Single transverse spin asymmetry of forward neutrons

Abstract

We calculate the single transverse spin asymmetry $A_N(t)$, for inclusive neutron production in $pp$ collisions at forward rapidities relative to the polarized proton in the energy range of RHIC. Absorptive corrections to the pion pole generate a relative phase between the spin-flip and non-flip amplitudes, leading to a transverse spin asymmetry which is found to be far too small to explain the magnitude of $A_N$ observed in the PHENIX experiment. A larger contribution, which does not vanish at high energies, comes from the interference of pion and $a_1$-Reggeon exchanges. The unnatural parity of $a_1$ guarantees a substantial phase shift, although the magnitude is strongly suppressed by the smallness of diffractive $\pi p\to a_{1}p$ cross section. We replace the Regge $a_1$ pole by the Regge cut corresponding to the $\pi \rho$ exchange in the $1^{+}S$ state. The production of such a state, which we treat as an effective pole $a$, forms a narrow peak in the $3\pi$ invariant mass distribution in diffractive $\pi p$ interactions . The cross section is large, so one can assume that this state saturates the spectral function of the axial current and we can determine its coupling to nucleons via the PCAC Goldberger-Treiman relation and the second Weinberg sum rule. The numerical results of the parameter-free calculation of $A_N$ are in excellent agreement with the PHENIX data.

8 pages, 6 figures. Based on the talk given by B.K. at the Third International Workshop on Transverse Polarization Phenomena, Veli Losinj, Croatia, 29 August - 2 September 2011

References in corpus (3)

• Damping of forward neutrons in pp collisions
• Breakdown of PCAC in diffractive neutrino interactions
• Neutrino-production of single pions: dipole description

Cited by in corpus (14)

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• Inclusive cross section and single-transverse-spin asymmetry for very forward neutron production in polarized p+p collisions at sqrt(s)=200 GeV
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• Pion-pion cross section from proton-proton collisions at the LHC
• Exclusive vector meson production with a leading neutron in photon - hadron interactions at hadronic colliders
• Nuclear dependence of the transverse-single-spin asymmetry for forward neutron production in polarized $p$$+$$A$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV
• Nonperturbative features of the axial current
• Large single transverse spin asymmetry for forward neutrons in ultraperipheral polarized proton-nucleus collisions in explanation of measurements at RHIC-PHENIX
• Diffractive neutrino-production of pions on nuclei: Adler relation within the color-dipole description
• Transverse momentum dependent forward neutron single spin asymmetries in transversely polarized $p$$+$$p$ collisions at $\sqrt{s}=200$ GeV
• Transverse single spin asymmetries of forward neutrons in $p$$+$$p$, $p$$+$Al, and $p$$+$Au collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV as a function of transverse and longitudinal momenta
• Gluon saturation: survival probability for leading neutrons in DIS
• Leading Neutrons From Polarized Proton-Nucleus Collisions