II-4. Hadron Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Transverse momentum dependence of forward neutron single spin√
asymmetries in polarized p↑ + p collisions at s = 200 GeV†
B. Mulilo∗1,∗2 for the PHENIX Collaboration
The PHENIX Collaboration has, for the ﬁrst time,
explicitly measured the transverse momentum (pT )dependent single spin asymmetries (AN ) for inclusive neutrons produced in the forward region of thePHENIX detector with η > 6.8 using 2015 data. During this time, a proton with transverse
polarization col√lided with another proton at s = 200 GeV. Owing to
the limited acceptance and resolution of the detector,
the measured quantities were considerably smeared.
We, therefore, corrected for the smearing in the measured pT and azimuth (ϕ) using unfolding.1)
As the physics of forward neutron production is
not clearly understood, we performed detailed simulations using diﬀerent event generators as input to afull geant3 simulation.2) The generators dpmjet3.1,pythia6.1, and pythia8.2 were used because diﬀractive processes are very diﬀerently handled. Anothergenerator was an empirical distribution of forward neutrons in pT , mimicking a one-pion exchange (OPE)model in which a pion balancing the momentum between the incoming proton and outgoing neutron collided with the other proton beam using pythia 8.
Ultra-peripheral collisions (UPCs) also play a role inforward neutron production.3) The distribution of photons was, therefore, simulated using the starlight4)
generator, and the photons collided with the proton
beam using pythia 8. As all Monte Carlo (MC) generators were intrinsically spin independent, we simulated
spin eﬀects by re-weighting events as a function of thegenerated pT (pT,g ) and azimuth (ϕg ) with the spinstates (↑) and (↓) randomly assigned. Furthermore, as
the shape of pT -dependent AN is not precisely known,
we used three weight forms to provide as much ﬂexibility as possible. The weight (w) based on a polynomial of third order (Pol3), power law, and exponential
forms is given by Eqs. (1), (2), and (3), respectively,
with Pol3 being the most general one:)(w = a · pT,g +b · p2T,g +c · p3T,g sin(ϕg +λ · π), (1)Fig. 1. Overall pT -dependent AN values shown as blacksolid square points averaged over all parameterizations,MC, and unfolding. Shaded boxes display total uncertainties from the unfolding, choice of MC, and functional form. Light-green, brown, and yellow shaded regions show χ2 below 10 units for Pol3, power-law, andexponential forms, respectively, while the corresponding broken lines show the best matching parameterizations.

The performance of parameters, functional form, andMC generator in reproducing AN values was evaluatedby the minimum χ2 between the measured MC anddata asymmetries. The 2D spin-dependent neutron
yields in pT and ϕ were then unfolded using the TSVDUnfold class based on a singular value decomposition(SVD)1) of the smearing response matrix. Overall ANvalues were ﬁnally calculated from the unfolded yields
using the left-right AN formula5) after ﬁtting a sinemodulation having magnitude and phase as free parameters. In Fig. 1, overall AN values rapidly increaseat low pT (≲ 0.1 GeV/c) and slowly level oﬀ at high pT .
With this result, the ﬁrst reliable tests of mechanisms
producing these asymmetries can be performed.

where λ (±1) is the spin state and a, b, and c are freeparameters. Accordingly, the power-law weight is,)((2)
w = a · pbT,g sin(ϕg + λ · π),References

and the last parameterization is an exponential form,
which eventually decays asymptotically as)((3)
w = a 1 − eb·pT ,g sin(ϕg + λ · π).

†∗1∗21) A. Hocker et al., Nucl. Instrum. Methods Phys. Res. A372, (1996).
2) A. Adare et al., Phys. Rev. D 90, 012006 (2014).
3) C. Aidala et al., Phys. Rev. Lett. 120, 022001 (2018).
4) S. R. Klein et al., Comput. Phys. Com. 212, (2017).
5) A. Adare et al., Phys. Rev. D 88, 032006 (2013).

Condensed from article in Phys. Rev. D 103, 032007 (2021)
RIKEN Nishina Center
Department of Physics, Korea University- 55 -1