[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: The ALPHA collaboration, based at CERN, has recently succeeded in confining cold antihydrogen atoms in a magnetic minimum neutral atom trap and has performed the first study of a resonant transition of the anti-atoms. The ALPHA apparatus will be described herein, with emphasis on the structural aspects, diagnostic methods and techniques that have enabled antihydrogen trapping and experimentation to be achieved.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2014; 735:319-340. DOI:10.1016/j.nima.2013.09.043 · 1.22 Impact Factor
[Show abstract][Hide abstract] 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. DOI:10.1088/1367-2630/16/1/013037 · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Silicon Vertex Detector (SVD) is the main diagnostic tool in the ALPHA-experiment. It provides precise spatial and timing information of antiproton (antihydrogen) annihilation events (vertices), and most importantly, the SVD is capable of directly identifying and analysing single annihilation events, thereby forming the basis of ALPHA's analysis. This paper describes the ALPHA SVD and its upgrade, installed in the ALPHA's new neutral atom trap.
13th Vienna Conference on Instrumentation; 12/2013
[Show abstract][Hide abstract] 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. DOI:10.1063/1.4811527 · 1.61 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: One of the goals of synthesizing and trapping antihydrogen is to study the validity of charge-parity-time symmetry through precision spectroscopy on the anti-atoms, but the trapping yield achieved in recent experiments must be significantly improved before this can be realized. Antihydrogen atoms are commonly produced by mixing antiprotons and positrons stored in a nested Penning-Malmberg trap, which was achieved in ALPHA by an autoresonant excitation of the antiprotons, injecting them into the positron plasma. In this work, a hybrid numerical model is developed to simulate antiproton and positron dynamics during the mixing process. The simulation is benchmarked against other numerical and analytic models, as well as experimental measurements. The autoresonant injection scheme and an alternative scheme are compared numerically over a range of plasma parameters which can be reached in current and upcoming antihydrogen experiments, and the latter scheme is seen to offer significant improvement in trapping yield as the number of available antiprotons increases. (C) 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4801067]
Physics of Plasmas 04/2013; 20(4):043510. DOI:10.1063/1.4801067 · 2.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We describe the implementation of evaporative cooling of charged particles in the ALPHA apparatus. Forced evaporation has been applied to cold samples of antiprotons held in Malmberg-Penning traps. Temperatures on the order of 10 K were obtained, while retaining a significant fraction of the initial number of particles. We have developed a model for the evaporation process based on simple rate equations and applied it succesfully to the experimental data. We have also observed radial re-distribution of the clouds following evaporation, explained by simple conservation laws. We discuss the relevance of this technique for the recent demonstration of magnetic trapping of antihydrogen.
NON-NEUTRAL PLASMA PHYSICS VIII: 10th International Workshop on Non-Neutral Plasmas; 03/2013
[Show abstract][Hide abstract] ABSTRACT: Long term magnetic confinement of antihydrogen atoms has recently been demonstrated by the ALPHA collaboration at CERN, opening the door to a range of experimental possibilities. Of particular interest is a measurement of the antihydrogen spectrum. A precise comparison of the spectrum of antihydrogen with that of hydrogen would be an excellent test of CPT symmetry. One prime candidate for precision CPT tests is the ground-state
hyperfine transition; measured in hydrogen to a precision of nearly one part in 1012. Effective execution of such an experiment with trapped antihydrogen requires precise knowledge of the magnetic environment. Here we present a solution that uses an electron plasma confined in the antihydrogen trapping region. The cyclotron resonance of the electron plasma is probed with microwaves at the cyclotron frequency and the subsequent heating of the electron plasma is measured through the plasma quadrupole mode frequency. Using this method, the minimum magnetic field of the neutral trap can be determined to within 4 parts in 104. This technique was used extensively in the recent demonstration of resonant interaction with the hyperfine levels of trapped antihydrogen atoms.
NON-NEUTRAL PLASMA PHYSICS VIII: 10th International Workshop on Non-Neutral Plasmas; 03/2013
[Show abstract][Hide abstract] ABSTRACT: The ALPHA experiment, located at CERN, aims to compare the properties of antihydrogen atoms with those of hydrogen atoms. The neutral antihydrogen atoms are trapped using an octupole magnetic trap. The trap region is surrounded by a three layered silicon detector used to reconstruct the antiproton annihilation vertices. This paper describes a method we have devised that can be used for reconstructing annihilation vertices with a good resolution and is more efficient than the standard method currently used for the same purpose.
[Show abstract][Hide abstract] ABSTRACT: The goal of the ALPHA experiment is the production, trapping and spectroscopy of antihydrogen. A direct comparison of the ground state hyperfine spectra in hydrogen and antihydrogen has the potential to be a high-precision test of CPT symmetry. We present a novel method for measuring the strength of a microwave field for hyperfine spectroscopy in a Penning trap. This method incorporates a non-destructive plasma diagnostic system based on electrostatic modes within an electron plasma. We also show how this technique can be used to measure the cyclotron resonance of the electron plasma, which can potentially serve as a non-destructive measurement of plasma temperature.
