Measurement of transverse single-spin asymmetries of ${\pi }^0$ and electromagnetic jets at forward rapidity in 200 and 500 GeV transversely polarized proton-proton collisions

Abstract

The STAR Collaboration reports measurements of the transverse single-spin asymmetry (TSSA) of inclusive ${\pi }^0$ at center-of-mass energies ($\sqrt{s}$) of 200 GeV and 500 GeV in transversely polarized proton-proton collisions in the pseudo-rapidity region 2.7 to 4.0. The results at the two different energies show a continuous increase of the TSSA with Feynman-$x$, and, when compared to previous measurements, no dependence on $\sqrt{s}$ from 19.4 GeV to 500 GeV is found. To investigate the underlying physics leading to this large TSSA, different topologies have been studied. ${\pi }^0$ with no nearby particles tend to have a higher TSSA than inclusive ${\pi }^0$. The TSSA for inclusive electromagnetic jets, sensitive to the Sivers effect in the initial state, is substantially smaller, but shows the same behavior as the inclusive ${\pi }^0$ asymmetry as a function of Feynman-$x$. To investigate final-state effects, the Collins asymmetry of ${\pi }^0$ inside electromagnetic jets has been measured. The Collins asymmetry is analyzed for its dependence on the ${\pi }^0$ momentum transverse to the jet thrust axis and its dependence on the fraction of jet energy carried by the ${\pi }^0$. The asymmetry was found to be small in each case for both center-of-mass energies. All the measurements are compared to QCD-based theoretical calculations for transverse-momentum-dependent parton distribution functions and fragmentation functions. Some discrepancies are found, which indicates new mechanisms might be involved.

after replying to referee's comments. minor changes

References in corpus (13)

• PYTHIA 6.4 Physics and Manual
• A Unified Picture for Single Transverse-Spin Asymmetries in Hard Processes
• Single transverse-spin asymmetry in high transverse momentum pion production in pp collisions
• Azimuthal and Single Spin Asymmetries in Hard Scattering Processes
• Forward Neutral Pion Transverse Single Spin Asymmetries in p+p Collisions at \sqrt{s}=200 GeV
• Towards an explanation of transverse single-spin asymmetries in proton-proton collisions: the role of fragmentation in collinear factorization
• Collins Fragmentation and the Single Transverse Spin Asymmetry
• The RHIC SPIN Program: Achievements and Future Opportunities
• Universal Structure of Twist-3 Soft-Gluon-Pole Cross Sections for Single Transverse-Spin Asymmetry
• Azimuthal Asymmetric Distribution of Hadrons Inside a Jet at Hadron Collider
• Collins azimuthal asymmetries of hadron production inside jets
• Testing the universality of the Collins function in pion-jet production at RHIC
• Comparison of transverse single-spin asymmetries for forward ${\pi }^0$ production in polarized $pp$, $p\mathrm{Al}$ and $p\mathrm{Au}$ collisions at nucleon pair c.m. energy $\sqrt{s_{\mathrm{NN}}}=200$ GeV

Cited by in corpus (14)

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• Comparison of transverse single-spin asymmetries for forward ${\pi }^0$ production in polarized $pp$, $p\mathrm{Al}$ and $p\mathrm{Au}$ collisions at nucleon pair c.m. energy $\sqrt{s_{\mathrm{NN}}}=200$ GeV
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• Transverse Nucleon Single-Spin Asymmetry for Single-Inclusive Hadron and Jet Production at NLO Accuracy
• NLO corrections and factorization for transverse single-spin asymmetries
• Energy Independence of the Collins Asymmetry in $p^{\uparrow }p$ Collisions
• Transverse single-spin asymmetry of charged hadrons at forward and backward rapidity in polarized $p$+$p$, $p$+Al, and $p$+Au collisions at $\sqrt{s_{NN}}=200$ GeV}
• Probing nucleon spin structure in deep-inelastic scattering, proton-proton collisions and Drell-Yan processes
• Triple Regge exchange and transverse single-spin asymmetries of the very forward neutral pion production in polarized $p+p$ collisions
• Fragmentation contributions to transverse nucleon spin observables in semi-inclusive deep-inelastic scattering at NLO
• Measurement of transverse single-spin asymmetries for dijet production in polarized proton-proton collisions at $\sqrt{\mathbf{s}}$ = 200 $\mathrm{GeV}$