Comparison of transverse single-spin asymmetries for forward $\pi^{0}$ production in polarized $pp$, $p\rm{Al}$ and $p\rm{Au}$ collisions at nucleon pair c.m. energy $\sqrt{s_{\mathrm{NN}}}= 200$ GeV

The STAR collaboration
Phys.Rev.D 103 (2021) 072005, 2021.

Abstract
The STAR Collaboration reports a measurement of the transverse single-spin asymmetries, $A_{N}$, for neutral pions produced in polarized proton collisions with protons ($pp$), with aluminum nuclei ($p\rm{Al}$) and with gold nuclei ($p\rm{Au}$) at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral pions are observed in the forward direction relative to the transversely polarized proton beam, in the pseudo-rapidity region $2.7<\eta<3.8$. Results are presented for $\pi^0$s observed in the STAR FMS electromagnetic calorimeter in narrow Feynman x ($x_F$) and transverse momentum ($p_T$) bins, spanning the range $0.17<x_F<0.81$ and $1.7<p_{T}<6.0$ GeV/$c$. For fixed $x_F<0.47$, the asymmetries are found to rise with increasing transverse momentum. For larger $x_F$, the asymmetry flattens or falls as ${p_T}$ increases. Parametrizing the ratio $r(A) \equiv A_N(pA)/A_N(pp)=A^P$ over the kinematic range, the ratio $r(A)$ is found to depend only weakly on $A$, with ${\langle}P{\rangle} = -0.027 \pm 0.005$. No significant difference in $P$ is observed between the low-$p_T$ region, $p_T<2.5$ GeV/$c$, where gluon saturation effects may play a role, and the high-$p_T$ region, $p_T>2.5$ GeV/$c$. It is further observed that the value of $A_N$ is significantly larger for events with a large-$p_T$ isolated $\pi^0$ than for events with a non-isolated $\pi^0$ accompanied by additional jet-like fragments. The nuclear dependence $r(A)$ is similar for isolated and non-isolated $\pi^0$ events.

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    A_N pAl 0.17<x_F<0.19

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    A_N pp 0.17<x_F<0.19

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    A_N pAu 0.17<x_F<0.19

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    A_N pAl 0.21<x_F<0.27

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    A_N pp 0.21<x_F<0.27

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    A_N pAu 0.21<x_F<0.27

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    A_N pAl 0.27<x_F<0.37

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    A_N pp 0.27<x_F<0.37

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    A_N pAl 0.47<x_F<0.61

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    A_N pAl 0.61<x_F<0.81

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    A_N pp 0.61<x_F<0.81

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    A_N for combined pp+pAl+pAu 1.5<P_T<2.0 GeV/c

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    A_N for combined pp+pAl+pAu 2.0<P_T<3.0 GeV/c

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    A_N for combined pp+pAl+pAu 3.0<P_T<4.0 GeV/c

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    A_N for combined pp+pAl+pAu 4.0<P_T<5.0 GeV/c

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    A_N for combined pp+pAl+pAu 5.0<P_T<7.0 GeV/c

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    Ratio A_N(pAu)/A_N(pp) 0.17<x_F<0.21

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    Ratio A_N(pAu)/A_N(pp) 0.21<x_F<0.27

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    Ratio A_N(pAu)/A_N(pp) 0.27<x_F<0.37

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    Ratio A_N(pAu)/A_N(pp) 0.37<x_F<0.47

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    Ratio A_N(pAu)/A_N(pp) 0.47<x_F<0.61

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    Ratio A_N(pAu)/A_N(pp) 0.61<x_F<0.81

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    Ratio A_N(pAl)/A_N(pp) 0.17<x_F<0.21

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    Ratio A_N(pAl)/A_N(pp) 0.21<x_F<0.27

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    Ratio A_N(pAl)/A_N(pp) 0.27<x_F<0.37

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    Ratio A_N(pAl)/A_N(pp) 0.37<x_F<0.47

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    Ratio A_N(pAl)/A_N(pp) 0.47<x_F<0.61

