Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

Analysis of fake asymmetry for the MTV experiment
F. Goto,∗1,∗2 H. Baba,∗2 J. A. Behr,∗3 T. Kajihara,∗4 H. Kawamura,∗5 M. Kitaguchi,∗1 C. D. P. Levy,∗3
H. Masuda,∗4 Y. Nakaya,∗4 K. Ninomiya,∗4 J. Onishi,∗4 R. Openshaw,∗3 S. Ozaki,∗4 M. Pearson,∗3 Y. Sakamoto,∗4
H. M. Shimizu,∗1 Y. Shimizu,∗4 R. Takenaka,∗4 K. Tamura,∗4 S. Tanaka,∗4 R. Tanuma,∗4 Y. Totsuka,∗4E. Watanabe,∗4 Y. Yamamoto,∗4 Y. Yamawaki,∗4 M. Yokohashi,∗1 and J. Murata4
The Mott polarimetry for T-Violation (MTV) experiment aims to search for a large time-reversal symmetry
violation (T-violation) in polarized 8 Li β decay. This
experiment was conducted at the TRIUMF Isotope Separator and Accelerator (TRIUMF-ISAC). T-Violation
may arise in triple-vector correlation (R-correlation) inbeta decay. R-correlation causes electron transverse polarization and it can be measured by detecting the angle
symmetry of electron Mott scattering using a cylindrical drift chamber (CDC). Figure 1 shows a schematicof how β rays are detected. A 8 Li beam is stopped at
the beam stopper placed at the center of CDC, and anelectron is emitted. CDC can detect the track of theelectron scattered on an analyzer foil.
The electron emission distribution of β − decay, ω, isexpressed as1)[]⃗
⃗ p⃗ep⃗e⟨J⟩⟨J⟩
×·+ . . . . (1)ωdEe dΩe ∝ 1 + R⃗σ ·+AJEeJ Ee

The deﬁnition of R-correlation can be found in this function, where J⃗ is the spin polarization of the parent nucleiand ⃗σ , p⃗e , and Ee are the spin polarization, momentum,and electron energy, respectively. A-correlation is theanisotrpy of β-ray emission. In the case of 8 Li, A = − 13 .
Both A- and R-correlation terms are proportional tothe parent nuclei’s polarization. Therefore, we adopt Rasymmetry (Mott Scattering angle asymmetry) divided
by A-asymmetry (β ray emission rate asymmetry) as ascale of coeﬃcient R.
In our previous studies, we recognized a systematiceﬀect that causes a large fake R-asymmetry. It is estimated that the source of this systematic eﬀect is accidental coincidence, and it shows an obvious dependence
on the width of the coincidence window and beam intensity.2) We assume an evaluation model function ofFig. 1. Schematic of CDC.
∗1
∗2
∗3
∗4
∗5Fig. 2. Beam-intensity and coincidence-window dependenceof Mott scattering asymmetry. The red line indicatesfitting with Eq. (2).

fake asymmetry Asym(x). The variable x is presented
as a product of the beam intensity and the width of thecoincidence window in arbitrary units.
Asym(x) =(2)The parameter a represents fake asymmetry caused byaccidental coincidence, b/x represents the ratio of thenumber of real Mott scattering events to accidental coincidence events, and c is the asymmetry of real Mott
scattering events. The limit of Asym(x) as x approaches
0 is c. This means that the y-intercept of this functionis the coeﬃcient R, which we want to know. Figure 2
shows 1183 measured asymmetry data points acquired in2017. The measurement time was 400 s per plotted datapoint. The beam intensity was about 105−7 pps, and thecoincidence window varies from 100 ns to 3000 ns. Thisshows that our model is suitable for evaluating the scaleof fake asymmetry as a function of the product of beam
intensity and the width of the coincidence window.
The accidental coincidence model we use is assumed
with consideration of the double coincidence of β rays,
but there can be triple or more coincidence events. Multiple coincidence could cause other eﬀects of fake asymmetry. We need to evaluate this factor to draw a conclusion from our data.
References

Department of Physics, Nagoya University
RIKEN Nishina Center
TRIUMF
Department of Physics, Rikkyo University
Cyclotron and Radioisotope Center, Tohoku Universityac+.
1 + xb
1 + xb1) J. D. Jackson, S. B. Treiman, H. W. Wyld Jr., Phys. Rev.
106, 517 (1957).
2) J. Murata et al., Hyperfine Interact. 237, 125 (2016).

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