Single spin asymmetries A_N in the inclusive production of charged pions in the reactions $\pi^{-}p^{\uparrow}\rightarrow\pi^{\pm}X$ were measured by the SPASCHARM experiment [1] at the U-70 accelerator complex in Protvino. The single-spin asymmetry in the reaction $\pi^{-}p^{\uparrow}\rightarrow\pi^{\pm}X$ was measured for the first time.

The single-spin asymmetries in both reactions, averaged over the transverse momentum range from 0.2 to 1.5 GeV/c, have the same positive sign. Taking into account systematic errors they are equal to each other in magnitude of ~20% and differ from zero by 3σ. Previously, asymmetries of similar sign and magnitude were measured in inclusive π⁰-meson production in various reactions [2].

Such a possible behavior was discussed earlier and predicted by the model of chromomagnetic polarization of quarks [3] and twist-3 model of asymmetry [4].

Dependence of A_𝑁 on x_F (left) and 𝑝_𝑇 (right) in the reactions $\pi^{-}p^{\uparrow}\rightarrow\pi^{\pm}X$ at 26.3 GeV (only statistic errors are shown, systematic errors are estimated as 5÷7 %).

1. V.V. Abramov et al, Phys. Part. Nucl. 54(1), 69 (2023).

2. V.D. Apokin et al.  Sov. J. Nucl. Phys. 49, 103 (1989); V.V. Mochalov et al, Phys. Atom. Nucl. 73(12), 2017 (2010); V.V. Abramov et al, Phys. Atom. Nucl. 77(5), 595 (2014).

3. V.V. Abramov, Phys. Atom. Nucl. 72(11), 1872 (2009).

4. A.V. Efremov, V.M. Korotkiyan, and O.V. Teryaev, Phys. Lett. B 348, 577 (1995).

V.V. Moiseev, V.V. Abramov, I.G. Alekseev et al.
JETP Letters 123, issue 11 (2026)