Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 53

Unfolding the transverse momentum distribution for very forward√neutron production in pAu collisions at sN N = 200 GeV
B. Mulilo∗1,∗2 for the PHENIX collaboration
Smearing response matrix
0.3

True Pt [GeV/c]

The PHENIX collaboration measured that when atransversely polarized proton√ with spin up collides
with unpolarized proton at s = 200 GeV, they generate neutrons predominantly to the right.1) In 2011,
theorists explained this result in terms of the interference of pion and a1 reggeon exchanges.2) But in 2015
using run 15 pAu data, we observed that when a po√
larized proton collides with a gold nucleus at sN N
= 200 GeV, they generate more neutrons to the left3)
contrary to theoretical predictions.2) This nuclear dependence of the asymmetry (AN ) has, therefore, attracted a massive interest in nuclear physics.
We are now studying AN as a function of the transverse momentum (P t). We begin with an understanding that our measurements are limited by known eﬀects
such as the detector resolution and detection eﬃciency
among others. Our technique is, therefore, to employ
a method known as unfolding to remove these known
eﬀects and recover the distribution.
We proceed by parametrizing measurement eﬀects
using the response matrix in Fig. 1 from Monte
Carlo.4) What this matrix does is to map the binnedtrue spectrum in the magenta line onto the smeared
spectrum in the green line of Fig. 2. For the smeared
and true distribution bins Ri and Tj , respectively, thesmearing matrix element Sij gives the fraction of entries from bin Tj that end up being reconstructed inbin Ri .
The unfolding has been performed using the singularvalue decomposition method5) contained in CERN’s
ROOT toolkit. Since our smearing matrix is not perfectly diagonal, we unfolded with a parameter, alias
Kreg,5) which determines the regularization of the unfolding. The unfolded spectrum is the distribution corresponding to an optimum regularization parameter,Kreg = 6 as depicted in Fig. 2.
We are now optimizing and extending the ideas ofthe one dimensional unfolding to two dimensional unfolding of transverse momentum (P t) in azimuthal angle (Φ). The transverse single spin asymmetry (AN )for very forward neutron production will then be calculated as a function of the unfolded P t distribution.

0.25
0.2

0.15
0.1
0.05
000.05

0.1

0.15

0.2
0.25
0.3
Smeared Pt [GeV/c]

Fig. 1. Smearing response matrix mapping the binned true
P t spectrum to the smeared P t spectrum.
Data, unfolded, true and smeared Pt distributions (Kreg = 6)

Yield×103

200

Data Pt spectrum

180

Unfolded Pt spectrum

160

True Pt spectrum

140

Smeared Pt spectrum

1201008060
4020000.05

0.1

0.15

0.2

0.25
0.3
Pt [GeV/c]

Fig. 2. Superposition of the experimental data, unfolded,
true and smeared P t distributions. The P t distribution
in the green line is smeared from Monte Carlo whilethat in the dark blue line is the experimental data. TheP t distribution in the magenta line is generated using
Monte Carlo event generator.4)

References∗1∗21) A. Adare et al., Phys. Rev. D 88, 032006 (2013).
2) B. Z. Kopeliovich et al., Phys. Rev. D 84, 114012 (2011).
3) A. Aidala et al., Phys. Rev. Lett. 120, 022001 (2018).
4) G. Mitsuka, Phys. Rev. C 95, 044908 (2017).
5) Nucl. Instrum. Methods Phys. Res. A 372, 469 (1996).

RIKEN Nishina Center
Department of Physics, Korea University- 77 -