Abstract
Symmetries under the parity transformation (P), charge-conjugation (C) and time reversal (T) are of fundamental importance in nuclear and elementary particle physics. Studies of the observables violating the combined CP symmetry constitute precise tests of the Standard Model. However, CP violation was observed to date only for systems involving quarks, raising the importance of searches its manifestations e.g. in purely leptonic systems. The 3γ decay of spin-aligned ortho-positronium atoms (o-Ps) can be used to test CP invariance in such a purely leptonic system. The Jagiellonian Positron Emission Tomograph (J-PET) detection system enables experimental tests of CP and CPT through measurement of the expectation values of angular correlation operators odd under these transformations and constructed from (i) spin vector of the ortho-positronium atom, (ii) co-planar momentum vectors of photons originating from the decay of the positronium atom, and (iii) linear polarization direction of annihilation photons. Precise experimental symmetry tests with J-PET are possible thanks to a dedicated reconstruction technique of 3γ ortho-positronium decays and a positronium production chamber including a highly porous aerogel target, whose setup allows for determining the orthopositronium spin polarization without the use of an external magnetic field.
Highlights
A novel detector based on strips of polymer scintillators is being developed by the Jagiellonian Positron Emission Tomograph (J-PET) Collaboration [1, 2]
The preliminary reconstruction results indicate that the current version of J-PET detector gives a realistic chance to improve the best present limits established for the both CP and CPT symmetry violation tests in the decays of positronium atom by more than one order of magnitude
The selection criteria have been set as that only event candidates with two back to back γ interactions were chosen with the azimuthal angle between the two hit detector modules in the range of 180 ± 20◦ and having the time over threshold of the recorded PMT signals between 10 ns and 30 ns, corresponding to energies deposited by 511 keV photons in Compton scattering
Summary
A novel detector based on strips of polymer scintillators is being developed by the J-PET Collaboration [1, 2]. The J-PET detector has high timing properties which allow us to investigate the fundamental physics phenomena like discrete symmetries in the purely leptonic system [3]. The J-PET detector consists of 192 (EJ-230) plastic scintillator strips of dimensions 500×19×7 mm, which are forming three concentric layers (48 modules of 425 mm radius, 48 modules of 467.5 mm radius and 96 modules of 575 mm radius [4]). The preliminary reconstruction results indicate that the current version of J-PET detector gives a realistic chance to improve the best present limits established for the both CP and CPT symmetry violation tests in the decays of positronium atom by more than one order of magnitude
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