Experimental set-up

To achieve our physics goal, we have developed the experimental apparatus shown in Fig. 1. It consists of an ultra-slow antiproton beam source (MUSASHI)16, a positron accumulator17, ultra-low energy beam transport lines and the cusp trap, which is a superposition of the strong magnetic field gradient provided by the superconducting anti-Helmholtz coil and an electrostatic potential10. For the preparation of antiproton and positron plasmas Penning–Malmberg type traps are used. They comprise a homogeneous strong axial magnetic field and an electrostatic potential. In all cases MRE18 are introduced into the respective magnets for the electrostatic manipulation of the trapped plasmas. Two scintillator modules are placed on each side of the cusp magnet, which are utilized to track pions produced by antiproton annihilation. This pion-tracking detector provides information about the antiproton annihilation position distribution. Downstream of the cusp trap a hyperfine spectrometer line is placed. It consists of a microwave cavity to induce spin flips, a superconducting sextupole magnet for spin-state analysis and an antihydrogen detector19. In the experiments described in this article, the microwave cavity is not used and the detector is placed 2.7 m downstream of the production region.

Figure 1: Schematic view of our experimental apparatus. Arrows represent 1 m in each direction. Antiprotons delivered from the AD via the RFQD are trapped, electron-cooled and radially compressed in the MUSASHI. Moderated positrons from a 22Na source are prepared and cooled in the positron accumulator and then are transported to the cusp trap. The cusp trap consists of an MRE and superconducting anti-Helmholtz coils. After positrons are accumulated near the maximum magnetic field region, antiprotons are injected from the MUSASHI and mixed with positrons synthesizing antihydrogen atoms. Antihydrogen atoms in low-field-seeking states are focused downstream of the cusp trap due to the strong magnetic field gradient, while those high-field-seeking states are de-focused. Thus, a polarized antihydrogen beam is produced. On both sides of the cusp trap, scintillator modules labelled as I–IV are mounted, which are used to track charged pions produced by annihilation reactions. Downstream of the cusp trap a spectrometer line is placed, which involves a sextupole magnet and an antihydrogen detector. Full size image

Antiprotons at 5.3 MeV from the AD pass a radio-frequency quadrupole decelerator (RFQD) that reduces their energy to 115 keV. Subsequently, the particles transmit through thin degrader foils (two 90 μg cm−2 biaxially oriented polyethylene terephtalate foils)20 and are trapped in the MUSASHI antiproton trap. The unique scheme of sequential combination of the RFQD and the MUSASHI trap enables us to use 5–50 times more antiprotons (over 106 in number) per AD cycle than other antihydrogen experiments16. In the MUSASHI trap the antiprotons are electron-cooled and radially compressed by a rotating electric field21. This preparation procedure enables the efficient transfer of an ultra-low energy antiproton beam to the cusp trap16.

Positrons are provided from a 22Na source with an activity of 0.6 GBq. The particles are moderated by Ne ice grown on a small cone in front of the source22,23. By interaction with a N 2 /CF 4 gas mixture the positrons are trapped and further thermalized in the MRE of the positron accumulator. Subsequently, the accumulated particles are transferred to the cusp trap. By repeating this sequence, typically 3 × 107 positrons are loaded into the cusp magnet in ~10 min.

Antihydrogen production in the cusp trap

For the production of antihydrogen atoms a nested well configuration is employed24,25,26, as shown in Fig. 2a, located in the local maximum of the cusp magnetic field. Simulations predict that when the mixing of positrons and antiprotons is performed at this position, the cusp magnetic field enhances the polarization of atoms, which flow out towards the downstream direction and pass the magnetic field minimum. About 3 × 105 antiprotons from the MUSASHI trap are injected into the positron plasma stored in the nested well. The kinetic energy of the antiproton beam is adjusted to be slightly above the potential energy of the plasma (Fig. 2a) in order to avoid significant heating induced by the antiproton injection. In contrast to the charged particles confined in the nested well, electrically neutral antihydrogen atoms escape from this potential configuration.

Figure 2: Antihydrogen synthesis. (a) Illustration of the direct injection scheme, which is used to produce antihydrogen atoms. A positron plasma is confined and compressed at the centre of the nested well (black solid line). The potential is opened (red solid line) when antiprotons with low energy spread are injected into the positron plasma. The antiproton kinetic energy is adjusted to slightly higher than the potential energy of the positron plasma (yellow solid line), which ensures efficient mixing of antiprotons and positrons. To prolong the interaction time during mixing, an rf drive (not shown in the figure) is applied, which drives the axial oscillation of the antiprotons. (b) The number of antihydrogen atoms field-ionized downstream of the nested trap as a function of time. The filled black squares are from an experiment when direct injection was applied. The filled red circles represent results obtained from the rf-assisted direct injection scheme. Error bars show s.d. of the mean. By applying the rf drive the yield of field-ionized antihydrogen atoms was increased by a factor of 3.5. Full size image

