Atomic Spectroscopy And Collisions Using Slow Antiprotons Research Articles

Slow positron production and storage for the ASACUSA-Cusp experiment

The ASACUSA (atomic spectroscopy and collisions using slow antiprotons) Cusp experiment requires the production of dense positron plasmas with a high repetition rate to produce a beam of antihydrogen. In this work, details of the positron production apparatus used for the first observation of the antihydrogen beam, and subsequent measurements, are described in detail. This apparatus replaced the previous compact trap design resulting in an improvement in the positron accumulation rate by a factor of $52\pm 3$ .

Upgrade of the scintillator detector for particle tracking in experiments with antiprotons

Experiments with antiprotons often require the tracking of charged particles emerging from the annihilation process. The Atomic Spectroscopy And Collisions Using Slow Antiprotons (ASACUSA) collaboration at the CERN Antiproton Decelerator (AD) used several panels of scintillating bars placed around the interaction region to detect the passage of charged pions and determine the annihilation vertex position and time. The panels were composed by extruded scintillating bars and the light was collected using WaveLength Shifting (WLS) fibers and multi-anode PhotoMultiplier Tubes (PMTs). After operating for several years, the fiber-PMT coupling quality had degraded and a major upgrade of the light readout system was planned. The PMTs will be replaced by Silicon PhotoMultipliers (SiPMs) and the front-end electronics changed accordingly. An improvement is expected in the efficiency and the uniformity of the detector response. In this contribution the commissioning of the upgrade will be described, including the results of preliminary tests with cosmic rays.

Upgrade of ASACUSA’s antihydrogen detector

The goal of the ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) CUSP experiment at CERN’s Antiproton Decelerator is to measure the ground state hyperfine splitting of antihydrogen in order to test whether CPT invariance is broken.The ASACUSA hodoscope is a detector consisting of two layers of 32 plastic scintillator bars individually read out by two serially connected silicon photomultipliers (SiPMs) on each end. Two additional layers for position resolution along the beam axis were scintillator fibres, which will now be replaced by scintillating tiles placed onto the existing bars and also read out by SiPMs. If the antiproton of antihydrogen annihilates in the centre of the hodoscope, particles (mostly pions) are produced and travel through the various layers of the detector and produce signals.The hodoscope was successfully used during the last data taking period at CERN. The necessary time resolution to discriminate between particles travelling through the detector from outside and particles produced in the centre of the detector was achieved by the use of waveform digitisers and software constant fraction discrimination. The disadvantage of this readout scheme was the slow readout speed, which was improved by two orders of magnitude. This was done by omitting the digitisers and replacing them with TDCs reading out the digital time-over-threshold (ToT) signal using leading edge discrimination.

Measurement of the principal quantum number distribution in a beam of antihydrogen atoms

The ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration plans to measure the ground-state hyperfine splitting of antihydrogen in a beam at the CERN Antiproton Decelerator with initial relative precision of 10^{-6} or better, to test the fundamental CPT (combination of charge conjugation, parity transformation and time reversal) symmetry between matter and antimatter. This challenging goal requires a polarised antihydrogen beam with a sufficient number of antihydrogen atoms in the ground state. The first measurement of the quantum state distribution of antihydrogen atoms in a low magnetic field environment of a few mT is described. Furthermore, the data-driven machine learning analysis to identify antihydrogen events is discussed.Graphic

Single-photon laser spectroscopy of cold antiprotonic helium

Some laser spectroscopy experiments carried out by the Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) collaboration to measure the single-photon transition frequencies of antiprotonic helium (overline {p}text {He}^{+}equiv overline {p}+text {He}^{2+}+e^{-}) atoms are reviewed. The overline {p}text {He}^{+} were cooled to temperature T = 1.5–1.7 K by buffer-gas cooling in a cryogenic gas target, thus reducing the thermal Doppler width in the single-photon resonance lines. The antiproton-to-electron mass ratio was determined as M_{overline {p}}/m_{e}=u20091836.1526734(15) by comparisons with the results of three-body quantum electrodynamics calculations. This agreed with the known proton-to-electron mass ratio.

Recent progress of laser spectroscopy experiments on antiprotonic helium.

The Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) collaboration is currently carrying out laser spectroscopy experiments on antiprotonic helium [Formula: see text] atoms at CERN's Antiproton Decelerator facility. Two-photon spectroscopic techniques have been employed to reduce the Doppler width of the measured [Formula: see text] resonance lines, and determine the atomic transition frequencies to a fractional precision of 2.3-5 parts in 109 More recently, single-photon spectroscopy of buffer-gas cooled [Formula: see text] has reached a similar precision. By comparing the results with three-body quantum electrodynamics calculations, the antiproton-to-electron mass ratio was determined as [Formula: see text], which agrees with the known proton-to-electron mass ratio with a precision of 8×10-10 The high-quality antiproton beam provided by the future Extra Low Energy Antiproton Ring (ELENA) facility should enable further improvements in the experimental precision.This article is part of the Theo Murphy meeting issue 'Antiproton physics in the ELENA era'.

