{-3}$ and are sensitive to the polarized gluon parton
distribution function, $\Delta g(x)$, in the region of Bjorken $x\lesssim$10$^{-3}$.
When combined with other measurements as part of a global analysis, these data are
expected to help constrain the contribution to the spin of the proton from polarized
gluons at low $x$.

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Physics Goals:

The spin 1/2 of the proton is now known to be composed of the following components:
quark/antiquark spin, gluon spin, and orbital angular momentum. The quark/antiquark
spin contribution is ~25%, the gluon spin contribution is still coming into focus,
and the orbital angular momentum has yet to be measured. Regarding the gluon spin
contribution, it is roughly 0.2 integrated over the high-x range of 0.05 to 1,
however it is not well-constrained at lower values of x.

This analysis focuses on neutral pion production in the forward pseudorapidity range
of 2.6-4.0, a process which is dominated by high-x quarks colliding with low-x soft
gluons; consequently, analyzing forward particle production provides access to the
low-x gluon polarization contribution to the proton spin. The asymmetry presented in
this analysis, the longitudinal double-spin asymmetry A_LL, is sensitive down to
x~0.001. In turn, it will help to provide low-x constraints on the gluon
polarization.

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Figures:

(to enlarge any figure, right click and open image as new tab)

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Conclusion:

The longitudinal double-spin asymmetry measurement presented in this analysis is
consistent with zero, within the shown statistical and systematic uncertainties. The
uncertainties form a constraint on the size of this asymmetry, which can be used,
along with measurements of A_LL in intermediate and central regions, in a global fit
analysis to improve constraints on gluon polarization at low-x.

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Main Presentations:

- Preliminary Release at SPIN2014 ( presentation + proceedings )
- STAR Collaboration Meeting -- Winter 2016 @ BNL
- STAR Collaboration Meeting -- Summer 2016 @ OSU
- STAR Collaboration Meeting -- Spring 2017 @ BNL

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Documentation:

- Analysis Note / Thesis --- in progress
- Analysis Code --- currently in a private gitlab repository; contact PAs for access