CALIOPE, or CP(T) Aberrant Leptons in ortho-Positronium Experiment, is a tabletop search for fundamental symmetry violations, including CP-and CPT-violation, in ortho-positronium (o-Ps). We use a tagged 22Na source adjacent to a cylinder of aerogel to generate o-Ps at the center of a cylindrical array of 24 NaI(Tl) bars. We search for CPT-violating angular correlations in the gamma rays emitted in the decay of o-Ps. With an angular acceptance of 75% of 4π, and the ability to acquire statistics over a longer period of time, CALIOPE will improve upon the limit set by previous experiments. The experimental setup can also be used in a search for CP-violation in o-Ps with the addition of an electromagnet. We describe the design of the experiment, results from a characterization of the systematics for the CPT-and CP-violating measurements, and a demonstration of the DAQ

Bartram, C.

Henning, R.

Carolina Digital Repository

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing LtdXV International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 1342 (2020) 012106IOP Publishingdoi:10.1088/1742-6596/1342/1/0121061Caliope: a search for CPT -violation in o-psC Bartram and R HenningDepartment of Physics and AstronomyUniversity of North Carolina at Chapel HillPhillips Hall, CB #3255120 E. Cameron Ave.Chapel Hill, NC 27599-3255E-mail: cbartram@live.unc.edu, rhenning@unc.eduAbstract. CALIOPE, or CP (T ) Aberrant Leptons in ortho-Positronium Experiment, is atabletop search for fundamental symmetry violations, including CP - and CPT -violation, inortho-positronium (o-Ps). We use a tagged 22Na source adjacent to a cylinder of aerogel togenerate o-Ps at the center of a cylindrical array of 24 NaI(Tl) bars. We search for CPT -violating angular correlations in the gamma rays emitted in the decay of o-Ps. With an angularacceptance of 75% of 4π, and the ability to acquire statistics over a longer period of time,CALIOPE will improve upon the limit set by previous experiments. The experimental setupcan also be used in a search for CP -violation in o-Ps with the addition of an electromagnet.We describe the design of the experiment, results from a characterization of the systematics forthe CPT - and CP -violating measurements, and a demonstration of the DAQ.1. IntroductionPositronium (Ps) is an unstable bound state of an electron and positron, which exists in bothtriplet and singlet states. Due to charge conjugation parity, the triplet state (ortho-positronium,or o-Ps) decays primarily to three gamma rays and the single state (para-positronium, or p-Ps)decays primarily to two gamma rays. Bound by a central potential, Ps is an eigenstate of parity.Free from QCD effects, it is a system fully described by quantum electrodynamics with smallweak corrections. These properties make it a clean system for fundamental symmetry violationsearches. CPT -violating searches in decay processes are potential sources for new physics beyondthe Standard Model. Matter/antimatter tests have constituted the majority of CPT -violationsearches in the past. As such, decay processes present an interesting potential source of CPT -violation. CPT -conservation is a foundation of theoretical physics, so its discovery in the leptonwould upend our current understanding of physics. These proceedings will focus on the CPT -violation search, but some discussion of progress towards a CP -violation search will also beincluded. CP -violation searches in the lepton sector are a burgeoning field since more CP -violation is necessary to explain the preponderance of matter over antimatter in the observableuniverse [1].We search for CPT -violation by measuring the following CPT -odd observable [2]:(~S· ~k1× ~k2)(1)where ~S is the spin of the positronium, ~k1 is the highest energy gamma ray emitted in theXV International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 1342 (2020) 012106IOP Publishingdoi:10.1088/1742-6596/1342/1/0121062decay of o-Ps and ~k2 is the second highest energy gamma ray emitted in the decay of o-Ps. Weknow from the position of the source relative to the aerogel and the array that some fractionof the spin is aligned along the z-axis. In our reconstruction of this observable, we assume anet spin polarization along the z-axis. We calculate the up/down asymmetry, or the number oftimes our observable is positive (N+) or negative (N−) viaA =N+ −N−N+ +N−(2)To compare with previous experiments, we then compute the CPT -violating term C =A/ 〈P 〉, where 〈P 〉 = 0.41 is the average polarization for 22Na. We aim to make animprovement a previous, similar experiment, which found no CPT -violation and measuredC = 0.0026±0.0031 [3].Figure 1. Left: Aerogel before being placed into the source holder. Right: Custom-builtattenuator board for the DAQ.2. MethodologyCALIOPE uses the APEX array [4], which consists of 24 NaI(Tl) bars arranged cylindrically.Each NaI(Tl) bar is bookended by PMTs. The positronium is generated at the center of the arrayusing a 22Na source flush against a solid cylinder of aerogel. The aerogel and source are containedwithin a holder that sits inside of a carbon fiber tube mounted inside the array. Silica aerogelis highly porous and serves as an effective positronium generator. We flow nitrogen throughthe carbon fiber tube to purge the aerogel, which helps eliminate pick-off annihilation, wherethe positron in the positronium annihilates with an electron in the ambient material. Positronsemitted in the decay of 22Na thermalize in the aerogel and eventually interact with an electronfrom the surroundings to form positronium. Some fraction of the positrons form positroniumin the triplet state (o-Ps), which decays to three gamma rays. These three gamma rays