UK Positron Meeting 2024

Europe/London
Harrie Massey Lecture Theatre (University College London)

Harrie Massey Lecture Theatre

University College London

David B Cassidy, Andreas Lanz (UCL - Department of Physics and Astronomy), Samuel Reeder (UCL), Krzysztof Habdas (Department of Physics and Astronomy, University College London)
Description

Bloomsbury Campus, University College London

The UK positron meeting will be held from 4th to 5th of April 2024 at UCL. The aim of the workshop is to provide an opportunity for exchange of the latest results and scientific information concerning physics involving positrons. 

 

The program for the workshop will include contributed talks and a poster session.

  • Thursday, April 4
    • 1:00 PM 1:25 PM
      Welcome desk Harrie Massey Lecture Theatre

      Harrie Massey Lecture Theatre

      University College London

    • 1:25 PM 1:30 PM
      Welcome words Harrie Massey Lecture Theatre

      Harrie Massey Lecture Theatre

      University College London

      Convener: David B Cassidy
    • 1:30 PM 3:30 PM
      Positronium Physics Harrie Massey Lecture Theatre

      Harrie Massey Lecture Theatre

      University College London

      Convener: David B Cassidy
      • 1:30 PM
        Measurement of the positronium $2\,^3\mathrm{P}_J$ fluorescence decay rate 30m

        Positronium (Ps) decay rate measurements can be used to test QED theory [1], and in the search for new physics. Previous experiments have measured the annihilation decay rates of the ground states [2,3], and the excited $2\,^3\mathrm{S}_1$ state [4] of Ps. We report a measurement of the fluorescence decay rate of Ps atoms in the $2\,^3\mathrm{P}_J$ level using a new technique. By applying an electric field to metastable $2\,^3\mathrm{S}_1$ Ps atoms, we produced Stark-mixed states with both $2\,^3\mathrm{S}_1$ and $2\,^3\mathrm{P}_J$ components, and controlled their relative populations by varying the strength of the field. A larger electric field was subsequently applied to induce rapid quenching, and annihilation, of the mixed states. The number of quenched atoms across various mixing field strengths was measured, and these measurements, along with Monte Carlo simulations, were used to determine the decay rate of the $2\,^3\mathrm{P}_J$ state.

        [1] S. G Karshenboim, Precision study of positronium: Testing bound state QED theory, Int. J. Mod. Phys. A 19, 3879 (2004).
        [2] Al-Ramadhan A. H. and Gidley D. W., New precision measurement of the decay rate of singlet positronium, Phys. Rev. Lett., 72 1632 (1994).
        [3] Nico J. S., Gidley D. W., Rich A. and Zitzewitz, Precision measurement of the orthopositronium decay rate using the vacuum technique, P. W., Phys. Rev. Lett., 65, 1344 (1990).
        [4] R. E. Sheldon et al., Measurement of the annihilation decay rate of $2\,^3\textrm{S}_1$ positronium, EPL 132 13001 (2020).

        Speaker: Rebecca J Daly (University College London)
      • 2:00 PM
        Positronium density measurement technique using Polaritonics 30m

