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Volume 352 - XXVII International Workshop on Deep-Inelastic Scattering and Related Subjects (DIS2019) - Working Group 2: Low-x and Diffraction
Transverse Spin Asymmetries in the $p^{\uparrow}p \to p\pi^{0} X$ Process at STAR
C. Dilks* on behalf of the STAR collaboration
*corresponding author
Full text: pdf
Pre-published on: 2019 July 19
Published on: 2019 October 04
A significant sample of $p^{\uparrow}p\to p\pi^0 X$ events has been observed at STAR
in $\sqrt{s}=200$ GeV transversely polarized $pp$ collisions, where an isolated
$\pi^0$ is detected in the forward pseudorapidity range $2.65<\eta<3.9$ along
with the forward-going proton $p$, which scatters with a near-beam forward
pseudorapidity into Roman Pot detectors. The sum of the $\pi^0$ and the
scattered proton energies is consistent with the incident proton energy of 100
GeV, indicating that no further particles are produced in this direction. It is
postulated that the forward incident proton may have fluctuated into a $p+\pi^0$
system, with an angular momentum correlated with the initial proton spin. The
backward-going proton interacts with the $p+\pi^0$ system, which then separates
such that the $\pi^0$ has a transverse momentum of ${\sim}2$ GeV/$c$ and the
proton has a transverse momentum of ${\sim}0.2$ GeV/$c$, while the backward
proton shatters into the remaining particles $X$. Correlations between the
$\pi^0$ and scattered proton will be presented, along with single-spin
asymmetries which depend on the azimuthal angles of both the pion and the
proton. This is the first time that spin asymmetries have been explored for
this process, and a model to explain their azimuthal dependence is needed.
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