Search

High-momentum configurations of nucleon pairs at short-distance are probed using measurements of the $^{12}$C$(e,e'p)$ and $^{12}$C$(e,e'pN)$ reactions (where $N$ is either $n$ or $p$), at high-$Q^2$ and $x_B>1.1$. The data span a missing-momentum range of 300--1000 MeV/c and are predominantly sensitive to the transition region of the strong nuclear interaction from a Tensor to Scalar interaction. The data are well reproduced by theoretical calculations using the Generalized Contact Formalism with both chiral and phenomenological nucleon-nucleon ($NN$) interaction models. This agreement suggests that the measured high missing-momentum protons up to $1000$ MeV/c predominantly belong to short-ranged correlated (SRC) pairs. The measured $^{12}$C$(e,e'pN)$ / $^{12}$C$(e,e'p)$ and $^{12}$C$(e,e'pp)$ / $^{12}$C$(e,e'pn)$ cross-section ratios are consistent with a decrease in the fraction of proton-neutron SRC pairs and increase in the fraction of proton-proton SRC pairs with increasing missing momentum. This confirms the transition from an isospin-dependent tensor $NN$ interaction at $\sim 400$ MeV/c to an isospin-independent scalar interaction at high-momentum around $\sim 800$ MeV/c as predicted by theoretical calculation., Accepted for publication in Physics Letters B. 7 pages, 3 figures, and online supplementary materials

The antihydrogen program of the ASACUSA collaboration at the antiproton decelerator of CERN focuses on Rabi-type measurements of the ground-state hyperfine splitting of antihydrogen for a test of the combined Charge–Parity–Time symmetry. The spectroscopy apparatus consists of a microwave cavity to drive hyperfine transitions and a superconducting sextupole magnet for quantum state analysis via Stern–Gerlach separation. However, the small production rates of antihydrogen forestall comprehensive performance studies on the spectroscopy apparatus. For this purpose a hydrogen source and detector have been developed which in conjunction with ASACUSA’s hyperfine spectroscopy equipment form a complete Rabi experiment. We report on the formation of a cooled, polarized, and time modulated beam of atomic hydrogen and its detection using a quadrupole mass spectrometer and a lock-in amplification scheme. In addition key features of ASACUSA’s hyperfine spectroscopy apparatus are discussed. The antihydrogen programme of the ASACUSA collaboration at the antiproton decelerator of CERN focuses on Rabi-type measurements of the ground-state hyperfine splitting of antihydrogen for a test of the combined Charge-Parity-Time symmetry. The spectroscopy apparatus consists of a microwave cavity to drive hyperfine transitions and a superconducting sextupole magnet for quantum state analysis via Stern-Gerlach separation. However, the small production rates of antihydrogen forestall comprehensive performance studies on the spectroscopy apparatus. For this purpose a hydrogen source and detector have been developed which in conjunction with ASACUSA's hyperfine spectroscopy equipment form a complete Rabi experiment. We report on the formation of a cooled, polarized, and time modulated beam of atomic hydrogen and its detection using a quadrupole mass spectrometer and a lock-in amplification scheme. In addition key features of ASACUSA's hyperfine spectroscopy apparatus are discussed.t

In order to test CPT symmetry between antihydrogen and its counterpart hydrogen, the ASACUSA collaboration plans to perform high precision microwave spectroscopy of ground-state hyperfine splitting of antihydrogen atom in-flight. We have developed an apparatus (“cusp trap”) which consists of a superconducting anti-Helmholtz coil and multiple ring electrodes. For the preparation of slow antiprotons and positrons, Penning- Malmberg type traps were utilized. The spectrometer line was positioned downstream of the cusp trap. At the end of the beamline, an antihydrogen beam detector was located, which comprises an inorganic Bismuth Germanium Oxide (BGO) single-crystal scintillator housed in a vacuum duct and surrounding plastic scintillators. A significant fraction of antihydrogen atoms flowing out the cusp trap were detected.