        Positronium (Ps) is an electron-positron bound state forming in fact a meta-stable two-body atomic system. As Ps is bosonic, a long term goal of Ps physics has been the formation of an ensemble of Ps atoms cold/dense enough to create a Bose-Einstein Condensate (BEC). Its primary motivation is that such a system may exhibit the phenomenon of stimulated annihilation, allowing for the creation of a gamma-ray laser. Recent experimental advances in Positronium physics have made it possible to produce dense Ps ensembles confined within the voids of porous materials, paving the way to the realization of a Ps BEC. In order to achieve this latter goal it would be advantageous to develop new methods to measure Ps densities in real-time. Here we describe a possible approach to do this exploiting concepts of cavity quantum electrodynamics (CQED), the field that investigates the interaction between dipolar active transitions in atoms, molecules or other materials, and single photons inside an optical cavity. When the light-matter coupling strength becomes larger than the loss rates of the light and matter excitations, the system enters the so-called strong coupling (SC) regime, and it can be described only in terms of the light–matter hybrid eigenmodes of the coupled system, often named polaritons. Using realistic experimental parameters we demonstrate that a dense Ps gas, can be strongly coupled to the photonic field of a distributed Bragg reflector microcavity. In this strongly coupled regime, the optical spectrum of the system is composed of two hybrid Ps polariton resonances separated by the vacuum Rabi splitting, which is proportional to the square root of the optically active atoms, and as such can serve as a direct measurement of the Ps density. This phenomenology is demonstrated for the 1S ↔ 2P optical transition in the UV spectrum by employing AlGaN/AlN DBRs, and for transitions between Rydberg states (n > 10) coupled to a high-quality-factor DBR cavity in the mid- to far-infrared range. Given that polaritons can be created on a sub-cycle timescale, a spectroscopic measurement of the vacuum Rabi splitting could be used as an ultra-fast Ps density measurement in regimes relevant to Ps BEC formation. Moreover, we show that Ps-polaritons could also enter the ultrastrong light-matter coupling regime potentially exhibiting its rich phenomelogy, such as the presence of virtual excitations in the ground state and the possibility of modifying the Ps wavefunction.
        In conclusion, the new found connection between positronium research and polaritonics not only solves one important problem in the path towards the achievement of Ps BEC, that is the ultrafast real-time measurement of the Ps population, but also offers to polaritonic scientists the unique opportunity of exploring non perturbative light-matter phenomena on a novel powerful polaritonic platform.

        Speaker: Erika Cortese (University of Southampton)
      • 2:30 PM
        Similarity of the near-threshold cross-sections for positronium formation and photoionisation in polyatomic molecules 30m

        This work reveals a remarkable similarity between the near-threshold energy dependence of the positronium (Ps) formation and photoionisation cross sections for several polyatomic molecules. The Ps-formation cross sections have been measured using a high resolution ($\sim$35 meV FWHM) trap-based positron beam for energies within a few eV of the Ps-formation threshold, $E_\text{thr} = E_I - |E_\text{Ps}|$, where $E_I$ is the target ionisation energy, and $E_\text{Ps}=-6.8$ eV is the Ps ground-state energy. Here we present the cross sections for aniline (C$_6$H$_5$NH$_2$), pyridine (C$_5$H$_5$N), and cyclopentane (C$_5$H$_{10}$). The cross section magnitude for pyridine are in agreement with earlier low-resolution data [1]. The Ps-formation cross sections are compared with measured photoionisation cross sections shifted to the Ps-formation threshold, with magnitudes scaled by a constant factor. This comparison shows that for each molecule, the two cross sections have a nearly identical dependence on the excess energy within a 1$-$2 eV of the threshold. This similarity is contrary to the significant difference between the two processes. In Ps formation, the interaction between the slow Ps and the residual cation is of short range, while in photoionisation, the electron moves in the attractive Coulomb field of the ion. As a result, at the basic level, the Ps-formation cross section is expected to obey the Wigner threshold law $\sigma \propto (E-E_\text{thr})^{1/2}$, while the photoionisation cross section must have a step-like onset with $\sigma =\text{const}$ at threshold. We discuss possible reasons for this similarity. In particular, it appears that the observed near-threshold behaviour is governed by vibrational excitations of the molecular cation and Franck-Condon factors, rather than the energy dependence of the underlying leptonic cross sections. For pyridine, this explanation is supported directly by comparisons with high-quality theoretical data for the vibronic excitation spectrum intensities [2]. We also discuss where the similarity breaks down and in what other targets it can be expected.

        We are grateful to A. B. Trofimov for providing the results of their calculations in numerical form, and for numerous useful discussions.

