The positronium atom, a bound state of electron and positron, is a suitable leptonic test site for Charge-Parity (CP) discrete symmetry research. According to the Standard Model, the photon–photon interaction in the final state due to the vacuum polarization may mimic CP violation of the order of 10$^{-9}$, while weak interaction effects lead to a violation of the order of 10$^{-14}$. So far, the experimental limits on CP symmetry violation in the decay of o-Ps are set at the level of 10$^{-3}$. The J-PET detector can be used to explore discrete symmetries by looking for probable non-zero expectation values of the symmetry-odd operators, constructed from spin of ortho-Positronium (o-Ps) and momentum, and polarization vectors of gamma (γ) quanta resulting from o-Ps annihilation. The upgraded version of the J-PET detector, with an additional fourth layer of detection modules increases signal acceptance, which allows to triple the efficiency of γ quanta detection for CP discrete symmetry studies

Eliyan, Kavya V.

Moskal, Paweł

Skurzok, Magdalena

Acta Physica Polonica B Proceedings Supplement

English

Jagiellonian Univeristy Repository

Acta Physica Polonica B Proceedings Supplement 15, 4-A10 (2022)PERSPECTIVE OF CP VIOLATION SEARCHBY MODULAR J-PET DETECTOR INTHE ORTHO-POSITRONIUM DECAY∗Kavya V. Eliyan†, Magdalena Skurzok, Paweł Moskalon behalf of the J-PET CollaborationFaculty of Physics, Astronomy and Applied Computer ScienceJagiellonian University, Łojasiewicza 11, 30-348 Kraków, PolandandCenter for Theranostics, Jagiellonian University, Kraków, PolandReceived 3 November 2022, accepted 13 December 2022,published online 27 December 2022The positronium atom, a bound state of electron and positron, is a suit-able leptonic test site for Charge-Parity (CP) discrete symmetry research.According to the Standard Model, the photon–photon interaction in thefinal state due to the vacuum polarization may mimic CP violation of theorder of 10−9, while weak interaction effects lead to a violation of the or-der of 10−14. So far, the experimental limits on CP symmetry violationin the decay of o-Ps are set at the level of 10−3. The J-PET detector canbe used to explore discrete symmetries by looking for probable non-zeroexpectation values of the symmetry-odd operators, constructed from spinof ortho-Positronium (o-Ps) and momentum, and polarization vectors ofgamma (γ) quanta resulting from o-Ps annihilation. The upgraded versionof the J-PET detector, with an additional fourth layer of detection modulesincreases signal acceptance, which allows to triple the efficiency of γ quantadetection for CP discrete symmetry studies.DOI:10.5506/APhysPolBSupp.15.4-A101. IntroductionCharge conjugation (C) and parity (P) operators are combined to createone of the discrete symmetries in the Universe, known as the CP discretesymmetry [1, 2]. The Standard Model value of CP symmetry violation inweak interactions seems insufficient to explain the observed predominanceof matter over antimatter in the Universe [3–5]. Strong and electromagnetic∗ Presented at the 4th Jagiellonian Symposium on Advances in Particle Physics andMedicine, Cracow, Poland, 10–15 July, 2022.† Corresponding author: kavya.eliyan@doctoral.uj.edu.pl(4-A10.1)4-A10.2 K.V. Eliyan, M. Skurzok, P. Moskalinteractions are both symmetric under the operators C and P, proving thatthey are likewise symmetric under the product CP [1, 2]. In positroniumdecay, the photon–photon interactions in the final state due to the vacuumpolarization may mimic CP symmetry violation of the order of 10−9 ac-cording to the Standard Model prediction [2, 6, 7]. Many particle physicsexperiments were conducted in the search of CP symmetry violation effectsin hadrons [8, 9] and in leptonic systems [10–12]. The most recent experi-mental limits on CP violation in positronium decays are set at the level of10−3 [13] and CPT violation limits in the o-Ps decay are set at the levelof 10−4 [14, 15]. Searches for the CP discrete symmetry violations are stillgoing on and we are aiming at improving these experimental symmetry vi-olation limits using our new modular J-PET detector [14].2. Jagiellonian-Positron Emission TomographyThe Jagiellonian-Positron Emission Tomography (J-PET) is the firstPET scanner designed using plastic scintillator strips, which makes it in-expensive for uses in both scientific research and medical applications [16–25]. The capacity of the J-PET detector to monitor the polarization andmomentum direction of the annihilation photons is one of its distinguish-ing properties [2, 21]. Recently, the J-PET Collaboration has constructedthe modular J-PET detector which offers greater statistics of γ quanta in ashorter measurement period to considerably improve the precision of exper-imental measurement values [14, 28–32].2.1. 