The ASACUSA CUSP experiment plans a high precision spectroscopy of the ground-state hyperfine splitting of antihydrogen to test CPT symmetry. To perform this, a Rabi-like antiatomic beam method has been developed with a novel double-cusp trap. Antihydrogen atoms were synthesized in the double-cusp trap by directly injecting antiprotons from an accumulation trap into a positron plasma. By adjusting the energy difference between the incoming antiproton beam and the positron plasma, a high-rate production of antihydrogen atoms was observed., JPS Conference Proceedings, 18, Proceedings of the 12th International Conference on Low Energy Antiproton Physics (LEAP2016), ISBN:978-4-89027-125-2

We report the progress of the H¯ beam production experiment and recent developments of the double cusp trap to improve of the beam intensity of the H¯ atomic beams, the ASACUSA Micromegas tracker to monitoring the antihydrogen synthesis and the antihydrogen beam detector., JPS Conference Proceedings, 18, Proceedings of the 12th International Conference on Low Energy Antiproton Physics (LEAP2016), ISBN:978-4-89027-125-2

In this article, the first experimental evidence is presented for the reflection of a conspicuous fraction (∼ 30%) of a beam of low energy antiprotons by an aluminum wall. This rare evidence comes from the new analysis of a set of annihilations of antiprotons stopped in rarefied helium gas after hitting the end wall of the apparatus. These results, supported by Monte Carlo simulations, show that the interaction between the antiprotons at low energies ( $\lesssim 10$ keV) and the solid surface is dominated by Rutherford-type scattering on aluminum nuclei, and contradict the common belief according to which the interactions between matter and antimatter are dominated by the reciprocally destructive phenomenon of annihilation.

The study of the antiproton ( $\bar {p}$ ) annihilation cross section on nuclei at low energies (eV-MeV region) has implications for fundamental cosmology as well as for nuclear physics. Concerning the former, different models try to explain the matter/antimatter asymmetry in the universe assuming the existence of the so-called “islands” where antinucleon-nucleon annihilations occur in the border region (Cohen et al. Astrophys. J. 495, 539–549, 1998), while, from the nuclear physics point of view, the annihilation process is a valuable tool to evaluate the neutron/proton ratio in order to probe the external region of the nucleus (Gupta et al. Nucl. Phys. B 70(3), 414–424, 1974). The existing data of antinucleon-nucleon (or -nucleus) annihilation cross-sections are mainly confined to energies above ≈1 MeV, while the cross section measured at LEAR in the 80’s-90’s (mostly with light targets Agnello et al. Phys. Lett. B 256, 349–353, 1991; Bertin et al. Phys. Lett. B 369, 77–85, 1996; Bertin et al. Phys. Lett. B 414, 220–228, 1997; Zenoni et al. Phys. Lett. B 461, 405–412, 1999; Bianconi et al. Phys. Lett. B 481, 194–198, 2000; Bianconi et al. Phys. Lett. B 492, 254–258, 2000) showed an unexpected behaviour for energies below 1 MeV (Bianconi et al. Phys. Lett. B 483, 353–359, 2000; Bianconi et al. Phys. Rev. C 62, 014611-7, 2000; Batty et al. Nucl. Phys. A 689, 721–740, 2001). The results showed a saturation with the atomic mass number against the A 2/3 trend which is observed for higher energies (being A the target mass number). The ASACUSA collaboration at CERN recently measured antiproton annihilation cross section on different kinds of nuclei with a $\bar {p}$ kinetic energy of 5.3 MeV (Bianconi et al. Phys. Lett. B 704, 461–466, 2011; Corradini et al. Nucl. Instr. Methods A 711, 12–20, 2013). Such results proved compatibility with the black-disk model with the Coulomb correction. But till now experimental difficulties prevented the investigation at energies below ≈1 MeV. In 2012, the 100 keV region has been investigated for the first time (Aghai-Khozani et al. Eur. Phys. J. Plus 127, 125–128, 2012). We present here the first preliminary results of this experiment.

The aim of the ASACUSA-CUSP experiment at CERN is to produce a cold, polarised antihydrogen beam and perform a high precision measurement of the ground-state hyperfine transition frequency of the antihydrogen atom and compare it with that of the hydrogen atom using the same spectroscopic beam line. Towards this goal a significant step was successfully accomplished: synthesised antihydrogen atoms have been produced in a CUSP magnetic configuration and detected at the end of our spectrometer beam line in 2012 [1]. During a long shut down at CERN the ASACUSA-CUSP experiment had been renewed by introducing a new double-CUSP magnetic configuration and a new semi-cylindrical tracking detector (AMT) [2], and by improving the transport feature of low energy antiproton beams. The new tracking detector monitors the antihydrogen synthesis during the mixing cycle of antiprotons and positrons. In this work the latest results and improvements of the antihydrogen synthesis will be presented including highlights from the last beam time.