        References

        1. D. Stevens, T. J. Babij, J. R. Machacek, S. J. Buckman, M. J.
          Brunger, R. D. White, G. García, F. Blanco, L. Ellis-Gibbings, J. P.
          Sullivan, Positron scattering from pyridine, J. Chem. Phys.
          148, 144308 (2018).
        2. A. B. Trofimov, A. D. Skitnevskaya, E. K. Grigoricheva, E. V.
          Gromov, H. Köppel; Vibronic coupling in the ground and excited
          states of the pyridine radical cation, J. Chem. Phys. 153,
          164307 (2020).
        Speaker: Gleb Gribakin (Queen's University Belfast)
      • 3:00 PM
        Multiple Compton scattering of entangled photons produced by ground-state parapositronium disintegration 30m

        The effect of entanglement on the scattering distributions of Bell-state photons participating in multiple Compton scattering events is an open question. This study addresses this question by proposing a methodology to compute cross sections that, in turn, allow predictions to be made about the scattering distributions. The framework is applied to the Compton scattering of Bell states created through the disintegration of parapositronium in the ground state. The focus is on calculating the cross section for 3-Compton scattering events, where one of the photons undergoes intermediate Compton scattering, and then both photons are detected using Compton polarimeters operating in coincidence mode. The correlation amplitude that governs the strength of the azimuthal correlations between the scattered photons is analysed with respect to the intermediate scattering angle. The correlation amplitude exhibits various characteristics, including enhancement, reduction, sign inversion, or vanishes at specific intermediate scattering angles.

        Speaker: Dr Peter Caradonna (University of York)
    • 3:30 PM 4:00 PM
      Coffee Break 30m E3/7 (UCL-Dept. of Physics and Astronomy)

      E3/7

      UCL-Dept. of Physics and Astronomy

    • 6:00 PM 8:00 PM
      Poster Session E3/7 (UCL-Dept. of Physics and Astronomy)

      E3/7

      UCL-Dept. of Physics and Astronomy

      • 6:00 PM
        Laser-driven positrons sources for Positron Annihilation Lifetime Spectroscopy 2h

        Positron Annihilation Lifetime Spectroscopy (PALS) is one of the most effective material analysis techniques at detecting sub-nanometer defects in materials. Current conventional PALS facilities use positron beams of low keV energies and long durations, resulting in poor penetration depth and durations similar to the annihilation lifetime (~150ps), therefore giving poor resolution. By using laser-driven positrons, tuneable MeV-scale energies can be attained with short durations (~30ps), improving both the resolution and allowing for volumetric studies. Here, preliminary experimental results using taranis at QUB will be shown, along with an outline of future investigations at ALFA

        Speaker: Temour Foster (Queen's University Belfast)
      • 6:00 PM
        Measuring the Orthopositronium Annihilation Decay Rate 20m

        As a purely leptonic system, precise measurement of the properties of positronium (Ps) offers a unique avenue for testing bound-state QED theory and physics beyond the standard model if experimental uncertainties can match or exceed those of theoretical results. The decay rate of the triplet ground-state (o-Ps) due to annihilation has been determined to an experimental uncertainty of 100 ppm [1], two orders of magnitude larger than current theory (2.7 ppm) [2]. Previous experimental methods [1, 3, 4] have required detailed models of the interaction of Ps with matter and fields, that ultimately limit the possible precision. Here we investigate an alternative technique, wherein an energetic o-Ps beam produced via positron-gas collisions in a gas cell [5, 6] is allowed to decay in free-space. The surviving fraction is determined for various flight times using an adjustable micro-channel plate detector. Initial results are presented and improvements required to achieve competitive precision are discussed.

        References:
        [1] Y. Kataoka, S. Asai, and T. Kobayashi, Phys. Lett. B, 671 219 (2009)
        [2] G. S. Adkins, D. B. Cassidy and J. Perez-Rios, Phys. Rep., 975 1 (2022)
        [3] R. S. Vallery, P. W. Zitzewitz, D. W. Gidley, Phys. Rev. Lett. 90 203403 (2003)
        [4] S. Asai, O. Jinnouchi, T. Kobayashi, Int. J. Mod. Phys. A 19 3927 (2004)
        [5] A. Ozen, A. J. Garner, G. Laricchia, Nucl. Instrum. and Meth. in Phys. Res. B 171 172 (2000)
        [6] D. M. Newson, T. J. Babij, D. B. Cassidy, Rev. Sci. Instrum., 94 083201 (2023)