Precision laser spectroscopy experiments on antiprotonic helium

At CERN‘s Antiproton Decelerator (AD) facility, the Atomic Spectroscopyand Collisions Using Slow Antiprotons (ASACUSA) collaboration is carrying out precise laser spectroscopy experiments on antiprotonic helium (p̅He+ ≡ p̅+He2++e−) atoms. By employing buffer-gas cooling techniquesin a cryogenic gas target, samples of atoms were cooled to temperatureT = 1.5–1.7 K, thereby reducing the Doppler width in the single-photon resonance lines. By comparing the results with three-body quantum electrodynamics calculations, the antiproton-to-electron mass ratio was determined as Mp̅/me = 1836.1526734(15). This agreed with the known proton-to-electron mass ratio with a precision of 8 . 1010. Further improvements in the experimental precision are currently being attempted. The high-quality antiproton beam provided by the future Extra Low Energy Antiproton Ring (ELENA) facility should further increase the experimental precision.

Numerical simulations of hyperfine transitions of antihydrogen

One of the ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration’s goals is the measurement of the ground state hyperfine transition frequency in antihydrogen, the antimatter counterpart of one of the best known systems in physics. This high precision experiment yields a sensitive test of the fundamental symmetry of CPT. Numerical simulations of hyperfine transitions of antihydrogen atoms have been performed providing information on the required antihydrogen events and the achievable precision.

Segmented scintillation detectors with silicon photomultiplier readout for measuring antiproton annihilations

The Atomic Spectroscopy and Collisions Using Slow Antiprotons experiment at the Antiproton Decelerator (AD) facility of CERN constructed segmented scintillators to detect and track the charged pions which emerge from antiproton annihilations in a future superconducting radiofrequency Paul trap for antiprotons. A system of 541 cast and extruded scintillator bars were arranged in 11 detector modules which provided a spatial resolution of 17 mm. Green wavelength-shifting fibers were embedded in the scintillators, and read out by silicon photomultipliers which had a sensitive area of 1 × 1 mm(2). The photoelectron yields of various scintillator configurations were measured using a negative pion beam of momentum p ≈ 1 GeV/c. Various fibers and silicon photomultipliers, fiber end terminations, and couplings between the fibers and scintillators were compared. The detectors were also tested using the antiproton beam of the AD. Nonlinear effects due to the saturation of the silicon photomultiplier were seen at high annihilation rates of the antiprotons.

Near-infrared laser spectroscopy of antiprotonic helium atoms

The ASACUSA (Atomic Spectroscopy and Collisions Using Slow Antiprotons) collaboration is currently attempting to observe a laser-induced resonant transition of antiprotonic helium atoms at wavelength of 1154.9 nm. Motivations to observe this transition and a nanosecond near-infrared laser prepared for this work are presented.

Beam profile monitor for antiproton-nucleus annihilation cross section measurements

The ASACUSA (the Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration is planning to measure the antiproton-nucleus annihilation cross sections at kinetic energy 120 keV on targets of various mass numbers (C, Ni, Sn, and Pt) using the Antiproton Decelerator (AD) of CERN. No previous measurement exists in this region where the A-dependence of this cross section is expected to deviate from the A(2/3) as reported by the Obelix collaboration. For this measurement, a beam profile monitor based on secondary electron emission with a grid of electrode pads fabricated on an FR4-type glass-epoxy circuit board was developed. The advantage of this detector is that it is simple, lightweight, and low cost. It was used to measure the spatial profile of 100-ns-long beam pulses containing > 6 × 104 antiprotons with an active area of 40 mm × 40 mm and a spatial resolution of 4 mm.

Beam profile monitor for annihilation cross section measurements of antiprotons at 100 keV

The ASACUSA (the Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration is planning to measure the cross sections of antiproton annihilations at kinetic energy 100 keV on targets of various mass numbers (C, Ni, Sn, and Pt) using the Antiproton Decelerator (AD) of CERN. No previous measurement exists in this region where the A-dependence of the cross section is expected to deviate from the A\(^{\frac{2}{3}}\) (Batty et al, Nucl Phys A 689:721, 2001) as reported by the Obelix collaboration. A beam profile monitor based on secondary electron emission with a grid of electrode pads fabricated on an FR4-type glass-epoxy circuit board was developed for this measurement. The advantage of this kind of detector is that it is simple, lightweight, and low cost. It was used to measure the spatial profile of 100-ns-long beam pulses containing > 6 × 104 antiprotons with an active area of 40 mm × 40 mm and a spatial resolution of 4 mm.

Development of Beam Profile Monitor for Antiproton Annihilation Cross Section Measurements by the ASACUSA Collaboration

The ASACUSA (the Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration has developed various kinds of monitors which both destructively and non-destructively measure the spatial profiles of pulsed antiproton beams at the Antiproton Decelerator (AD) of CERN. These include detectors consisting of foil and wire electrodes based on gas ionization or secondary electron emission, which are compatible with ultra-high vacuum conditions. A new beam profile monitor with a grid of electrode pads fabricated on a normal glass-epoxy circuitboard was adapted for the measurement of antiproton annihilation cross sections. This monitor has an active area of 40 mm × 40 mm and a spatial resolution of 4 mm. It can measure the spatial profile of 40-50-ns-long beam pulses containing > 104 antiprotons striking each anode strip.