To monitor antihydrogen synthesis, we prepare a field ionization well 20 cm downstream of the mixing region11. An antihydrogen atom in a Rydberg state with principal quantum number n is field-ionized if n⩾(3.2/ε)1/4 × 102 is satisfied27, where ε (V cm−1) is the electric field strength. The average field strength is 139 V cm−1 (93 V cm−1 on axis), which can field-ionize antihydrogen atoms with n≳39. Here we define the average electric field strength, which is the mean value of the field averaged over the entire trap radius. Resulting antiprotons are trapped in the field-ionization well11,26. When this well is opened, the particles escape from the trap, annihilate and are counted by the pion-tracking detector. In such a direct injection scheme typically 75 field-ionization counts are obtained in a time interval of 80 s. To investigate the time evolution of antihydrogen formation, during the mixing process the field ionization well is opened and closed periodically. Results of that measurement are shown in Fig. 2b (filled squares). A maximum is reached after ~20 s, followed by a slow decrease explained by the axial separation of antiprotons and positrons. This separation is due to two possible processes: one process is the energy loss of the antiprotons by interaction with electrons formed in annihilation with the background gas; the other is the reduction of the antiproton’s axial energy due to collisional relaxation. When the axial energy of the antiprotons drops below the positron potential energy, the synthesis is stopped. This axial separation model is based on information obtained from our position-sensitive pion-tracking detector11.

To counteract the axial separation and to prolong the antihydrogen production period, an rf-assisted direct injection scheme was developed. During the mixing process an rf drive at 420 kHz is applied to one of the ring electrodes of the MRE, which excites the axial oscillation of the trapped antiprotons28. The filled red circles in Fig. 2b represent a typical result obtained from such an experiment. More than 260 antiprotons are counted in the time-window of 80 s, which is a factor of 3.5 more than without rf.

Detection in a magnetic field-free environment

The antihydrogen detector placed at the end of the spectrometer line is made out of a bismuth germanium oxide (Bi 4 Ge 3 O 12 , BGO) single-crystal. This scintillating material was selected because of its high density (7.13 g cm−3), high photon yield (8–10 per keV energy deposit) and ultra-high vacuum compatibility. The BGO crystal has a diameter of 10 cm and a thickness of 5 mm. It is placed inside a vacuum chamber with its centre on the beam axis. Outside the chamber, five plastic scintillator plates (thickness 10 mm, total solid angle coverage 49% of 4π) are installed to detect annihilation pions. Each scintillator is read-out by a photomultiplier tube. The BGO signal is recorded by a waveform digitizer while the timing of the plastic scintillator signal is read-out by time-to-digital converters. The signal of the BGO scintillator was energy calibrated by comparing measured cosmic rays with simulations using GEANT4 (ref. 29) and the CRY package30.

Antiproton annihilations originating from antihydrogen atoms hitting the crystal surface yield on average three charged pions with a mean momentum of ~300 MeV c−1 (ref. 31), which are detected by the plastic scintillators. In order to reduce background events from antiprotons annihilating upstream or from cosmic rays, a coincidence between the BGO and the plastic scintillators is required. Owing to the high multiplicity of the annihilation products, coincidence with at least two plastic scintillators can be used. According to our simulations this coincidence condition reduces the background event rate by three orders of magnitude while the signal count rate is only reduced by about a factor of 2. Using even higher multiplicities leads to an additional decrease of the signal by at least a factor of 3. Thus double coincidence events are used in the following analysis.

To investigate the principal quantum number of the antihydrogen atoms that reach the detector, voltages of either −400 or −2,000 V are applied to a set of field-ionization electrodes located in front of the BGO scintillator, which corresponds to average electric fields of 94 V cm−1 (n≳43) and 452 V cm−1 (n≳29), respectively (hereafter called scheme 1 and scheme 2). In other words, the antihydrogen atoms with n≲43 (scheme 1) or n≲29 (scheme 2) reach the BGO scintillator, while the antiprotons originating from field-ionized antihydrogen are repelled by the electric field.

The unshaded histogram bordered by the solid line in Fig. 3a shows the distribution of energy deposited in the BGO scintillator for double coincidence events up to 200 MeV for scheme 1. According to simulations, 200 MeV is the maximum energy deposited on the BGO scintillator when an antiproton annihilates on its surface. The shaded histogram shows results where antiprotons are trapped and cooled with electrons instead of positrons (referred to as a background run hereafter). The obtained energy spectrum represents events originating from cosmic rays and secondary particles, especially charged pions, which are produced by annihilation of antiprotons trapped in the nested well. The annihilation rate during the background run is expected to be comparable to the background annihilation rate of schemes 1 and 2. As in the low-energy region the annihilation cross section S is proportional to 1/ν (Langevin cross section), the annihilation rate, which is proportional to νS, does not depend on the antiproton energy, where ν is the relative velocity between the antiproton and a residual gas atom32. A GEANT4 simulation predicts that events due to antihydrogen annihilation along the vacuum tube upstream of the BGO scintillator are negligibly small. Figure 3a shows that the number of events with energy deposition above 40 MeV are significantly larger than those of the background run, that is, the threshold energy E th =40 MeV can be chosen to identify antihydrogen atoms annihilating on the BGO scintillator. The total accumulation times for the schemes 1 and 2, and a background run were 4,950, 2,100 and 1,550 s, respectively.