[Show abstract][Hide abstract] ABSTRACT: The ALPHA project is an international collaboration, based at CERN, with the experimental goal of performing precision spectroscopic measurements on antihydrogen. As part of this endeavor, the ALPHA experiment includes a silicon tracking detector. This detector consists of a three-layer array of silicon modules surrounding the antihydrogen trapping region of the ALPHA apparatus. Using this device, the antihydrogen annihilation position can be determined with a spatial resolution of better than 5 mm. Knowledge of the annihilation distribution was a critical component in the recently successful antihydrogen trapping effort. This paper will describe the methods used to reconstruct annihilation events in the ALPHA detector. Particular attention will be given to the description of the background rejection criteria.
[Show abstract][Hide abstract] ABSTRACT: Precision spectroscopic comparison of hydrogen and antihydrogen holds the promise of a sensitive test of the Charge-Parity-Time
theorem and matter-antimatter equivalence. The clearest path towards realising this goal is to hold a sample of antihydrogen
in an atomic trap for interrogation by electromagnetic radiation. Achieving this poses a huge experimental challenge, as state-of-the-art
magnetic-minimum atom traps have well depths of only ∼1T (∼0.5K for ground state antihydrogen atoms). The atoms annihilate
on contact with matter and must be ‘born’ inside the magnetic trap with low kinetic energies. At the ALPHA experiment, antihydrogen
atoms are produced from antiprotons and positrons stored in the form of non-neutral plasmas, where the typical electrostatic
potential energy per particle is on the order of electronvolts, more than 104 times the maximum trappable kinetic energy. In November 2010, ALPHA published the observation of 38 antiproton annihilations
due to antihydrogen atoms that had been trapped for at least 172ms and then released—the first instance of a purely antimatter
atomic system confined for any length of time (Andresen etal., Nature 468:673, 2010). We present a description of the main components of the ALPHA traps and detectors that were key to realising this result.
We discuss how the antihydrogen atoms were identified and how they were discriminated from the background processes. Since
the results published in Andresen etal. (Nature 468:673, 2010), refinements in the antihydrogen production technique have allowed many more antihydrogen atoms to be trapped, and held
for much longer times. We have identified antihydrogen atoms that have been trapped for at least 1,000s in the apparatus
(Andresen etal., Nature Physics 7:558, 2011). This is more than sufficient time to interrogate the atoms spectroscopically, as well as to ensure that they have relaxed
to their ground state.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] ABSTRACT: The ALPHA experiment has succeeded in trapping antihydrogen, a major milestone on the road to spectroscopic comparisons of antihydrogen with hydrogen. An annihilation vertex detector, which determines the time and position of antiproton annihilations, has been central to this achievement. This detector, an array of double-sided silicon microstrip detector modules arranged in three concentric cylindrical tiers, is sensitive to the passage of charged particles resulting from antiproton annihilation. This article describes the method used to reconstruct the annihilation location and to distinguish the annihilation signal from the cosmic ray background. Recent experimental results using this detector are outlined.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 08/2012; 684:73–81. DOI:10.1016/j.nima.2012.04.082 · 1.22 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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. DOI:10.1088/1367-2630/14/1/015010 · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Over the last decades it has become routine to form beams of positrons and antiprotons and to use them to produce trapped samples of both species for a variety of purposes. Positrons can be captured efficiently, for instance using a buffer-gas system, and in such quantities to form dense, single component plasmas useful for antihydrogen formation. The latter is possible using developments of techniques for dynamically capturing and then cooling antiprotons ejected from the Antiproton Decelerator at CERN. The antiprotons can then be manipulated by cloud compression and evaporative cooling to form tailored plasmas. We will review recent advances that have allowed antihydrogen atoms to be confined for the first time in a shallow magnetic minimum neutral atom trap superimposed upon the region in which the antiparticles are held and mixed. A new mixing technique has been developed to help achieve this using autoresonant excitation of the centre-of-mass longitudinal motion of an antiproton cloud. This allows efficient antihydrogen formation without imparting excess energy to the antiprotons and helps enhance the probability of trapping the anti-atom.
THE 17TH INTERNATIONAL CONFERENCE ON ATOMIC PROCESSES IN PLASMAS (ICAPIP); 01/2012
[Show abstract][Hide abstract] 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]  Andresen,
G. B., et al (ALPHA), Phys. Rev. Lett. 106, 025002 (2011). 
Andresen, G. B., et al (ALPHA), Phys. Lett. B 695, 95-104 (2011).
[Show abstract][Hide abstract] 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