II-4. Hadron Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Transverse single spin asymmetry in charged pion production at
midrapidity in polarized p + p collisions at 200 GeV
J. H. Yoo∗1 for the PHENIX Collaboration
One of the main goals of the RHIC spin program isthe determination of the transverse-spin structure of theproton, which can in turn provide some insight into theangular-momentum component of partons. For reaching the goal, we measure Transverse Single-Spin Asymmetries (TSSAs) which are left-right asymmetries of ﬁnal stage particles produced in the transversely polarized p + p collision.1) The TSSAs (AN ) of pion at midrapidity in p + p collision is expected to be helpful tounderstand the transversity distributions of quark andgluons, the transverse intrinsic parton momentum, initial and ﬁnal state eﬀects. In PHENIX the charged pionsat mid-rapidity can be measured by the central arm detectors that consist of the Electro-Magnetic Calorimeter(EMCal), Ring-Imaging Cherenkov Detec tor (RICH),
Drift Chamber (DC), Pad Chamber (PC). Trigger ﬁred
if pions deposited energy over than 1.4 GeV on EMCal and we can detect charged pion’s track and energy. In mid-rapidity charged pion measurement, thebackground is categorized into two sources which are
hadron background(kaon and proton) and electron background(electron and positron). Pions with momentum
above 5 GeV/c create Cherenkov light in the RICH but
kaons and protons can’t create Cherenkov light under
E/p

510

0.2 0.4 0.6 0.8 1 1.2 1.45104

510 10105

104 104

104

33310 10103

222

10Counts

E/p0.2 0.4 0.6 0.8 1 1.2 1.41010 101021010 101011050.20.40.6

T0.8

11.21.40.20.40.6

T0.8

11.2104103103 103

103102102 102

1021010 101017 GeV/c < p < 8 GeV/c 1 1 8 GeV/c < p < 11 GeV/cCountse±

T
0.2 0.4 0.6 0.8
1 1.2 1.40.2

0.4

0.60.811.2

1.4 5101050.2

104104 104310103 103210102 102

1010 10TT

0.40.60.8

11.2

11.4

DATA: π±MC: π±+e±
MC: π±±MC: e
Fit to Data: π±+e±
Fit to Data: π±
Fit to Data: e±±
Fit to Sim : e

0.2 0.4 0.6 0.8 1 1.2 1.4rπ,e0.0436
0.06880.13750.11920.21540.00652
0.06569
0.16460.85600.9178Aπ,Sig(1 + rπ ) − Ae,Sigrπ (1 + re )NN
1 − r π reeNMC
N π,BgAeDATA
lumi_scaledrπ ≡ π,Sig = e×π+eN
AMClumi_scaled
NDATA

AπN =(1)(2)

Aπ,Sig, Ae,Sig
are asymmetry in the pion and electronNNenhanced sample, respectively. AeDATA , AeMClumi_scaled
are amplitude of the Gaussian centered at around one
in the data and the luminosity scaled MC simulation,π,eπ,eand NMC
are number ofrespectively. NDATA
lumi_scaled
entries in the E/p distributions for the data and the luminosity scaled MC simulation in pion enhanced sample,respectively. Figure 1 shows E/p distribution of pion
data and distributions of pion and electron in MC, respectively. Background fraction tended to increase withincreasing pT (Table 1). These values are used to calculate corrected AN for charged pion.
References

Fig. 1. E/p distribution in pion enhancement sample for five
PT bins.
Department of Physics, Korea University

pT bin (GeV/c)
5–6
6–7
7–8
8–11
11–15
5–6
6–7
7–8
8–11
11–15

16 GeV/c and 30 GeV/c, respectively. Only electron
background remained an object to subtract. In EMCal, electrons lost most of their energy by electromagnetic interactions. Therefore, primary electron’s energy/momentum (E/p) distributed around 1. A secondary electron track from photon conversion would beincorrectly reconstructed with large momentum. Therefore secondary electron’s energy/momentum would bein the low area. because we can know only backgroundfraction but we can’t distinguish electron event frompion event, Eq. (1) is used for background correctionof the asymmetries.2) By using Monte Carlo simulation,
we calculated electron background fraction in both pion
enhancement sample (0.2 < E/p < 0.8) and electron
enhancement sample (E/p < 0.2 and 0.8 < E/p).

1.4104 104

Countsπ±5105 100.2
0.2 0.4 0.6 0.8 1 1.2 1.4
0.4 0.6 0.8 1 1.2 1.4 105

1 11 GeV/c < p < 15 GeV/c 1 1∗1particle

15 GeV/c < p < 6 GeV/c 1 1 6< p0.8 < 71 GeV/c0.2 GeV/c0.4
0.6
1.2
1.4104105

Table 1. Background fraction of two samples.

1) C. Aidala et al. (PHENIX Collaboration), Phys. Rev. D95, 112001 (2017).
2) T. B. Moon, Analysis Note No. 1268 in PHENIX Collaboration.

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