        Speaker: Ivneet Bhangoo (University College London)
      • 6:00 PM
        Polarisation Sensitive Microwave Spectroscopy of the $2\,^3$S$_1 \rightarrow 2\,^{2S+1}$P$_1 (S = 0, 1)$ Transitions in Positronium 2h

        Precision spectroscopy of the positronium (Ps) $n = 2$ fine structure intervals has been performed several times to test bound state QED [1]. All previous measurements have used microwave waveguides with a fixed polarisation (e.g. [2,3,4]), and the effect of polarisation on these transitions has not been explored. The polarisation can change the subset of transitions driven by the radiation, changing Zeeman and Stark shifts, creating unwanted systematic effects.

        We have used a horn antenna [5], which can be rotated to change the polarisation of the microwave radiation, to measure line shapes of the $2\,^3$S$_1 \rightarrow 2\,^{2S+1}$P$_1~(S = 0, 1)$ transitions. This was done in a large magnetic field to create large Zeeman splitting between the states, and to allow the $S = 0$ transition, which is forbidden by charge conjugation symmetry [6]. We report the effects of microwave polarisation on the resonance frequency and strength of the measured transitions. This work is a precursor to precision waveguide experiments which will measure these energy intervals as well as test for possible charge symmetry violation in the $S = 0$ transition.

        [1] Precision spectroscopy of positronium: Testing bound-state QED theory and the search for physics beyond the Standard Model. G. S. Adkins, D. B. Cassidy, J. Pérez-Ríos; Phys. Rep. 975, 1 (2022).

        [2] Measurements of the $2\,^3$S$_1 \rightarrow 2\,^3$P$_J~(J = 0, 1, 2)$ fine-structure splittings in positronium. S. Hatamian, R. S. Conti, and A. Rich; Phys. Rev. Lett. 58, 1833 (1987).

        [3] Spectroscopy of excited state positronium. R. Ley, D. Hagena, D. Weil, G. Werth, W. Arnold, and H. Schneider; Hyperfine Interactions 89, 327–341 (1994).

        [4] Observation of asymmetric line shapes in precision microwave spectroscopy of the positronium $2\,^3$S$_1 \rightarrow 2\,^3$P$_J~(J = 0, 1, 2)$ fine-structure intervals. L. Gurung, T. J. Babij, J. Pérez-Ríos, S. D. Hogan and D. B. Cassidy; Phys. Rev. A. 103, 042805 (2021).

        [5] Microwave spectroscopy of positronium atoms in free space. R. E. Sheldon, T. J. Babij, S. H. Reeder, S. D. Hogan, and D. B. Cassidy; Phys. Rev. A 107, 042810 (2023).

        [6] Tests of the discrete symmetries C, P, and T in one-photon transitions of positronium. R. S. Conti, S. Hatamian, and A. Rich; Phys. Rev. A 33(5), 3495 (1986).

        Speaker: Ross E. Sheldon (UCL)
      • 6:00 PM
        Status of laser spectroscopy of antiprotonic helium at CERN 2h

        Metastable antiprotonic helium is a neutral three-body atom [1-6] that contains a helium nucleus, an electron occupying the 1s state, and an antiproton in a Rydberg state of large principal (n≈38) and orbital angular momentum (l=n-1) quantum numbers. Whereas spectroscopy of antihydrogen atoms probes the interaction between an antihadron and antilepton, the antiprotonic helium atom is a hadron-antihadron quantum bound system having the longest known lifetime that can be readily produced. The ASACUSA collaboration at CERN will utilize the unprecedented high-quality beam of the ELENA facility and the latest laser metrology techniques to carry out sub-Doppler two-photon laser spectroscopy of narrow resonances of antiprotonic helium atoms. By comparing the results with three-body QED calculations the antiproton-to-electron mass ratio and upper limits on exotic forces that can arise between the constituent particles will be precisely determined.

