Test of the CPT symmetry in positronium
annihilations at sub-permil
precision using the J-PET tomography device
August 22nd 2021

Aleksander Gajos
on behalf of the J-PET Collaboration
Jagiellonian UniversityMotivation: discrete symetry tests with o-Ps → 3γ decays●Discrete symmetries are scarcely tested with leptonic systems●

Prominent results from neutrinos oscillation experiments●●●

Dirac phase, δCP ~3σ level [T2K, Nature 580 (2020) 339]

Electron EDM < 1.1x10-29 [ACME, Nature 562 (2018) 355]
Positronium – the lightest purely leptonic bound state,the only system consisting of charged leptons used for tests of CP and CPT to date

How can we test discrete symmetries in the positronium system?
●●Searches for prohibited positronium
annihilations●

Certain SME-based searches for CPT
violation were proposed with positronium
spectroscopy [Phys. Rev. D92 (2015) 056002]

Searches for non-vanishing symmetry-odd correlations
CCP = (1.3±2.1±0.6)⨯10-3
PRL 104 (2010) 083401

10-1 10-2 10-3 10-4

Physical sensitivity limit:
false asymmetries fromγγ interactions in the final state

6 OOM ofunexplored precision

10-9precision

CCPT = (2.6±3.1)⨯10-3

PRL. 91 (2003) 263401
22.08.2021

A. Gajos, 20th Lomonosov Conference2Testing discrete symmetries withangular correlations in o-Ps → 3γ decays
for an odd operator

Using ortho-positronium spin
Requires either:● polarization● spin control● spin estimation

operator

Using photon polarization

C+

P
–T+

CP
–CPT–++–+–+–––++–––+++–+–+–+––[ W. Bernreuther et al., Z. Phys. C41 (1988) 143 ][ P. Moskal et al., Acta Phys. Polon. B47 (2016) 509 ]22.08.2021

A. Gajos, 20th Lomonosov Conference3o-Ps → 3γ operators involving spin
Presently studied with J-PET:
T & CPT-violation sensitive
CP-violation sensitive

T & CP-violation sensitive but
requires o-Ps tensor polarization→ not available with the current
J-PET approach

Event-by-event spin estimation
Using an extensive-size o-Ps production andannihilation medium

Effective polarization depends
on o-Ps→3γ vertex resolution
22.08.2021

A. Gajos, 20th Lomonosov Conference4

Reconstruction of o-Ps→3γ decays in J-PET
1. Find the decay plane containing the 3 hits in the J-PET barrel

2. Transform the hit coordinates to a
2D coordinate system in the decay plane

3. For each of the recorded γ hits,
define a circle of possible origin pointsof the incident γ assuming o-Ps decay at time t

4. The decay point (x',y') in the decay plane and time t is anintersection of 3 such circles:[A. Gajos et al., NIM A 819 (2016), 54-59]
22.08.2021

A. Gajos, 20th Lomonosov Conference5J-PET vs previous measurements
Gammasphere

CCPT = (2.6±3.1)⨯10-3

Yamazaki et al.

PRL 104 (2010) 083401

PRL. 91 (2003) 263401

CCP = (1.3±2.1±0.6)⨯10-3

Limiting positron emission direction
1 Mbq β+ emitter activity
4π detector but low angular resolution

Recording multiple
geometrical configurations
e+ spin estimatedevent-by-event

22.08.2021

Polarized o-Ps using external B field
Inclusive measurement
Only certain angular configurations

Plastic scintillators = fast timing→ using high β+ emitter activity(tested up to 10 Mbq)
● Recording all 3 annihilation photons● Angular resolution at 1o level●A. Gajos, 20th Lomonosov Conference6o-Ps production in J-PET with anextensive size annihilation chamberβ+ source

porous silica

Tomographic images of the chamber
obtained using γγ annihilations:

●

●

●

Extensive-size chamber, R=12 cm
Walls coated with porous silica
material enhancing o-Ps formation
10 MBq β+ emitter (22Na) placed in
the center of the chamber

22.08.2021

A. Gajos, 20th Lomonosov Conference7

Identification of o-Ps → 3γ events in J-PET
Using total Time Over Threshold (TOT)of PMT signals from a scintillator strip→ a measure of ɣ deposited energy

22Ne* de-excitation

511 keV
Comptonedge

1274 keV
Comptonedgeo-Ps →3γ

22.08.2021

Confirming o-Ps presence withpositron lifetime distribution

t3γ – tprompt

A. Gajos, 20th Lomonosov Conference

o-Ps

8

Treatment of main background sources
Secondary Compton
scatterings●●Subsequent
scattering events

Secondary Compton-scatteredphotons may be recordedby J-PET again
For each pair of annihilation
photon candidates
i and j (i,j=1,2,3) we compute:o-Ps → 3ɣ2ɣ from the β+source setup●●Using angular topologyof the event
in XY detector plane
Considering all hypotheticalback-to-back 2ɣ pairs(tomographic “Lines Of Response”)
22.08.2021

A. Gajos, 20th Lomonosov Conference9Evaluation of the CPT-asymmetric observable
Efficiencies evaluated with MC
are symmetric in cosθStandard asymmetry:is generalized by the mean value of cosϑ:

