C. So

University of California, Berkeley, Berkeley, California, United States

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Publications (26)158.94 Total impact

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    ABSTRACT: The properties of antihydrogen are expected to be identical to those of hydrogen, and any differences would constitute a profound challenge to the fundamental theories of physics. The most commonly discussed antiatom-based tests of these theories are searches for antihydrogen-hydrogen spectral differences (tests of CPT (charge-parity-time) invariance) or gravitational differences (tests of the weak equivalence principle). Here we, the ALPHA Collaboration, report a different and somewhat unusual test of CPT and of quantum anomaly cancellation. A retrospective analysis of the influence of electric fields on antihydrogen atoms released from the ALPHA trap finds a mean axial deflection of 4.1±3.4 mm for an average axial electric field of 0.51 V mm−1. Combined with extensive numerical modelling, this measurement leads to a bound on the charge Qe of antihydrogen of Q=(−1.3±1.1±0.4) × 10−8. Here, e is the unit charge, and the errors are from statistics and systematic effects.
    Nature Communications 06/2014; 5. · 10.74 Impact Factor
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    ABSTRACT: We demonstrate a novel detection method for the cyclotron resonance frequency of an electron plasma in a Penning-Malmberg trap. With this technique, the electron plasma is used as an in situ diagnostic tool for measurement of the static magnetic field and the microwave electric field in the trap. The cyclotron motion of the electron plasma is excited by microwave radiation and the temperature change of the plasma is measured non-destructively by monitoring the plasma's quadrupole mode frequency. The spatially-resolved microwave electric field strength can be inferred from the plasma temperature change and the magnetic field is found through the cyclotron resonance frequency. These measurements were used extensively in the recently reported demonstration of resonant quantum interactions with antihydrogen.
    New Journal of Physics 01/2014; 16:013037. · 4.06 Impact Factor
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    ABSTRACT: Knowledge of the residual gas composition in the ALPHA experiment apparatus is important in our studies of antihydrogen and nonneutral plasmas. A technique based on autoresonant ion extraction from an electrostatic potential well has been developed that enables the study of the vacuum in our trap. Computer simulations allow an interpretation of our measurements and provide the residual gas composition under operating conditions typical of those used in experiments to produce, trap, and study antihydrogen. The methods developed may also be applicable in a range of atomic and molecular trap experiments where Penning-Malmberg traps are used and where access is limited.
    The Review of scientific instruments 06/2013; 84(6):065110. · 1.52 Impact Factor
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    ABSTRACT: Physicists have long wondered whether the gravitational interactions between matter and antimatter might be different from those between matter and itself. Although there are many indirect indications that no such differences exist and that the weak equivalence principle holds, there have been no direct, free-fall style, experimental tests of gravity on antimatter. Here we describe a novel direct test methodology; we search for a propensity for antihydrogen atoms to fall downward when released from the ALPHA antihydrogen trap. In the absence of systematic errors, we can reject ratios of the gravitational to inertial mass of antihydrogen >75 at a statistical significance level of 5%; worst-case systematic errors increase the minimum rejection ratio to 110. A similar search places somewhat tighter bounds on a negative gravitational mass, that is, on antigravity. This methodology, coupled with ongoing experimental improvements, should allow us to bound the ratio within the more interesting near equivalence regime.
    Nature Communications 04/2013; 4:1785. · 10.74 Impact Factor
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    ABSTRACT: The injection of antiprotons into positron plasma during antihydrogen synthesis in ALPHA is simulated numerically and compared with experimental measurements. The antiprotons and positrons are initially confined in adjacent axial potential wells in a nested Penning-Malmberg trap. The antiproton plasma is excited autoresonantly and partially injected into the adjacent positron plasma, creating antihydrogen. The excitation and injection process is modeled numerically with a hybrid code in which the antiproton plasma responds to the autoresonant drive fully dynamically, and the positrons respond quasi-statically. The strong axial magnetic field suppresses radial transport on the timescales of interest. The antiproton plasma is thus assumed to consist of concentric cylindrical tubes within which antiprotons move only in the axial direction, and the evolution of the phase space distributions in each tube obeys a one-dimensional Vlasov equation. The antiproton self-field is obtained by solving the Poisson equation in two-dimensions, thereby coupling the tubes. Alternative injection schemes and the effect of varying antiproton number and temperature are also examined.
