Periodic Reporting for period 1 - OPSVIO (Ortho-positronium decay and the search for CP and CPT violation in leptonic sector)
Periodo di rendicontazione: 2021-05-01 al 2023-04-30
In our universe, we observe asymmetry between the matter and antimatter. In 1967 A. Sahkarov proposed a solution requiring, among others, the existence of CP symmetry violation (C - charge conjugation, P - parity). This was indeed observed in the quark sector but at the magnitude which is not large enough to solve the matter-antimatter asymmetry problem. Since the CP violation is allowed by some leptogenesis models, the search for additional CP symmetry violation was extended to leptonic sector. A recent experiment involving neutrino oscillations reported the indication of CP violation but with no decisive conclusion.
A complementary approach is to use the decay of Ortho-Positronium (o-Ps), which decays into three gammas. o-Ps has well-defined C, P and T (T - time reversal) properties, what makes it suitable to probe for the violation of individual symmetries and their combinations (CP and CPT) which are related to a particular angular correlation between the direction of the Positronium spin and the momenta of gammas. Any deviation of the angular correlation amplitudes from zero would indicate discovery of a new physics.
Within this action we will build a new experimental set-up and perform measurement of CP and improve the precision level compared to the state-of-art results. The key part of the experimental set-up will be a ring of detectors consisting of 24 BaF2 crystals for detection of 3 gammas. The data acquisition system will be based on fast pulse digitizers, what will increase the rate and the amount of collected data but will also increase the complexity of the data analysis. Data collected in this measurement will also contain information on two CPT amplitudes. We will determine both, and one of them will be determined for the first time. Since no suitable polarimeter for annihilation gamma rays exists at the moment, we will study polarization sensitivity of a single-layer Compton polarimeter based on 8x8 GaGG(Ce) crystal array, which will be read out by a matching SiPM array.
A complementary approach is to use the decay of Ortho-Positronium (o-Ps), which decays into three gammas. o-Ps has well-defined C, P and T (T - time reversal) properties, what makes it suitable to probe for the violation of individual symmetries and their combinations (CP and CPT) which are related to a particular angular correlation between the direction of the Positronium spin and the momenta of gammas. Any deviation of the angular correlation amplitudes from zero would indicate discovery of a new physics.
Within this action we will build a new experimental set-up and perform measurement of CP and improve the precision level compared to the state-of-art results. The key part of the experimental set-up will be a ring of detectors consisting of 24 BaF2 crystals for detection of 3 gammas. The data acquisition system will be based on fast pulse digitizers, what will increase the rate and the amount of collected data but will also increase the complexity of the data analysis. Data collected in this measurement will also contain information on two CPT amplitudes. We will determine both, and one of them will be determined for the first time. Since no suitable polarimeter for annihilation gamma rays exists at the moment, we will study polarization sensitivity of a single-layer Compton polarimeter based on 8x8 GaGG(Ce) crystal array, which will be read out by a matching SiPM array.
To build the experimental set-up several components needed to be designed, build, and procured. First, extenders were designed for mounting the 24 BaF2 detectors to form a ring without gaps. Housings for larger BaF2 crystals were designed and build allowing for an upgrade of the detectors. The laboratory acquired a license for the use of the class 5 radioactive sources and two 22Na radioactive sources were procured (1 MBq and 3.7 MBq). Two 0.5 T neodymium magnets were procured and iron yoke for the magnets was designed and build. Special positron tagging detectors were designed and build, which included procurement of plexiglass light guides, thin plastic scintillators and different mechanical holders for positron tagging detectors.
Since the positron tagging detectors are based on the 0.1 mm plastic scintillators the light yield out of these detectors is small. Several assembly methods and wrapping with aluminum foil were tried out to have the light signals at both ends of the light guide, which allowed for reading them out in coincidence and in turn reduce the background. The data acquisition program was optimized and tested with different data file types. Several test measurements with the completed experimental set-up where performed. The programs for data analysis based on ROOT framework were developed. Geant4 simulation for the study of the background was developed.
Several measurements with single-layer Compton polarimeters based on 8x8 GaGG(Ce) crystal array were performed. The polarization modulation of 2-gamma annihilation was studied first. The data was taken for back-to-back gammas and when one gamma scattered inside an intermediate scatterer at a given angle. The polarization modulation was determined for all angles and compared with results of two different types of Compton polarimeters. The paper describing the experiment and the result is prepared and uploaded to arXiv and is submitted to a journal.
Since the positron tagging detectors are based on the 0.1 mm plastic scintillators the light yield out of these detectors is small. Several assembly methods and wrapping with aluminum foil were tried out to have the light signals at both ends of the light guide, which allowed for reading them out in coincidence and in turn reduce the background. The data acquisition program was optimized and tested with different data file types. Several test measurements with the completed experimental set-up where performed. The programs for data analysis based on ROOT framework were developed. Geant4 simulation for the study of the background was developed.
Several measurements with single-layer Compton polarimeters based on 8x8 GaGG(Ce) crystal array were performed. The polarization modulation of 2-gamma annihilation was studied first. The data was taken for back-to-back gammas and when one gamma scattered inside an intermediate scatterer at a given angle. The polarization modulation was determined for all angles and compared with results of two different types of Compton polarimeters. The paper describing the experiment and the result is prepared and uploaded to arXiv and is submitted to a journal.
In the case of fundamental symmetry violation study (CP and CPT), a measurement of the angular correlation amplitude different from zero would indicate a symmetry breaking and a discovery of new physics in leptonic sector. If it turns out to be consistent with zero, it will further constrain the level conservation of this symmetry in leptonic sector.
The study of single-layer Compton polarimeters will help to build cost-effective alternatives for future PET systems, and by measuring the polarization modulation in 2- and 3-gamma annihilation it is possible to study quantum entanglement.
The study of single-layer Compton polarimeters will help to build cost-effective alternatives for future PET systems, and by measuring the polarization modulation in 2- and 3-gamma annihilation it is possible to study quantum entanglement.