The ASACUSA collaboration of CERN has recently irradiated antiprotonic helium atoms with two counter-propagating laser beams. This excited some non-linear two-photon transitions of the antiproton at the deep UV wavelengths λ = 139.8-197.0 nm. The counterpropagating geometry of the laser beams reduced the thermal Doppler broadening of the observed resonances. Their narrow spectral lines allowed the measurement of three transition frequencies with fractional precisions of 2.3-5 parts in 10 9 . By comparing the results with three-body QED calculations, the antiproton-to-electron mass ratio was derived as 1836.1526736(23).

Exotic molecule tests fundamental symmetry Spectroscopy of exotic molecules can offer insight into fundamental physics. Hori et al. studied the transition frequencies of an unusual helium atom in which one of the two electrons was substituted by an antiproton, the negatively charged antiparticle partner of the proton (see the Perspective by Ubachs). The antiprotonic helium was cooled down to low temperatures to allow the frequencies to be measured with high precision. The extracted mass of the antiproton (relative to the electron mass) was in good agreement with previous measurements of the proton mass. This finding is in keeping with the implications of the combined charge, parity, and time-reversal symmetry of physical laws. Science , this issue p. 610 ; see also p. 546

We describe the instrumentation for an experiment to measure the cross sections of antiprotons with kinetic energies of 130±10 keV annihilating on carbon, palladium, and platinum target foils of sub-100 nm thicknesses. A 120 ns long pulsed beam containing 10 5 – 10 6 antiprotons was allowed to traverse the foils, and the signal annihilations that resulted from this were isolated using a time-of-flight method. Backgrounds arose from Rutherford scattering of the antiprotons off the target foils, their annihilations in the target chamber walls, and π → μ → e decay of the charged pions that emerged from the annihilations. Some antiprotons slowed down and annihilated in the contamination on the target surfaces. This reduced the signal-to-background ratio of the measurement.

We report the successful commissioning and testing of a dedicated field-ioniser chamber for measuring principal quantum number distributions in antihydrogen as part of the ASACUSA hyperfine spectroscopy apparatus. The new chamber is combined with a beam normalisation detector that consists of plastic scintillators and a retractable passivated implanted planar silicon (PIPS) detector., Comment: 5 pages, 2 figures, 6th International Symposium on Symmetries in Subatomic Physics (SSP2015)

The ASACUSA collaboration has developed a detector consisting of a large size BGO crystal to detect an atomic antihydrogen beam, and performed the direct detection of antihydrogen atoms. Energy spectra from antihydrogen annihilation on the BGO crystal are discussed in comparison to simulation results from the GEANT4 toolkit. Background mainly originating from cosmic rays were strongly suppressed by analyzing the energy deposited in the BGO and requiring a multiplicity of charged pions. Thus antihydrogen events were identified.

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.

Data on p¯ slowing down and capture in helium at 1 and 0.2 mb at room temperature are presented and compared to the corresponding previously publicated data in molecular hydrogen and deuterium. A Monte Carlo simulation containing a low-energy extrapolation of measured p¯electronic stopping power in helium gas,screened Rutherford collisions, and simple cascade mechanisms is able to reproduce the gross features of the data, but cannot explain some nontrivial details of the measured distributions.

none 46 The stopping power in helium from 1 keV kinetic energy is evaluated. Contrary to the effect observed around and below the maximum, Obelix data indicate a stopping power higher than that for proton, the difference being of the order of 15±5% at ≈700 keV. The result contributes to assert the fundamental difference between stoppings in the simplest gases (He, H2) and in solid targets below some MeV. none E. Lodi Rizzini; A. Bianconi; M.P. Bussa; M. Corradini; A. Donzella; M. Leali; L. Venturelli; N. Zurlo; M. Bargiotti; A. Bertin; M. Bruschi; M. Capponi; S. De Castro; R. Dona; L. Fabbri; P. Faccioli; B. Giacobbe; F. Grimaldi; I. Massa; M. Piccinini; N. Semprini-Cesari; R. Spighi; S. Vecchi; M. Villa; A. Vitale; A. Zoccoli; O.E. Gorchakov; G.B. Pontecorvo; A.M. Rozhdestvensky; V. Tretyak; M. Poli; C. Guaraldo; C. Petrascu; F. Balestra; L. Busso; O.Yu. Denisov; L. Ferrero; R. Garfagnini; A. Grasso; A. Maggiora; G. Piragino; F. Tosello; G. Zosi; G. Margagliotti; L. Santi; S. Tessaro E. Lodi Rizzini; A. Bianconi; M.P. Bussa; M. Corradini; A. Donzella; M. Leali; L. Venturelli; N. Zurlo; M. Bargiotti; A. Bertin; M. Bruschi; M. Capponi; S. De Castro; R. Dona; L. Fabbri; P. Faccioli; B. Giacobbe; F. Grimaldi; I. Massa; M. Piccinini; N. Semprini-Cesari; R. Spighi; S. Vecchi; M. Villa; A. Vitale; A. Zoccoli; O.E. Gorchakov; G.B. Pontecorvo; A.M. Rozhdestvensky; V. Tretyak; M. Poli; C. Guaraldo; C. Petrascu; F. Balestra; L. Busso; O.Yu. Denisov; L. Ferrero; R. Garfagnini; A. Grasso; A. Maggiora; G. Piragino; F. Tosello; G. Zosi; G. Margagliotti; L. Santi; S. Tessaro