3-layer J-PET detectorThe 192 plastic scintillator strips (EJ230, 500× 19× 7 mm3, form con-centric layers of 48 modules on a radius of 425 mm, 48 modules on a radiusof 467.5 mm, and 96 modules on a radius of 575 mm) make up the three lay-ers of the 3-layer J-PET detector (Fig. 1 (left)) [16–20]. Each scintillator inthe J-PET scanner is optically connected with Hamamatsu R9800 vacuumtube photomultipliers at each end, which read out the optical signals fromthe scintillators [16–20]. The sides of scintillator strips are wrapped withreflective foil to reduce photon losses [20–24, 26, 27]. Although the J-PETdetector was designed for medical imaging applications, its outstanding angleresolution of roughly one degree and time resolution of about 250 ps allowsus to measure relative angles between the interacting photons [2, 29–33].These features enable the J-PET detector to investigate ortho-Positronium(o-Ps) to 3γ events with a reduction of e+e− → 2γ background events bya factor of 104, while rejecting only 3% of signal events [2, 20, 28, 34–36].Perspective of CP Violation Search by Modular J-PET Detector . . . 4-A10.3Figure 1 (right) shows o-Ps → 3γ decay and the momentum of 3 primaryannihilation photons are marked with ~k1, ~k2, ~k3 (where |~k1| > |~k2| > |~k3|),while the secondary scattered photon is denoted with ~k′1 [2, 28].Fig. 1. Left: Photograph of 3-layer J-PET detector [20] and right: its cross-sectionalview with a point-like 22Na source at the center (red) covered in the XAD-4 porouspolymer (blue) [20]. The solid arrows show o-Ps (black dot) → 3γ decay andthe momentum of 3 primary annihilation photons are marked with ~k1, ~k2, ~k3 andthe secondary scattered photon is denoted with ~k′1 [2, 35]. The red dotted arrowindicates a prompt γ quantum from de-excitation of the daughter nucleus 22Neoriginating from the 22Na decay [15, 37].2.2. Modular J-PET detectorThe modular J-PET detector (Fig. 2 (left)) is a newly developed, flexible,and portable variant of the J-PET detector that can be fitted within the3-layer J-PET detector as shown in Fig. 2 (right) [14, 20].Fig. 2. Left: Photograph of the modular J-PET detector. Right: Cross-sectionalview of the modular layer J-PET detector (blue rectangles) fitted inside the 3-layerJ-PET detector.4-A10.4 K.V. Eliyan, M. Skurzok, P. MoskalThis 4-layer arrangement of detection modules in the modular J-PET de-tector increases the acceptance of signal events to evaluate the CPT and CPdiscrete symmetry violations in the o-Ps annihilation process [14, 29]. Themodular J-PET detector consists of 24 detection modules and each moduleis comprised of 13 strips of BC404 plastic scintillators with dimensions of6× 25× 500 mm3 and they are wrapped in Vikuiti ESR and Pokalon 100Bfoils [19]. Each scintillator strip is read out at both ends by arrays of 1matrixHamamatsu Silicon PhotoMultipliers (SiPMs) [30]. Inserting of the modulardetector layer inside the 3-layer J-PET prototype reduces the photons lossesthrough gaps and is capable of performing positronium decay imaging withincreased multi-photon registration probability [14, 29, 33]. The enhancedo-Ps to 3γ decay events detection efficiency by the modular J-PET detectorallows for the discrete symmetry violation study at a precision level of 10−5[14, 28].3. CP Discrete symmetry study in the leptonic sectorIn 1933, Anderson discovered the positron in the cosmic ray showers,which is an antiparticle of an electron [38, 39]. An ortho-Positronium (o-Ps)is a bound state formed between a positron and an electron with spin equalto 1 (figure 3) [2, 39].Fig. 3. Interaction between e−–e+ pair leads to direct annihilation into prompt γor creation of a bound state called positronium, which can be formed in two spinconfigurations and later get annihilated into either odd or even number of photons(o-Ps → 3γ, 5γ, . . . and p-Ps → 2γ, 4γ, . . .) [1, 2, 39].Positronium exists in the singlet state (p-Ps) and triplet (o-Ps) stateswith spin equal to zero (0) and one (1), respectively [2]. In vacuum, themean lifetime value of the ground states of p-Ps is 0.125 ns and o-Ps isabout 142 ns [2]. In a medium, o-Ps pick-off annihilation process may occurwhen a positron (e+) from o-Ps annihilates directly with an electron fromPerspective of CP Violation Search by Modular J-PET Detector . . . 