        [1]: V.I. Korobov, L. Hilico, J.-P. Karr, Phys. Rev. A, 032511 (2014).
        [2]: A. Sótér, H. Aghai-Khozani, D. Barna, A. Dax, L. Venturelli, M. Hori, Nature 603, 411 (2022).
        [3]: V.I. Korobov, L. Hilico, J.-P. Karr, Phys. Rev. A, 032511 (2014).
        [4]: M. Hori et al., Science 354, 610 (2016).
        [5]: M. Hori et al., Nature 475, 484 (2011).
        [6]: M. Hori, H. Aghai-Khozani, A. Sótér, A. Dax, D. Barna, Nature 581, 37 (2020).

        Speaker: Masaki Hori (Imperial College London)
      • 6:20 PM
        Positronium spectroscopy with a beam of excited-state atoms 20m

        Precision measurements of simple atomic systems [1] can be used to test fundamental theories and place constraints on physics beyond the standard model. Positronium is one such system and an attractive candidate as it is described to high-precision by bound-state quantum electrodynamics (QED) [2]. If one assumes QED is accurate in its current form, determination of lifetimes and intervals in positronium can constrain physics not included in the theory. Despite such promise, only a few properties of positronium have been determined precisely [3,4,5]. Application of instrumentation and techniques (e.g. [6]), novel to positronium, are required to at least match current theoretical uncertainties. In this work I describe the characterisation of an energetic beam of excited-state positronium atoms [7], towards precision microwave spectroscopy of the n=2 fine-structure using the method of separated fields [8]. Results of initial spectroscopic studies and recent progress are presented.


        [1] S. G. Karshenboim Phys. Rep. 422, 63 (2005)
        [2] G. S. Adkins et al. Phys. Rep. 975, 1 (2022)
        [3] M. S. Fee et al. Phys. Rev. A 48, 192 (1993)
        [4] Vallery et al. Phys. Rev. Lett. 90, 203402 (2003)
        [5] M. W. Ritter et al. Phys. Rev. A 30, 1331 (1984)
        [6] A. C. Vutha and E. A. Hessels Phys. Rev. A 92, 052504 (2015)
        [7] D. M. Newson et al. Rev. Sci. Instrum. 94, 083201 (2023)
        [8] T. J. Babij and D. B. Cassidy, Eur. Phys. J. D 76, 121 (2022)

        Speaker: Donovan Newson
      • 6:40 PM
        A scheme for testing antimatter gravity with positronium using Rydberg atom interferometry 20m

        Measurements of the acceleration due to Earth’s gravity of positronium (Ps) atoms complement tests of antimatter gravity with antihydrogen at CERN [1], and extend tests of the Weak Equivalent Principle to purely leptonic systems. In its ground state, the annihilation lifetime of Ps of 142ns precludes precise measurements of g. However, when excited to Rydberg states annihilation is suppressed and such excited atoms can have lifetimes of >10µs [2]. These extended lifetimes can be exploited to perform interferometric measurements of g using a scheme we have developed, which is an electric analogue of Stern-Gerlach interferometry [3]. This is implemented by preparing the atoms in superpositions of Rydberg states with different static electric dipole moments, and using inhomogeneous electric fields to exert state dependent forces on them [4]. We will present the scheme and design of a full loop Rydberg-atom interferometer of this kind to be implemented to measure g for helium, and which can subsequently be extended to experiments with Ps.

        [1] Anderson, E.K., Baker, C.J., Bertsche, W. et al., Nature 621, 716–722 (2023)
        [2] A. Deller, B. S. Cooper, S. D. Hogan and D. B. Cassidy, Phys. Rev. A 93, 062513 (2016)
        [3] Y. Margalit, O. Dobkowski, Z. Zhou, O. Amit, Y. Japha, S. Moukouri, D. Rohrlich, A. Mazumdar, S. Bose, C. Henkel, R. Folman, Sci. Adv 7, 22 (2021)
        [4] J. E. Palmer and S. D. Hogan, Phys. Rev. Lett 122, 250404 (2019); J. D. R. Tommey and S. D. Hogan, Phys. Rev. A 104, 033305 (2021)

        Speaker: Stephen Hogan
    • 10:00 AM 10:30 AM
      Morning coffee 30m E3/7 (UCL-Dept. of Physics and Astronomy)