π-4He interactions at 106MeV, with two secondary charged particles in the final state, have been studied using a self-shunted streamer chamber in a 0.65T magnetic field and equipped with two CCD videocameras for obtaining digitized stereo-images of nuclear events occurring within the fiducial volume of the streamer chamber. Branching ratios for the various interaction channels are deduced. Evidence is presented for a reaction channel involving a γ produced in the final state, as well as for the first observation of direct Δ- -resonance production. Cross-section distributions for the elastic and quasi-elastic interaction channels are also reported.

T. Kobayashi1 †, D. Barna1,2, R. S. Hayano1, Y. Murakami1, K. Todoroki1, H. Yamada1, A. Dax3, L. Venturelli4,5, N. Zurlo4,5, D. Horvath2,6, H. Aghai-Khozani7,8, A. Soter7, and M. Hori7,1 1Department of Physics, University of Tokyo, Hongo, Bunkyo-ku 113-0033, Japan 2Institute for Particle and Nuclear Physics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, PO Box 49, Budapest, 1525, Hungary 3Paul Scherrer Institute, 5232 Villigen-PSI, Switzerland 4Dipartimento di Ingegneria dell Informazione, Universita di Brescia, I-25123 Brescia, Italy 5INFN, Gruppo Collegato di Brescia, I-25133 Brescia, Italy 6Institute of Nuclear Research, H-4001 Debrecen, Hungary 7Max-Planck-Institut fur Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748 Garching, Germany 8CERN, CH-1211 Geneva 23, Switzerland

We have been developing ground-state antihydrogen atomic beams to test CPT symmetry via in-flight hyperfine spectroscopy. A new antihydrogen beam detector has been developed. The overview of the experiment, the detail of the detector and latest results will be presented.

The first observation of in-flight antiproton-nucleus annihilation at ∼130 keV obtained with the ASACUSA detector has demonstrated that the measurement of the cross section of the process is feasible at such extremely low energies Aghai-Khozani, H., et al., Eur. Phys. J. Plus 127, 55 (2012). Here we present the results of the data analysis with the evaluations of the antiproton annihilation cross sections on carbon, palladium and platinum targets at ∼125 keV.

The ASACUSA (Atomic Spectroscopy and Collisions Using Slow Antipro- tons) 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.

The ASACUSA collaboration of CERN has recently carried out two-photon laser spectroscopy of antiprotonic helium atoms. Three transition frequencies were de- termined with fractional precisions of 2.3-5 parts in 10 9 . By comparing the results with three-body QED calculations, the antiproton-to-electron mass ratio was determined as 1836.1526736(23).

Up to now it has been observed that a fraction of about 3% of the p 's stopped in a dense He target survives for a surprisingly long time (tens of microseconds). As suggested by Condo in 1964 the long lifetime of p 's in He is explained by the existence of metastable states of the exotic atom p He + . An analysis of data from p 's stopped in He targets at 3 bar pressure by the Obelix apparatus at the LEAR decelerator at CERN is presented. We do not observe any significative amount of “prompt” annihilations (few nanoseconds) after the p 's capture.

The annihilation frequencies of 19 p p annihilation reactions at rest obtained in different target densities are analysed in order to determine the values of the P -wave annihilation percentage at each target density and the average hadronic branching ratios from P - and S -states. Both the assumptions of linear dependence of the annihilation frequencies on the P -wave annihilation percentage of the protonium state and the approach with the enhancement factors of Batty are considered. Furthermore the cases of incompatible measurements are discussed.