    10/2012;
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    ABSTRACT: The hydrogen atom is one of the most important and influential model systems in modern physics. Attempts to understand its spectrum are inextricably linked to the early history and development of quantum mechanics. The hydrogen atom's stature lies in its simplicity and in the accuracy with which its spectrum can be measured and compared to theory. Today its spectrum remains a valuable tool for determining the values of fundamental constants and for challenging the limits of modern physics, including the validity of quantum electrodynamics and--by comparison with measurements on its antimatter counterpart, antihydrogen--the validity of CPT (charge conjugation, parity and time reversal) symmetry. Here we report spectroscopy of a pure antimatter atom, demonstrating resonant quantum transitions in antihydrogen. We have manipulated the internal spin state of antihydrogen atoms so as to induce magnetic resonance transitions between hyperfine levels of the positronic ground state. We used resonant microwave radiation to flip the spin of the positron in antihydrogen atoms that were magnetically trapped in the ALPHA apparatus. The spin flip causes trapped anti-atoms to be ejected from the trap. We look for evidence of resonant interaction by comparing the survival rate of trapped atoms irradiated with microwaves on-resonance to that of atoms subjected to microwaves that are off-resonance. In one variant of the experiment, we detect 23 atoms that survive in 110 trapping attempts with microwaves off-resonance (0.21 per attempt), and only two atoms that survive in 103 attempts with microwaves on-resonance (0.02 per attempt). We also describe the direct detection of the annihilation of antihydrogen atoms ejected by the microwaves.
    Nature 03/2012; 483(7390):439-43. · 38.60 Impact Factor
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    ABSTRACT: Recently, antihydrogen atoms were trapped at CERN in a magnetic minimum (minimum-B) trap formed by superconducting octupole and mirror magnet coils. The trapped antiatoms were detected by rapidly turning off these magnets, thereby eliminating the magnetic minimum and releasing any antiatoms contained in the trap. Once released, these antiatoms quickly hit the trap wall, whereupon the positrons and antiprotons in the antiatoms annihilate. The antiproton annihilations produce easily detected signals; we used these signals to prove that we trapped antihydrogen. However, our technique could be confounded by mirror-trapped antiprotons, which would produce seemingly identical annihilation signals upon hitting the trap wall. In this paper, we discuss possible sources of mirror-trapped antiprotons and show that antihydrogen and antiprotons can be readily distinguished, often with the aid of applied electric fields, by analyzing the annihilation locations and times. We further discuss the general properties of antiproton and antihydrogen trajectories in this magnetic geometry, and reconstruct the antihydrogen energy distribution from the measured annihilation time history.
    New Journal of Physics 01/2012; 14(1):015010. · 4.06 Impact Factor
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    ABSTRACT: In the ALPHA apparatus, low temperature antiprotons (=p) and positrons (e^+) are prepared adjacent to each other in a nested Penning trap. To create trappable antihydrogen (=H), the two species must be mixed such that some resultant =H atoms have sub-Kelvin kinetic energy. A new simulation has been developed to study and optimize the autoresonant mixing [1,2] in ALPHA. The =p dynamics are governed by their own self- field, the e^+ plasma field, and the external fields. The e^+'s are handled quasi-statically with a Poisson-Boltzmann solver. =p's are handled by multiple time dependent 1D Vlasov-Poisson solvers, each representing a radial slice of the plasma. The 1D simulatiuons couple through the 2D Poisson equation. We neglect radial transport due to the strong solenoidal field. The advantages and disadvantages of different descretization schemes, comparisons of simulation with experiment, and techniques for optimizing mixing, will be presented [4pt] [1] Andresen, G. B., et al (ALPHA), Phys. Rev. Lett. 106, 025002 (2011). [2] Andresen, G. B., et al (ALPHA), Phys. Lett. B 695, 95-104 (2011).
