Abstract
Discrete symmetries such as parity (P), charge-conjugation (C) and time reversal (T) are of fundamental importance in physics and cosmology. Breaking of charge conjugation symmetry (C) and its combination with parity (CP) constitute necessary conditions for the existence of the asymmetry between matter and antimatter in the observed Universe. The presently known sources of discrete symmetries violations can account for only a tiny fraction of the excess of matter over antimatter. So far CP and T symmetries violations were observed only for systems involving quarks and they were never reported for the purely leptonic objects. In this article we describe briefly an experimental proposal for the test of discrete symmetries in the decays of positronium atom which is made exclusively of leptons. The experiments are conducted by means of the Jagiellonian Positron Emission Tomograph (J-PET) which is constructed from strips of plastic scintillators enabling registration of photons from the positronium annihilation. J-PET tomograph together with the positronium target system enable to measure expectation values for the discrete symmetries odd operators constructed from (i) spin vector of the ortho-positronium atom, (ii) momentum vectors of photons originating from the decay of positronium, and (iii) linear polarization direction of annihilation photons. Linearly polarized positronium will be produced in the highly porous aerogel or polymer targets, exploiting longitudinally polarized positrons emitted by the sodium 22 Na isotope. Information about the polarization vector of orthopositronium will be available on the event by event basis and will be reconstructed from the known position of the positron source and the reconstructed position of the orthopositronium annihilation. In 2016 the first tests and calibration runs are planned, and the data collection with high statistics will commence in the year 2017.
Highlights
The violation of the combined symmetries of charge conjugation and spatial parity (CP) was discovered 52 years ago in the decays of neutral K mesons [1]
A test of the discrete symmetries in the decays of positronium into photons is limited by the photon-photon interaction which is expected to mimic the symmetry violation at the level of about 10−9 only [10, 11]
In this article we briefly describe a new proposal for tests of the T, CP as well as CPT symmetries in the decays of the positronium
Summary
The violation of the combined symmetries of charge conjugation and spatial parity (CP) was discovered 52 years ago in the decays of neutral K mesons [1]. At present mesons, which are systems of quark and antiquark bound via strong interaction, remain the only objects in which violations of T and CP symmetries were observed. A test of the discrete symmetries in the decays of positronium into photons is limited by the photon-photon interaction which is expected to mimic the symmetry violation at the level of about 10−9 only [10, 11]. This is still about six orders of magnitude smaller than the present experimental limits equal to about 3×10−3 [12, 13]. For the detailed description of the motivation to perform such experiments the interested reader is referred to the recent article [15]
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