Antiprotonic Helium Atoms

At CERN's antiproton decelerator (AD), the ASACUSA (atomic spectroscopy and collisions using slow antiprotons) collaboration, a Japan–Austria–Hungary–Italy–Denmark–Germany group led by the author, has been working on the precision laser spectroscopy of antiprotonic helium atoms since 2000 [1]. By comparing the measured laser transition frequencies with three-body quantum electrodynamics (QED) calculations [2–4], we have determined the antiproton-to-electron mass ratio m p /me to the level of 2 × 10−9 [5]. Together with the atomic transition frequencies in hydrogen and deuterium, our results (assuming CPT symmetry, i.e., the equality of particle-antiparticle masses) yield information on the Rydberg constant, the proton and deuteron charge radii, and the relative atomic mass of the electron, in the CODATA recommended values of the fundamental physical constants 2006 [6].

Efficient Accumulation of Antiprotons and Positrons, Production of Slow Mono-Energetic Beams, and Their Applications

Recent progress of ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) project, particularly the antiproton trapping and slow antiproton production, is discussed. An RFQD (Radio Frequency Quadrupole Decelerator) installed in the ASACUSA beam line has an excellent deceleration efficiency of 25% providing 10-130keV antiprotons, which improves the final accumulation efficiency at least one and half orders of magnitude. The decelerated antiprotons are then injected in a large volume multiring trap, stored, and electron-cooled. About 1 million antiprotons are successfully accumulated per one AD shot and 10-500eV antiprotons are extracted as a mono-energetic beam. A UHV compatible positron accumulation is newly developed combining electron plasma and an ion cloud, which yields an accumulation rate as high as 400e + /s/mCi, two and a half orders of magnitude higher than other UHV compatible schemes. A new scheme to synthesize a spin-polarized antihydrogen beam is also discussed, which will play a vital role in determining the magnetic moment of antiproton with high precision.

Intense Ultra Slow Antiproton Beam and its Application

In the ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) collaboration, the Trap group has been working on an efficient accumulation of antiprotons and production of ultra slow mono-energetic antiproton beam, which is realized by a combination of an RFQD (Radio Frequency Quadrupole Decelerator) and a large multi-ring trap (MRT) installed in a super-conducting solenoid. We have succeeded to accumulate several million antiprotons. A mono-energetic antiproton beam of 10 eV has been extracted and transported through a specially designed beam line, which has a high transport efficiency and at the same time enabling differential pumping of more than six orders of magnitude between the MRT and a collision chamber. This configuration was adopted to make atomic collision experiments like ionization and antiprotonic atom formation processes and also to study spectroscopic nature of various meta-stable antiprotonic atoms under single collision conditions, which has never been possible. A new scheme of efficient positron accumulation has been invented employing high-density electron plasma and an ion cloud, which fits quite well with the UHV requirements of antihydrogen synthesis as well as other applications. A so-called cusp trap configuration has been proposed as a new synthesizer of antihydrogen, where antiprotons and positrons can co-exist in the same place even at low temperature. Because of the inhomogeneous magnetic field distribution of the cusp trap, (1) a low energy component of the antihydrogen atom so formed can be trapped for a macroscopic time, and (2) a high energy component emerges as an intensity-enhanced and energy-filtered spin polarized antihydrogen beam, which is suitable for measurements of the hyperfine splitting of antihydrogen, and could provide the magnetic moment of antiproton with some ppm accuracy.

Production of ultra slow antiprotons, its application to atomic collisions and atomic spectroscopy – ASACUSA project

The Atomic Spectroscopy And Collisions Using Slow Antiprotons (ASACUSA) project aims at studying collision dynamics with slow antiprotons and high precision spectroscopy of antiprotonic atoms. To realize these purposes, the production of high quality ultra slow antiproton beams is essential, which is achieved by the combination of antiproton decelerator (AD) from 3 GeV to 5 MeV, a radio frequency quadrupole (RFQ) decelerator from 5 MeV to 50 keV, and finally an electromagnetic trap from 50 keV to 10 eV. From the atomic physics point of view, an antiproton is an extremely heavy electron and/or a negatively charged proton, i.e., the antiproton is a unique tool to shed light on collision dynamics from the other side of the world. In addition to this fundamentally important feature, the antiproton has also a big practical advantage, i.e., it annihilates with the target nuclei emitting several energetic pions, which provides high detection efficiency with very good time resolution. Many-body effects which are of great importance to several branches of science will be studied through ionization and antiprotonic atom formation processes under single collision conditions. Various antiprotonic atoms including protonium (p̄p) are expected to be meta-stable in vacuum, which is never true for those in dense media except for antiprotonic helium. High precision spectroscopy of protonium will for the first time become feasible benefited by this meta-stability. The present review reports briefly the production scheme of ultra slow antiproton beams and several topics proposed in the ASACUSA project.