    11/2011;
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    ABSTRACT: ALPHA is an experiment at CERN, whose ultimate goal is to perform a precise test of CPT symmetry with trapped antihydrogen atoms. After reviewing the motivations, we discuss our recent progress toward the initial goal of stable trapping of antihydrogen, with some emphasis on particle detection techniques.
    05/2011;
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    ABSTRACT: Atoms made of a particle and an antiparticle are unstable, usually surviving less than a microsecond. Antihydrogen, made entirely of antiparticles, is believed to be stable, and it is this longevity that holds the promise of precision studies of matter-antimatter symmetry. We have recently demonstrated trapping of antihydrogen atoms by releasing them after a confinement time of 172 ms. A critical question for future studies is: how long can anti-atoms be trapped? Here we report the observation of anti-atom confinement for 1000 s, extending our earlier results by nearly four orders of magnitude. Our calculations indicate that most of the trapped anti-atoms reach the ground state. Further, we report the first measurement of the energy distribution of trapped antihydrogen which, coupled with detailed comparisons with simulations, provides a key tool for the systematic investigation of trapping dynamics. These advances open up a range of experimental possibilities, including precision studies of CPT symmetry and cooling to temperatures where gravitational effects could become apparent.
    Nature Physics 04/2011; 7(7). · 19.35 Impact Factor
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    ABSTRACT: Charges in cold, multiple-species, non-neutral plasmas separate radially by mass, forming centrifugally separated states. Here, we report the first detailed measurements of such states in an electron-antiproton plasma, and the first observations of the separation dynamics in any centrifugally separated system. While the observed equilibrium states are expected and in agreement with theory, the equilibration time is approximately constant over a wide range of parameters, a surprising and as yet unexplained result. Electron-antiproton plasmas play a crucial role in antihydrogen trapping experiments.
    Physical Review Letters 04/2011; 106(14):145001. · 7.73 Impact Factor
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    ABSTRACT: Sources of positrons and antiprotons that are currently used for the formation of antihydrogen with low kinetic energies are reviewed, mostly in the context of the ALPHA collaboration and its predecessor ATHENA. The experiments were undertaken at the Antiproton Decelerator facility, which is located at CERN. Operations performed on the clouds of antiparticles to facilitate their mixing to produce antihydrogen are described. These include accumulation, cooling and manipulation. The formation of antihydrogen and some of the characteristics of the anti-atoms that are created are discussed. Prospects for trapping antihydrogen in a magnetic minimum trap, as envisaged by the ALPHA collaboration, are reviewed.
    Journal of Physics Conference Series 01/2011; 262(1):012001.
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    ABSTRACT: Spectroscopy of antihydrogen has the potential to yield high-precision tests of the CPT theorem and shed light on the matter-antimatter imbalance in the Universe. The ALPHA antihydrogen trap at CERN’s Antiproton Decelerator aims to prepare a sample of antihydrogen atoms confined in an octupole-based Ioffe trap and to measure the frequency of several atomic transitions. We describe our techniques to directly measure the antiproton temperature and a new technique to cool them to below 10K. We also show how our unique position-sensitive annihilation detector provides us with a highly sensitive method of identifying antiproton annihilations and effectively rejecting the cosmic-ray background. KeywordsAntihydrogen–Antimatter–CPT–Penning trap–Atom trap
    Hyperfine Interactions 01/2011; 199(1):39-48. · 0.21 Impact Factor
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    ABSTRACT: We demonstrate controllable excitation of the center-of-mass longitudinal motion of a thermal antiproton plasma using a swept-frequency autoresonant drive. When the plasma is cold, dense, and highly collective in nature, we observe that the entire system behaves as a single-particle nonlinear oscillator, as predicted by a recent theory. In contrast, only a fraction of the antiprotons in a warm plasma can be similarly excited. Antihydrogen was produced and trapped by using this technique to drive antiprotons into a positron plasma, thereby initiating atomic recombination.
