III-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

A LYSO calorimeter prototype for muonic X-ray detection
S. Kanda∗1 and K. Ishida∗2The Weinberg angle is an energy-dependent parameter that describes the mixing of electromagnetic andweak interactions. Measurements of the Weinberg angle
at various energy scales are important as a precision testof the standard model and a search for new physics. TheWeinberg angle can be determined via neutral currentinteractions. The most precise result was obtained at a
low-energy scale by observing the atomic parity violation (APV) in cesium atoms.1)
When a nuclear Coulomb potential captures a negative muon, the muon forms an exotic bound state
called a muonic atom. An APV experiment using
muonic atoms provides a unique opportunity to searchfor physics beyond the standard model. In the 1990s,
several experiments were conducted at the Paul Scherrer Institute.2) However, no parity-odd transition wasobserved because of diﬃculties in the experiment.
A new experiment using a high-intensity pulsed muon
beam and a segmented calorimeter has been proposed
to revisit this topic. The process of interest is the transition from the 2S state to the 1S state, which involves
a one-photon emission. In the experiment, a segmented
calorimeter consisting of Ce: LYSO crystals and silicon
photomultipliers (SiPMs) detects X-rays from muonicatoms. The target energy resolution is 10% (FWHM)
at 75 keV. As the ﬁrst stage of the project, a calorimeter prototype was developed and tested at Port4 of theRIKEN-RAL muon facility.
Figure 1 illustrates the experimental setup. Muonic
X-rays and decay electrons from a graphite target weredetected using the prototype detector. The detector has
two layers, each consisting of a 2-cm-square, 3-mm-thickcrystal attached to four SiPMs (Hamamatsu S133603050CS) with an active area of 3-mm-square. The signals were ampliﬁed and shaped using EASIROC-based
electronics and recorded with a waveform digitizer using

Fig. 1. Experimental setup. The inset shows the structureof the prototype.
∗1∗2

Institute of Materials Structure Science, KEK
RIKEN Nishina CenterFig. 2. Electron and X-rays time spectra. The prompt electrons were detected by the beam monitor.

Fig. 3. X-ray energy spectrum obtained by the calorimeter
prototype.

DRS4.
Figure 2 shows the time spectra of electrons and Xrays. Since electrons penetrate the two detector layers, the coincidence analysis allowed us to distinguishbetween electron and X-ray events. Figure 3 shows anenergy spectrum of muonic X-rays. The energy was calibrated with the 81 keV line of 133 Ba. The peak structure
was consistent with K X-rays of muonic carbon atoms.
While the prototype demonstrated a suﬃcient time resolution, there is room for improvement in the energyresolution. A new version with improved light collection
eﬃciency is under fabrication.
The authors thank Dr. Adrian Hillier and Ms. Bethany
Hampshire for their kind assistance in preparation for
the experimental setup at RAL. This experiment wasconducted under the user program RB1970126 at theRIKEN-RAL muon facility. This work was supported
by Japanese JSPS KAKENHI Grant Number 19H04618.
References

1) J. Guena et al., Phys. Rev. A 71, 042108 (2005).
2) K. Kirch et al., Phys. Rev. Lett. 78, 4363 (1997).
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