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    Ratio A_N(pAl)/A_N(pp) 0.61<x_F<0.81

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    A_N(pA))/A_N(pp) 0.17<x_F<0.19

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    A_N(pA))/A_N(pp) 0.21<x_F<0.27

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    A_N(pA))/A_N(pp) 0.27<x_F<0.37

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    A_N(pA))/A_N(pp) 0.37<x_F<0.47

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    A_N(pA))/A_N(pp) 0.47<x_F<0.61

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    A_N(pA))/A_N(pp) 0.61<x_F<0.81

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    Average over all p_{t} with Type 2 Fit

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    Average over low p_{T} Type 1 Fit

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    Average over high p_{T} Type 1 Fit

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    Non-Isolated pi0 in pp 0.17<x_F<0.21

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    Isolated pi0 in pp 0.17<x_F<0.21

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    Non-Isolated pi0 in pp 0.21<x_F<0.27

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    Isolated pi0 in pp 0.21<x_F<0.27

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    Non-Isolated pi0 in pp 0.27<x_F<0.37

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    Isolated pi0 in pp 0.27<x_F<0.37

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    Non-Isolated pi0 in pp 0.37<x_F<0.47

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    Isolated pi0 in pp 0.37<x_F<0.47

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    Non-Isolated pi0 in pp 0.47<x_F<0.61

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    Isolated pi0 in pp 0.47<x_F<0.61

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    Non-Isolated pi0 in pp 0.61<x_F<0.81

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    Isolated pi0 in pp 0.61<x_F<0.81

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    Non-Isolated pi0 in pAl 0.17<x_F<0.21

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    Isolated pi0 in pAl 0.17<x_F<0.21

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    Non-Isolated pi0 in pAl 0.21<x_F<0.27

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    Isolated pi0 in pAl 0.21<x_F<0.27

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    Non-Isolated pi0 in pAl 0.27<x_F<0.37

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    Isolated pi0 in pAl 0.27<x_F<0.37

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    Non-Isolated pi0 in pAl 0.37<x_F<0.47

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    Isolated pi0 in pAl 0.37<x_F<0.47

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    Non-Isolated pi0 in pAl 0.47<x_F<0.61

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    Isolated pi0 in pAl 0.47<x_F<0.61

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    Non-Isolated pi0 in pAl 0.61<x_F<0.81

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    Isolated pi0 in pAl 0.61<x_F<0.81

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    Non-Isolated pi0 in pAu 0.17<x_F<0.21

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    10.17182/hepdata.100167.v1/t70

    Isolated pi0 in pAu 0.17<x_F<0.21

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    Non-Isolated pi0 in pAu 0.21<x_F<0.27

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    Isolated pi0 in pAu 0.21<x_F<0.27

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    Non-Isolated pi0 in pAu 0.27<x_F<0.37

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    Isolated pi0 in pAu 0.27<x_F<0.37

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    Non-Isolated pi0 in pAu 0.37<x_F<0.47

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    Isolated pi0 in pAu 0.37<x_F<0.47

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    Non-Isolated pi0 in pAu 0.47<x_F<0.61

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    10.17182/hepdata.100167.v1/t78

    Isolated pi0 in pAu 0.47<x_F<0.61

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    10.17182/hepdata.100167.v1/t79

    Non-Isolated pi0 in pAu 0.61<x_F<0.81

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    Subset of Data from Figure 12

    10.17182/hepdata.100167.v1/t80

    Isolated pi0 in pAu 0.61<x_F<0.81

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    Subset of Data from Figure 13

    10.17182/hepdata.100167.v1/t81

    Isolated Type 2 Fit. Power P vs x_F for Nuclear Law Dependence A_N(pA)/A_N(pp)=A^P

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    Subset of Data from Figure 13

    10.17182/hepdata.100167.v1/t82

    Non-Isolated Type 2 Fit. Power P vs x_F for Nuclear Law Dependence A_N(pA)/A_N(pp)=A^P

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