    Physical Review Letters 01/2011; 106(2):025002. · 7.73 Impact Factor
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    ABSTRACT: Precision comparisons of hyperfine intervals in atomic hydrogen and antihydrogen are expected to yield experimental tests of the CPT theorem. The CERN-based ALPHA collaboration has initiated a program of study focused on microwave spectroscopy of trapped ground-state antihydrogen atoms. This paper outlines some of the proposed experiments, and summarizes measurements that characterize microwave fields that have been injected into the ALPHA apparatus. KeywordsAntihydrogen–CPT–Hyperfine splitting–Penning-Malmberg trap–Neutral atom trap
    Hyperfine Interactions 01/2011; 212(1-3):1-10. · 0.21 Impact Factor
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    ABSTRACT: In efforts to trap antihydrogen, a key problem is the vast disparity between the neutral trap energy scale ( ~ 50\upmueV\sim\!50\,\upmu\mathrm{eV}), and the energy scales associated with plasma confinement and space charge (~1 eV). In order to merge charged particle species for direct recombination, the larger energy scale must be overcome in a manner that minimizes the initial antihydrogen kinetic energy. This issue motivated the development of a novel injection technique utilizing the inherent nonlinear nature of particle oscillations in our traps. We demonstrated controllable excitation of the center-of-mass longitudinal motion of a thermal antiproton plasma using a swept-frequency autoresonant drive. When the plasma is cold, dense and highly collective in nature, we observe that the entire system behaves as a single-particle nonlinear oscillator, as predicted by a recent theory. In contrast, only a fraction of the antiprotons in a warm or tenuous plasma can be similarly excited. Antihydrogen was produced and trapped by using this technique to drive antiprotons into a positron plasma, thereby initiating atomic recombination. The nature of this injection overcomes some of the difficulties associated with matching the energies of the charged species used to produce antihydrogen. KeywordsAntihydrogen–Plasma–Nonlinear–Dynamics
    Hyperfine Interactions 01/2011; · 0.21 Impact Factor
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    ABSTRACT: Antihydrogen spectroscopy promises precise tests of the symmetry of matter and antimatter, and can possibly offer new insights into the baryon asymmetry of the universe. Antihydrogen is, however, difficult to synthesize and is produced only in small quantities. The ALPHA collaboration is therefore pursuing a path towards trapping cold antihydrogen to permit the use of precision atomic physics tools to carry out comparisons of antihydrogen and hydrogen. ALPHA has addressed these challenges. Control of the plasma sizes has helped to lower the influence of the multipole field used in the neutral atom trap, and thus lowered the temperature of the created atoms. Finally, the first systematic attempt to identify trapped antihydrogen in our system is discussed. This discussion includes special techniques for fast release of the trapped anti-atoms, as well as a silicon vertex detector to identify antiproton annihilations. The silicon detector reduces the background of annihilations, including background from antiprotons that can be mirror trapped in the fields of the neutral atom trap. A description of how to differentiate between these events and those resulting from trapped antihydrogen atoms is also included.La spectroscopie de l'anti-hydrogène promet des tests précieux de la symétrie entre matière et antimatière dans l'univers. Cependant, l'anti-hydrogène est difficile à synthétiser et il n'est produit qu'en petite quantité. Le groupe de collaborateurs ALPHA poursuit donc des travaux pour capturer de l'anti-hydrogène froid afin de permettre l'utilisation d'outils précis de mesure en physique atomique pour comparer l'anti-hydrogène avec l'hydrogène. Nous montrons comment ALPHA s'est attaqué à cette tâche et comment le contrôle du volume de plasma a aidé à diminuer l'influence des champs multipolaires utilisés dans le piège pour atomes neutres et ainsi abaisser la température des atomes produits. Finalement, nous discutons le premier essai systématique pour identifier l'anti-hydrogène piégé dans notre système. Ceci inclut des techniques spéciales pour relâcher rapidement les anti-atomes piégés, ainsi qu'un détecteur au silicium pour identifier l'annihilation de l'anti-proton. Nous avons utilisé le détecteur au silicium pour réduire le fond d'annihilation, incluant celui produit par les anti-protons qui peuvent être dans le piège miroir des champs du piège à atomes neutres. Nous décrivons aussi comment nous pouvons différentier entre ces événements et ceux qui résultent des atomes d'anti-hydrogène piégés.