      E3/7

      UCL-Dept. of Physics and Astronomy

    • 10:30 AM 12:30 PM
      Material Science Harrie Massey Lecture Theatre

      Harrie Massey Lecture Theatre

      University College London

      Convener: Mark Law (University of Aberdeen)
      • 10:30 AM
        Laser-driven positrons sources for Positron Annihilation Lifetime Spectroscopy 30m

        Positron Annihilation Lifetime Spectroscopy (PALS) is one of the most effective material analysis techniques at detecting sub-nanometer defects in materials. Current conventional PALS facilities use positron beams of low keV energies and long durations, resulting in poor penetration depth and durations similar to the annihilation lifetime (~150ps), therefore giving poor resolution. By using laser-driven positrons, tuneable MeV-scale energies can be attained with short durations (~30ps), improving both the resolution and allowing for volumetric studies. Here, preliminary experimental results using taranis at QUB will be shown, along with an outline of future investigations at alfa

        Speaker: Temour Foster (Queen's University Belfast)
      • 11:00 AM
        Detection and Identification of Vacancy Defects in Antimony Selenide 30m

        Antimony selenide (Sb$_2$Se$_3$) is photovoltaic material with an optimal bandgap and a high optical absorption coefficient comprising of earth abundant elements. Solar cell power conversion efficiencies initially increased markedly but more recently the rate of increase has slowed. There is a large open circuit voltage consistent with the presence of detrimental concentrations of point defects. Here we report the results of variable energy positron annihilation lifetime measurements and related two component density functional theory calculations of positron lifetimes. Measurements have been performed on single crystal and thin film samples exhibiting p-type, intrinsic and n-type conductivity. The results provide evidence for the presence of both monovacancy and divacancy defects. They are consistent with both the Se and the Sb vacancies exhibiting negative charges states in intrinsic and n-type samples.

        Speaker: Prof. David Keeble (University of Dundee)
      • 11:30 AM
        Vacancy defect identification in lead halide perovskite materials 30m

        There is intensive focus on the development of lead halide perovskite for a range of photonic devices including solar cells and gamma-ray detectors. Lead occupies the perovskite B-site where it is octahedrally coordinated by halide anions, while the larger A-site accommodates a small organic molecular ion, for example, methylammonium (MA) (CH$_3$NH$_3$$^+$). The rapid development has, in part, been attributed to reports of modest defect densities, there is nevertheless intense efforts to identify and quantify point defects in these materials. The Vacancy defects are a centrally important class hence positron annihilation spectroscopies are of directly relevance. Here we report the results of variable energy positron annihilation lifetime spectroscopy measurements performed on a range of lead halide perovskites using the mono-energetic positron source beamline at Helmholtz–Zentrum Dresden–Rossendorf. Measurements have been performed on single crystal MAPbI$_3$, MAPbBr$_3$, MAPbCl$_3$, and on formamidinium (FA) lead bromide, FAPbBr$_3$. Comparison with two-component density functional theory calculated lifetime for positron states localised at vacancy defects provide evidence for the identification of both Pb vacancy and MA-vacancies.

        Speaker: Mr Aryaveer Singh (Physics, SUPA, School of Science and Engineering, University of Dundee, Dundee DD1 4HN, United Kingdom)
      • 12:00 PM
        Neural Network Variational Monte Carlo for Positronic Chemistry 30m

        In recent years, several authors have demonstrated that deep neural networks can produce excellent representations of the many-body wavefunction for use in variational Monte Carlo calculations of the ground-state properties of atoms, small molecules, and simple solids. In this talk, I will briefly introduce the principle of neural network variational Monte Carlo, and discuss how it is quite straightforward to extend the neural network wavefunction to the description of multi-component (positrons and electrons) wavefunctions. I will share our initial, quite promising, results for the positron binding energy and annihilation rate of several well-studied molecules, and discuss why NNVMC may be particularly well suited for this problem.

        Speaker: Gino Cassella (Imperial College)
    • 12:30 PM 12:45 PM
      Closing remarks Harrie Massey Lecture Theatre

      Harrie Massey Lecture Theatre

      University College London

      Convener: David B Cassidy