areour signal. The hits are registered in the NaI(Tl) bars. Position and energy reconstruction isachieved using the charge amplitudes from the PMTs at either end of a bar. The light from anionizing interaction in the bar falls off exponentially as it reaches the end of the bar:A± =EγPE0exp(−µ(L/2±x)) (3)where µ is the attenuation coefficient, L is the length of the bar, P is the quantum efficiencyof the PMT, Eγ is the energy deposited by the gamma ray, and E0 is the energy deposited perlight photon created in the scintillator. The energy is then proportional to the square root ofthe product of the charge amplitudes at either end of the bar, and the position is proportionalto the natural log of the ratio of the two charge amplitudes [5].XV International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 1342 (2020) 012106IOP Publishingdoi:10.1088/1742-6596/1342/1/0121063Figure 2. Position reconstruction uses the relative amplitudes of the pulses at either end of aNaI(Tl) bar as shown above.Figure 3. Front view of the APEX array at TUNL.3. DAQCALIOPE uses a VME-based DAQ consisting of CAEN QDCs and TDCs to measure the chargedeposition and timing. Signals from the PMTs are split into high and low gain channels andamplified. Signals for the low gain channels are attenuated before reaching the QDC.We have successfully calibrated all bars in the array and have been able to demonstrateenergy and position reconstruction. The PMTs were calibrated using a 10µCi source locatedbetween two lead cylinders which serve as a collimator. The lead collimator is housed inside ofa delrin tube attached to a 1.07 m rod that sits inside of an aluminum pipe. The rod is etchedevery 0.5 cm, enabling us to position the source with the same level of precision. The leadcylinders constrain the gamma rays such that they only emerge from the gap, which is typicallyonly about 1 mm in width. Using this collimated source, we were able to demonstrate positionand energy reconstruction capabilities along the length of the bar. See fig 4 below.4. SimulationWe have developed a Geant4 [6] Monte Carlo simulation to characterize systematics effects ofour experiment. Because Geant4 does not include the ability to model positronium physics, wewrote custom code to generate the o-Ps decay kinematics. The APEX array was modeled inits entirety, and the carbon fiber structure which holds the positronium generator was included.An example of our simulation results both with and without CPT -violation is shown. The codehas the ability to study different systematic effects such as variations in the bar thickness andtranslations and rotations of the source holder.One example of a systematic that did not yield any fake asymmetry is a translation of thesource holder. A translation of the source holder would have the effect of shifting the decayvertex of the o-Ps. At first glance, this seems to yield a potential systematic. As shown in toXV International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 1342 (2020) 012106IOP Publishingdoi:10.1088/1742-6596/1342/1/0121064Figure 4. Left: Energy spectrum from a collimated 10µCi 22Na and 1 µCi 133Ba source sittingon an individual APEX bar. Right: Position reconstruction for a 10µCi 22Na collimated sourcepositioned 10 cm towards the front of the array.1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1o-Ps Events0100200300400500600700800900310×cos(theta)1− 0.8− 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1o-Ps Events0100200300400500310×cos(theta)Figure 5. Left: Exaggerated example of CPT-violation using the positronium generator code.Right: Example of no CPT-violation, using the same code. θ is the angle between the spin axis(z-axis) and the normal to the decay plane.the left of fig 6, an assumption that the source holder is centered (red ( ~k1) and blue ( ~k2) gammarays) yields a negative triple correlation term, when in reality the true triple correlation term ispositive (green ~k1 and violet ~k2). There is, however, no preferred orientation for ~k1 as opposedto ~k2, so for every positive triple correlation term flipped to negative, there will be a negativetriple correlation term flipped to positive. In other words, ~k1 could equally well be blue insteadof red and violet instead of green. Thus, from geometrical considerations alone, a translationshould not cause a systematic effect.Shown below is a Geant4 rendering of the APEX array, with the carbon fiber tube and,therefore, source holder, with an exaggerated rotation about the x-axis of the array. This wasshown to not contribute any source of systematic error.XV International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 1342 (2020) 012106IOP Publishingdoi:10.1088/1742-6596/1342/1/0121065Figure 6. Left: Rotated source holder inside APEX array using Geant4. Right: Example ofan event which flips sign when the source holder is translated.5. ConclusionCALIOPE will search for CPT -violating angular correlations between gamma rays emitted ino-Ps decay. We have demonstrated the capabilities of the DAQ and developed the simulation.Data-taking will commence this fall.5.1. AcknowledgmentsThe authors wish to thank the TUNL staff, especially Matthew Busch and Brogan Thomas forthe design of the support structure needed to mount the source inside the array, and MohammadAhmed, for his help with the DAQ.References[1] Dune collaboration http://www.dunescience.org/ accessed: 2017-10-20[2] Bernreuther W, Low U, Ma J and Nachtmann O 1988 Z.Phys. C41 143[3] Vetter P and J Freedman S 2003 91 263401[4] Betts R 1989 Nucl. Instr. and Meth. 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Caliope: A search for CPT-violation in o-ps

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