    Canadian Journal of Physics 12/2010; 89(1):7-16. · 0.90 Impact Factor
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    ABSTRACT: We present the results of an experiment to search for trapped antihydrogen atoms with the ALPHA antihydrogen trap at the CERN Antiproton Decelerator. Sensitive diagnostics of the temperatures, sizes, and densities of the trapped antiproton and positron plasmas have been developed, which in turn permitted development of techniques to precisely and reproducibly control the initial experimental parameters. The use of a position-sensitive annihilation vertex detector, together with the capability of controllably quenching the superconducting magnetic minimum trap, enabled us to carry out a high-sensitivity and low-background search for trapped synthesised antihydrogen atoms. We aim to identify the annihilations of antihydrogen atoms held for at least 130 ms in the trap before being released over ~30 ms. After a three-week experimental run in 2009 involving mixing of 10^7 antiprotons with 1.3 10^9 positrons to produce 6 10^5 antihydrogen atoms, we have identified six antiproton annihilation events that are consistent with the release of trapped antihydrogen. The cosmic ray background, estimated to contribute 0.14 counts, is incompatible with this observation at a significance of 5.6 sigma. Extensive simulations predict that an alternative source of annihilations, the escape of mirror-trapped antiprotons, is highly unlikely, though this possibility has not yet been ruled out experimentally.
    Physics Letters B 11/2010; 695:95. · 4.57 Impact Factor
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    ABSTRACT: Antimatter was first predicted in 1931, by Dirac. Work with high-energy antiparticles is now commonplace, and anti-electrons are used regularly in the medical technique of positron emission tomography scanning. Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN (the European Organization for Nuclear Research) since 2002. Antihydrogen is of interest for use in a precision test of nature's fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom (about two parts in 10(14) for the frequency of the 1s-to-2s transition), subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter. However, so far experiments have produced antihydrogen that is not confined, precluding detailed study of its structure. Here we demonstrate trapping of antihydrogen atoms. From the interaction of about 10(7) antiprotons and 7 × 10(8) positrons, we observed 38 annihilation events consistent with the controlled release of trapped antihydrogen from our magnetic trap; the measured background is 1.4 ± 1.4 events. This result opens the door to precision measurements on anti-atoms, which can soon be subjected to the same techniques as developed for hydrogen.
    Nature 11/2010; 468(7324):673-6. · 38.60 Impact Factor
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    ABSTRACT: The comparison of the 1S-2S energy levels of hydrogen and antihydrogen will yield a stringent test of CPT conservation. Necessarily, the antihydrogen atoms need to be trapped to perform high precision spectroscopy measurements. Therefore, an approximately 1 T deep neutral trap, about 0.7 K for ground state (anti)hydrogen atoms, has been superimposed on a Penning-Malmberg trap in which the antiatoms are formed. The antihydrogen atoms, which are required to have a low enough kinetic energy to be trapped, are produced following a number of steps. A bunch of antiprotons from the CERN Antiproton Decelerator are caught in a Penning-Malmberg trap and subsequently sympathetically cooled down and then compressed using rotating wall electric fields. A positron plasma, formed in a separate accumulator, is transported to the main system and also compressed. Antihydrogen atoms are then formed by mixing the antiprotons and positrons. The velocity of the antiatoms, and their binding energies, will strongly depend on the initial conditions of the constituent particles, for example their temperatures and densities, and on the details of the mixing process. In this talk the complete lifecycle of antihydrogen atoms will be presented, starting with the production of the constituent particles and the description of the manipulations necessary to prepare positrons and antiprotons appropriately for antihydrogen formation. The latter will also be described, as will the possible fates of the antiatoms.
    Journal of Physics Conference Series 11/2010; 257126162.

Publication Stats

48 Citations
158.94 Total Impact Points

Institutions

  • 2010–2014
    • University of California, Berkeley
      • Department of Physics
      Berkeley, California, United States
  • 2012
    • York University
      • Department of Physics and Astronomy
      Toronto, Ontario, Canada
  • 2011
    • The University of Tokyo
      • Department of Physics
      Tokyo, Tokyo-to, Japan
    • University of Zurich
      Zürich, Zurich, Switzerland
  • 2010–2011
    • Aarhus University
      • Department of Physics and Astronomy
      Aars, Region North Jutland, Denmark