Potential of the J-PET Detector for Studies of Discrete Symmetries in Decays of Positronium Atom --- A Purely Leptonic System

Abstract

The Jagiellonian Positron Emission Tomograph (J-PET) was constructed as a prototype of the cost-effective scanner for the simultaneous metabolic imaging of the whole human body. Being optimized for the detection of photons from the electron-positron annihilation with high time-and high angular-resolution, it constitutes a multi-purpose detector providing new opportunities for studying the decays of positronium atoms.Positronium is the lightest purely leptonic object decaying into photons. As an atom bound by a central potential it is a parity eigenstate, and as an atom built out of an electron and an anti-electron it is an eigenstate of the charge conjugation operator. Therefore, the positronium is a unique laboratory to study discrete symmetries whose precision is limited in prin-(1) arXiv:1602.05226v1 [nucl-ex] 16 Feb 2016 2 ciple by the effects due to the weak interactions expected at the level of (∼ 10 −14 ) and photon-photon interactions expected at the level of (∼ 10 −9 ).The J-PET detector enables to perform tests of discrete symmetries in the leptonic sector via the determination of the expectation values of the discrete-symmetries-odd operators, which may be constructed from the spin of ortho-positronium atom and the momenta and polarization vectors of photons originating from its annihilation. In this article we present the potential of the J-PET detector to test the C, CP, T and CPT symmetries in the decays of positronium atoms.

A feasibility study of ortho-positronium decays measurement with the J-PET scanner based on plastic scintillators

“…J-PET detector has been already built [1]. The mechanical construction for the next phases J-PET+1 and J-PET+2 is also prepared and the hardware upgrade is planned within the next 2 yearsLayer numberLayer radius with respect to the center of scintillator (cm)Number of scintillators in the layerAngular displacement of scintillatorJ-PET   142.5048    246.7548    357.5096 J-PET+2   142.5048    246.7548    350.9096    453.3096    557.5096 J-PET+1   142.5048    246.7548    353.3096    457.5096 J-PET-full   143.0400    245.0437 ...…”

“…Therefore, it is well suited for the studies of these discrete symmetries in the leptonic sector. These symmetries may be studied by the measurement of the expectation values of various operators (odd with respect to the studied symmetry) constructed from the momenta of photons and the spin of the ortho-positronium [1]. Such studies are limited by the photon–photon interaction, however it was estimated that the vacuum polarisation effects may mimic the CP and CPT symmetries violation only at the level of 10 [2], which is still by six orders of magnitude less than the presently best known experimental limits for CP and CPT violations in the positronium decays which are at the level of 0.3 % [3, 4].…”

“…In fundamental physics, studies of the three gamma annihilation allows not only to test the discrete symmetry violation [14] but also enables searches of physics beyond the Standard Model: extra dimensions [15], dark matter [16] and a new light vector gauge boson [17]. Since a detailed physics program of J-PET and its motivation is described elsewhere in a dedicated article [1], here as an example we would like only to discuss briefly experimental approach to determining the expectation value of the odd operator for the CPT symmetry, whose violation has not been observed so far. As it was recently shown [18] the J-PET detector allows for a spin direction () determination of o-Ps created in cylindrical target.…”

Trilateration-based reconstruction of ortho -positronium decays into three photons with the J-PET detector

“…This polarization is to large extent preserved during the thermalization process and transferred to the positronium [28,29]. How- The ability to reconstruct the o-Ps decay vertex allows for a different way of polarization estimation, presented schematically in Figure 4 [14]. If a β + source is located inside a vacuum-filled cylinder covered with a layer of a medium for positronium formation (such as aerogel), then reconstructed position of the o-Ps→ 3γ decay can be used to obtain the positron momentum direction.…”

“…Detailed description of potential of the J-PET detector for studies of discrete symmetries in decays of positronium atom can be found in Ref. [14].…”