We report here experimental evidence of the reflection of a large fraction of a beam of low energy antiprotons by an aluminum wall. This derives from the analysis of a set of annihilations of antiprotons that come to rest in rarefied helium gas after hitting the end wall of the apparatus. A Monte Carlo simulation of the antiproton path in aluminum indicates that the observed reflection occurs primarily via a multiple Rutherford-style scattering on Al nuclei, at least in the energy range 1-10 keV where the phenomenon is most visible in the analyzed data. These results contradict the common belief according to which the interactions between matter and antimatter are dominated by the reciprocally destructive phenomenon of annihilation., 5 pages with 5 figures

The resonance transition $(n,l)=(40,36)\to(41,35)$ of the antiprotonic helium ($\overline{\mathrm{p}}^{4}$He$^{+}$) isotope at a wavelength of 1154.9 nm was detected by laser spectroscopy. The population of $\overline{\mathrm{p}}^{4}$He$^{+}$ occupying the resonance parent state $(40,36)$ was found to decay at a rate of $0.45\pm0.04$ $\mu$s$^{-1}$, which agreed with the theoretical radiative rate of this state. This implied that very few long-lived $\overline{\mathrm{p}}^{4}$He$^{+}$ are formed in the higher-lying states with principal quantum number $n\ge41$, in agreement with the results of previous experiments.

The p Ne annihilation cross section is measured for the first time in the momentum interval (53÷63) MeV/c. About 9000 pictures collected by the Streamer Chamber Collaboration (PS179) at LEAR-CERN have been scanned. Four events are found, corresponding to σ ann =2210±1105 mb. The result is compared to the set of measurements presently available in the region of low p momentum.

The p 3He annihilation cross section is measured for the first time in the momentum interval (50÷60) MeV/c. About 9000 pictures collected by the Streamer Chamber Collaboration (PS179) at LEAR–CERN have been scanned. Six events are found, corresponding to σann=1850±700 mb. The result is compared to the set of measurements presently available in the region of low p momentum.

The hadronic annihilation branching-ratios in quasi-two-body final states have been obtained from the observation of the reactions pbarp→π+π−π0, K+K−π0,KπK0 at rest in hydrogen targets at different densities. The enhancement or suppression of specific hadronic channels connected to dynamical selection rules is observed in the production of different resonances both in protonium S and P–waves, systematically investigated for the first time. Besides the well known ρ(770)π and a2(1320)π enhancement from 3S1 and 1S0 partial waves, the dominance of one isospin source in K*(892)Kbar production, well established from S–waves, is confirmed also in P–waves (1S0, I = 0; 3S1, I = 1; 1P1, I = 0; 3P1, I = 0; 3P2, I = 1). In addition, the experimental data clearly show a strong suppression of φ(1020)π and a0(980)π final states from P–wave which has a remarkable coincidence with K*(892)Kbar pattern production.

We measured the in flight annihilation frequencies and cross sections of reactions n p → π + π 0 ,π + η and K+KS in the antineutron momentum range between 50 and 400 MeV/c. The annihilation frequencies of these channels from the different allowed initial states were calculated and some information about the n p annihilation dynamics were obtained. The first determination of the D-wave contribution in this momentum range was also obtained.

The partial wave analysis of the reaction p p → π + π − π 0 at rest has been performed for the first time by using high-statistics data sets collected in hydrogen targets at three different densities. The different mixtures of partial waves corresponding to our samples allow a reliable determination of each contribution. This technique is crucial in the identification of the exotic candidate f2(1565) which can be produced by 1S0, 3P1 and 3P2 initial states. The amplitude analysis was performed in the frame of the K-Matrix and P-Vector approach. It requires presence of the exotic candidates f0(1500) and f2(1565) with the following masses and total widths: (1449 ± 20)MeV, (114 ± 30)MeV and (1507 ± 15)MeV, (130 ± 20)MeV respectively. In addition to f0(980), f0(1300) f2(1270) and ϱ(770), a clear evidence of I = 1, JP = 1− signal, which we identify with ϱ(1450), is obtained.