4-A10.5the material outside, which may reduce the o-Ps mean lifetime even up toa few nanoseconds [2, 39–41]. As a bound system constrained by a centralpotential, positronium is an example with a parity operator (P) eigenstate,and it is built of an electron and an anti-electron (positron) so, positroniumis an eigenstate of charge conjugation operator (C), therefore, it is also a CPeigenstate [2, 40, 41]. So, o-Ps is a perfect leptonic bound system to studydiscrete symmetries. The unique feature of the J-PET detector which allowsthe study of CP symmetry violation is the measurement of the polarizationdirection of o-Ps annihilation photons [2]. Distinguishing the geometry andproperties of the J-PET detector allows us to design the positronium sourceso that the vector polarization of generated o-Ps can be identified [2, 20,28]. Due to parity violation in the β decay, the positron emitted from the22Na source will be longitudinally polarized [2] and they will interact withelectrons in the cylindrical layer of the XAD-4 porous material target toform the spin-linear polarized ortho-Positronium [2, 20, 28]. The timingand position information of o-Ps annihilation photon interactions in thescintillator strips allow us to reconstruct the annihilation position using thetrilateration method, and hence their momentum and polarization vector [2,14, 34]. The reconstructed annihilation photon momentum and polarizationvector can be used then to study CP symmetry violation by determiningthe expectation values of the CP symmetry odd operator listed in Table 1[2, 6, 7, 42].Table 1. Discrete symmetry odd operator constructed using linear polarizationdirection (~ε1 = ~k1 × ~k′1) of the most energetic annihilation photon and momen-tum directional vector (~k2) of the second (in decreasing energy order) annihilationphoton from the same o-Ps decay event [2, 42].Operator C P T CP CPT~ε1 · ~k2 + − − − +The value of the CP operator (Table 1) is given by equation (1) [2, 35]cos(α) =(~ε1 · ~k2)/|~ε1|∣∣∣~k2∣∣∣ , (1)where (α) is the angle between the linear polarization direction of the mostenergetic annihilation photon (~ε1) and momentum directional vector of thesecond annihilation photon (~k2) from the same o-Ps decay event. The meanof cos(α) values is used as a measure of the CP symmetry violation [2, 35].4-A10.6 K.V. Eliyan, M. Skurzok, P. MoskalSo far, the experimentally reached non-zero expectation value limit forCP and CPT symmetry violation in the o-Ps annihilation has a precisionbetween 10−2 and 10−3, respectively [13, 15, 28]. The statistical uncer-tainty of the number N of recorded o-Ps into 3γ decay events and analyzingpower of the detector roughly reflects the sensitivity to the discrete sym-metry violation parameter [28]. J-PET’s objective to reach a sensitivityvalue exceeding 10−3, which demands the value of N at least of the orderof 107 was achieved with three months of continuous measurements usinga 10MBq sodium source [14, 28, 35]. In the year 2021, the limitations ofthe previous experiments were overcome by the 3-layer J-PET detector dueto its much higher granularity which improved the world result and reachesthe statistical precision of the order of 10−4 for CPT and CP discrete sym-metry [14, 29, 35]. The reported result is the present best upper limit onthe discrete symmetry violation in the decay of o-Ps, leaving us 5 orders ofmore statistical sensitivity to be explored in this aspect [14, 29, 35]. With anincrease in source activity or measurement duration, the most recently up-dated modular version of the J-PET detector may realistically increase theacquired photon statistics by a factor of 100, which is required to achieve thesensitivity level of 10−5 for our CP symmetry violation studies [39, 42, 43].4. ConclusionThe enhanced multiphoton detection efficiency of the modular versionof the J-PET detector will help to improve the o-Ps annihilation γ quantaregistration probability and will provide larger statistics of o-Ps decay eventsin a shorter duration of time for the CP discrete symmetry study. The testsof CP symmetry with the modular J-PET detector are planned for the year2023. We will aim at achieving the sensitivity at the level of 10−5 for the CPdiscrete symmetry test by combining the J-PET detector with an additionalmodular layer of fully digital readout system-based SiPMs, which wouldenable three times higher single photon detection efficiency.The authors acknowledge support by the TEAM POIR.04.04.00-00-4204/17program, the National Science Centre, Poland (NCN) grant Nos. 2019/35/B/ST2/03562, 2021/42/A/ST2/00423, and the SciMat and qLife PriorityResearch Areas budget under the program Excellence Initiative — ResearchUniversity at the Jagiellonian University, Jagiellonian University underproject No. CRP/0641.221.2020, and the Ministry of Education and Sci-ence through grant No. SPUB/SP/490528/2021.Perspective of CP Violation Search by Modular J-PET Detector . . . 4-A10.7REFERENCES[1] M.S. Sozzi, «Discrete Symmetries and CP Violation», Oxford UniversityPress 2008.[2] P. Moskal et al., Acta Phys. Pol. B 47, 509 (2016).[3] M. Kobayashi, T. Maskawa, Prog. 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Perspective of CP violation search by modular J-PET detector in the ortho-positronium decay

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Perspective of CP violation search by modular J-PET detector in the ortho-positronium decay

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