Single spin asymmetries in transversely polarized proton(antiproton) - proton inclusive processes

TL;DR

The paper extends a QCD-based framework to include intrinsic transverse momentum and a Sivers-like spin-dependent distribution, fixing its parameters from $p^\uparrow p \to \pi X$ data. It then applies the model to $\bar{p}^\uparrow p \to \pi X$, $p^\uparrow (\bar{p}^\uparrow) p \to \gamma X$, and $p^\uparrow p \to K X$, obtaining parameter-free predictions for several channels and showing reasonable agreement with existing measurements. The study highlights how fragmentation-function choices, especially for kaons, influence predicted SSAs and proposes specific measurements to test the mechanism. Overall, it supports a higher-twist, $k_{\perp}$-dependent extension of factorization as a viable description of SSA in high-energy hadronic processes.

Abstract

We consider several single spin asymmetries in inclusive, transversely polarized proton(antiproton) - proton processes as higher twist QCD contributions, taking into account spin and intrinsic transverse momentum effects in the quark distribution functions. This approach has been previously applied to the description of the single spin asymmetries observed in transversely polarized proton - proton -> pion + X reactions and all its parameters fixed: we give here predictions for new processes, which agree with experiments for which data are available, and suggest further possible measurements.

Figures (16)

• Figure 1: Fit of the data on $A_N$ for the process $p^\uparrow p\to\pi\,X$exp1, with the parameters given in Eq. (\ref{['par']}); the upper, middle, and lower sets of data and curves refer respectively to $\pi^+$, $\pi^0$, and $\pi^-$.
• Figure 2: Single spin asymmetries $A_N$ for the process $\bar{p}^\uparrow p\to\pi\,X$exp2; the lower, middle, and upper sets of data and curves refer respectively to $\pi^+$, $\pi^0$, and $\pi^-$.
• Figure 3: Single spin asymmetry for the process $p^\uparrow p\to\pi^0\,X$ at fixed $x_F$, as a function of $p_T$; experimental data, at $|x_F| \leq 0.15$, are from Ref. exp3; the solid curve shows our corresponding theoretical prediction at $x_F=0$.
• Figure 4: Single spin asymmetry for the process $p^\uparrow (\bar{p}^\uparrow) \, p\to\gamma\,X$; experimental data, at $|x_F| \leq 0.15$ and $2.5 < p_T < 3.1$ GeV/$c$, are from Ref. exp4. The curves show our corresponding theoretical predictions at $p_T=2.5$ GeV/$c$; the solid curve refers to the $p^\uparrow p\to\gamma\,X$ process, the dashed curve to the $\bar{p}^\uparrow p\to\gamma\,X$ case.
• Figure 5: Predicted single spin asymmetries for the process $p^\uparrow p\to K\,X$, with the set of kaon FF's BKK1 bkk1; kinematical conditions are the same as for the pion case, at $p_T=1.5$ GeV/$c$. The solid, dashed, dot-dashed, double dot-dashed curves refer respectively to the $K^+$, $K^-$, $K^0$, $K^0_S$ cases. Results for $\bar{K}^0$ meson are very similar to those for $K^-$ case.