New measurements of the pp total annihilation cross-section at very low momenta (around 50 MeV/c) confirm the previous results obtained by the OBELIX collaboration. The measured values are in general agreement with a scattering length approximation of the annihilation cross-section where the pp scattering lengths are obtained from measurement of the protonium shifts and widths and which accounts for the Coulomb interaction. Partial annihilation cross-sections and annihilation frequencies for some channels have been measured too. The values of the total and partial cross sections at the lowest p momentum show an unexpected behaviour

The study of the I = 1, 0 ++ states with the OBELIX detector in the channel pp → K ±K 0 sπ ∓ at three different target densities is reported. We discuss in particular the results about a o (980) and the evidence of an extra scalar state with mass 1.29 ± 0.01 GeV/c 2 and width 0.080 ± 0.005 GeV/c 2

We report in this paper the results of the study of the ƒ 0 (1500)/ƒ 2 (1565) resonances in the in-flight annihilation reaction np → 2 π + π − , taking into account a possible dependence on the n momentum.

We report our latest results from our two-photon spectroscopy of antiprotonic helium (a bound state of a helium nucleus, an electron and an antiproton). By measuring three transition frequencies with a precision of 2.3−5×10−9 and comparing these to 3-body QED calculations, we derived the antiproton-electron mass ratio 1,836.1526736(23),which agrees with the proton-electron mass ratio (known to a three-times better precision) within the errors. Since the (anti)proton's atomic mass is known to a better precision than that of the electron, assuming mp = mp this result can be used to determine the electron's atomic mass as me=0.0005485799091(7) u.

Recently, the production of low energy anti-hydrogen atoms in the cusp trap with the use of 150eV antiproton beams from the MUSASHI trap was reported. The purpose of producing low energy anti-hydrogen atoms in a cusped magnetic field is to extract a polarized anti-hydrogen beam to a field free region, so that the hyperfine structure of anti-hydrogen atoms can be measured.

We describe an experiment performed in the ATHENA apparatus in which there is evidence that the antiproton-proton bound state, protonium, has been produced at very low energies in vacuum following the interaction of cold antiprotons with a trapped cloud of molecular hydrogen ions. The latter were confined in a centrifugally separated belt outside a positron plasma used for antihydrogen formation. Studies have been performed at low positron plasma temperatures in which the protonium annihilation signal has been identified along with that from antihydrogen, and we discuss how their contributions can be disentangled. With the positron plasma heated to around 10,000 K the ions become distributed in the positrons, and the majority of the annihilation signal can be explained in terms of protonium formationion, as antihydrogen creation is heavily suppressed. In this case we compare the observed protonium formation rate with expectations from theory and find reasonable accord, when experimental systematics are taken into account. The effect on the annihilation signals of the passage of an electron current through a pre-loaded positron plasma has been studied in detail, and the results are presented here for the first time.

We report stopping powers of hydrogen and helium for antiprotons of kinetic energies ranging from about 0.5 keV to 1.1 MeV. The Barkas effect, i.e., a difference in the stopping power for antiprotons and protons of the same energy in the same material, shows up clearly in either of the gases. Moreover, below [approx]0.5 keV there is indirect evidence for an increase of the antiproton stopping power. This nuclear'' effect, i.e., energy losses in quasimolecular interactions, shows up in fair agreement with theoretical predictions.

Experimental data on p slowing down at very low energy in a low density (0.2 mbar pressure) Xe target evidence a very low capture cross section with respect to He, H 2 and D 2 in agreement with specific phenomenological previsions by Briggs et al. [Hyperfine Interact. 119 (1999) 235; J. Phys. B 32 (1999) 197].

ASACUSA collaboration has been making a path to realize high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atom in flight for stringent test of the CPT symmetry. Recently, we have succeeded in synthesizing our first cold antihydrogen atoms employing a CUSP trap. It is expected that synthesized antihydrogen atoms in the low-field-seeking states are preferentially focused along the cusp magnetic field axis whereas those in the high-field-seeking states are not focused, resulting in the formation of a spin-polarized antihydrogen beam. We report the recent results of antihydrogen atom synthesis and beam production developed with the CUSP trap.

ASACUSA collaboration has been making a path to realize high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atom in flight for stringent test of the CPT symmetry. Recently, we have succeeded in synthesizing our first cold antihydrogen atoms employing a CUSP trap. It is expected that synthesized antihydrogen atoms in the low-field-seeking states are preferentially focused along the cusp magnetic field axis whereas those in the high-field-seeking states are not focused, resulting in the formation of a spin-polarized antihydrogen beam. We report the recent results of antihydrogen atom synthesis and beam production developed with the CUSP trap.

