Studies of CPT symmetry in positronium decays with 192 plastic strip J-PET detector

TL;DR

This study presents a direct CPT-symmetry test in ortho-positronium decays using the J-PET detector, reconstructing the o-Ps emission direction without external fields and measuring the CPT-odd angular correlation $O_{CPT}$. Through 356 days of data, 47.8 million $\mathrm{o o 3\gamma}$ events were analyzed, yielding a polarization-corrected CPT amplitude $C_{CPT} = (-0.00029 \pm 0.00022)$, consistent with CPT invariance and representing a fourfold improvement over the previous best. The analysis combines precise event reconstruction, background suppression, and systematic checks, establishing the most stringent limit to date on CPT-violating effects in o-Ps decays. The results bolster CPT invariance in QED and set the stage for future, higher-sensitivity measurements with next-generation J-PET facilities to probe CPT and related SME parameters more tightly.

Abstract

A direct test of the CPT symmetry is performed for the electromagnetic decays of ortho-positronium using the J-PET tomograph. We present the precise measurement of the CPT-sensitive angular correlation entailing the positronium spin and the momenta of its annihilation photons, surpassing previous studies utilizing the same detection system. Positrons originating from a $^{22}$Na source are emitted from the detector's center and subsequently form positronium atoms within the spherical chamber covered with porous material. Reconstruction of annihilation locations using the 192-strip J-PET detector makes it possible to determine the positronium emission direction, which defines the quantization axis along which positronium is polarized, without the application of external magnetic fields. The measurements were performed in total for 356 days resulting in an identification of 47.8 million events with ortho-positronium decays into three photons. The results are consistent with the exactness of CPT symmetry with measured asymmetry amplitude -0.00029 $\pm$ 0.00022 (stat.) and with statistical error four times smaller than the previous best measurement.

Figures (7)

• Figure 1: The experimental setup for CPT symmetry test.(a) Photograph of the 192-strip plastic scintillator J-PET with an annihilation chamber in its center. (b) Schematic representation of the detector where three photons from o-Ps annihilation (arrows in red color) on the chamber's walls are emitted. (c) Photograph of the positronium production medium used in the setup where the spherical annihilation chamber is placed inside a cylindrical tube enclosed by the endcaps at its two ends. (d) The $^{22}$Na source sandwiched between Kapton foil (in yellow) is supported through strings and four bolt-like structures and placed on the source holder. This source holder is fixed at the center of the spherical chamber. The chamber setup is centered in the detector.
• Figure 2: Hit-based analysis criteria for selection of o-Ps $\to$ 3$\gamma$.(a) Distribution of a total number of $\gamma$ interactions in the detector referred to as hit multiplicity in an event. The events with a hit multiplicity greater than or equal to three are selected. (b) Time over Threshold (TOT) distribution for each hit in the selected event. The accepted region up to the Compton edge at 67 ns is for the annihilation events. (c) The smallest value of $\delta$d$_{ij}$ = $\mid$$\vec{r_{i}}$ - $\vec{r_{j}}\mid - c \mid \vec{t_{i}}$ - $\vec{t_{j}}\mid$ is plotted for each selected event. The distribution of min $\delta$d$_{ij}$ is compared for experimental data and o-Ps signal events from Monte-Carlo simulations. The hits with a greater time of interaction out of the hit pairs in the region $\delta$d$_{ij}$$<$ 23 cm are discarded assuming those are the secondary Compton scatterings of the annihilation photons.
• Figure 3: Event-based analysis criteria for the selection of o-Ps $\to$ 3$\gamma$ events. The events with a hit multiplicity equal to three are considered after the hit-based selection criteria. (a) The sum of the two smallest relative angles between the photons vs. the smallest distance between the hypothetical annihilation point of 2$\gamma$ on the Line of Response (LOR) and the center of detector. The angles $\theta$ are calculated assuming that photons originate from the center of the detector. The highly concentrated region around $\theta_{1}$ + $\theta_{2}$$=$ 180$^{\circ}$ is the 2$\gamma$ annihilation events from the direct annihilation in the source. The other concentrated region around 200$^{\circ}$ is the contribution from events like p-Ps annihilation in the porous material at the wall of the annihilation chamber and secondary Compton scatterings of annihilation photons. (b) Events in the region $\theta_{1}$ + $\theta_{2}$$>$ 204$^{\circ}$ are identified as originating from the o-Ps annihilation.
• Figure 4: Identification of o-Ps events from Monte-Carlo simulations. Distribution of sum of two smallest angles vs. smallest of distances from LOR is compared for (a) Total Monte-Carlo simulations that include the signal and different kinds of background in the study, (b) only signal events with three annihilation photons from o-Ps atom, and (c) three hit background events that can mimic the signal events in the study. The background event of 2$\gamma$ from direct annihilations is not shown here, therefore no structure is present around $\theta_{1}$ + $\theta_{2}$$=$ 180$^{\circ}$ in (a) and (c) unlike in the experimental data Fig. \ref{['fig:analysis_event_based']}a. Distribution (a), (b) and (c) are shown in different color scale to enhance the visibility of signal.
• Figure 5: Data consistency check. One year of experimental data is divided into four independent sub-samples. The single point represents the mean with its statistical error for the CPT odd operator for a sub-sample consisting of three months of data. The red and yellow lines represent the final mean and its error of $\langle$O$_{\text{CPT}}$$\rangle$ for the whole data sample.