J-PET is sensitive to the full range of this operator

Expected effect withCPT-asymmetric
Simulations(exaggerated violation)[see also Symmetry 12(2020) 8, 1268]

22.08.2021

A. Gajos, 20th Lomonosov Conference10Evaluation of the experiment’s sensitivity●●●

MC-simulating same statistics as experimental data●
Artificially inducing different levels of CPT violation
Applying identical analysis as used on data
Testing observed level of violation (CCPT)

Preliminary

Different colors denoteindependent simulations

22.08.2021

A. Gajos, 20th Lomonosov Conference

11Results of the CPT test
Using 2 x 106 of identifiedo-Ps → 3ɣ annihilations3ɣ image of the o-Ps production chamberin the tranverse view of the detector

Preliminary

Preliminary

The first image ofan extensive-size object
obtained with o-Ps→3γannihilations
22.08.2021<cosϑ> statistical uncertainty: 3.3 x 10-4
systematic uncertainty 1.4 x 10-4
Analyzing power S = 37.4 %(polarization-dominated)
A. Gajos, 20th Lomonosov Conference12Summary and further perspectives●

The J-PET detector is capable of exclusive registration of o-Ps→3γ annihilations●●●

Full event recontruction including determination of the annihilaiton point
in an extensive-size medium●Estimation of o-Ps spin on an event-by-event basis●The first image of an extensive-size object otained solely with o-Ps annihilations

Sub-permil precision of the CPT test reached with the first J-PET measurement
J-PET aims at the sensitivity of the CP and CPT symmetry tests
at the level of 10-5 with the pending improvements to the setup:[Symmetry 12 (2020) 8, 1268]
22.08.2021

A. Gajos, 20th Lomonosov Conference

13Thank you for your attention!

This work is supported in the framework of the TEAM POIR.04.04.00-00-4204/17 Programmeof the Foundation for Polish ScienceBackup Slides

22.08.2021

15

Testing discrete symmetries with ortho-positronium
operator

If polarization direction of thephotons (ϵ) can be estimated,a new class of operators becomes
available for measurement!

C+P
–T+CP
–CPT–++–+–+–––++–––+++–+–+–+––[W. Bernreuther et al., Z. Phys. C41 (1988) 143]
[P. Moskal et al., Acta Phys. Polon. B47 (2016) 509]

J-PET can determine the scattering planein events with secondary Compton scatterings!
22.08.2021

A. Gajos, 20th Lomonosov Conference

Compton scattering angle

Eur. Phys. J. C 78 (2018) 970

Angle between ϵ and the scattering plane
16The J-PET Detector●●●Constructed at theJagiellonian University
Fist PET device using stripsof plastic scintillators
At the same time:a robust photon detector
for fundamental research!

A new inner layer
with digital readout→ See talks by:
G. Korcyl
Sz. NiedźwieckiOn Thursday
22.08.2021

A. Gajos, 20th Lomonosov Conference17

O-Ps creation and decay

22.08.2021

A. Gajos, 20th Lomonosov Conference18Distinguishing o-Ps→3γ and e+e-→2γ[J-PET: P.Kowalski, P.Moskal, in preparation]
22.08.2021

A. Gajos, 20th Lomonosov Conference

19Resolution dependence on γ hit time resolution
The resolution of o-Ps decay obtained with the presented reconstruction method
depends predominantly on the timing resolution of γ hits in scintillator strips.

spatial22.08.2021

O-Ps decay resolution

A. Gajos, 20th Lomonosov Conference

time20Ortho-positronium life time resolution
For each event of o-Ps decay, the positronium decay time can be estimated as:

where t0 is the o-Ps decay time reconstucted with the presented method andLγdeexc. is calculated using reconstructed o-Ps decay point.

tγdeexc.
t0

22.08.2021

Lγdeexc.

A. Gajos, 20th Lomonosov Conference21Scheme of the experiment22.08.2021

A. Gajos, 20th Lomonosov Conference

22Time-Over-Threshold as a measureof deposited γ energy
Using total Time Over Threshold (TOT)of PMT signals from a scintillator strip

PMT B

PMT AγTOT(B)TOT(A)The relation between TOT and energy deposited bya photon in Comton scattering
is under an extensive study right now.

[S. Sharma et al.,EJNMMI Phys. 7, 39 (2020)]

22.08.2021

A. Gajos, 20th Lomonosov Conference23Angular topology of the 3γ events
Angles defined in the transverse plane only

2-photon[D. Kamińska et al., Eur. Phys. J.
C76 (2016) no.8, 445 ]22.08.2021

o-Ps[M. Mohammed, PhD thesis, 2020]
A. Gajos, 20th Lomonosov Conference24Signal & background events

22.08.2021

A. Gajos, 20th Lomonosov Conference25Detector improvements

22.08.2021

A. Gajos, 20th Lomonosov Conference

26Expected sensitivity[Symmetry 12 (2020) 8, 1268]
22.08.2021

A. Gajos, 20th Lomonosov Conference27

Control of detector asymmetries●●Insensitive to CPT violation in absenceof o-Ps tensor polarization
No B field used in the current experiment=> we expect <0> unlessspurious asymmetries originate
from detector/chamber geometry<O> = ( 0.99 +/- 1.7 ) x 10-4

22.08.2021

A. Gajos, 20th Lomonosov Conference

28