A study of\(\bar np\) in-flight annihilations is presented. The 4π invariant mass spectrum resulting from the analysis of the five-prong exclusive events, following the annihilation of ∼ (100–297) MeV/c\(\bar n\), reveals, besides a large structure centred at ∼ 1500 MeV/c2, some evidences for a new state around 1660 MeV/c2.

The annihilation frequency of the reaction p p → π 0 π 0 has been measured, for the first time by stopping antiprotons in a NTP hydrogen target. The obtained result ƒ(π 0 π 0 ) NTP = (12.7±0.5±2.0)×10 −4 is discussed in the frame of the study of the initial angular momentum of the pp system.

We report the first measurement of the K+K−, π+π− frequencies for p annihilations at rest in a gaseous H2 target at low pressure (5 mbar). We have obtained: fππ = (4.26±0.11)×10−3, fKK = (0.46±0.03)×10−3. The ratio R = fKK/fππ has also been measured at a pressure of 2 mbar. Comparison with previous measurements shows that fKK decreases monotonically with the target density but its value at 5 mbar is still higher than the value with L X-ray in coincidence.

Physical laws are believed to be invariant under the combined transformations of charge, parity and time reversal (CPT symmetry). This implies that an antimatter particle has exactly the same mass and absolute value of charge as its particle counterpart. Metastable antiprotonic helium ($\bar{p}{\rm He}^+$) is a three-body atom consisting of a normal helium nucleus, an electron in its ground state and an antiproton ($\bar{p}$) occupying a Rydberg state with high principal and angular momentum quantum numbers, respectively $n$ and $\ell$, such that $n\sim\ell\sim 38$. These atoms are amenable to precision laser spectroscopy, the results of which can in principle be used to determine the antiproton-to-electron mass ratio and to constrain the equality between the antiproton and proton charges and masses. Here we report two-photon spectroscopy of antiprotonic helium, in which $\bar{p}{\rm ^3He^+}$ and $\bar{p}{\rm ^4He^+}$ isotopes are irradiated by two counter-propagating laser beams. This excites nonlinear, two-photon transitions of the antiproton of the type $(n,\ell)\rightarrow (n-2,\ell-2)$ at deep-ultraviolet wavelengths ($\lambda$=139.8, 193.0 and 197.0nm), which partly cancel the Doppler broadening of the laser resonance caused by the thermal motion of the atoms. The resulting narrow spectral lines allowed us to measure three transition frequencies with fractional precisions of 2.3-5 parts in $10^9$. By comparing the results with three-body quantum electrodynamics calculations, we derived an antiproton-to-electron mass ratio of 1,836.1526736(23), where the parenthetical error represents one standard deviation. This agrees with the proton-to-electron value known to a similar precision.

A semi-empirical model is presented for the excitation of collective nuclear resonances induced by pions on nuclei in the Δ(1232) energy region. The physical features of the collective states have been extracted according to the model by fitting data from a collection of resonant π-nucleus elastic scattering cross-sections. The contributions to the total binding energy and to the width of a collective state per each additional nucleon involved in the resonance have been extracted. The interaction between the collective resonances and the surrounding nuclear medium is discussed.

Measurement of the in-medium modifications of the Δ resonance, signatures of the excitation of nuclear collective states and the first experimental evidence for a thermal emission of photons have been obtained from the analysis of new π±4He data at Tπ = 106 MeV at PAINUC experiment.

The analysis of new data on π±4 He interaction at T π =106 MeV provided information on the in-medium modifications of the Δ resonance and revealed signatures of the excitation of a nuclear collective state. Evidences for a reaction channel with thermal emission of photons are also presented.

The stopping power of H, for p S was measured in the p energy interval 1 keV + 120 keV using a low density gas target ( p pρ ≈ l0 −3 + 10 −2 ÷ pρ being the Hz density at STP) with the OBELIX spectrometer at the LEAR of CEBN. Furthermore, the formation of metastable states in the p 4 He interaction at rest, having a mean life of the order of 3 μs, was investigated at six target gas pressures (3 atm and 400,250,200,150,50 mbar). The influence of contaminants (like Ha, Ne, Ar, N2 and Xe) on the life of such metastable states was examined at the target gas pressure of 3 atm.

We report here the first successful synthesis of cold antihydrogen atoms employing a cusp trap, which consists of a superconducting anti-Helmholtz coil and a stack of multiple ring electrodes. This success opens a new path to make a stringent test of the CPT symmetry via high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atoms.